HSE
Health & Safety
Executive
Novel mobile and portable methods
for detecting rock failure
Prepared by Rock Mechanics Technology Ltd
and the University of Exeter for the
Health and Safety Executive 2004
RESEARCH REPORT 248
HSE
Health & Safety
Executive
Novel mobile and portable methods
for detecting rock failure
David Bigby BSc. (Hons.), PhD., MIMMM CEng
Alan Bloor BSc. (Hons.), PhD.
Rock Mechanics Technology Ltd
Bretby Business Park, Ashby Road
Burton-on-Trent, Staffordshire, DE15 0QD
Chris Chester BSc. (Hons.), MSc., MCSM
Camborne School of Mines, University of Exeter
Redruth, Cornwall, TR15 3SE
At the outset of the Project, there was no portable non-contacting technique available for determining the
condition of a mine tunnel roof. Such a technique would be of great benefit to Health and Safety in the mining
industry as it could allow unsafe roof to be identified without the investigator having to be positioned within the
hazardous area. A suitable technique could be of particular use in the cut-out area of a tunnel excavation, prior
to support placement, and could facilitate dynamic decision making on the timing of support placement.
The Project investigated a number of physical phenomena, such as acoustic response, ultrasound emissions,
electromagnetic emissions and thermal imaging which were considered to have a potential to provide warning of
failing roof rock, to indicate when initial or additional support placement might be required. A practical instrument
utilising one of these phenomena would have to be capable of machine mounting or remote operation. Initial
tests using these techniques had already produced encouraging results.
The research indicated that the detection of emissions, ultrasonic, acoustic or electromagnetic, as an indication
of microfracturing and the imminent failure of a rock mass, has a number of inherent problems relating to its use
in a mining environment, in particular problems of filtering background noise.
The Project went on to examine how more conventional means of rock failure detection could be modified to
become remote methods. This involved considering the different responses of a rock mass, which had failed,
but not detached completely from the surrounding mass, to exterior stimuli such as vibration or a thermal
gradient.
It was concluded that the most promising principles for development of a mobile or portable, non-intrusive
instrument for detecting failing and/or failed rock in a working mining environment are; measurement of induced
vibration using a laser vibrometer or similar device and detection of an induced thermal gradient using a thermal
imaging system. However both principles require further research and development before a practical tool can
be developed.
This report and the work it describes was jointly funded by the Health and Safety Executive (HSE), the
European Coal and Steel Community (ECSC) and several mine operators. Its. Its contents, including any
opinions and/or conclusions expressed, are those of the authors alone and do not necessarily reflect HSE
policy.
HSE BOOKS
© Crown copyright 2004
First published 2004
ISBN 0 7176 2866 3
All rights reserved. No part of this publication may be
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Applications for reproduction should be made in writing to:
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ii
CONTENTS
EXECUTIVE SUMMARY
1
1.
INTRODUCTION
1.1 Background to Project
1.2 Aims
1.3 Objectives
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5
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6
2.
LITERATURE REVIEW
2.1 Electromagnetic Emissions
2.2 Thermal Response
2.3 Ultrasonic / Acoustic Emissions
2.4 Induced Vibration
2.5 Laser Displacement Measurements
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7
9
13
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3.
LABORATORY WORK
3.1 Introduction
3.2 Electromagnetic Emissions
3.3 Ultrasonic Emissions
3.4 Thermal Response
3.5 Laser Vibrometer
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4.
FIELD TRIALS
4.1 Introduction
4.2 Thermal Response Trials
4.3 Laser Vibrometer Trials
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5.
CONCLUSIONS
5.1 Ultrasonic / Acoustic Emissions
5.2 Electromagnetic Emissions
5.3 Thermal Response
5.4 Laser Vibrometer
5.5 Overall Conclusions
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35
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36
37
6.
RECOMMENDATIONS
39
7.
REFERENCES
41
8.
FIGURES
43
iii
LIST OF TABLES Table 1
Table 2
Results of 2-D slab modelling calculations, after Kononov 2000.
Potential ‘hard’ applications of the AEM with assigned confidence
levels, after Piper et al, 2002.
iv
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18
LIST OF FIGURES
Figure 1 Figure 2
Figure 3 Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Figure 9
Figure 10
Figure 11
Figure 12
Figure 13
Figure 14
Examples of Data Collected From Underground Experiments,
(a) Shows a Clear Electromagnetic Anomaly Prior to a Seismic
Event,
(b) Shows No Clear Anomaly Prior, During or After a Seismic
Event, Possibly Due to the Level of Background EMR
Temperature Profile In A Simulated Slab (1-d Fin Model) After
Kononov, 2000
Plotted b-values from the Underground Research Laboratory,
Canada
(a) During a Seismically Quiet Period, and
(b) a Seismically Active Period
Acoustic Energy Meter.
(a) Polytec Portable Digital Vibrometer (PDV)
(b) Schematic Representation Of A Michelson-Morley
Interferometer.
(c) Schematic Drawing Of A Heterodyne Interferometer, With
Bragg Cell.
(a) Schematic Of Instrumentation Configuration
(b) Photograph Of Instrumentation And Rock Press.
Screen Dumps Showing the Measurements of Background Noise
Seen Inside the Faraday Shield
Reduction in Background Noise Achieved Using the RC Filters.
(a) Schematic Representation Of The Instrumentation
Configuration, After Modification
(b) Photograph Of Instrumentation And Rock Press.
(a) Modifications Made to Thermal Test Blocks, Position of Four
Thermistors Embedded Into Slab,
(b) Enlarged Image of Embedded Thermistor
Thermal Response of Loose and Solid Test Blocks to a Period of
Heating Followed by a Period of Cooling.
Laboratory Thermal Response Trials, Using Liquid CO2, to Lower
the Temperature of Simulated Failed and Intact Rock Mass.
Computer Simulation of the Thermal Response of Loose and Solid
Rock Mass to Cooling After an Initial High Temperature (Natural),
Computer Simulation of the Thermal Response of Loose and Solid Rock
Mass Heated Then Subsequently Allowed to Cool (Enhancement).
Figure 15
Figure 16
Vibrometer Trace From Simulated Loose and Solid Rock Mass,
Vibrometer Trace Parameter Plotted Against Acoustic Energy
Meter (AEM) Value
Figure 17 Surface Temperature Profiles of a Pillar at a Dimension Stone
Mine, Near Bath.
(a) Shows the Surface Temperature Immediately After a Heating
Period of Four Minutes.
(b), the Surface Temperature Difference After Four Minutes of
Cooling
(c), the Surface Temperature Difference After 12 Minutes of
Cooling.
Figure 18 Acoustic Energy Meter Value Plot For The Same Pillar. Failed
Rock Mass Represented By The High AEM Values.
Figure 19 (a), Approximate Roadway Profile (Not to Scale),
v
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45
46
47
48
49
50
50
51
52
53
54
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55
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56
57
58
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59
Figure 20
Figure 21
Figure 22
Figure 23
Figure 24
(b), Temperature Variation Across Roof Profile.
Vibrometer Trace Parameter Value Plotted Against AEM Value,
From the Dimension Stone Mine
Vibrometer Trace for Failed and Intact Rock From the Dimension
Stone Mine.
Vibrometer Traces for One Target Excited From Different
Locations at Derbyshire Limestone Mine.
Algorithm Value Versus AEM Values For Different Separations
From Limestone Stone Mine.
Algorithm Value Versus AEM Values For Different Separations
From Dimension Stone Mine.
vi
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60
61
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62
EXECUTIVE SUMMARY
This 3 year HSE research project, 4273/R33.097, entitled “Novel Mobile and Portable Methods
for Detecting Rock Failure” was co-funded by a European Coal and Steel Community funded
RTD Project, 7220-PR092, entitled “Advanced Geotechnical Instrumentation for Detecting
Rock Failure and Monitoring Support Loads”. The HSE Project commenced on 1st April 2001
and was completed on 31 March 2004. The HSE Project was undertaken by Rock Mechanics
Technology Ltd of Burton on Trent, Staffs. The ECSC Project was undertaken by 5 partners
drawn from 4 Western European coal mining countries (France, Germany, Spain and the UK).
Some of the work reported here was undertaken in collaboration with Camborne School of
Mines, Exeter University, under an Associate Contract to the ECSC Project.
At the outset of the Project, there was portable non-contacting technique available for
determining the condition of a mine tunnel roof. Such a technique would be of great benefit to
Health and Safety in the mining industry as it could allow unsafe roof to be identified without
the investigator having to be positioned within the hazardous area. A suitable technique could
be of particular use in the cut-out area of a tunnel excavation, prior to support placement and
could facilitate dynamic decision making on the timing of support placement.
There were a number of physical phenomena, such as acoustic response, ultrasound emissions,
electromagnetic emissions and thermal imaging that were considered to have a potential to
provide warning of failing roof rock, to indicate when initial or additional support placement
might be required. A practical instrument utilising one of these phenomena would have to be
capable of machine mounting or remote operation. Initial tests using these techniques had
already produced encouraging results.
The research indicated that the detection of emissions, ultrasonic, acoustic or electromagnetic,
as an indication of microfracturing and the imminent failure of a rock mass, has a number of
inherent problems relating to its use in a mining environment. Emission strength is a key
problem. When background noise is also considered, the problem of detection becomes even
more difficult. In a working underground environment there are a number of sources of
acoustic, ultrasonic and electromagnetic emissions, which create significant background noise.
However, the detection of such emissions, especially acoustic and to some extent
electromagnetic, as an indication of failing rock mass, is important for research purposes.
Under laboratory conditions, where variables such as background noise and failure rate can be
controlled, such emissions can provide a great deal of information about the failure process.
Due to these inherent problems, the researchers went on to examine how more conventional
means of rock failure detection could be modified to become remote methods. This involved
considering the different responses of a rock mass, which had failed, but not detached
completely from the surrounding mass, to exterior stimuli such as vibration or a thermal
gradient.
Technology and experience already gained during the development of the Acoustic Energy
Meter, a contact device capable of quantifying the integrity of a surface, were applied. Research
was conducted into the transient vibrational characteristics of failed rock, and how this could be
stimulated and measured remotely. Using a laser vibrometer, measurements of transient
vibration were taken from both intact and failed rock mass in the laboratory and underground.
Results showed that, with sufficient vibration, distinguishing characteristics could be seen
between the two rock mass classes. Mathematical algorithms applied to the results, could
1
quantify the integrity of the surface under investigation and give an indication of its potential to
cause harm.
Further work on induced vibration is recommended in the following areas: x Safe and mine worthy techniques for the delivery of sufficient energy to a surface under
investigation should be the main focus due to its critical role in the whole concept.
Without sufficient vibration, measurements are inaccurate and unreliable, hence the
requirement for extra research.
x Increasing the return signal strength is also important. Without a good return signal the
measurements are inaccurate and unreliable. During the field trials, reflective tape was
placed on the target surface; this however falls short of a wholly remote method. It is
recommended that further work should examine the potential of the paintball marker,
initially used unsuccessfully for the delivery of vibrational energy, as a method of
placing reflective paint onto the target surface.
The thermal response of intact and loose rock mass to two different temperature environments,
either naturally occurring or enhanced by man in the underground environment was also
researched. Initial laboratory work demonstrated that by heating a surface, simulated loose rock
mass could be distinguished from solid rock mass by its different thermal response. Field trials,
however, failed to provide similar positive results, with only minor success in identifying loose
rock mass seen at one location in an evaporate mine. It is the authors’ opinion that this remains
a viable concept and requires further research, and the following further work is recommended:
x In the authors’ opinion the concept is most suited to deep excavations where virgin rock
temperatures are high, hence research should be focused here, rather than shallow
excavations where more energy is required to develop a thermal gradient. Work should
be conducted into ways in which normal mining practices can be modified in order to
enhance a temperature difference to detectable levels. The following are some
suggestions of how a temperature difference could be enhanced between intact and
loose rock mass: ƒ Increasing ventilated airflow in a development for a few hours prior to a survey.
ƒ Use of an auxiliary cooling fan to lower ventilated air temperature in a
development prior to a survey.
ƒ Stop ventilating a development for an over night period, causing the rock mass to
reach an equilibrium closer to virgin rock temperature. After the over night
period, restart the ventilation, whilst surveying the area with thermal imaging
equipment.
ƒ Spraying hot rock with cooled water. This could be particularly successful in
deep, hot mining environments where cooling water is often already available for
other purposes.
In summary, it was concluded that the most promising principles for development of a mobile or
portable, non-intrusive instrument for detecting failing and/or failed rock in a working coal
mining environment are; measurement of induced vibration using a laser vibrometer or similar
device and detection of an induced thermal gradient using a thermal imaging system. Both
principles require further research and development before a practical tool can be developed.
