SITE CHARACTERIZATION & INSTRUMENTATION
MODULE 4
Module 4: Site Investigation using Non-Destructive Tests
Topics:
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Introduction
Electrical Methods
Magnetic Methods
Gravity Methods
Acoustic Emission Methods
Seismic Methods
Keywords:NDT,GPR, MASW, SASW, Down-hole shear waves
4.1 Introduction:
Non-destructive testing are analysis techniques used in science and technology to
evaluate the properties of a material, component or system without causing damage.
NDT methods may rely upon use of electromagnetic radiation, sound and inherent
properties of the material to examine samples. The need for applying these tests
during site investigation is their ability to non-destructively and rapidly screen sites at
relatively low cost and to target significant and possibly unexpected subsurface
features (i.e. hazards) that represent majority of the engineering project.
The great difference between the methods used in geophysics and civil engineering
lies in the difference in geometrical scales. In geophysics, the characteristic distances
are kms to thousands of kms. In civil engineering, the characteristic distances are tens
of meters to several kms.
Geophysical techniques offer the chance to overcome some of the problems inherent
in more conventional ground investigation techniques. Many methods exist with the
potential of providing profiles and sections, so that (for example) the ground between
boreholes can be checked to see whether ground conditions at the boreholes are
representative of that elsewhere.
Geophysical techniques also exist which can be of help in locating cavities, backfilled
mineshafts, and dissolution features in carbonate rocks, and there are other techniques
which can be extremely useful in determining the stiffness properties of the ground.
Many mineral explorations will be centred upon deep deposits, where ground
conditions are spatially relatively uniform, and geological structures are large.
Geophysical techniques are relatively cheap and are highly regarded in such a
speculative environment.
Geophysical techniques can contribute greatly to the process of ground investigation
by allowing an assessment, in qualitative terms, of the lateral variability of the nearsurface materials beneath a site.
Geophysical techniques can also be used for vertical profiling. Here the objective is to
determine the junctions between the different beds of soil or rock, in order to either
correlate among boreholes or to infill between them. Techniques used for this purpose
include electrical resistivity depth profiling, seismic methods, the surface wave
technique, and geophysical borehole logging.
Sectioning is carried out to provide cross-sections of the ground, generally to give
details of beds and layers. It is potentially useful when there are marked contrasts in
the properties of the ground.
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4.1.1 Test used to Measure Shear wave velocity
The field tests or the in situ tests measure the dynamic soil properties without altering
the chemical, thermal or structural condition of the soil. The field test can be broadly
divided into two–low strain and large strain tests.
Low Strain Tests: The strain levels in these types of tests will be around 0.0001%.
Some of the important low strain tests are discussed below.
1. Seismic Reflection Test: This test is used to evaluate the wave propagation velocity
and the thickness of soil layers. The test setup will consist of a source producing a
seismic impulse and a receiver to identify the arrival of seismic waves. The travel
time from source to receiver is measured. Based on these measurements, the thickness
of soil layer can be evaluated.
2. Seismic Refraction Test: This test will use the arrival time of the first seismic wave at
the receiver. Using the results obtained from this test, the delineation of major
stratigraphic units is possible.
3. Suspension logging test: This test is used to measure the wave propagation velocity
and it is commonly used in petroleum industry. This is very effective at higher depths
(up to 2 km)
4. Steady state vibration test: In this test, the wave propagation velocities are measured
from steady state vibration characteristics. However, these tests can be useful for
determining the near surface shear wave velocity and they fail to provide the details of
highly variable soil profiles.
5. Seismic cross hole test: In seismic cross hole test, the wave velocities are measured
using more than one bore hole. In the simplest case, two bore holes are used –one
with an impulse source and the other with a receiver and both are kept at the same
depth. The test is repeated at various depths to get the soil profile. Generation of body
waves dependent upon source type the seismic wave generated would be P, SV, or SH
body waves
6. Seismic down hole (up hole) test: This test is used to measure the travel time of
seismic waves from source to receiver. It is performed using a single borehole. In
seismic down hole test, the receiver is kept at the ground surface and the impulse
source is kept at different depths. The up hole test is done with receiver at the ground
surface and the impulse source in the borehole. This test is not effective for depths
greater than 30 to 60 m.
4.2 Types of Geophysical methods based control of input:
Geophysical methods may be divided into two groups.
4.2.1. Passive techniques: The anomalies measured by the technique pre-exist. They cannot
be varied by the investigator. Repeated surveys can be carried out to investigate the effects of
variations of background ‗noise‘, but apart from varying the time of the survey and the
equipment used, no refinement is possible. Generally, these techniques involve measurements
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of local variations in the Earth‘s natural force fields (for example, gravity and magnetic
fields).
4.2.2. Active technique: These techniques measure perturbations created by an input, such as
seismic energy or nuclear radiation. Signal-to-noise ratio can be improved by adding together
the results of several surveys (stacking), or by altering the input geometry.
In general, interpretation is more positive for active than for passive techniques, but the cost
of active techniques tends to be greater than for passive techniques.
4.3 Types of Geophysical methods based type of measurement:
Some geophysical techniques detect the spatial difference in the properties of the
ground. Such differences lead to perturbations of the background level of a particular
measurement which are measured, and must be interpreted. These perturbations are
termed ‗anomalies‘.
Other geophysical techniques measure particular events and during interpretation
these measurements are converted into properties (in this case, seismic shear wave
velocity).
A particular geophysical technique will make measurements of only a single type.
Techniques that are commonly available measure:
1. Seismic wave amplitude, as a function of time;
2. Electrical resistivity or conductivity;
3. Electromagnetic radiation;
4. Radioactive radiation;
5. Magnetic flux density; and
6. Gravitational pull
Gravity methods respond to differences in the mass of their surroundings, which
results either from contrasts in the density of the ground, or from variations in
geometry (cavities and voids, embankments, hills, etc.).
Magnetic methods detect differences in the Earth‘s magnetic field, which are
produced locally by the degree of the magnetic susceptibility (the degree to which a
body can be magnetized) of the surroundings. Such methods will primarily detect the
differences in the iron content of the ground, whether its natural or artificial.
The seismic method relies on the differences in velocity of elastic or seismic waves
through different geological or man-made materials. An elastic wave is generated in
the ground by impact force (a falling weight or hammer blow) or explosive charge.
The resulting ground motion, is detected at the surface by vibration detectors
(geophones). Measurements of time intervals between the generation of the wave and
its reception at the geophones enable the velocity of the elastic wave through different
media in the ground to be determined.