The areas requiring most attention are;
x a remote means to impart sufficient vibrational energy into the rock,
2
x a means of improving the return signal strength to a laser vibrometer in a mining
environment,
x a means of producing a sufficient thermal gradient; this is most likely to be achieved in
a deep mining environment
3
4
1 INTRODUCTION
1.1 BACKGROUND TO PROJECT
This 3 year HSE research project, 4273/R33.097, entitled “Novel Mobile and Portable Methods
for Detecting Rock Failure” was co-funded by a European Coal and Steel Community funded
RTD Project, 7220-PR092, entitled “Advanced Geotechnical Instrumentation for Detecting
Rock Failure and Monitoring Support Loads”. The HSE Project commenced on 1st April 2001
and was completed on 31 March 2004. The HSE Project was undertaken by Rock Mechanics
Technology Ltd of Burton on Trent, Staffs. The ECSC Project was undertaken by 5 partners
drawn from 4 Western European coal mining countries (France, Germany, Spain and the UK).
Some of the work reported here was undertaken in collaboration with Camborne School of
Mines, Exeter University, under an Associate Contract to the ECSC Project.
The ability to detect/predict the onset of rock mass failure has been the goal for a considerable
number of geotechnical researchers over the past decades, with earthquake prediction being the
main driving force behind the research. Such research has produced a number of novel ways in
which to detect rock failure, all with different degrees of success, and with none being 100%
accurate. In recent years the same methods have been developed for use in the mining
environment, with the aim of predicting / detecting mining induced, rock mass failure.
Seismic techniques first developed for earthquake prediction, have seen the most use in the
mining environment, with a number of deep, hard rock mines using the technique as a method
of identifying seismically “quiet” periods. These periods are often seen as fore runners to a
rockburst or bump, a dangerous hazard in deep seated mines.
In mining terms the ability accurately to detect failed or failing rock mass would have two
significant advantages; firstly safety. Areas where rock mass failure has occurred, or is
currently occurring, could be identified and the correct action taken before an accident, or
fatality occurs. The second advantage would come from increased production, a result of less
“down time” caused by falls of ground and other rock mass failure problems, and the accidents
and fatalities which accompany them.
Devices are currently available which can give an indication of the integrity of the rock mass.
They are however often crude and rely heavily on the operator’s experience and judgement.
These available devices also require direct contact with the surface under investigation, a
situation, which puts the operator at increased risk. An example of such a device is the scaling
or sounding bar, used to sound an excavation in search of loose rock, often after a round of
explosives has been fired. Such a technique is subjective as it is not able to quantify the
integrity of a surface, leaving the decision as to the level of risk to the operator. A device
capable of detecting failed or failing rock mass and quantifying its integrity from a remote
location, would remove such risks, making the underground environment a safer more
productive place.
1.2 AIMS
The primary aim of the work undertaken under this Report was to develop a portable or semiportable means of detecting areas of rock mass failure in a mine, by using measurements of
emissions and/or responses to certain stimuli that failed or failing rock mass might display. The
technique(s) developed, would be capable of detecting failed rock mass from a remote location,
and not require entry into an area under investigation by the operator.
5
A secondary aim was the design and manufacture of a product capable of fulfilling the primary
aim, for economic, commercial application in the mining and extractive industries. This had to
be considered when researching and developing concepts and techniques as it imposes a number
of constraints on any potential device. Consideration was also be given to the following when
assessing possible devices and their operation.
x
x
x
x
x
x
Intrinsic safety
Mine worthiness, (robust, capable of handling extreme environments)
Useable during normal mining activities Portability
Potential for use as a survey tool and / or as a continuous monitoring device Short survey time.
1.3 OBJECTIVES
The objectives were as follows:
x Complete an extensive literature survey into research undertaken by other authors in the
field of rock mass failure prediction / detection. In particular, examine work focused
towards the mining and extractive industries and the problem of rock mass failure found
in underground environments
x Using the knowledge gained from the literature review and from the experience held by
the Partners, identify and acquire sensory devices and instrumentation, which showed
potential of fulfilling the primary aim.
x Undertake laboratory trials of the selected prototype devices on simulated rock mass
conditions, and make appropriate modifications to the devices and operating procedures
if required. The laboratory experiments must simulate underground conditions as
closely as possible.
x After assessing the results of the laboratory experiments, undertake field trials of the
device(s) which demonstrated the most potential for fulfilling the stated aims. It may be
necessary to use laboratory facilities to further develop the operating procedures for the
devices under investigation before their trial in an underground environment.
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2 LITERATURE REVIEW
A great deal of research has been undertaken into how rock mass characteristics change during
the failure process, with most research looking at the emissions and responses produced during
and after failure. In recent times this research has advanced to another level, with the progress
in computer technology and sensory devices enabling vast quantities of data to be collected
during the failure process. From such data a better understanding of what occurs during the
failure process has been achieved with much of what has been learnt used in the construction of
increasingly complex computer modelling software. Sensory devices have also improved, not
only in their range but also their sensitivity and size, enabling easy placement around test
samples, and accurate measurements from around their circumferences.
This Chapter provides an overview of work undertaken by other researchers in this field in
recent times, with emphasis on work directed towards the mining industry and the
environmental conditions therein.
2.1 ELECTROMAGNETIC EMISSIONS
Also known as electromagnetic radiation (EMR), the first observation of electromagnetic
emissions from fractured material under stress was made in 1933 (Urusovskaja, 1969). Since
then a number of authors have investigated the emissions from different materials under
laboratory conditions and have witnessed high counts of EMR readings prior to failure. The
exact origin of electromagnetic emission produced during the fracture process is not truly
understood, with a number of different theories being proposed. The most accepted theory
among them states that the fracturing of atomic bonds during the failure process is the likely
cause of the emissions. The theory is substantiated by the mirroring of electromagnetic
emissions by acoustic emissions, whose origin has long been known to be micro fracturing
during the failure process.
The frequency range over which emissions have been witnessed from failing rock mass is
between 1 kHz and 10 MHz with wavelengths from 30m to +300 km. Cress et al (1987) found
that maximum EMR occurred over the frequency range 0.5 - 1 MHz, during their studies using
rock samples and a loading frame to induce failure. Earlier research by Hanson et al (1981) had
detected EMR during the catastrophic failure of quartz rich rocks. A more important finding
from Hanson et al’s research was that fracture size is directly proportional to the amplitude of
the emissions. This finding prompted Hanson to state that “the most important application for
electromagnetic emissions observation is the monitoring of unstable rock faces in mines”,
(Hanson, 1981). He went on to say that a portable detection system, which would not require
direct contact to the rock surface like geophones, could be produced for use in mining
situations. The device would simply detect increases in the number or amplitude of EMR
events and warn of imminent rock failure.
While studying solar activity using an array of world-wide radio receivers for cosmic radio
noise, Warwick et al., 1982, witnessed large, strange, fluctuating signals, which lasted 20
minutes on the 16th May 1960. A terrestrial source for the emissions was deemed the most
probable origin due to the emissions being some magnitudes larger than normal background
solar emissions. Five days later on the 21st May 1960 a series of earthquakes struck along the
Chilean fault zone, devastating large areas of Chile. Warwick realised the emissions he had
witnessed five days earlier, could be the result of stress-induced micro-fracturing of quartz
bearing rocks along the Chilean fault prior to mass failure and the subsequent earthquake.
Although the direct evidence obtained by Warwick falls somewhat short of identifying the
7
Chilean fault as the source of the emissions, it was possible to calculate that the source would be
of similar dimensions to the fault and lie some distance from the antennae.
Other researchers have looked into EMR as a precursor to earthquakes and other seismic
activity. A controversial group often referred to as VAN (Panayiotis Varotsos et al), have
claimed success in measuring EMR prior to earthquake activity in Greece. However various
other authors have presented papers criticising the VAN group’s methods, Stravakakis (1998)
and Geller (1997) have both argued against it. The arguments against the VAN method include
claims that no actual EMR has been recorded at the time of an earthquake and neither has
seismic activity been recorded at the time when the EMR precursors are claimed to be
measured. Also many EMR measurements were only detected at one VAN measuring station
and not at others in Greece. Finally they claim evidence that the source of much of the EMR
was digital radio-telecommunications transmitters and other industrial sources rather than the
proposed failing rocks.
More recently, Rabinovitch et al, 2000, conducted a series of tests on chalk. They failed a
number of samples while simultaneously recording electromagnetic emissions. Sophisticated
equipment and techniques were employed to block / filter out background electromagnetic
radiation so that only emissions from the failing sample were recorded. From the tests,
Rabinovitch, like Hanson in 1981, noted that the amplitude of the EMR emissions increased as
the fracture length grew, a feature they put down to the severing of atomic bonds as the fractures
increased in length. This theory has since become the most accepted for the cause of EMR.
Assuming a constant crack velocity (Vcr), Rabinovitch also stated that the time from the origin
of the pulse to its maximum amplitude envelope (Tl) is proportional to the crack length l.
Therefore as the crack length increases the time between the emission’s origin and the
maximum of its envelope increases proportionally. This is represented in the following
equation:
T'
l
vcr
[2.1]
A further observation was made that the frequency of the EMR emissions (Z) was related to the
fracture width (b) and could be represented by the following equation:
Y
Svel
b
[2.2]
Where vel is the Rayleigh wave speed.
The combination of the two relationships above means the fracture area can be calculated using
equation [2.3], providing the velocities are known:
T'
Y
1
S
Sv cr v el
[2.3]
Where S = l x b = the fracture area.
The equations can be used for both the determination of the exact fracture area or comparison of
different fracture areas. The advantage of this mathematical approach is that it is less affected
8
by external background radiation, which may overprint the actual EMR produced by the failing
sample. The use of the equation above in a device used to measure EMR in a mining
environment would allow an interpretation of the extent of fracturing that has occurred to be
made. Once a limit of fracture area has been reached and failure is imminent, the EMR
detection device could sound a warning to avert disaster.
Research has also been conducted in mining environments for the detection of electromagnetic
radiation caused by rock failure due to mining activity. Such underground experiments allow
for a more realistic testing environment, for which a device would be used. Also background
levels of EMR in mines are drastically reduced allowing measurements of fracture induced
EMR to be better observed.
(The following is from a private communication with the Safety in Mines Research Advisory
Committee SIMRAC)
One such experimentation was conducted in an actively failing coal mine in South Africa. Both
EMR and seismic activity were recorded using a radio receiver and antenna, a pocket radio and
a geophone all connected to a multi-seismometer and ruggedised laptop for data recording. The
frequency over which the antenna and radio receiver were set, changed periodically to increase
the overall range in which measurements were taken. Only EMR and / or seismic activity above
a set background levels was recorded by the seismometer and all background nose was
disregarded.
At a frequency of 4.92 MHz the EMR was observed most clearly and occurred just prior to 80%
of confirmed seismic events during the period that frequency was used. Figures 1 (a) and (b)
illustrate the type of data recorded from the experiments. Figure 1(a) clearly shows an EMR
emission prior to a seismic event. Figure 1(b) however shows a clear seismic event of similar
magnitude to that in figure 1(a) but no discernible EMR emission. The level of seismic and
electromagnetic background noise encountered in a mining environments can also be clearly
seen in figure 1(a) & (b), it is believed that this noise could be masking any small EMR
emissions prior to seismic events.
2.2 THERMAL RESPONSE.
The use of thermal imaging and remote temperature measuring devices in industry has evolved
over the last few decades, with a growing number of applications, helped by the development of
more sophisticated equipment. The mining industry is one of the industries which have applied
the technology in a number of its key areas, however it is only in recent years that researchers
have looked at the possibilities of using remote temperature measurement technology to detect
areas of failed rock mass.
Two different methods of using thermal response in mining situations to detect failed rock mass
have been developed in recent years. The one that has seen most interest involves looking at
temperature differences between solid and failed rock mass. The other method has seen less
mining related research, with authors concentrating on using the method for the prediction of
seismic activity. The method involves using satellite based thermal imagery to locate areas of
increased stress, highlighted by higher temperatures, caused by increased friction. Both
applications will be discussed further in the following sections.
2.2.1 Loose rock detection
As with many new ideas, this phenomenon was discovered by accident during work by the
United States Bureau of Mines (USBM) on roof beam failure in a limestone mine in 1958. The
original experiment involved the widening of a room till failure was induced; Merrill and
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Morgan (1958). However due to cost and safety implications it was decided not to mine until
failure occurred, but to use compressed air forced into the separation between the lowest roof
beam and the rock above to induce failure. It was during this phase of the work when the roof
beam had detached from the overlying rock that a difference in temperature was noticed
between loose (beam) and solid rock mass. The temperature gradient was sufficient for it to be
detected by feel alone using the palm of the hand.
The cause of the noticeable difference is that two different temperature environments act on the
rock mass within an excavation. Firstly, there is the virgin rock temperature at which the rock
lies before excavation. Secondly, there is the ventilated air temperature, which can be
significantly lower than virgin rock temperature. If rock mass failure occurs an air gap or
separation may form, creating a thermal barrier. Heat from surrounding rock, which lies at
higher virgin rock temperatures, would be restricted from conducting into the cooler failed rock
mass, which lie at a temperature between virgin rock temperature and ventilated air temperature.
If the flow of heat by conduction is restricted sufficiently, a noticeable difference should be seen
between failed and solid rock mass. At the time of Merrill and Morgan’s (1958) discovery the
technology available for remote thermal imagery / temperature measurement was unable to
detect the small changes in temperature between failed and solid rock mass.
In 1970 Merrill and Stateham revisited the idea, with the advantage of more sensitive
instrumentation capable of detecting temperature differences of 0.2 oC. During the experiments,
a single point measurement device, call an infrared pyrometer, and a thermal imager, which
displayed a visual indication of an objects temperature, were used.
Trials using the equipment were conducted in a number of localities in order to witness the
variety of air to rock temperature conditions found in mines. In some of these localities the air
temperature and flow was controlled in an effort to determine the time taken for failed or loose
rock mass to adjust to changes in temperature compared to solid rock mass. Although few
figures and details relating to the conditions to which the excavations were subjected are given,
Merrill and Stateham (1970) state that the rocks studied took, on average, between 15 and 30
minutes to develop a detectable temperature difference.
An example of the trials conducted involved the stopping of all ventilation and the sealing of a
mine for an overnight period, allowing the excavation to form a natural equilibrium.
Measurements taken the following morning with the pyrometer and imager showed no
discernable difference in temperature between loose and solid rock mass. The ventilation
system was then switched on. After 15 to 30 minutes of normal ventilation conditions a
difference between 1 and 7oC, depending on the ventilated air temperature that day, was
witnessed between loose and solid rock mass. Merrill and Stateham also discovered that freshly
blasted faces produced temperature differences of 1 to 1.5 oC between intact and loose rock,
compared to the 0.2 to 0.3oC difference shown by pillars only 30ft away.
Since Merrill and Morgan (1958) first discovered the concept and then Merrill and Stateham
(1970) follow up, little work has been done on the subject, with the most significant work
undertaken in the last decade. Yu et al (1990) conducted a more systematic approach to
examining the concept, by using a 350 kg lump of rhyolite which contained loose “flakes” of
rock, Yu et al were able to repeat experiments without encountering any significant changes.
Yu et al also enclosed the test block in a wooden box in order to reduce the effects of the
surrounding environment. The block was initially heated using thermocouple wires attached to
the solid rock and the tip of a loose flake. Once a predetermined temperature was reached, air
was blown across the block to simulate ventilated air, the temperature and velocity of which was
varied. Again an infrared pyrometer and thermal imager with sensitivities of 0.2 oC were used
to obtain visual and numerical data for analysis of trends.
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Yu et al’s (1990) initial experiments provided the expected conclusions, that loose rock cools
faster than solid rock mass and creates a discernable temperature difference between the two
rock mass conditions. Field trials, conducted also by Yu et al at Kidd Creek Mine, again
confirmed that a temperature difference was observable between areas of loose and solid rock
mass. However from the trial, Yu concluded that the age of an excavation has a negative effect
on the observable temperature difference, with older excavations demonstrating few detectable
temperature differences. To answer this problem Yu et al developed a number of techniques to
improve or enhance the temperature difference. The following enhancement methods were
used: x
x
x
x
Hot diesel exhaust from an LHD machine blowing against the roof for 20- 30 minutes.
Airflow rate acceleration using a local fan.
Radiant heat from an IR source.
Evaporation of water from the rock surface.
The hot air from the LHD exhaust provided the best results according to Yu et al, with a more
significant temperature difference seen due to this method than any of the others. Yu et al took
the research a step further by mounting a thermal imaging scanner onto a mechanical scaler, the
intention being to aid the operator during the scaling of a roof. To enhance the conditions the
diesel exhaust of the scaler was directed towards the roof being inspected.
The effect of using the equipment was to cut the time taken for the roof inspection, with a 30m
section of tunnel taking 30.4 minutes using the IR equipment. This compared with 64.4 minutes
taken to inspect the same area without the IR equipment. However Yu et al also state that the
checking of areas scanned using the IR scanner, using a scaling bar demonstrated that a
significant amount of loose rock had been missed by the IR method, a problem that may be
alleviated using other temperature enhancement methods.
Kononov (2000) conducted the most recent work on the subject with his work for Safety in
Mines Research Advisory Committee (SIMRAC), South Africa. The research focused more on
the theoretical aspects, with estimations on heat transfer between tunnel surface and airflow
being the main aim. Calculations on the conduction of rock were performed using a 1-D “fin”
type model, designed to simulate a slab of rock, the aim being to see how heat dissipates along
the slab (“fin”). The affect of ventilated air on the temperature of the slab was also taken into
account, by including convection calculations in the simulation. The results, figure 2, from the
calculations clearly reiterate what Yu et al (1990) had observed that better results are found in
newly excavated areas. Also, with time the surface temperature of both solid and failed rock
mass become closer to that of the ventilated air temperature.
Kononov (2000) also produced more sophisticated 2-D models using more realistic geometry
and global assumptions in the calculations. Four variations of the calculations were conducted
with different air velocities being the main variable, slab dimension being the other. The results
of the calculations are shown in Table 1.
The results highlight the effect that ventilated air velocity has on the temperature difference
between loose and solid rock mass, with slower velocities producing the best conditions for
loose rock detection. The 2-D results again illustrated the effect of time on the temperature
difference, with a marked contrast between the year old slab and the 1 month old slab. One new
conclusion drawn from the modelling relates to the effect the size of the slab has on the results
with a thinner slab showing a larger temperature difference compared to a thicker slab with the
same variables.
11
Table 1. Results of 2-D slab modelling calculations, after Kononov 2000 Air velocity
(m/sec)
0.5
1
5
0.5
5
Slab dimensions
(w.l.t)
(m)
0.8 x 1 x 0.4
0.8 x 1 x 0.4
0.8 x 1 x 0.4
0.8 x 1 x 0.3
0.8 x 1 x 0.4
Calculated temperature
difference (oC)
Time since simulated
excavation
3.47
3.26
1.75
3.72
2.48
1 year
1 year
1 year
1 year
1 month
Like Merrill and Stateham (1970), and Yu et al (1990), Kononov conducted field trials to
corroborate the results of the modelling. Two South African gold mines were used as test
locations, with an IR radiometer (thermometer) for remote temperature measurements, Kononov
also used a device to measure air velocity and temperature.
The results of the field trials again reiterated the findings of Merrill and Stateham (1970) and Yu
et al (1990) that a significant temperature difference between rock and ventilated air is the most
important factor for the concept to work. Kononov stated that this is the reason why such a
concept isn’t viable in the United States, where the temperature gradient is not significant. The
deeper South African gold mines where conditions are more favourable should, in Kononov’s
view, prove more suitable. Other recommendations drawn from the research include the
development of a mine worthy, intrinsically safe IR radiometer that is capable of simultaneous
measurements of air and rock temperature and air velocity.
2.2.2 Stress induced thermal radiation
This section discusses the application of infrared thermal imagery in the detection of areas of
rock mass subjected to abnormal stress loads. In their paper on infrared radiation features of
coal and rocks under loading, Wu and Wang (1998) discuss how abnormal rises in surface
temperature could be detected by the thermal imager of NOVA climate satellites in 1990. The
abnormalities, according to two other Chinese scientists, Qiang and Ning (paper unavailable),
witnessed several days before and after strong earthquakes had occurred, were caused by the
changing stress fields in the rock, releasing infrared radiation.
The two scientists have since gone on to predict 68 earthquakes based on the thermal infrared
anomaly theory, 50% of which Wu and Wang (1998) state gave “good” results. Although the
research in this area is predominantly focused towards earthquake prediction, some authors have
realised its potential in a mining situation. Again Wu and Wang (1998) present further evidence
for stress induced thermal radiation, but within a mining environment. They state that mining
engineers have experienced abnormal high temperatures in some mines running up to gas bursts
and other major changes in the stress field. An example is given of a coal mine in Britain,
where the temperature of outburst coal was as high as 60oC.
The mechanism behind the infrared radiation discharged by highly stressed rocks is not
completely understood. Wu and Wang (1998) suggest both inner structure reconfiguration of
the rock material and associated physical and chemical phenomena during loading as the causes
of the temperature change. These “phenomena” according to Brady and Rowell (Nature 321),
include inner fracturing, ionisation, energy accumulation, energy consumption, electrical
resistance variation etc., the result of which is the conversion of mechanical energy, which is
causing the failure, into thermal energy and electromagnetic radiation, as discussed above.
12
Wu and Wang undertook their own experiments with a series of tests on coal and sandstone
samples loaded to failure in a loading frame, whilst recording the changes in infrared radiation.
From these simple experiments, Wu and Wang (1998) discovered that a forewarning of failure
occurs in the form of infrared radiation (IR) change. However this change in IR is not uniform,
with three separate categories of IR change witnessed by Wu and Wang, as follows: Type I: Forewarning by low temperature
Type II: Forewarning by high temperature
Type III: Forewarning by continuous high temperature.
(After Wu and Wang, 1998)
A further finding from the research was that during loading of a “fat” coal sample the surface
temperature increased significantly more than the internal temperature, whilst the opposite was
found when loading a blind coal sample.
In 2000, Wu et al (2000), produced a paper on the new subject of Remote Sensing Rock
Mechanics (RSRM), in which they laid out the background to the new subject and an overview
of research that had gone on previously. The paper also contains an interesting framework
which they believe the RSRM studies and applications should follow. Over an 8 year period
Wu and his team have developed this subject and conducted a large quantity of research in the
new area. To detect, measure, record and analyse any electromagnetic radiation being emitted
by rock while it is subjected to stress the group has used a variety of sensory equipment. In this
paper Wu et al sum up their research with the following significant conclusions: 1.
2.
3.
Both infrared radiation temperature and infrared spectral radiation temperature
increase with load.
Infrared radiation temperature increases with the increase of rock strength.
Infrared radiation temperature increases with the depth inside the sample
surface.
2.3 ULTRASONIC / ACOUSTIC EMISSIONS
An ultrasonic emission is defined as physical vibrations in matter occurring at frequencies
above 20 kHz, which is approximately the limit of human hearing. Many prolific users of
ultrasonic sounds are found in nature, with the bat being one of the most well known users of
ultrasonics as a system for nocturnal navigation. Dolphins also use ultrasonics in the
underwater environment for navigation, the detection of food and for a basic communication.
Ultrasonics has been adapted for a variety of uses in the human environment, including
medicine (echocardiography), engineering (ultrasonic fracture detection, NDT) and military
(submarine sonar ping). Ultrasonics has also been developed for use in seismology (or
microseismology) where it is known under the term acoustic emission. A more detailed
description of an acoustic emission is a transient elastic wave emitted from small-scale cracks
(microcracks) that form due to changes in stress (Hazzard, 1999). Lockner (1993) states that the
frequency range of these emissions is 100 kHz to 2000 kHz.
In order to avoid confusion for the reader the following definitions will be used when discussing
this subject: - Acoustic Emission (AE) will be used to describe emissions (frequency 100 kHz 2000 kHz) detected by devices (e.g. piezoelectric receivers) attached directly to the surface of
the rock sample. The term Ultrasonics Emission (frequencies > 20 kHz), on the other hand will
be used to describe emissions detected with devices that have no contact with, or surfaces
adjoining the rock from which the emissions originated. For such devices to work, the emission
13
must travel through the rock material, across the air - rock boundary, through the air and still be
detectable by a device.
Whilst researching the background to ultrasonic emissions and acoustic emissions it became
clear that little or no research had been done on ultrasonic emissions as a precursor to rock mass
failure. However research into acoustic emissions, as a precursor to rock mass failure is wide
and extensive, with detailed observation being made on all scales. The reason for this
imbalance is described by Obert (1941) in his paper for the United States Bureau of Mines,
where the theory of AE is first put forward. It was during some seismic investigations in a mine
that Obert (1941) noticed that the normally “quiet” mine had become “noisy” (seismic noise).
The noise continued to increase for about 15 minutes until it terminated with a rockburst, which
occurred within 50 feet of the point of observation. The observation prompted further research
into what Obert (1941) refers to as sub-audible noise for use in the prediction of rock failure
(rockbursts).
In order to detect the sub-audible noise Obert bypassed the idea of using a non-contact device
and opted for a device that had contact with the rock surface. The reason for the decision is
mainly due to the large signal impedance at the rock to atmosphere boundary. When a
microfracture forms, an acoustic emission is released, radiating outwards from its origin.
Sudden changes in density of the medium through which the AE is travelling, will cause loss of
energy, refraction and reflection of the emission. In the case of acoustic emissions, they must
travel across the rock to atmosphere boundary, a massive density change, which creates a huge
loss of emission energy, between 80-90%. Such a large loss of energy lessens the chance of
detection using a non-contact device, located some distance from the emission origin.
The absorbing characteristics of the atmosphere, through which an ultrasonic emission would
travel from source to detector, would also hinder the performance of such a technique. An
analogy of the problem can be given by a thunderstorm. At the centre of the storm the
thunderclap is sharp, short and loud, but at some distance the atmosphere absorbs the high
frequencies of the thunderclap. The result being a low drawn out rumble caused by sound
reflecting off the surrounding landscape. For a non-contact device to work it would need to be
sensitive enough to detect the weakened emissions and positioned close enough to capture them
also. Due to these reasons, Obert, and those who have followed, have opted for contact devices
to detect the emissions from failing rock mass.