A seismic disturbance in elastically homogenous ground, whether natural or
artificially induced, will cause the propagation of four types of elastic wave, which
travel at different velocities. These waves are as follows:
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1. Longitudinal waves
2. Transverse waves
3. Rayleigh waves
4. Love waves
4.4 Electrical Methods:
Some methods use the naturally occurring electrical fields and others require
artificially introduced electrical currents. Electrical methods are mostly used to search
metals and minerals at relatively shallow depths, down to 500 m. In cultural stone,
natural electric currents probably do not exist so the electrical field has to be
artificially introduced in testing stone with electrical methods.
Three methods which already have been applied or should have a potential to be
applied in the study of cultural stone have been selected. These are the radar,
resistivity and electromagnetic methods.
4.4.1 Ground penetrating radar (GPR):
RADAR (RAdio Detection And Ranging) was initially developed as a means of using
microwaves to detect the presence of objects, typically aircraft and ships, and to
derive their range from the transmitter. This process was achieved by transmitting
pulses of radiation and recording the reflections.
Ground penetrating radar operates at frequencies between 1 and 2500 MHz and is
capable of penetrating the ground to depths of more than 30m.However, in the case of
ground with a relatively high conductivity (for example, saturated clay) the depth of
penetration may be reduced to less than 1 m.
The radar unit produces a pulsed electromagnetic wave, which travels through the
ground at a velocity controlled by the electrical properties of the ground. Differences
in relative permittivity (dielectric constant) or electrical conductivity resulting from
changes in soil type or groundwater chemistry will result in, the waves being
reflected. The signals reflected from subsurface interfaces or buried objects are
received by the same antenna as that for transmission.
The receiving electronics amplifies and digitizes the reflected signals which are stored
on disk or tape for complete post- processing.
Thus once the return signal is received by the antenna, the radar system acts in a
similar manner as a seismograph, in providing an accurate timebase for storing and
displaying the radar record. Figure 4.1 shows operation concept of ground penetrating
radar (GPR) system.
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Figure 4.1: Operation of a ground penetrating radar (GPR) system.
One major difference between a seismograph and a radar system is in the time-base
resolution. For a radar system, the resolution is measured in tens of picoseconds,
whereas the resolution for a seismograph may be several hundred nanoseconds.
Immediate on-site results may be viewed on a graphics display.
Electrical properties affecting GPR wave propagation are di-electric value and
electrical conductivity.
4.4.1.1 Principles of GPR image:
Reflection and polarity:
When a GPR signal propagates from medium 1 to medium 2 and medium dielectricity values are E1 and E2, the reflection amplitude will be:
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On the basis of the formula, the polarity of the reflection changes if ε1 is smaller than
ε2, which is usually the basic situation in road and soil structures (moisture content is
getting higher when getting deeper). If ε1 is bigger than ε2, then the polarity of the
reflected wave remains the same as the progressive wave‘s polarity at the interface.
However, unlike other GPR road surveys, the polarity information is not so critical in
site investigations and still in many cases, GPR site investigations data is presented in
a way that does not show the sign of the polarity.
Depth penetration, resolution and interface depth:
Achievable depth penetration with ground penetrating radar depends on what antenna
frequency is used and therefore the signal wave length. The attenuation increases
when GPR central frequency increases. A highly conductive medium results in an
increase in the amount of energy scattering objects, when the wavelength gets shorter.
Similarly, the penetration depth gets smaller as the frequency gets higher. Figure 4.2
shows Principle of the ground penetrating radar.
On the other hand, the resolution gets better at the same time. The resolution also
improves when dielectric value increases. In site investigations this means that if
target for the survey is to obtain information from as deep as possible, then antenna
central frequencies between 50 – 200 MHz should be used. If the target is closer to
the surface, i.e. 3 – 6 m, then frequencies higher than 200 MHz can be used.
Resolution refers to how close interfaces can be to one another and can still be
identified as separate interfaces. This applies to both directions, horizontal and
vertical. The vertical dimension‘s resolution of the pulse can be calculated from the
following formula:
Where
c = the speed of light in vacuum (0.3 m/ns)
τ = the pulse length (ns)
εr = medium‘s relative di-electricity
The depth to an observed interface can be calculated from this formula (for monostatic antennas):
Where
twt = two way travel time of the wave
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Figure 4.2 Principle of the ground penetrating radar (Finnish Geotechnical Society,
1992)
4.4.1.2 GPR Data Processing and Interpretation:
The primary goal of the GPR surveys in site investigations is to provide information
for geotechnical design, find reasons for certain failures or collect basic information
for instance from ground water conditions. In most of the cases,
information
collected with GPR will be transferred to different CAD systems.
If the data collection has been carried out properly with appropriate equipment, the
information described earlier can be produced with careful and professional
processing and interpretation. The following describes the general guidelines for data
processing and interpretation.
Pre-processing:
In this case, pre-processing means GPR data editing and combining other site
investigation or geological data, acquired in different ways. Data editing includes
operations which do not change the original information content of the data. This
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means distance scaling, joining and splitting of different lines and reversing directions
(if needed).
Linking the distance coordinates means that the survey has a starting point and all the
data is scaled to this starting point with equal scale. However, this is suitable in
special cases where surface coordinates are not important.
Many times, the coordinates at certain points on a survey line are known and these
points should be linked to the data the way that their z-coordinates are also stored.
Recently, in most cases, coordinates are collected simultaneously during the surveys,
using real time GPS systems.
Data processing and interpretation:
Ground penetrating radar data basic processing usually includes scaling, the definition
of the surface level and ―background removal‖ filtering operations. If needed, vertical
high pass and low pass filtering and gaining can be used.
Ground penetrating radar data interpretation includes defining the critical layer
interfaces and calculating their depths and making annotations on the soil and bedrock
types, their material properties and, for instance, position of ground water table.
If the layer interfaces are weak and the interpretation is uncertain, interface quality
codes must be used. These interface codes are:
a) A clear, strong and reliable interface (solid line)
b) Unclear, but probable interface (a line longer than the interval)
c) Unclear interface (interval longer than the line)
The limitation of this method is that, it can only be applied to non-conducting
materials. Both global and local damages are possible to detect with this method. It is
used in determination of the depth of rock and ground water, localisation of sand and
gravel deposits, localisation of blocks, investigations of rods and archaeological
investigations.
4.4.2 Electrical resistivity methods:
The methods of electrical resistivity are used to measure the apparent resistivity of the
ground. The variations of the apparent resistivity are due to the variations in the
composition of the ground.
The electrical resistivity of a material is defined as the resistance of a cylinder with a
cross section of unit area and with unit length. If the resistance of a conducting
cylinder having a length, L, and cross section area, A, is R, then the resistivity, ρ, is
expressed by the formula
The electrical resistance of the cylindrical body R (Ω), is defined by the Ohm‘s law as
follows:
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with V being the potential (V) and I is the current (A). Electrical characteristic is also
commonly described by the conductivity value σ, equal to the reciprocal of the soil
resistivity. Thus:
In a homogeneous and isotropic half-space, electrical equipotential are hemispherical
when the current electrodes are located at the soil surface. The current density J
(A/m2) has then to be calculated for all the radial directions with :
Where 2πr2 is the surface of a hemispherical sphere of radius r. The potential V can
then be expressed as follows:
Measurement of electrical resistivity usually requires four electrodes: two electrodes
called A and B that are used to inject the current (‗‗current electrodes‘‘), and two
other electrodes called M and N that are used to record the resulting potential
difference (‗‗potential electrodes‘‘). The potential difference ΔV measured between
the electrodes M and N is given by the equation:
Where AM, BM, ANand BN represent the geometrical distance between the
electrodes A and M, B and M, A and N, and B and N, respectively. The electrical
resistivity is then calculated using:
where K is a geometrical coefficient that depends on the arrangement of the four
electrodes A, B, M and N. The current electrodes A, B and the potential electrodes M
and N can be placed in the field at the soil surface, or in boreholes. As compared with
the surface methods, the cross borehole methods present the advantage of a high
resolution with depth (Slater et al., 2000). This technique requires nevertheless,
intrusion into the studied bodies for the insertion of the electrodes. At the laboratory
scale this technique can also be applied by placing the electrodes around the soil
sample at various depths.
The porosity and chemical content of the water filling the pore spaces are more
important in governing the resistivity than the conductivity of the mineral grains of
which the rock is composed. The salinity of the pore water is probably the most
critical factor determining the resistivity. The range of resistivity among rock
materials is enormous extending from 10-5 to 10-15 Ω-m.
The method as it is normally used in geophysics is based on the introduction of a
direct current, I, through electrodes at the surface of the material. The electrical
potential, U associated with this current is measured between two other electrodes on
the same surface. Typical arrangement of 1-D resistivity test is shown in Figure 4.3
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Figure 4.3: Arrangement of a resistivity test
The potential, U, is measured and the apparent resistivity is calculated with the help of
equations mentioned above.
If the electrodes are laid out in a line and their distances are increased in a systematic
manner, it is possible to determine the variation of the resistivity with depth.
One-, two- and three-dimensional surveys:
One-dimensional survey:
One-dimensional arrays using four-electrode cells A, B, M, and N are commonly used
in the laboratory for electrical resistivity calibration and in the field for vertical
electrical sounding (VES). The latter consists of electrical measurements where
distances between the electrodes are successively increased. At each step, the depth
and volume of soil investigated increases and the measurement displays the variation
of soil resistivity with depth without taking into account the horizontal variation. For
VES data interpretation, it is usually assumed that the subsurface consists of several
horizontal layers.
Two-dimensional survey:
Two-dimensional multi-electrode arrays provide a two-dimensional vertical picture of
the sounding medium. The current and potential electrodes are maintained at a regular
fixed distance from each other and are progressively moved along a line at the soil
surface. At each step, one measurement is recorded. The set of all these measurements
at this first inter electrode spacing gives a profile of resistivity values. The interelectrode spacing is increased then by a factor n = 2 and a second measurement line is
done.
This process (increasing the factor n) is repeated until the maximum spacing between
electrodes is reached. One can notice that larger the n-values, greater the depths of
investigation. As distribution of the current also depends on the resistivity contrasts of
the medium, the depth of investigation deduced from the spacing is called the
‗‗pseudo-depth‘‘. The data is then arranged in a 2D ‗‗pseudo-section‘‘ plot that gives
a simultaneous display of both horizontal and vertical variations in resistivity.
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Depending on the respective position of the potential electrodes and on the current
electrodes, several array configurations can be defined. They are:
1. Wenner,
2. Wenner–Schlumberger,
3. Dipole–dipole,
4. Pole–pole,
5. Pole–dipole.
The latter configuration is an asymmetrical array, in which two directions (forward
and reversed) are considered.
Depending on the array configuration, the geometrical factor K differs. Figure 4.4
shows summarizes for different 2D array configurations and compares the following
characteristics for all the arrays:
1. The sensitivity of the array to horizontal and vertical heterogeneities,
2. The depth of investigation,
3. The horizontal data coverage and
4. The signal strength.
Figure 4.4: 2-D electrical resistivity section
The different orientations of heterogeneity can be vertical for heterogeneities such as
dykes, cavities, preferential flow, or horizontal such as sedimentary layers. The depth
of investigation is determined for homogeneous ground, but gives a prior indication of
the depth of investigation in heterogeneous ground.
All the different array types have specific advantages and limitations. The choice of
the array configuration then depends on the type of heterogeneity to be mapped and
also on the background noise level; the characteristics of an array have to be taken
into account (Table 4.1).
Table 4.1: Characteristics of different 2D arrays configurations types
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Sensitivity of array
horizontal structures
++++
Wennerschlumberger
++
Dipoledipole
+
Polepole
++
Poledipole
++
Sensitivity of array
vertical structures
+
++
++++
++
+
Depth of investigation
+
++
+++
++++
+++
Horizontal data
coverage
Signal strength
+
++
+++
++++
+++
++++
+++
+
++++
++
Three-dimensional survey:
Two methods can be used to obtain a three dimensional electrical resistivity
acquisition. The first method, consists of building a three dimensional electrical
picture by the reconstruction of a two dimensional network of parallel pseudosections. An accurate three-dimensional electrical picture is thus, recorded if electrical
anomalies are preferentially oriented and if the in-line measurement electrodes are
perpendicular to the orientation of the anomalies. Figure 4.5 shows 2D in line
electrodes array configuration and 3D electrode device.
Figure 4.5: 2D in line electrodes array configuration and 3D electrode
device
The second method consists of using a square array of four electrodes. Such an array,
provides a measure of resistivity less orientation-dependent than that given by an inline array.
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Through the combined results of theoretical studies and of field surveys, one of their
conclusions pointed out the advantage of using a circular array configuration in the
detection of heterogeneities in an archaeological context. Apparent anisotropy effects
also provide useful information related to the direction of electrical anomalies for 3D
electrical survey.
Errors in electrical resistivity methods:
1. Electro-magnetic coupling between the potential and current electrode cables
2. Interference from high(or low) tension electrical cables or electrified railway lines
3. Highly heterogeneous ground
Interpretation:
Quantitative interpretation of electrical sounding data is possible using various curve
matching techniques. The measured values of apparent resistivity are plotted as a
function of current electrode spacing. For Wenner configurations, ρa is plotted as a
function of a (1/3 current electrode spacing) and for Schlumberger configurations ρa is
plotted as a function of L (1/2 current electrode spacing). Logarithmic scales are
normally employed as this facilitates direct curve matching techniques since there are
no problems in comparison of data due to the use of different scales.