Obert conducted his experiments using sensitive vibration microphones, referred to as
geophones, placed 100 feet apart at the end of drill holes in an active part of a mine. The early
geophones were connected to an amplifier, a recording device and a pair of earphones, some
distance from the active area. These initial trials highlighted the problem of background noise
created by the everyday mining activities of drilling, blasting, ore shoot movement etc. Obert
(1941) states that the background noise could be identified by the different amplitude,
frequencies and wavelengths, however at the time no equipment was available to filter it out.
In 1942 Obert and Duvall (1942) returned to the same mine with more sophisticated equipment
purposely built for the job of recording acoustic emissions under normal mining conditions.
Mining induced noise was recorded and the resulting waveforms were analysed. Combinations
of filters were then employed to remove the mining induced noise and leave just AE.
Unfortunately Obert and Duvall discovered that much of the mining induced noise has a
frequency which overlaps with that of acoustic emissions causing them to be obscured. Rock
drilling was found to be particularly effective, not surprising as drilling is the controlled
fracturing of rock mass.
14
In conclusion Obert and Duvall (1942) state that the equipment then available was unable to
filter out mining noise, in effect restricting the use of such a method to periods when mining
was not active. Also, more importantly, a rockburst often followed a period of increased
acoustic activity, which could be deemed a precursor. It was however impossible to determine
at what point, after what length of time or after how many AEs would a rockburst occur,
seconds, minutes, hours or even days.
Since the discovery and early trials by Obert (1941) and Obert and Duvall (1942) the use of
acoustic emissions as a precursor to rock mass failure has advanced, with most trials in this area
now conducted on laboratory scale. By conducting trials on a lab scale researchers have more
control of the variables, enabling experiments to be repeated with the same parameters. It has
also long been known that the results from laboratory trials can be scaled up. Even with the
advantages of laboratory based experiments and the development of sophisticated detection,
recording and analysis equipment, the ability accurately to predict rock mass failure using
acoustic emission techniques still eludes researchers. Progress has been made to an extent, with
acoustic emission techniques having some success at predicting the onset of rockbursts. The
following are two techniques used in the mining and research industry to both predict rockbursts
to some degree and monitor failure patterns.
2.3.1 Gutenberg - Richter (b-value) relationship.
Also known as the frequency magnitude relationship, this well known relationship was first
discovered through the research of earthquake fore and aftershocks and relates the frequency of
earthquake occurrence to the earthquake magnitudes. The relationship can be shown using the
following equation: -
log N
a bM
Where
N
=
a&b =
Number of earthquakes greater than magnitude M
Constants (the b value can however vary from one earthquake
region to another but is usually close to one, (Lockner, 1993)).
The relationship was first used in earthquake prediction when Scholz et al (1973) proposed that
a significant change in b-value might be used as an indication of impending large earthquakes.
Due to the close relationship between earthquakes and acoustic emissions the relationship has
been applied to AE analysis. In 1987 Hirata (1987) discovered a power law frequencymagnitude relation for acoustic emission events, which corresponds to the Gutenberg-Richter
relationship. Laboratory AE experiments have since shown a decrease in b-value with increased
stress in both uniaxial and biaxial directions during deformation of intact samples, Meredith et
al, (1990).
Due to these findings the relationship has been employed in several South African deep hard
rock mines. Using geophones placed throughout the mine, acoustic emissions are monitored.
The results are then plotted on a log frequency-magnitude graph and interpreted. With a build
up of stress, which could lead to a rockburst, a change in b-value should occur causing a visible
change in the gradient of the plotted data. The b-value is often displayed at the shaft top for
miners to take note of an impending rockburst, and high risk areas are closed to all personnel
until after the seismic event occurs, or the b-value returns to normal. Figures 3 (a) and 3 (b)
show b-value plots produced from data collected by Young and Collins, (1997) at the
underground research laboratory (URL), Canada, which is discussed in the following section.
Figure 3 (b) illustrates the b-value plot for data taken during a period of acoustic activity prior to
larger seismic activity, while figure 3 (a) illustrates the b-value plot for a more stable period.
15
A clear difference can be seen between the two illustrations with a much steeper gradient
formed during the period of increased acoustic activity prior to a larger seismic event, than the
seismically quiet period. Another difference lies in the magnitude of seismic events with the
seismically active period having seismic events up to a magnitude of 2.3 compared to the
maximum magnitude of 1.6 seen during the seismically quiet period. Although these
illustrations look like an ideal method for predicting seismic activity and hence rock mass
failure, they are in fact idealised examples, with a lot of b-value plots showing little to no
difference in slope. Another problem arises from the time frames involved; b-value plots may
give an indication of the onset of increased seismic activity, but no indication of when an event
may take place can be gleaned from the plots.
2.3.2 Acoustic monitoring
The monitoring and analysis of acoustic emissions is another way that researchers are using
acoustic emissions to study rock mass failure with the hope of some day predicting it. A major
research area, which has invested heavily in acoustic monitoring, is the search for safe
repositories for spent nuclear fuel. The research on the repositories covers everything from the
most appropriate geological setting to the type of storage containers, in which the spent fuel
rods would be kept. The problem caused by fluid ingress and groundwater contamination is one
of the major areas of research. The repositories must have low fluid flow rates to avoid
groundwater contamination in the event of a fuel rod storage container failure.
The simple answer to the problem would seem to be to build the repository in unfractured
crystalline rock i.e. deep seated igneous plutonic rock. It is however important to take into
account the effect mining has upon the stress field and in turn the structure of the surrounding
rock. Acoustic monitoring is used to study these effects as experimental repositories are
excavated in a number of localities throughout the world. One example, mentioned previously,
is the Underground Research Laboratory (URL), Canada, where acoustic monitoring techniques
have been used and developed during extensive experimentation. Young and Collins (1997)
used acoustic emissions and microseismics (MS) to assess the effect of a shaft extension. Four
piezoelectric geophones were placed into four inclined boreholes located around the planned
extension of the mineshaft. Acoustic and microseismic events were monitored and recorded
during the quiet periods following each blast to assess the impact of the excavation. By using
four sensors carefully placed and located in the shaft wall it was possible for Young and Collins
(1997), to locate the source position of each event. Each event can be plotted relative to the
excavations, and areas of serious concern can be identified early. Event magnitude can be
shown also.
Although the method is restricted to quiet periods, when no mining is taking place, it is
invaluable as a method of observing failure in rock mass, and could lead to a greater
understanding of the process. Such a system would, however, prove difficult to use in an active
mine with its use restricted by background noise and its inability to predict failure accurately.
2.4 INDUCED VIBRATION
An alternative to monitoring vibrations occurring naturally in the rock is to induce a vibration
and then monitor how the rock responds to that vibration. Seismic exploration techniques apply
this principle, using explosives or other seismic sources to impart energy to the rock, and rely
upon reflection and refraction of seismic waves from interfaces within the rock and differences
in transmission velocities between beds to build up a tomographic image of the solid rock.
On a much smaller scale, a contact instrument, known as the Acoustic Energy Meter (AEM),
(Figure 4) was developed by Rock Mechanics Technology (Altounyan and Minney, 2000). The
instrument was first designed to locate voids behind concrete tunnel linings. It has later been
16
developed and tested on a variety of failed rock mass conditions, and has proved successful in
identifying surface “looseness” in almost all. The device uses a geophone placed on the surface
under investigation to measure the transient vibrations caused by a hammer blow to the same
surface. By processing the geophone signal a value, quantifying the integrity of the surface is
displayed on an LCD screen. A traffic light style indication is also shown, with a green light
illuminated to symbolise solid rock mass. The instrument effectively provides a more objective
version of the “sounding bar” technique that has been used by miners for hundreds of years to
detect loose rock. It overcomes the reliance of the older technique on experience and good
hearing, an sense which has often been impaired in experienced miners working in noisy
conditions. The algorithm applied to the geophone signal to determine a single parameter for
display effectively calculates the time that the struck surface continues to vibrate or “ring”.
The AEM has been positively assessed for application in small mines in the UK (Hurt,
MacAndrew, Bigby 2000), South African coal mines (Altounyan, Clifford and MacAndrew,
1999), shotcrete lined tunnels (Cartwright, Clifford, Armanen and Vuori, 2001), and South
African gold and platinum mines (Piper, Bron, van Rooyan, Goldbach and Clifford, 2002) and it
is currently being assessed for application in Australian coal mines. The conclusions of the
assessment for South African gold and platinum mines, which was undertaken independently of
the manufacturers during the period of this Project, are summarised below.
The work was divided into four areas, i) Theoretical evaluation, ii), AEM calibration under
controlled laboratory conditions, iii) AEM surface field trials, and iv) AEM underground trials
in gold and platinum mines.
The theoretical analysis showed that such “seismic” techniques have the potential to detect
changes in hanging wall integrity in gold and platinum mines and that the AEM has application
potential in gold and platinum mines provided that the correct frequencies are introduced into
the rock mass and that the instrument’s specification is such that the required frequencies are
measured to correctly interpret the reflected seismic energy. Although the AEM had been
successfully applied in coal mines, where the nature of the hazards is much simpler than the
complex fracturing expected in gold and platinum, its performance was difficult to predict in the
gold and platinum environments. Thus the degree of success in the various potential
applications had to be established in a detailed field evaluation programme.
As a result of the work it was concluded that the applications for the AEM fell into two main
categories; ‘hard’ and ‘soft’. The hard applications are where the meter is used as a scientific
tool (see Table 2). The soft applications are where it is used as part of a documentation and
management control system rather than placing all the emphasis on the readings themselves. For
example, it was shown that the AEM has the potential to identify the leading edge of dome
structures. In the application of the AEM against the hanging wall the obvious presence of a
dome is likely to be recognised by the operator. Therefore a management control system can be
put in place which will ensure that the operator checks all critical parts of the hanging wall and
provides necessary proof of the work in the form of a completed survey sheet. Furthermore, the
AEM could be used as a training tool to assist mine personnel in confirming the presence of
loose blocks which could be identified visually in the future.
The accuracy of the AEM was determined using a calibration table. The influence of the
following parameters was analysed: i) hammer weight, ii) distance of hammer from AEM, iii)
intensity of hammer blow, iv) hammer side. The most consistent results were obtained when the
intensity of hammer blow was constant, the distance between hammer and AEM was 200mm,
the ball-point side of the hammer was used and a 0.75kg hammer was used. Except for the rock
volume and distance between the hammer and the AEM, all the other parameters are a function
of the person conducting the survey and the hammer used. The influence of these factors could
17
be eliminated by introducing a constant impact device. An in-situ calibration of the AEM
should be conducted at each site before conducting a survey.
Surface trials, which were undertaken on unweathered andesite rock showed that the AEM
detects slightly open discontinuities up to 0.8m inside the rockwall, but does not detect tight
discontinuities beyond 0.3m into the rockwall.
Table 2. Potential ‘hard’ applications of the AEM with assigned confidence levels
Potential application
Detection of loose rock
Leading edge of dome structures
Indicate de-bonding of shotcrete support from rock surface
Monitor deterioration of hanging wall over time
Determine opening on planes of weakness within 1st metre of rock
surface
Indicate ‘hot spots’ within a stope panel as a function of ground
conditions
Indicate high damage geotechnical areas relative to other
geotechnical areas in seismically active areas
Indicate effectiveness of pre-conditioning
Determine opening on planes of weakness beyond 1m into the rock
surface
Confidence level
High
High
High
Medium to high
Medium to high
Medium to high
Medium to high (if large number
of readings taken)
Medium to low
Very low
The AEM results from the gold mines showed that the AEM could detect slightly open
discontinuities up to 0.5m into the hanging wall at Tau Lekoa, 0.65m into the hanging wall at
Mponeng and approximately 0.7m into the hanging wall at South Deep. The AEM results from
the platinum mines showed that the AEM could detect open discontinuities up to approximately
0.7m depth at Bleksop and Eastern Platinum, 0.9m depth at Frank Shaft and 0.3m depth at
Waterval. A comparison between the AEM and Infrared Thermography was conducted but
showed no correlation between the two techniques.
Table 2 gives a summary of the potential ‘hard’ applications of the AEM in hard rock mines and
the confidence level to which it can be applied. The confidence levels are high, medium, low
and very low and are based on using a 0.75kg hammer. Due to the variability of the hammer
blow, some of the applications have a lower confidence level.
From the above, it would seem that the principles employed by the Acoustic Energy Meter are
highly suitable for development into a remote reading device for detecting failed rock, if it were
possible to impart energy into the rock and measure the resulting vibration without direct
contact with the rock in the potentially hazardous zone.
2.5 LASER DISPLACEMENT MEASUREMENTS
The use of lasers as a remote method for the accurate measurement of distance has increased
dramatically recently, with many new applications being discovered. Most laser products, such
as the laser speckle pattern interferometer, designed to measure the displacement field from
objects with rough surfaces, are lab based only. However other products, such as laser range
finders and laser doppler vibrometers, have been developed into workshop and field instruments
capable of making accurate distance measurements. These two devices were identified during
the literature review stage as showing some potential for detecting rock mass failure. They were
identified when assessing conventional methods of detecting rock mass failure, such as the
sounding bar and Acoustic Energy Meter, and how they might be adapted to become remote
methods. It is unknown if either of the devices have been used previously for such an
18
application, as no published information was discovered. The following section discusses each
device in turn, describing their specification, some of their applications and how they could
replace conventional methods
2.5.1 Laser vibrometer
Laser Vibrometers measure the vibration of a surface remotely as the component of surface
velocity, by measuring the Doppler effect of a reflected laser speckle pattern. Accurate
measurements of velocities above 500 mm/sec, equal to 100mm of displacement can be
obtained using modern, small, portable devices. Figure 5(a) shows a portable laser vibrometer
produced by Polytec of Germany.
Laser Vibrometers or Laser Doppler Vibrometers are based on the principal of the detection of
the Doppler shift of coherent laser light, that is scattered from a small area of the target object.
The object scatters or reflects light from the laser beam and the Doppler frequency shift is used
to measure the component of velocity, which lies along the axis of the laser beam.
The basic principle of a laser (light amplification by stimulated emission of radiation), is the
induced emission of photons. These emitted photons possess identical properties and thus
produce the coherent light of the same wavelength, which is necessary for this application. The
laser used for Laser Doppler Vibrometers including the PDV 100 by Polytec, is a helium neon
(He-Ne) laser, which produces a visible red laser beam (O = 0.6328 Pm).
Due to the laser light’s very high frequency, a direct demodulation of the light is not possible
and an optical interferometer is therefore employed. This mixes the scattered light coherently
with a reference beam. The photo detector measures the intensity of the mixed light, whose
(beat) frequency is equal to the difference in frequency between the reference and the
measurement beam. The Michelson-Morley, see figure 5(b), is a typical interferometer used in
LDV. A laser beam is divided at a beam splitter into a measurement beam and a reference
beam, which propagates in the arms of the interferometer.
Due to the sinusoidal nature of the photo-detector signal, the direction of the vibration is
ambiguous. Two methods have been developed to introduce a directional sensitivity to LDV.
x Introduction of an optical frequency shift into one arm of the interferometer to obtain a
virtual velocity offset.
x Adding polarization components and an additional photo receiver in such a way, that at
the interferometer output a second homodyne signal occurs being in quadrature to the
primary photodetector output.
The most common method used to solve the problem of directional ambiguity is solution 1
listed above. This method involves the incorporation of an acoustic optic modulator (Bragg
cell) into one arm of the interferometer. The Bragg cell is driven at frequencies of 40 MHz or
higher and generates a carrier signal. The signed object velocity determines sign and amount of
frequency deviation with respect to the centre frequency. This type of interferometer is called a
heterodyne interferometer, see figure 5(c), and has a number of significant advantages. As only
high frequency AC signals are transmitted there is no disturbance from hum and noise,
introduced from all types of power supplies. The high efficiency achievable by Bragg cells,
produce less losses than the polarizers needed for solution 2 listed above. As almost all
vibrometers use the Bragg cell method including the vibrometer used in this trial, it is not
necessary to give details of the other solution to the problem of directional ambiguity.
19
As mentioned above Polytec laser vibrometers use heterodyne interferometers with an acousto
optic modulator in one arm of the interferometer. This generates a frequency modulated carrier
signal in the RF region, whose centre frequency is identical to that of the acousto optical
modulator drive signal. The directional sensitive Doppler information is thus contained in the
RF carrier. The signed object velocity determines sign and amount of frequency deviation with
respect to the centre frequency. The Doppler frequency is proportional to the surface velocity
and the phase change with respect to the phase of the reference signal is proportional to the
displacement of the object.
The complicated physics used to decode this RF carrier signal and its reference beam are
beyond the scope of this description into the workings of a laser vibrometer. It is however
worth noting that the output signal from the decoder can be both analogue or digital and give
results as velocity and/or displacement
The remote capabilities and accurate measurement achievable with such a device mean it has
been adopted by a number of industries for a variety of uses. Industries, which have adopted the
laser vibrometer technology, use its unique ability to measure vibration remotely, often without
the need for any surface preparation. By examining the vibration of a rotating gearbox, for
example, manufacturers can check for faults with any of its components. Neural networks can
also be developed to recognise the different vibrations caused by specific faults, within the
gearbox. Using the vibrometer and neural network technology together, manufacturers have a
fast and effective method of conducting quality control on their products as they leave the
assembly areas. Researchers also use the technology to study a wide range of subjects from the
beating of an insect’s wing, to the workings of the inner ear.
The existence of laser vibrometer technology was discovered during research into the Acoustic
Energy Meter (AEM) and how this device / technology could be developed into a remote
device.
It is not known by the authors whether an instrument such as the laser vibrometer has been
previously used in such a way. Neither of the two main manufactures of laser vibrometer,
Polytec and B&K, had heard of previous work in this field. The concept of using a laser
vibrometer to identify areas of failed rock mass by their vibrational characteristics, would
appear to be “blue sky” in it’s thinking and deserves further research.
2.5.2 Laser range finder
Laser range finders use the travel time of a reflected laser beam to measure the distance between
the reflecting object and the range finder, accurately. Large distance can be measured
depending on the laser power and the reflecting surfaces. Range finders used in “total station”
for surveying purposes can measure large distances with the help of a reflecting mirror on the
sighting poll. The accuracy of the devices can be a high as 0.02 mm, again depending on the
strength of the reflecting surface.
The laser range finder was identified whilst investigating conventional strata displacement
measurement equipment and how the technology could be developed into a novel remote
device. Equipment such as the tell-tale extensometer and Magnesonic extensometer measure
the movement in both roof and rib strata to give an indication and forewarning of failure in the
rock mass, which surrounds the instrument. With the use of a laser range finder it was
suggested that the dimensions of an excavation could be monitored remotely, with any changes
in rock mass integrity being detected due to the change in excavation shape.
Although such an instrument is capable of measuring the dimensions of an excavation and
detecting any significant changes, a number of problems arise with such a technique. The lack
20
of a stable reference point, to which the laser range finder could be placed, is the main problem.
Without a stable point of reference it would be impossible to tell what movement was being
measured, either the surface under investigation or that of the range finder. The use of
gyroscopes was investigated as a possible means of eliminating the problem of an unstable
reference point. However the “random walk” factor, which gyroscopes suffer unless up dated
from a stable reference, meant a range finder fitted with such a device would not solve the
problem. It was decided that no further research should be conducted into the area and that
other areas such as the laser vibrometer offered more potential for success.
21
22
3 LABORATORY WORK
3.1 INTRODUCTION
Using the knowledge gained from the literature survey of work completed by other authors in
this field, and from the experience of the Partners, four areas were pinpointed for further
research and trial. Electromagnetic and ultrasonic emission detection, thermal response of
failed rock mass and the transient vibrational characteristic differences between intact and failed
rock mass were the areas selected for further investigation. Laboratory trials were used to test
each of the concepts, as to their capabilities of fulfilling the primary aim of this research. This
chapter discusses the aims, experimental procedure, results and conclusions of the laboratory
trials conducted into each concept.
3.2 ELECTROMAGNETIC EMISSIONS
3.2.1 Aims
Conduct experimentation under laboratory conditions, to identify whether electromagnetic
emissions are detectable during the controlled failure of rock samples common to coal mine
roofs.
3.2.2 Experimental procedure
Using two different sandstone samples, both similar to those found in many coal mine roofs, a
series of trial experiments was conducted to observe for electromagnetic emissions during the
failure of the samples. A calibrated, 1000 kN, servo controlled rock press was used to induce
failure of the rock samples, while a high gain antenna with in built amplifier was used to
detected the emissions. The antenna with a frequency range of 1 MHz to 2 GHz used an in built
amplifier allowing only EMR to be amplified. A digital oscilloscope, a Fluke Scopemeter, was
used to capture and store the amplified emissions.
Hollington sandstone, quarried at Hollington, Staffordshire, for use as a building stone, was one
of the sandstones used in the trials. This a coarse grained, flesh pink coloured, fairly
homogeneous sandstone, with well rounded, well sorted, poorly cemented grains, which
crumble easily and could be described as weak (UCS 20 MPa and Young’s modulus 3.97 GPa).
Horton sandstone, quarried at Horton, Staffordshire was the other sandstone used in the trials.
Again quarried for dimension stone, it is medium grained, light camel brown coloured, with
some small patches of hematisation, well consolidated, fairly homogeneous and could be
considered hard sandstone (UCS 67 MPa and Young’s modulus 12.52 GPa). Both samples
were prepared to ISRM guidelines, their dimensions being 42.3 mm (average) diameter and 105
- 126 mm in length.
Figure 6 (a & b) show a schematic representation of the experimental set up and a photo of the
load frame, antenna and oscilloscope. Once the equipment and the sample to be tested were in
position, a measurement of the background noise was taken; the automatic trigger on the
oscilloscopes was then set just above that level. The samples were then subjected to a slow rate
of loading (1 kN/sec) whilst being monitored for electromagnetic emissions, as they gradually
approached failure.
The initial trials proved unsuccessful with no electromagnetic emissions detected during or after
the failure process. Due to these initial findings, further alterations were made to the testing
procedure. These involved the building of a Faraday shield around the loading frame, using
aluminium foil attached to the safety shielding of the loading frame. A Resistance (5k:)-
23
Capacitor (1000 micro Farad) (RC) filter was also constructed with the aim of reducing the
background noise further, so as to reveal underlying emissions. The filter was connected
between the oscilloscope and the antenna in its own aluminium casing, before the experiments
were conducted a second time.
3.2.3 Results and discussion
During the second phase of the laboratory experiments into electromagnetic emissions, it
became clear that the Faraday shield was ineffective. The level of background noise recorded,
see figure 7, within the Faraday shield itself, although reduced, was still too high, indicating the
ineffectiveness of the shield. The captured radiation displayed a clear frequency, which could
be the result of FM radio broadcasting and which may also contain an RDS signature. The RC
filter did achieve a small reduction in the level of noise. This can be seen by comparing figures
7 and 8. Again, however, the reduction was not sufficient, with no emissions observable above
even the reduced radiation level.
The reason for the ineffectiveness of the Faraday shield became clear following investigation of
the loading frame construction. It was found that the frame, a Dartec 1000 kN press, contains
an earth wire in the centre of one of the four legs. The earth wire, which is also connected to the
mains earth, acts as a large aerial, drawing in electromagnetic radiation and interfering with the
function of the Faraday shield.
3.3 ULTRASONIC EMISSIONS
3.3.1 Aims
To examine whether ultrasonic emissions created by the micro fracturing of sandstone during
the failure process can be detected, and used as a precursor of rock mass failure.
3.3.2 Experimental procedure
The experiments were conducted along similar lines to those of the electromagnetic radiation
experiments. The Hollington and Horton sandstones were used as samples again and the
loading frame used to provide a means of inducing failure. The sensory device was a subaudible or ultrasonic sound amplifier, with variable volume gain and a frequency range of 20 to
130 kHz. After later modifications it was installed in an aluminium enclosure and the inbuilt
microphone was placed onto a 60 cm length of coaxial cable. The oscilloscope was again used
as a capture and storage device for any emissions detected by the ultrasonic amplifier.
During the experiments the microphone was initially situated within 0.15m of the sandstone
sample as it was loaded to failure. Due to problems with background ultrasonic noise however,
the microphone, after the modifications described above, was attached to the sample itself for
the later experiments. Figure 9 shows both a schematic representation and a photograph of the
experimental set up.
As with the electromagnetic radiation experiments, the oscilloscope was set with a trigger level
just above the background ultrasonic noise level. The frequency of the ultrasonic amplifier was
set at 90 kHz, a frequency where background ultrasonic radiation was at its least. The sample
was then loaded at a rate of 1 kN/sec, whilst being monitored for ultrasonic emissions, until total
failure occurred.
3.3.3 Results and discussion
As with the electromagnetic emission experiments, background noise, this time ultrasonic,
served to mask or obscure all but the strongest emissions during the rock failure process. The
emissions, which were detected by the amplifier and captured by the oscilloscope, occurred at
the point of sample failure, which is often explosive. No emissions were detectable above the
24
level of background noise during any other part of the loading process. The origin of the
emissions, which were captured, i.e. at the point of sample failure, cannot be identified as being
the result of failing rock mass. At the point of failure a number of ultrasonic sources of noise
are present. Not only the failing rock sample, but also the loading mechanism tripping out and
the loading platens striking the loading frame base are all potential sources at that moment in
time. This made drawing any conclusions as to the source of the emissions very difficult.