These master curves are matched with the field curves. The master curve which best
matches the field curve can be used to determine the apparent resistivity and the
depths of each layer detected in the field. Figure 4.6 shows some field data on which a
best-fit master curve has been superimposed. The field curve will never exactly match
the master curve because of undulating or dipping interfaces, which cannot be
represented in a finite set of master curves.
Preliminary electrical soundings are therefore, carried out where possible close to the
site of a borehole or near an exposure to provide a control over interpreting
subsequent electrical soundings. Data processing techniques now allow a more
objective approach to be made in finding a physical model which best represents the
field prototype.
One limitation of the method is that different soil profiles can give the same
experimental results, i.e. the same potential U.
Applications:
Only global damages can be detected by this method.
The method is well established in geophysics particularly for detecting the depth to
the ground water surface, depth to the rock surface, quality of water, spreading
plumes of contaminants and many other applications.
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Figure 4.6: Interpretation of electrical resistivitysounding data.
4.4.3 Electro-magnetic methods:
The electromagnetic method as used in geophysics is mostly applied to mineral
prospecting. For that purpose, it is the most widely used method.
The method is based on the induction of electrical currents in buried conductors by
electro-magnetic waves generated at the surface of the ground. The waves are
generated by alternating currents, which are passed through loops of wire. The
frequencies are in the range of a few hertz to a few megahertz. When the waves pass
through a conducting body, they induce currents in the body. These currents become a
source for new electromagnetic waves, which can be detected by another coil.
There are several types of electromagnetic methods. Two of them are the VLF method
and the Slingram method.
In the VLF (very low frequency) method, the sources are distant stationary emitters,
which are situated at several places around the world. The frequencies are in the range
of 15 - 25 kHz.
In the Slingram method, the emitter and the receiver coils are carried together. The
frequencies emitted are often in the range of 800 – 18,000 Hz.
The quantities measured are the electric potentials.
A limitation of the method is that it is difficult to make quantitative interpretations.
Many electrical configurations in the ground may yield the same experimental results.
Only locally occurring metal objects will be found with this method. The method is a
well-established method in geophysics where it is primarily used to study clay
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deposits, salinity of water, depth of weathering in rock, weak zones in the rock
basement etc.
4.5 Magnetic Methods:
Magnetic methods are based on the measurement of local variations in the Earth‘s
magnetic field. Such variations are associated with differences in magnetic
susceptibility of rocks and soils. Since magnetic methods measure variations in a
natural force field, the resulting data cannot be readily interpreted in a quantitative
manner.
Magnetic techniques are particularly useful in locating localized subsurface features
of engineering interest such as abandoned mineshafts, sink holes, and buried services.
The main advantage of the method is the fact that magnetic measurements can be
made extremely fast and hence the use of the method is reasonably cheap.
The measurements made in magnetic surveying may be of the vertical component of
the Earth‘s magnetic field or of the Earth‘s total magnetic field strength.
Measurements of the vertical component of the Earth‘s magnetic field are made
mechanically using magnetic balances. The total field strength is measured using
fluxgate or proton instruments.
Ideal sites for the use of magnetic methods are on open little-developed land, free
from extraneous interference.
Advantages:
The advantage of magnetic methods is the speed at which measurements can be taken.
With a proton magnetometer it is possible to cover an area of 1500 m2 in a day taking
measurements on a 1 m grid. It is possible to cover 1000 m2 in 60 mm taking
measurements on a 2 m grid with the less sensitive fluxgate magnetometer.
Since essentially only one correction is applied to the field measurements, data
reduction, presentation and interpretation can be carried out rapidly.
Clearly the magnetic methods can be most cost effective in site investigations that
require localized features at shallow depths.
The main use of magnetic methods in site investigation appears to be for the location
of abandoned mineshafts.
4.6 Gravity Methods:
Gravity methods involve measuring lateral changes in the Earth‘s gravitational field.
Such variations are associated with near-surface changes in density and hence may be
related to changes in soil or rock type. Because gravity methods involve the
measurement of a natural force field, ambiguous data (in terms of interpretation) are
common and hence the interpretation of field data is qualitative.
The effects of rapid near-surface changes in density limit the use of this method in
practice to the mapping of large- scale geological structures. This can be of great
value in oil exploration but on the very much smaller scale of engineering site
investigations it seriously restricts the use of gravity measuring techniques.
Gravity methods may be used for the location of large faults and to find the extent of
large buried channels.
Gravity measuring instruments must therefore be extremely sensitive. A gravity meter
comprises sensitive balances such that small variations are magnified by mechanical
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or mechanical and optical methods to enable readability. Only the vertical component
of gravity is measured with these instruments.
Measured values of the vertical component of the gravitational field are not only a
function of density but also a function of latitude, elevation, local topography, and
tidal effects. The effects of these on gravity measurements can be determined and
corrections can be made.
Every field measurement must be reduced by applying the following corrections.
1. Latitude correction
2. Elevation correction
3. Terrain correction
4. Instrumental drift and tidal correction
The corrected gravity data are normally presented as a contoured gravity map.
In general, gravity methods are too slow and expensive to be cost effective in
conventional site investigations. Only in rare circumstances, the use of gravity
method is justified, particularly as justification must normally be based on the limited
information available at an early stage of the investigation.
4.7 Acoustic Emission Methods:
When a brittle material is stressed or strained, elastic waves are generated. These
waves are called as acoustic emission (AE) or micro seismic activity (MA).
The stress waves i.e. P and S-waves propagate to the surface where they can be
recorded and analysed. This is the same situation when earthquake sources deep in the
ground produce stress waves, which are recorded by seismological observatories. The
signals are mostly measured by piezoelectric transducers.
By using three or more transducers it is possible to localise the source. Mostly the
numbers of events per unit time are recorded or the number of counts per unit time.
Each event is formed by a number of counts, which are the peaks above the
background noise threshold of the recorded event.
The different methods of AE/MA offer a possibility to investigate different stress
situations.
The method may also be used in the evaluation of the effectiveness of conservation
treatments; specifically the use of consolidants and protectors. The AE technique can
be very sensitive.
The quantities measured are of different kinds. They may be arrival times and
acceleration time histories from which it is possible to deduce the number of counts,
duration and frequency content of the signal.
The equipment needed are accelerometers and a data acquisition system, which are
commercially available.