The observed background noise was produced predominantly by the loading mechanism of the
rock press and proved impossible to filter out. This is due to the noise occurring across the
whole ultrasonic range, for which the ultrasonic amplifier was designed, including the frequency
at which most rock failure noise occurred. Even later modifications, described above in section
3.3.2, to the ultrasonic amplifier, which were designed to reduce the effect of background noise,
were insufficient. Again background noise continued to obscure any precursory emissions. The
probable source of the background noise, witnessed after the alterations were made, was the
loading mechanism, transmitting ultrasonics through the loading platens to the sample and the
attached microphone.
3.4 THERMAL RESPONSE
3.4.1 Aims
Investigate under laboratory conditions whether a temperature difference between loose and
solid rock mass can be detected and or enhanced by means of applying a heating or cooling
element, in order to use it as a method of distinguishing the two rock mass conditions.
3.4.2 Experimental procedure
In order to conduct these experiments a set of test blocks, designed to simulate failed (loose)
and unfailed (solid) rock mass, were built. This was achieved using heavy duty domestic
paving slabs, cemented together to form a solid mass, with the upper most slab being left loose
on one block in order to simulate a loose or failed section of rock mass.
The next stage involved the heating of the test blocks using a 1.5 kW infrared heater suspended
directly above the thermal test blocks at a distance of 1.5m. A Raytek MX 2 G, series infrared
thermometer was used to remotely measure the surface temperature at the centre of the two
slabs during both the heating and cooling processes, when the IR source was removed. The
device had the following specification: -30 -900 oC measurement range, Laser sighting circle, defines measurement area, K-type thermocouple for emissivity calibration, and air temperature measurements,
LCD display, with current temperature display and graphical representation of last 10 readings,
x 0.1oC resolution,
x Accuracy of ±1 oC or ± 0.75% of reading, and
x Measurement spot size of 29mm at a distance of 1.5 m. x
x
x
x
The two test blocks were heated until the surface temperature of the two blocks had first
developed a thermal gradient, and then had stabilised at a higher temperature. At this point the
IR source was removed and the blocks allowed to cool. Readings were taken every minute and
recorded for later entry into a spreadsheet program.
The test blocks were modified for a further series of tests. The blocks were rebuilt with
thermistors placed within the surface of the four uppermost slabs (see figure 10), including the
25
top surface, which was then skimmed with a layer of the cement adhesive. A Campbell data
logger and multiplexer were used to interrogate all the thermistors during the heating and
cooling process. The logger stored the data for later download onto a PC, for analysis. The
modified test blocks were designed to give a better understanding of heat transfer through the
entire block instead of just the surface temperature as examined in the initial experiments.
A further change was made to the experimental procedure in the later trials. The test blocks,
now with embedded thermistors, were also subjected to cooling using liquid CO2, at a
temperature of -80 degrees. As for the initial trials, the blocks were subjected to different
periods of cooling (15, 30 and 45 sec), over which time their internal and surface temperatures
were observed using both the in built thermistors and the infrared thermometer. The CO2 was
delivered from a 25 kg dip cylinder via an insulated hose and nozzle which was directed at the
surface of the test blocks and moved slowly across their whole area.
Measurements were taken before, during and after the cooling period in order to observe at
which point, if at all, during the whole process, a temperature difference between the two blocks
appeared most notable. Periods of time to which the blocks were subjected to the liquid CO2,
were kept deliberately short. In an underground environment it would be unwise to release
large amounts of CO2, and so the laboratory trials were designed to follow the same restrictions.
3.4.3 Results and discussion
The initial experiments proved successful, with an observable temperature difference seen
between the loose and solid simulated test blocks, see figure 11. The surface temperature
difference between the two blocks became clear almost immediately, although a small initial
difference in temperature of 0.1 oC was present. The maximum thermal gradient was seen after
only 4 minutes of heating, at which point a 0.5 oC difference in surface temperature was
observed. After a further 4 minutes the gradient had gradually reduced until no detectable
difference was seen. At this point the heat source was removed, allowing observations to be
made during the cooling process. A period of 3 minutes elapsed before a temperature difference
began to reappear, with the largest gradient, of 0.2 oC, occurring 15 minutes into the experiment
i.e. after 7 minutes of cooling. A thermal difference of 0.1 oC continued for a further 5 minutes,
when measurements were ceased.
The second phase of the laboratory experiment proved less successful than the previous
laboratory trials. Using CO2 to lower the temperature of the test blocks, it was envisaged that a
difference in surface temperature would appear during cooling and or re-heating by the ambient
conditions of the surface. Figure 12 shows the resulting average surface temperature for the
loose and solid test blocks before, during and after being subjected to 30 sec of cooling by the
liquid CO2. The temperature at 0.04m depth is also shown for both the simulated failed and
intact rock mass blocks. Similar results were observed after the 15 and 45 sec cooling periods.
At no point during or after the cooling periods, can a significant difference in surface
temperature be seen, between that of the simulated failed rock mass and that of the simulated
solid rock mass. Small differences in the temperature do appear, see figure 12, but they do not
occur for any significant length of time or to any significant amount. Figure 12 also
demonstrates how only the immediate surface is affected by the liquid CO2, with no change in
average temperature seen at a depth of 0.04m. The simulated failed rock mass block has the
uppermost slab loose, forming an air gap at a depth of 0.04m into the block. It is this air gap,
which, in theory, should cause the surface temperature to return to equilibrium at a different rate
than the surface temperature of the solid test block. However figure 12 demonstrates that the
cooling effect did not penetrate that deep, hence the effects of the air gap were unseen. A longer
period of cooling may return better results; this however would not be possible with liquid CO2
due to the underground constraints of its use as discussed above.
26
The trials conducted here involved relatively simple simulations of failed rock mass, and the
results may not accurately mirror what would be seen during underground trials of such a
technique. The results from these initial trials should, therefore, not be taken as evidence
against the use of such a technique, but as useful data for designing further underground trials.
3.4.4 Computer simulations
As part of the thermal response experiments, 2D computer models where constructed using
basic 2D thermodynamics and heat transfer equations, with Microsoft Excel used as the
modelling program. Both a thermal enhancement technique, similar to that used in the
laboratory experiments, and a naturally occurring thermal gradient were modelled. Figure 13, a
plot of data produced from the Excel spreadsheet model, demonstrates the effect of heating two
rock surfaces, one solid the other loose, for two hours and then allowing them to cool. A clear
temperature difference can be seen during and after the heating process between the solid and
loose surfaces, with the loose surface heating faster and cooling more slowly than the solid rock.
Figure 14, is a plot of data also produced from the Excel spreadsheet model, but this time
demonstrating a naturally occurring temperature gradient. Again two surfaces are modelled,
one loose and the other solid; they are initially given a high temperature, similar to virgin rock
temperature in deep mines. The model then simulates excavation, causing the surfaces under
investigation to come into contact with the cooler ventilated air. The model is left to run for a
sufficient length of time to simulate the gradual cooling effect of the ventilation air. From
figure 14 a clear difference in temperature can be seen between the loose and intact rock after
only a short period of time.
3.5 LASER VIBROMETER
3.5.1 Aims
To investigate the potential of laser Doppler vibrometery as a method of identifying failed rock
mass remotely, by way of its different transient vibration characteristics.
3.5.2 Experimental procedure
The laboratory and field experiments with the laser vibrometer were conducted during two
periods of 1 week and a later period of 1 month, due to limited availability of the instrument.
The laser Doppler vibrometer, a PDV 100, see figure 5 (a), used in the trials is produced by
Polytec of Germany and is designed to be a portable unit. The unit uses the frequency Doppler
shift of a reflected laser beam from an object under investigation, and by processing the return
signal it is possible to measure the vibration of the object remotely. The PDV 100 is one of the
more basic units produced by Polytec, and has the following features: x
x
x
x
x
x
x
3 velocity range settings (±20mm/s, ±100mm/s, ±500mm/s).
Frequency range 0 to 22 kHz.
Working distance 0.2m to >30m.
3 digital low pass filter ranges, 1 kHz, 5 kHz, 22 kHz.
Analogue high pass filter, 100 kHz.
5 hours working time using Li-ion batteries.
Output is through a BNC socket, which can be connected to an oscilloscope for capture
and storage.
The laboratory based experiments, using the laser vibrometer, were initially designed to test if such a device was capable of detecting the vibrations, created by a light hammer blow to a rock
mass. This was tested using a large concrete mass, which simulated solid rock mass conditions. 27
Once it was established that a strong, clear signal could be produced from striking a simulated
solid rock mass, it was clear that a vibration could be measured from less solid / looser
materials. When this important characteristic had been confirmed, the next task was to develop
and trial a testing procedure, which could identify loose rock remotely , on surface before taking
it underground for field trials.
A laboratory test site was designed and built using concrete paving slabs attached to a solid wall
with different amounts of cementitious grout applied to the slabs. By covering only a third of a
slab with adhesive and attaching it to the wall, it was possible to simulate a failed rock mass.
An intermediate slab, half covered, and a solid slab, which had a full covering of adhesive, were
also produced. Using two different slab types, it was also possible to examine the effect of
surface texture and colour on the return signal strength of the laser vibrometer, an essential
element for a clear, accurate measurement.
In order to measure the degree of looseness simulated by the slabs, an Acoustic Energy Meter
(AEM), see figure 4, capable of quantifying the integrity of a surface, was used. The instrument
measures the attenuation of a seismic impulse created by a hammer blow to the area under
investigation. The seismic signal is measured using a geophone placed on the surface and then
processed in real time. A value representative of the degree of looseness is displayed, along
with a “traffic light” style indication with green representing solid rock mass. The values
obtained using the AEM were used to assess the effectiveness of the vibrometer.
A paintball “gun”, which fires paint filled gelatine spheres at a velocity of 100 m/sec was used
to provide a means of imparting vibrational energy to the surface under investigation from a
remote location. The “gun” could also be used to place retro-reflective paint, remotely, onto a
surface under investigation in order to improve the return signal strength of the vibrometer. The
experiments consisted of firing a paintball at one of the slabs, while targeting the laser
vibrometer at the centre of the same slab. The transient vibration measurement was then
captured using the trigger capabilities of a digital oscilloscope. The laser and paintball marker
were positioned 5 metres from the target surface, and the shot was delivered at an angle
perpendicular to the target surface.
3.5.3 Results and discussion
Figures 15 shows transient vibration traces for both the loose and solid slab, after been struck by
a paintball from a distance of 5m. Figure 15 clearly illustrates a difference in vibrational
characteristics between the loose slab and the solid. The initial amplitude of the vibration of the
loose slab is much larger and it continues vibrating long after the solid slab has stopped
noticeably vibrating.
Numerous signal processing methods were investigated to derive a single parameter from the
LDV traces which could be related to the looseness of the samples tested. Eventually, a simple
algorithm, similar to that used for signal processing by the AEM, was derived and found to be
appropriate. Figure 16 shows the resulting values plotted against the AEM value.
The values produced using this algorithm and plotted in figure 16, show a distinct correlation
with the Acoustic Energy Meter data, values increasing as the integrity of the surface under
investigation decreases. For the purposes of the laboratory and later field trials, the algorithm
was applied to the signal by post processing the digital waveform captured from the LDV via a
Microsoft Excel spreadsheet.
The derived algorithm was as follows:
28
§ V 2 Vn
¨¨
© V max
Where :-
V
Vn
Vmax
Ri
Th
·
¸¸ V x
¹
= Velocity
= Background Noise (max from first 30 measurements)
= Velocity max
= Rock integrity
= Threshold value
In order to obtain a single quantitive value, IF logic was applied to the data, with the resulting
values being added together to give the single value required. The IF logic equation, applied
was as follows
R = 1, IF Vx • Th
R = 0, IF Vx < Th
Ri = SUM R
The equation can also be written in Microsoft Excel format, as follows:
IF (Vx < Th = 0, Vx > Th = 1) SUM = Ri
29
30
4 FIELD TRIALS
4.1 INTRODUCTION
After completion of the respective laboratory trials, each concept that had been experimented
upon was examined in detail as to its future potential. From these examinations and
considerations of practicality, it was decided that both the thermal response and transient
vibration response of failed rock mass, should be pursued to the field trial stage. Field
experimentation to test the two concepts under more realistic conditions was designed for
different underground locations, where failed rock mass conditions could be accessed under a
safe operating environment. A Bath dimension stone mine, a Derbyshire limestone mine, an
evaporate mine and a hard rock test mine in Cornwall, were the locations used during the field
trials.
4.2 THERMAL RESPONSE TRIALS
4.2.1 Aims
To conduct field based experimentation into the thermal response characteristics of failed and
unfailed rock masses, with the aim of identifying a difference in temperature between the two
rock classes. Also to examine the potential for the creation and/or enhancement of a preexisting thermal gradient for each of the two rock mass classes.
4.2.2 Location
Two locations were identified as suitable for field based experiments of thermal response, a
dimension stone mine in the Bath area, and an evaporate mine in the north of England. Both
localities demonstrated situations where a remote technique for the detection of failed rock mass
would be invaluable as an everyday survey tool to check on support performance. At the
dimension stone mine a practical technique would also provide a safety tool to check the
condition of old, unsupported developments.