The basic equipment for AE/MA monitoring is relatively simple; a transducer which
often is an accelerometer, an amplification system and a recording equipment. A filter
system is also included to take away the background noise.
One limitation of the method is that to perform a test is a rather complicated
procedure with relatively complicated instruments.
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4.8 Ultra-Sonic Method:
Ultrasonic methods are well established in flaw and crack detection in the mechanical
industry. It is probably appropriate to say that ultrasonic methods are the most
commonly used NDT-methods.
Both the exciters and the receivers are normally piezoelectric devices in ultrasonic
tests. Ultrasonic frequencies are those which are above audible sound i.e. above 20
kHz.
The ultra-sonic methods are similar to seismic methods. The quantities measured for
the different methods of ultra-Sonics are the corresponding quantities measured in the
seismic methods (which will be discussed later).
The equipment needed for ultrasonic measurements is specially designed equipment,
which is highly sophisticated and expensive. In principle, it contains of ordinary
transducers and a data acquisition system.
One of the limitations is the bulkiness of the equipment.
4.9 Seismic Methods:
Some of the seismic methods, particularly the transmission and the reflection method,
are standard methods in geophysics and are used since a long time.
Seismic methods use mechanical waves to obtain data from the tests. The seismic
surface wave method is strictly speaking a seismic transmission method.
4.9.1 Mechanical waves:
Mechanical wave motion means a collective phenomenon, which moves energy
through the material. The speed of that energy movement is the speed of the wave or
wave velocity. The individual particles of the material move around their points of
equilibrium and retain that position after the wave has passed.
This means that the particles move with a velocity, which changes with time. The
speed of the particles or particle velocity is normally in the order of mm/s and is much
lower than the speed of the wave which normally is in the order of km/s. Wave
motion thus does not mean transport of mass.
Mechanical waves in solid material can be of many different kinds. These can be
divided into two completely different types; the longitudinal waves and the transverse
waves. A longitudinal wave moves the particles parallel to the direction of
propagation while a transverse wave moves the particles normal (90°) to the direction
of propagation.
The longitudinal wave travels with a higher speed cp, than the transverse wave. It is
therefore called the primary wave or the P-wave. The transverse wave is called the
secondary wave or the S-wave. The speed of the S - wave, cs, is approximately half of
that of the P - wave.
There are two more waves which are referred to as surface waves. They are the love
waves and the Rayleigh waves.
Hence there are totally four different types of mechanical waves propagating through
a surface whenever a disturbance occurs. The speeds of the waves are supposed to be
correlated to the strength of the material.
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4.9.2 Seismic transmission method:
This method uses the transmission of body waves (P or S wave) from one point, the
emitter, to another point, the receiver, to test the material. The material may be
homogenous or if it is inhomogeneous, the result is the average values of the
investigated physical entities.
There are basically two different ways to perform this test. One is by a transient; a
relatively short pulse from the source, produced for example by a hammer blow.
Another way is to use steady-state vibrations i. e. vibrations during a relatively long
time, produced by a vibrator.
Both methods can measure P or S - waves by arranging the source and the transducers
in the adequate positions. For both methods there must be two free and accessible
surfaces opposite to each other in order to perform the tests.
The principle of the transient method is to measure the travel time between one
observation point to another observation point on the object. A schematic drawing of
a test is shown in Figure 4.7.
A hammer instrumented by an accelerometer excites the waves at one point and an
accelerometer receives them at another point of the object. The signals from both
transducers are recorded, either on paper or electronically, and the travel time, ΔT,
between the recordings is measured.
The travel time is the difference in time between the first arrived recordings i.e. where
the trace of the signal first departs from the zero line. The time difference between
peaks of the signals may be subjected to other time delays and will not give an
accurate result.
By measuring the distance, ΔL, between the emitter and the receiver it is possible to
calculate the average speed, ci, of the wave under consideration (i = P or S) by
Ci = ΔL/ ΔT
4.7: A schematic drawing of a seismic transmission test.
The principle of the steady-state method is to perform the measurements at different
frequencies and to measure the phase difference between two signals. For two
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harmonically varying signals the phase difference is the time interval between two
corresponding zero passages or corresponding peaks in terms of whole cycles; (points
with the same phase).
It is convenient to mathematically express one cycle as 2π radians instead of 360°.
From the phase difference,
, it is possible to calculate the average speed, ci, of the
wave under consideration (i = P or S). The equation is
Where f is the frequency in Hz of the vibration and
is the phase difference in
cycles, i.e.,
= 0.5 means 180° (or π radians) out of phase.
The quantities measured are in the case of the transient method; the length of the
travel path, the time difference and the damping.
In the case of the steady-state method the quantities measured are the length of the
travel path, the phase differences at different frequencies.
The equipment needed for this test method is an exciter, hammer with some protective
pads or a vibrator. The delay of the waves in the pads must be known. Two
accelerometers and a recorder to store the signals are also necessary.
For the steady state method a frequency analyser is needed.
The limitations of the method is that only an average value of the wave speeds along
the travel path will be obtained if not a topographical analysis is applied.
The damages which may be detected are of the global type. The method is not
common in NDT-geophysics but has been used for some particular projects as pillars
in mines. In destructive testing, as measuring between bore holes, cross - hole tests
are very common.
4.9.3 Seismic refraction method:
This method uses the refraction of P-waves to test the material. The material has to be
inhomogeneous with wave speeds increasing from the surface for the method to work.
The result is a profile or a map of the interior of the material. The source is commonly
a detonation or a big vibrator.
The principle is as following. For simplicity it is assumed that the stone under
consideration consists of only two different materials; a homogenous material with
depth H overlying a homogenous material, with greater P-wave velocity, of infinite
depth.
When a P- or S-wave travels in a homogenous medium its path is a straight line.
When it encounters a different medium the path changes its direction. The wave is
refracted. Another part of the wave is reflected back into the medium. The relations
between the incident, , the reflected,
and the refracted,
angles are the
well-known formulae by Snell‘s,
and
=
where
and
are the P-wave speeds of medium 1 and 2. One consequence of this
is that, for a particular angle of incidence the refracted ray will travel parallel to the
surface of discontinuity,
= 90° if the P-wave speed of material 2 is greater than
the P-wave speed of medium 1.
This wave will act as source for waves in medium 1. But as this source travels faster
than the speed of the P-waves in medium 1, the wave front in medium 1 will be a
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straight line. This wave, which travels from the surface of discontinuity out towards
the free surface and eventually will appear at the surface is called, head wave.
Once the head wave has appeared at the surface, it will travel with the speed of the Pwave in medium 2. It is thus possible to measure the speed of the P-wave in medium 2
by measuring the first arrival of signals recorded at the surface of the material.
The first arrival times are plotted in a time distance diagram called travel-time
diagram. The curve obtained in the diagram will for this special case contain one
break point denoted x*.
The second part of the curve if it is extrapolated will give an intercept with the time
axis, t0. By measuring either of these entities it is possible to calculate the depth of the
superficial layer.
The P-wave speeds,
and
of the layers are the inverted values of the slopes of
the curves in the travel time diagram. If the material consists of several layers
overlying the infinite medium, there will be as many break points as there are
layers.Figure 4.8 shows working principle of seismic refraction survey.
It is also possible to use S-waves for the test but it is more difficult since the signals
then might be obscured by the P-waves.
The quantities measured are the travel times and the locations (distances).
The equipment needed for the tests are transducers (seismometers or accelerometers)
and a multichannel data acquisition system.
The limitation of the method is that the P-wave velocity has to increase monotonically
with the distance from the surface.
Only global damages will be detected by this method.
The literature of this method is huge. The method is established since decades and has
been used extensively all over the world for investigating the ground for smaller
distances in civil engineering works and for very long distances, thousands of
kilometres, in geophysical research works.
Figure 4.8: The principle of seismic refraction
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4.9.4 Seismic reflection method:
This method uses the reflected waves from reflectors in the ground to perform the
tests. The reflectors may be surfaces of discontinuities or concentrated defects. In the
method of refraction, the transducers were located in a line from the source and
outwards, sometimes to very long distances. In this method, the transducers are
located around the point of excitation. The source is often a detonation of explosives
or a big vibrator. Figure 4.9 shows typical seismic reflection method.
When a mechanical wave hits a surface of discontinuity it will be reflected. How
much will be reflected depends on the angle of incidence and the specific impedance,
z, which has the dimension of N/m3. It can be written in three different ways i.e.,
Where G is the shear modulus of the material.
For vertically propagating and reflected waves the reflection co-efficient R, is
The reflection coefficient is defined as the ratio between the amplitudes of the
reflected and the incoming waves.
A travel time diagram of reflected waves is drawn. The curves are hyperbolas whose
equations are for the case with one layer with thickness, H, on top of an infinite layer:
where n is the number of reflections in the surface of discontinuity. is the speed of
the investigated wave (i = P or S).
A difficulty is that P-waves will give reflected P and S-waves and vice versa. The
equation is valid for those rays which are only P-waves or S-waves. After only a few
reflections the picture of P and S waves is very complex.
When there are several layers there will also be multiple reflections inside the
medium.
The signal processing of the recorded signals from seismic reflection tests is therefore
important and it has reached a sophisticated level particularly in the petroleum
prospecting industry.
The equipment needed for the tests are transducers, seismometers or accelerometers,
and a multichannel data acquisition system possibly containing some filter functions.
A limitation of the method is that the vertical axis in the travel time diagram is a time
axis. In order to transform that to a length axis, i. e. to transform it to a real sounding
of the ground, the P-wave speeds of the different layers must be known.
In most geophysical situations and for the geological interpretation of the result it is
sufficient to work with the travel time diagram.
The seismic reflection method can be used both for local and global damages. The
seismic reflection method is an established method in geophysics, particularly in the
petroleum industry.
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Figure 4.9: The method of seismic reflection and its travel time diagram
4.10 Surface Wave Methods:
Many geophysical methods are attempted for seismic site characterization, but widely
used methods are Spectral Analysis of Surface Waves (SASW) and Multichannel
Analysis of Surface Waves (MASW). SASW and MASW are surface wave methods
widely used for many civil and earth science applications
Historically, most of the surface wave applications have followed three fundamental
steps:
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1. Acquisition
2. Dispersion Analysis
3. Seeking the layered-earth model (Vs, Vp, h, r, etc.)
The main topics of development in recent history have been field procedures (data
acquisition) and data processing (dispersion and inversion analyses). Early pioneering
work in surface waves goes back to 1950s when the steady state method was first
used by Van der Pol (1951) and Jones (1955).
At this time, it was based on the fundamental-mode (M0)-only Rayleigh wave
assumption and all other types of waves higher modes, body waves, etc. were ignored.
This method then evolved later to be more-commonly called Continuous Surface
Wave (CSW) method (Matthews et al., 1996).
In the meantime, the soil site inversion theory was refined by Tokimatsu et al. (1991).
Since the very early stage of the surface wave application, pavement was found to be
more complex than soil (Sezawa, 1938; Press and Dobrin, 1956), with a special type
of guided wave called leaky waves that required a complex-domain approach in
solving wave equations (Jones, 1962; Vidale, 1964).
A modern computer approach was introduced later by Martincek (1994), but it still
produced limited results. 20th century when Jones (1961) and other investigators
used small vibrators as wave experienced a boom in the mid-1980s when digital
computers became popular. A brief coverage of this historical development can be
found in the 2005 special issue of JEEG (Journal of Environmental and Engineering
Geophysics) on the surface wave method. Another historical overview can be found
in Park and Ryden (2007).
4.10.1 Two-Receiver Approach (The SASW Method)
In early 80s, a two-receiver approach was introduced by investigators at the
University of Texas (UT), Austin, that was based on the Fast Fourier Transform
(FFT) analysis of phase spectra of surface waves generated by using an impulsive
source like the sledge hammer. It then became widely used among geotechnical
engineers and researchers. This method was called Spectral Analysis of Surface
Waves (SASW) (Heisey et al., 1982). Figure 4.10 shows Schematic representation of
overall procedure of the SASW method
The fundamental-mode (M0)-only Rayleigh wave assumption was used during the
early stages. Simultaneous multi-frequency (not mono-frequency) generation from
the impact seismic source and then separation by FFT during the subsequent data
processing stage greatly improved overall efficiency of the method in comparison to
earlier methods such as the continuous surface wave (CSW) method. Since then,
significant research has been conducted at UT-Austin (Nazarian et al., 1983; Rix et
al., 1991; Al-Hunaidi, 1992; Gucunski and Woods, 1992; Aouad, 1993; Stokoe et al.,
1994; Fonquinos, 1995; Ganji et al., 1998) and a more complete list of the
publications on SASW up to early 1990s can be found in ―Annotated bibliography on
SASW‖ by Hiltunen and Gucunski (1994). The overall procedure of SASW is as
follows.