The dimension stone mine at Bath provided an ideal locality to conduct trials using a thermal
enhancement technique to create a discernable difference in temperature between loose and
solid rock mass. The mine lies at a shallow depth of 20 - 30 m, and has a constant low
temperature throughout the year, hence the requirement to create an artificial temperature
gradient. A large support pillar in an old development was surveyed using the Acoustic Energy
Meter (AEM) discussed in section 2.4 (also see figure 4). One of the faces contained a large
loose flake, with areas of solid rock mass present on either side, ideal for a thermal response
trial.
The evaporate mine, due to its depth and high virgin rock temperature, provided a model
location to examine the theory of a naturally occurring temperature gradient between solid and
loose rock mass. It was hoped that interaction of the hot virgin rock temperature and the cooler
ventilated air, would create a discernable temperature gradient between the loose and solid rock
mass. The mine was suffering failure problems in the roof of their main longer term
developments, a situation where a remote device capable of surveying the roof at speed to detect
areas of failure would be ideal.
4.2.3 Experimental procedure
The first field trials on the thermal response theory, were conducted at the dimension stone mine
near Bath. A form of thermal enhancement was employed; with the aim of identifying a loose
flake on a pillar face by a discernable difference in temperature from the surrounding solid rock
31
mass. Using an infrared heater, placed 2m away, the pillar face was heated for a period of 4
minutes, after which time temperature measurements were taken on a grid pattern across the
pillar face. An infrared thermometer, with a sensitivity of 0.2 oC, was used to measure the
temperature remotely at 3 minute intervals.
A second trial conducted at the deep evaporate mine, required no enhancement techniques as a
steep temperature gradient already existed between the hot virgin rock and the cooler ventilated
air. A number of localities were visited throughout the mine, where failure had occurred both
recently and at sometime in the past and in both side wall and roof. Again the infrared
thermometer was used to measure the rock surface temperature remotely. Where failure was
visible in the roof or sidewalls the thermometer was used to scan across the area, while the
operator looked for any discernable temperature difference. The thermometer was also used to
measure air temperature using a thermocouple probe attachment.
4.2.4 Results and discussion
Figures 17 (a, b, & c) illustrate the temperature, at different time frames, of the pillar at the
dimension stone mine as it cooled after the initial heating period of 4 minutes. Figure 18 shows
the AEM values taken at the same positions on the pillar as the temperature measurements. High
values illustrate areas of loose rock mass. As the graphs demonstrate, no correlation can be seen
between the pillar surface temperature and the AEM values, which define the areas of loose
rock mass. The heater footprint is the only clear thermal feature in the surface temperature
plots.
Due to problems with the ventilation it was impossible to extend the heating period to more than
4 minutes, which prevented further results from being gathered. Another problem lay with the
infrared thermometer used, which caused the “blurring” of results. It took 2.5 minutes to take
all the readings required to produce each surface temperature plot. This led to a significant
offset in time between the first and last measurements of each scan. The effect is a blurring of
the results over a 2.5 minute period. The use of a thermal imaging camera would eliminate this
problem and would allow measurements to be taken throughout the heating and cooling period.
The field trials conducted at the deep evaporate mine, provided a variety of situations under
which the theory of a naturally occurring temperature gradient between loose and solid rock
mass could be assessed. In general, the roof temperature was found to be constant to within +/0.1 oC over its width, as were the sides and floor. The air temperature, as measured by the
thermocouple probe, was typically 2 oC below that of the rock.
However these conditions proved ineffective in producing a detectable temperature difference
between loose and solid rock mass. In only one area, a visible and recent shear failure in the
roof, was a detectable change in temperature from loose to solid rock mass observed. The shear
failure had created a wedge shape across half the width of the development, where also a small
portion of the wedge near the sidewall had fallen. This created a partially detached area with an
air-gap between it and the intact roof above. Figure 19 (a) shows a schematic cross section of
the structure observed.
A scan across the width of the development at this location produced a small but noticeable
temperature change, with a drop in temperature of 0.4 oC between the partially detached area
and the adjacent intact roof. The scan across the roof showed an increase of 0.2 oC to the
opposite sidewall, see figure 19 (b).
Although the temperature gradient is small and at the lower end of the instrument’s capabilities,
this result appears to confirm the theory behind the thermal response concept. Figure 19 (b)
clearly illustrates the change in temperature between the detached area and the adjacent intact
32
roof. However it must be noted that this change could also be due to the two surfaces having a
slightly different emissivity, which would also result in a small change in the temperature
reading.
4.3 LASER VIBROMETER TRIALS
4.3.1 Aims
To examine the potential of a laser vibrometer as a tool to detect failed rock mass by measuring
the transient vibration of a surface caused by the impact of a paintball.
4.3.2 Location
Three separate locations were used during the course of these field experiments. The dimension
stone mine near Bath, used during the thermal response trials, became the focus of most of the
work. The mine again provided a number of ideal test sites, which were safe enough to conduct
the trials but also contained areas of failed rock mass in both roof and wall on which
measurements could be taken. Measurements were taken at four different localities throughout
the mine, all of which had areas of rock failure adjacent to solid intact rock mass, again defined
using the Acoustic Energy Meter. Three of the localities were in the roof, whilst the fourth was
part of a large support pillar.
A hard rock test mine in Cornwall provided another location for experiments to take place. The
regular jointing pattern of the granite created suitable test situations, where blocks of granite
were loose but keyed in by their shape, preventing them from falling. A large block, loose but
still keyed into the sidewall provided an ideal test site.
The third location was a limestone mine in Derbyshire, where joint interaction created areas of
loose material adjacent to solid rock mass. A low level roof also created easy access for
alternative excitation methods, other than the paintball marker, to be examined.
4.3.3 Experimental procedure
As with the laboratory trials the initial procedure involved the firing of a paintball at a surface
whilst the laser vibrometer was focused on the same surface. The shot was fired to impact
within 0.15m of the laser spot of the vibrometer. A digital oscilloscope connected to the
vibrometer captured any transient vibration trace detectable above background signal noise
(electrical noise). Captured traces were then downloaded to a PC for later processing and
analysis. Again, an Acoustic Energy Meter was used to measure the integrity of the surfaces
under investigation and provide a value of looseness, with which the vibrometer measurements
could be compared.
The procedure was modified after the series of experiments at the dimension stone mine and the
hard rock test mine in Cornwall, as discussed in the following section. The modified procedure
involved the striking of previously installed roof bolts with a 2 kg hammer at varying distances
from the area targeted by the laser vibrometer. A second nut was placed on the end of the bolt
both to increase the area on which to strike with the hammer and also to protect the bolt from
damage. This new technique allowed more energy to be transferred to the target surface but still
from a remote location. The bolt struck by the hammer transmitted vibrational energy to the
surrounding rock mass more effectively than directly striking the rock mass.
4.3.4 Results and discussion
Figure 20 was produced from the traces generated during the initial field trials at the Bath
dimension stone mine. The same simple algorithm for processing the traces was used as during
the initial laboratory trials. Over 30 measurements were obtained during these trials at the
33
dimension stone mine and the Cornish hard rock mine. Figure 21 shows an example of the
traces from areas of solid and loose rock at the dimension stone mine.
The results from the initial set of tests, at both the dimension stone mine and the hard rock test
mine in Cornwall, were unclear, with traces showing little of the characteristics shown in the
initial laboratory trials. The reason for the unclear results was identified as laying with the
paintball marker used to provide a remote source of vibration. It appears that the energy
transferred into the rock mass by the impacting paintball was insufficient to cause a significant
vibration. Although the impact of a paintball had been calculated to be equal to a 1 kg mass
falling 1m (around 10 joules), other effects had not been considered. Atmospheric friction,
impact angle and the lateral dissipation of energy on impact all served to reduce the impact
energy delivered to the surface by a paintball.
A change in experimental procedure, described above in section 4.3.3, was made, and further
trials were conducted at the limestone mine in Derbyshire, and again, at the Bath dimension
stone mine. Over 50 measurements were obtained during this second series of experiments
using the alternative procedure of imparting energy into the rock by striking a roof bolt head
with a hammer.
Figure 22 (a & b) shows a sample of some of the traces produced using the alternative
experimental procedure at the limestone mine in Derbyshire. Four different traces are plotted
showing measurements taken at the same position by the laser vibrometer, with the vibrational
energy source (hammer blow) at four different distances from the target. The graph shows a
good representation of the type of traces obtained during this part of the trials.
By applying the mathematical algorithm to the traces, a single value was obtained, which was
plotted against the AEM values taken from the same target surfaces, see figures 23 and 24. The
two graphs are plots of the algorithm derived parameter values against the acoustic energy meter
values from both the Derbyshire limestone mine and the Bath dimension stone mine and also
show the distance between the energy source and the LDV target point. Figure 23 shows the
values obtained at the Derbyshire limestone mine with Figure 24 showing the values obtained
from the Bath dimension stone mine.
Figure 22 shows the effect that distance between hammer impact and the target surface had on
the vibration measurements. The traces produced when the impact was close to the target
surface are, clear, well formed and distinctly different for loose and solid rock mass. As the
distance increases, the traces become weak and unclear, and difficult to analyse.
Figures 23 and 24 also highlight the problem of insufficient vibrational energy. For the
Derbyshire limestone mine (Figure 23), when the separation distance is large, (4.4m) the
algorithm values obtained show little in the form of a trend when compared to the AEM value,
with only a slight rise in value from solid to loose rock mass. When the separation distance is
smaller (1.56m) a definite trend can be seen with algorithm values increasing in a similar trend
to those of the AEM. The figures also illustrate that the technique used to obtain and process
the data, produces better results when the rock mass being examined is of intermediate to
extremely loose condition, see figure 23, as at the Derbyshire limestone mine. The transition
from solid to intermediate rock mass conditions as at the Bath stone mine, see figure 24,
delivered more random algorithm values with no correlation between LDV and AEM
measurements. This again is likely to be due to the insufficient vibrational energy.
34
5 CONCLUSIONS
5.1 ULTRASONIC / ACOUSTIC EMISSIONS
The work undertaken in the laboratory, discussed in section 3.3, on ultrasonic emissions and the
findings of the literature study into the principles of the theory provided strong evidence against
such a device being a practical solution to the problem of detecting failing and/or failed rock.
Problems with signal strength restrict the distance over which a device could work and
background noise serves to obscure all but the largest of emissions. If such a device was
capable of overcoming these hurdles one final problem still remains. Science has not yet found
a reliable way of using precursory emissions either acoustic or ultrasonic successfully to predict
rock mass failure.
In conclusion, the measurement of ultrasonic emissions as a portable means of predicting local
rock mass failure in a mine does not appear practical and does not warrant further research at
this time. The use of acoustic emissions as a measure of rock mass failure is a more reliable and
proven technique.
5.2 ELECTROMAGNETIC EMISSIONS
Other authors in this field have successfully demonstrated how electromagnetic emissions can
be detected during the failure of rock mass. However the initial laboratory trials conducted into
this phenomenon demonstrated the problem of background radiation and its masking effect on
failure induced emissions.
Two different conclusions can be drawn from the initial laboratory trials; firstly the failure
mechanism of the sandstone sample may not have caused the fracture of atomic bonds and the
subsequent release of electromagnetic emissions. During the failure of the samples it was noted
that the Hollington sandstone tended to crumble, rather than fail dramatically, which could
result in no electromagnetic emissions being released. The failure of the Horton sample was
however more dramatic, yet again no electromagnetic emissions were detected during its failure.
It is therefore probable that the electromagnetic emissions released during the failure of the two
sandstone samples lay below the level of background noise.
As with ultrasonic emissions, if a device was capable of detecting electromagnetic emissions in
a working underground environment, a method of processing the emissions to give accurate
identification of failure would have to be found. This problem has not yet been resolved after
many years of research; with some scientists now believing such a method is unachievable, as
failure is inherently chaotic. From the work done under this Project it would be unfair to
dismiss the concept as unusable in a practical mining situation but it does not appear to be the
most likely technique to produce a successful and viable device.
5.3 THERMAL RESPONSE
Work completed under this project, both in the laboratory and field, and by other authors, into
the thermal response of failed rock mass and its detection has highlighted the problems involved
with such a concept. Computer simulations of the concept, conducted during this project and by
other authors, show the theory behind the concept to be sound.
The laboratory trials of the concept demonstrated that a simulated failed rock mass could be
distinguished by its difference in surface temperature when its surface is heated with an infrared
35
heat source for a period of only 20 minutes. The difference in temperature was seen twice,
during the heating period and also during the cooling period, when the IR source was removed.
When a cooling source, liquid carbon dioxide, was applied to the same simulated rock mass
conditions, no discernable difference in temperature was observed.
Field based experimentation, into the concept, like the lab based work, proved inconclusive with
only one area of failed rock mass detected using the concept. However only two sites were
available for the field trials, neither of which were ideal. The field and laboratory trials were
important in highlighting the problems with the concept, and in the development of a better
understanding of what is required for such a concept to be practically applied in the field.
5.4 LASER VIBROMETER
The results of the initial laboratory trials conducted on the simulated test slabs and the
preliminary work conducted on the large concrete mass, were able to demonstrate the potential
of the laser vibrometer. The results from these trials proved that, as with the AEM, loose or
failed rock mass can be identified from intact rock mass from its different vibrational
characteristics. The traces produced from the vibrometer were also successfully processed
using an algorithm similar to that used by the AEM and a single representative value was
returned.
Due to these clear conclusions it was recommended that further field trials, under realistic
conditions, should be undertaken. Under these conditions the experimental procedure was
unable to reproduce the same quality of results as were seen during the laboratory phase of the
trials. A lack of vibrational energy resulted in unclear traces and irregular mathematical
parameter values from the processing algorithms which were applied. An alteration in the way
the target surface was excited worked up to a distance of 1.5 - 2 m from the target surface but
only if no discontinuities lay between the roof bolt being struck and the target surface.
From these laboratory and field experiments it is concluded that failed rock mass demonstrates a
noticeable difference in transient vibration characteristics compared to that of solid rock mass.
Using a laser vibrometer it is possible to measure the transient vibrations caused by an impact to
a surface under investigation and identify these differences using mathematical algorithms
applied to the signal traces.
However, for reliable measurements to be made, two things must be achieved. Firstly sufficient
vibrational energy must be applied to the surface area under investigation, from a remote
position. The methods used during these experiments to excite the surface under investigation
were not effective in delivering sufficient vibrational energy from a distance of more than 2m.
Secondly a strong return signal from the surface under investigation is also required for reliable
measurements. To overcome the problem during these trials, retro-reflective tape was used to
obtain a good return signal from the target surfaces, which improved the accuracy of the
measurement and the reliability of the reading. The placement of such tape onto the surface
under investigation does not follow the brief, that the device should be used remotely from the
surface under investigation.
If a testing procedure can be developed, which fulfils the two requirements described above, it
is the authors’ opinion that such a concept would be capable of detecting and quantifying areas
of failed rock mass. The system would be completely remote, allowing operators to survey an
area suspected of being unsafe without having to enter it and put themselves at risk.
36
5.5 OVERALL CONCLUSIONS
The detection of emissions, ultrasonic, acoustic or electromagnetic as an indication of
microfracturing and the imminent failure of rock mass has a number of inherent problems
relating to its use in a mining environment. Emission strength is a key problem, when
attempting to detect such emissions. When background noise is also considered, the problem of
detection becomes even more difficult. In a working underground environment there are a
number of sources of acoustic, ultrasonic and electromagnetic emissions, which create
significant background noise. Such noise then serves to obscure the emissions released by the
failing rock mass, making detection almost impossible in a working underground environment.
However, the detection of such emissions, especially acoustic and to some extent
electromagnetic, as an indication of failing rock mass, is important for research purposes.
Under laboratory conditions, where variables such as background noise and failure rate can be
controlled, such emissions can provide a great deal of information about the failure process.
Even though it has not yet proved possible to use the emissions to give accurate and reliable
predictions of rock mass failure, a better understanding of the processes of failure has been
gained. This knowledge has since been used in the design of safer mines and excavations, to
improve their performance under high stress conditions, and reduce the risk of violent rock
mass failure.
Due to these inherent problems, the researchers went on to examine how more conventional
means of rock failure detection could be modified to become remote methods. This involved
considering the different responses of rock mass, which had failed, but not detached completely
from the surrounding mass. Technology and experience already gained during the development
of the acoustic energy meter, a contact device capable of quantifying the integrity of a surface,
were applied. Research was conducted into the transient vibrational characteristics of failed
rock mass, and how this could be stimulated and measured remotely. Using a laser vibrometer,
measurements of transient vibration were taken from both intact and failed rock mass. Results
showed that with sufficient vibration, distinguishing characteristics could be seen between the
two rock mass classes. Mathematical algorithms applied to the results, could quantify the
integrity of the surfaces under investigation and give an indication of its potential to cause harm.
Although the ability of the vibrometer to measure the different transient vibrations of rock mass
was proven, a safe, mine worthy, technique, by which, sufficient energy could be delivered to a
surface under investigation, must still be found. A method of improving the return signal of the
laser vibrometer, from surfaces under investigation, is another area, which must be resolved,
before such an approach would be suitable for further development.
The thermal response of intact and loose rock mass to two different temperature environments,
either naturally occurring or enhanced by man in the underground environment was also
researched. Initial laboratory work demonstrated that by heating a surface, simulated loose rock
mass could be distinguished from solid rock mass by its different thermal response. Field trials
however failed to provide similar positive results, with only minor success in identifying loose
rock mass seen at one location in an evaporate mine.
37
38
6 RECOMMENDATIONS From the work conducted under this Project it is recommended that the following areas deserve
further research into their potential as novel mobile and portable methods for detecting rock
failure.
(i)
The initial laboratory and field trials into the use of remote transient vibration
measurements and their use to identify failed rock mass, showed positive results. Further
work is recommended into the following areas: Safe and mine worthy techniques for the delivery of sufficient energy to a surface under
investigation should be the main focus due to its critical role in the whole concept.
Without sufficient vibration, measurements are inaccurate and unreliable, hence the
requirement of extra research.
Increasing the return signal strength by means of reflective material placed onto the target
surface by remote means. Without a good return signal the measurements are again
inaccurate and unreliable, hence the need for reflective material on the target surface to
ensure a strong return signal. During the field trials, reflective tape was placed on the
target surface; this however falls short of a wholly remote method. It is recommended
that further work should examine the potential of the paintball marker initially used
unsuccessfully for the delivery of vibrational energy, as a method of placing reflective
paint onto the target surface.
Calculations on the impact energy required to excite loose rock mass of different
dimensions and the displacements likely to be seen, would help in the analysis of the data.
Such work would help in defining the limitations of the concept and give an
understanding of what is possible with the current procedure.
(ii) The work conducted into the thermal response of failed rock mass identified the problems
relating to such a concept. Although previous researchers in this subject were able to
demonstrate how failed rock mass can be identified by a temperature gradient between it
and surrounding intact rock mass, it proved difficult to repeat the same findings outside of
the laboratory during this research. It is the authors’ opinion that this remains a viable
concept and requires further research, and the following further work is recommended: In the authors’ opinion the concept is most suited to deep excavations where virgin rock
temperatures are high, hence research should be focused here, rather than shallow
excavations where more energy is required to develop a thermal gradient. Work should
be conducted into ways in which normal mining practices can be modified in order to
enhance a temperature difference to detectable levels. The following are some
suggestions how a temperature difference could be enhanced between intact and loose
rock mass: x Increasing ventilated airflow in a development for a few hours prior to a survey.
x Use of an auxiliary cooling fan to lower ventilated air temperature in a
development prior to a survey.
x Stop ventilating a development for an over night period, causing the rock mass to
reach an equilibrium closer to virgin rock temperature. After the over night
39
period, restart the ventilation, whilst surveying the area with thermal imaging
equipment.
ƒ Spraying hot rock with cooled water. This could be particularly successful in
deep, hot mining environments where cooling water is often already available for
other purposes.
40
7 REFERENCES
Altounyan P.F.R. and Minney D. 2000, Field Experience of Measuring the Acoustic Energy
from a Hammer Blow to Coal Mine Roof and its relationship to Roof Stability. Proc. 19th
Conference on Ground Control in Mining, Morgantown USA. 8-10 Aug 2000. pp 12- 18. Altounyan P.F.R., Clifford B. and MacAndrew K.M. 1999. Assessing and evaluating acoustic techniques for testing roof conditions in coal mines. Final Report SIMRAC Project COL 610.
Brady, B.T., Rowell, G.A. 1986. Laboratory investigation of the electrodynamics of rock fracture. Nature 1980;321:488-492.
Cartwright P., Clifford B., Ärmänen E. and Vuori A. 2001. Application of the Acoustic Energy Meter for assessment of tunnel lining condition. Proc. ISRM Regional Symposium
Eurock 2001. Epso, Finland, 4-7 June 2001, pp333-338
Cress, G.O., Brady, B.T. and Rowell, G.A. 1987. Source of electromagnetic radiation from fracture of rock samples in the laboratory. Geophys Res Lett 1987;14:331-4.
Geller, R.J., 1997. VAN cannot predict earthquake – nor can anyone else. International centre for disaster-mitigation engineering (INCEDE) newsletter, vol5-4 Hanson, D.R. and Rowell, G.A. 1982. Electromagnetic radiation from rock failure. USBM RI
8594, 21p:,27cm, US Department of the Interior. Hazzard, J.F. 1999. Numerical modelling of acoustic emissions and dynamic rock behaviour.
PhD thesis, Keele University, Staffordshire, UK, 1999.
Hirata, T. 1987. Omori’s power law aftershock sequences of microfracturing in rock fracturing experiment. J. Geophys. Res. 92. Pp 6215-6221.
Kononov, V.A. 2000.
Pre-feasibility investigation of infrared thermography for the
identification of loose hangingwall and impending falls of ground. SIMRAC final report
GAP706. 26p.
Lockner, D. 1993. The role of acoustic emission in the study of rock fracture. Int. J. Rock.
Mech. Min. Sci. & Geomech. Abstr. Vol.30(7) 1993, pp883-899.
Meredith, P.G., Main, I.G., and Jones, C. 1990. Temporal variations in seismicity during
quasi-static and dynamic rock failure. Tectonophysics 175, pp249-268.
Merrill, R.H. and Morgan, T.A. 1958. Method of determining the strength of mine roof. U.S.
Bureau of mines, Report of Investigation R.I. 5406, 22p.
Merrill, R.H. and Stateham, R.M. 1970. Loose rock can be detected by infrared devices.
Mining engineering, November 1970, pp59-62.
Nitsan, V. 1977. Electromagnetic emissions accompanying fracture of quartz-bearing rocks.
Geophys Res Lett, 4(8):333-6.
Obert, L. 1941. Use of subaudible noise for prediction of rock bursts. U.S. Bureau of mines,
Report of Investigation R.I. 3555, 1941.
Obert, L. and Duvall, W. 1942. Use of subaudible noise for the prediction of rock bursts, part
II. U.S. Bureau of mines, Report of Investigation R.I 3654, 1942.
Piper P.S., Bron K.B., van Rooyan H., Goldbach O.D. and Clifford B., 2002. The
application of acoustic techniques for identifying rock-related hazards in gold and platinum
mine. Final Report SIMRAC Project GAP 822.
Rabinovitch, A., Frid, V., Bahat, D. and Goldbaum, J. 2000. Fracture area calculation from
electromagnetic radiation and its use in chalk failure analysis. Int J Rock Mech Min Sci
2000;37:1149-1154.
Stravrakakis, G. 1998. Claims of success in using geoelectrial precursors to predict earthquakes are criticised and defended. Physics today, January 1998.
Urusovskaja, A.A. 1969. Electric effects associated with plastic deformation of ionic crystals.
Sov Phys-Usp 1969;11:631-43.
41
Warwick, J.W., Stoker, C. and Meyer, T.R. 1982. Radio emissions associated with rock
fracture: possible applications to the Great Chilean earthquake of May 22, 1960. J Geophys Res 1982;87(b4):2851-6.
Wu, L., Cui, C., Geng., Wang, J. 2000. Remote sensing rock mechanics (RSRM) and
associated experimental studies. Int. J. Rock Mech. Min. Sci. Vol. 37(6) pp879-888.
Wu, L. and Wang, J. 1998. Infrared radiation features of coal and rocks under loading. Int. J.
Rock Mech. Min. Sci. Vol.35(7), 1998, pp 969-976.
Yu, T.R., Henning, J.G. and Croxall, J.E. 1990. Loose rock detection with infrared
thermography. CIM bulletin, May 1990, pp 46-52.
Young, R.P., Collins, D.S. 1997. Acoustic emission/microseismicity research at the
underground research laboratory, Canada (1987-1997). Report# RP037AECL, University Keele,
1997, p139.
42
8 FIGURES 43
Seismic anomaly.
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57
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200
300
400
500
AEM value
)LJXUH $OJRULWKPYDOXHYHUVXV$(0YDOXHVIRUGLIIHUHQWVHSDUDWLRQVIURP
OLPHVWRQHPLQH
Algorithm
Solid
Intermediate
100
90
80
70
60
50
40
30
20
10
0
0.73m
1.73m
2.71m
5.54m
0
20
40
60
80
AEM value
)LJXUH $OJRULWKPYDOXHYHUVXV$(0YDOXHVIRUGLIIHUHQW
VHSDUDWLRQVIURPGLPHQVLRQVWRQHPLQH
62
100
Printed and published by the Health and Safety Executive
C30 1/98
Printed and published by the Health and Safety Executive
C1.10 06/04
ISBN 0-7176-2866-3
RR 248
£15.00
9 78071 7 628667