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Figure 4.10: Schematic representation of overall procedure of the SASW method
a. Field setup with different separations (D‘s),
b. Data processing for phase velocity (Vph): Vph=2*pi*f / dp (dp=phase difference,
f=frequency, pi=3.14159265)), and
c. Wavelength (L) filtering criteria—compact dispersion curve
Earlier research of SASW method was focused on ways to enhance accuracy of the
fundamental-mode (M0) Rayleigh-wave dispersion curve through field procedure and
data processing efforts. Then soon came the speculation about the possibility of the
curve ―being more than M0‖ and subsequently higher modes (HM‘s) were included in
the studies (Roesset et al., 1990; Rix et al., 1991; Tokimatsu et al., 1992; Stokoe et al.,
1994). In consequence, the concept of ―apparent (or effective)‖ dispersion-curve
(Gucunski and Woods, 1992; Williams and Gucunski, 1995) was introduced that
accounts for the possible mixture of multiple influences rather than M0 alone.
Once multiple modes were recognized and included, the field approach and data
processing techniques attempted to account for the multiple-mode possibilities.
Pavement investigation by SASW was regarded quite challenging, especially for base
layers, and the possibility of multi-modal superimposition was speculated as being
responsible for this. Reported difficulties with SASW fit into the following three main
categories:
1. Higher modes (HM‘s) inclusion that was previously underestimated,
2. Inclusion of other types of waves (body, reflected and scattered surface waves, etc.)
(Sheu et al., 1988; Hiltunen and Woods, 1990; Foti, 2000) that was also
underestimated or not considered at all, and
3. Data processing, for example, phase unwrapping (Al-Hunaidi, 1992) during the
phase-spectrum analysis to construct a dispersion curve.
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4.10.2 Multichannel Approach (MASW)
In early 2000s, the MASW (Multichannel Analysis of Surface Waves) method came
into popular use among the geotechnical engineers. The term ―MASW‖ originated
from the publication made on Geophysics by Park et al. (1999).
The project actually started in mid-90s at the Kansas Geological Survey (KGS) by
geophysicists who had been utilizing the seismic reflection method—long used in the
oil industry to image the interior of the earth for depths of several kilometres. Called
the high-resolution reflection method, it was used to image very shallow depths of
engineering interest (e.g., 100 m or less).
It was in the mid-90s when KGS started a project to utilize surface waves. Knowing
the advantages with the multichannel method proven throughout almost half-century
of its history for exploration of natural resources, their goal was a multichannel
method to utilize surface waves mainly for the purpose of geotechnical engineering
projects.
From the extensive studies performed by SASW investigators, they acknowledged
that surface wave properties must be more complex than previously assumed or
speculated, and that the two-receiver approach had clearly reached its limitation to
handle the complexity.
Based on the normal notion that the number of channels used in seismic exploration
can directly determine resolving power of the method, they utilized diverse techniques
already available after a long history of seismic data analysis (Telford et al., 1976;
Robinson and Treitel, 1980; Yilmaz, 1987) and also developed new strategies in field
and data processing to detail surface wave propagation properties and characterized
key issues to bring out a routinely-useable seismic method.
The first documented multichannel approach for surface-wave analysis goes back to
early 80s when investigators in Netherlands used a 24-channel acquisition system to
deduce shear-wave velocity structure of tidal flats by analyzing recorded surface
waves (Gabriels et al., 1987). It first showed the scientific validity of the multi
channel approach in surface wave dispersion analysis and, in this regard, the study
can be regarded as a feasibility test of the approach for routine use in the future.
Then, using uncorrelated Vibroseis data, Park et al. (1999) highlighted the
effectiveness of the approach by detailing advantages with multichannel acquisition
and processing concepts most appropriate for the geotechnical engineering
applications. A subsequent boom in surface wave applications using the MASW
method for various types of geotechnical engineering projects has been observed
worldwide since that time. There were a few other applications of multichannel
approach to aid oil-exploration reflection surveys (Al-Husseini et al., 1981; Mari,
1984).
4.10.2.1 Multichannel analysis of surface waves:
First introduced in GEOPHYSICS (1999), the multichannel analysis of surface waves
(MASW) method is one of the seismic survey methods evaluating the elastic
condition (stiffness) of the ground for geotechnical engineering purposes. MASW
first measures seismic surface waves generated from various types of seismic sources
such as sledge hammer analyzes the propagation velocities of those surface waves,
and then finally deduces shear-wave velocity (Vs) variations below the surveyed area
that is most responsible for the analyzed propagation velocity pattern of surface
waves.
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o Shear-wave velocity (Vs) is one of the elastic constants and closely related to Young‘s
modulus. Under most circumstances, Vs is a direct indicator of the ground strength
(stiffness) and is therefore commonly used to derive load-bearing capacity. After a
relatively simple procedure, final Vs information is provided in 1-D, 2-D, and 3-D
formats.
Overall Procedure of MASW Survey :
The common procedure for (1-D, 2-D, and 3-D) MASW surveys usually consist of
three steps. Figure 4.11 shows MASW three step.
1. Data Acquisition - acquiring multichannel field records (commonly called shot
gathers in conventional seismic exploration)
2. Dispersion Analysis - extracting dispersion curves (one from each record)
3. Inversion - back-calculating shear-wave velocity (Vs) variation with depth (called
1-D Vs profile) that gives theoretical dispersion curves closest to the extracted curves
(one 1-D Vs profile from each curve).
Figure 4.11: Steps involved in MASW
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4.10.2.2 Advantages of the MASW Method
1. Unlike the shear-wave survey method that tries to measure directly the shear-wave
velocities which is notoriously difficult because of difficulties in maintaining favourable
signal-to-noise ratio (S/N) during both data acquisition and processing stages. MASW is
one of the easiest seismic methods that provide highly favourable and competent results.
2. Data acquisition is significantly more tolerant in parameter selection than any other
seismic methods because of the highest signal-to-noise ratio (S/N) easily achieved. This
most favourable S/N is due to the fact that seismic surface waves are the strongest seismic
waves generated that can travel much longer distance than body waves without suffering
from noise contamination.
3. Because of an increased ability to discriminate useful signal from harmful noise, the
MASW method assures an increased resolution when extracting signal in the midst of
noise that can be anything from natural or cultural activities (wind, thunder, traffic, etc.)
to other types of inherent seismic waves generated simultaneously (higher-mode surface
waves, body waves, bounced waves, etc.)
4. In consequence, overall field procedure for data acquisition and subsequent dataprocessing step becomes highly effective and tolerant, rendering a non-expert method.
5. The multichannel seismic concept is analogous to resolution in digital imaging
technology. As the higher number of bits available, a broader colour resolution is
achieved, whereas the higher image resolution is achieved as more pixels are used to
capture the image.The concept of number of channels plays similar roles to those by the
bit and pixel concepts in delineating the subsurface information.
4.11 Down/up hole Shear Wave Velocity
Steps involved in finding out the downhole shear wave velocity are
1. Anchoring System
2. Automated source
3. Polarized Wave
4. Downhole VS
In the down-hole test, an impulse source is located on the ground surface adjacent to
the borehole. A single receiver that can be moved to different depths, or a string of
multiple receivers at predetermined depths, is fixed against the walls of the borehole,
and a single triggering receiver is located at the energy source.
All receivers are connected to a high-speed recording system so that their output can
be measured as a function of time. The objective of the down-hole test is to measure
the travel time of the p and/or s waves from the energy source to the receivers.
By properly locating the receiver positions, a plot of travel time versus depth can be
generated. The slope of the travel time curve at any depth represents the wave
propagation velocity at that depth.
Figure 4.12 shows schematic representation of Downhole survey
With an SH wave source, the down-hole test measures the velocity of waves similar
to those that carry most seismic energy to the ground surface. Because the waves must
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travel through all materials between the impulse source and receivers, the down-hole
test allows detection of layers that can be hidden in seismic refraction surveys.
Figure 4.12: Downhole shear wave survey
(http://geosystems.ce.gatech.edu/Faculty/Mayne/Research/)
Potential difficulties with down-hole tests and their interpretation can result from
disturbance of the soil during drilling of the borehole, casing and borehole fluid
effects, insufficient or excessively large impulse sources, background noise effects.
The effects of material and radiation damping on wave forms can make identification
of s-wave arrivals difficult at depths greater than 30-60 m
4.11.1Automated Seismic Source
To improve upon the downhole testing program, an automatic seismic source was
developed for use in seismic piezocone testing. A new source, named the AutoSeis,
was initially tested at the national geotechnical experimentation site in Spring Villa,
Alabama and compared to available crosshole data to assess its ability to meet the
primary and secondary goals.
Later testing was conducted at two test sites in the Mid-America earthquake region
near Memphis. With reliable shear waves generated to a depth of 20m, the first
iteration of the AutoSeis has proven successful and has provided the necessary
information for the design of an improved version.
In order to improve the downhole testing program and accuracy of the geophysical
results, the decision was made to develop an automatic seismic source for use in
seismic piezocone testing. It was determined that the current source, which consists of
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a 2.3kg sledgehammer and steel beam, although mechanicallyadequate, could be
improved to increase both consistency and productivity.
This new source, named the AutoSeis, would have to meet certain design and
performance criteria. The AutoSeis would have to be small, portable, and easy to use.
It would also need to generate shear waves that are more uniform and repeatable. The
first iteration of the AutoSeis source and control box can be seen in the Figure 4.13.
Fig 4.13: AutoSeis components, from left, control box, source frame, typical seismic
cone, and internal chassis
4.11.2 Down/Up hole Shear Waves
The down-hole test is a method which, determines soil stiffness properties by
analyzing direct compressional and shear waves along a borehole. It is similar in
several respects to the Cross-hole seismic test method, but requires only one borehole.
An impulse source is used to generate a seismic wave train at the ground surface
adjacent to the borehole. Down-hole receiver(s) are used to detect the arrival of the
seismic wave train. The down-hole receivers may be positioned at selected test depths
in a borehole or advanced as part of the instrumentation (Fig: 4.14). All receiver(s) are
connected to a high-speed recording system so that their output can be measured as a
function of time. In up-hole test, a movable energy source is located in the borehole
with a receiver on the ground surface adjacent to borehole.
Measurements of the arrival times (travel time from source to sensor) of the generated
P and S- waves are then made so that in-situ P-wave and S-wave velocities can be
determined (Fig: 4.15). The calculated seismic velocities can be transformed to soil
stiffness. The slope of travel time curve at any depth represents the wave propagation
velocity at any depth.
S-waves can be generated much more easily in down-hole test by reversing the
direction of energy blow, which allows shear wave pattern to be recorded in the
reverse direction while the compression wave pattern is essentially unchanged. Thus,
the shear wave patterns are distinguished from compressional wave. However, in the
up-hole test, P-wave tends to be more predominant due to difficulty in generate shear
waves.
Features:
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1. Down-hole (up-hole) method is cheaper than cross-hole, since only one borehole is
required for testing.
2. Down-hole test allows detection of layers that can be hidden in seismic refraction
surveys.
3. Difficulties in interpretation of result, due to disturbance of soil during drilling of the
borehole, casing and borehole fluids effects, and ground water table effects.
Source
Receiver
Receiver
Source
(a)
(b)
Figure 4.14: (a) Seismic up-hole test, and (b) Seismic down-hole test
Figure 4.15: Travel time curve for down-hole test
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4.11.3 Down/Up hole Shear Waves
Shear-wave velocity profiles obtained from downhole surveys are routinely
incorporated in site response modeling for earthquake hazard evaluation and structural
design.
A downhole seismic survey (also called a borehole velocity survey) is conducted by
measuring the time for seismic waves generated by an impulsive source at the surface
to travel to a sensor located at a sequence of depths in the borehole.
The sensor consists of three geophones arranged in an X-Y-Z pattern. Two orthogonal
horizontal geophones are used to detect shear-wave (S-wave) arrivals and a vertical
geophone is used to detect compression-wave (P-wave)arrivals. At each measurement
level, the sensor assembly is locked to the borehole wall using a clamping mechanism
so that the geophones will couple with the seismic signals propagating in the earth.
The down/up hole P wave velocity log is derived using either a 12-or 24-channel
hydrophone array. This array is moved incrementally either up or down the borehole;
a surface source (commonly a 12-gauge Buffalo gun fired in a shallow hole) is placed
close to the borehole (3 to 6 m to one side, at 1 m depth).
The spacing between hydrophones is fixed at 0.5 meters; hence, incremental vertical
moves of the array in the order of 1 m between source records will yield considerable
redundancy of hydrophone locations. Travel-times between source and receivers are
individually picked for each shot record. The data redundancy is used to obtain best
estimates of interval velocities over short vertical intervals (Hunter et al., 1998). For
this compilation plot of P wave velocities are given at intervals of 0.5 meters
downhole. Usually 3 pt (over 1 m vertically) or 5 pt (over 2 m vertically) velocity fit
results are shown in Figure 13.5
Compressional (P) wave velocities are strongly affected by the presence or absence of
pore-water. Low velocities are exhibited above the water table and in areas of the
borehole where gas exists in the pore space. Most normally consolidated watersaturated soils have velocities close to that of water (1480 m/s). Over consolidation of
water-saturated soils ( with resulting reduction of porosity) is indicated by somewhat
higher velocities (e.g. a compacted coarse-grained basal till can yield velocities of
2500-3500 m/s. Lithification to rock, or ice-bonding of soils, results in velocities
which may range between 2500-5500 m/s. Empirical relationships between soil
porosity and P wave velocities have been developed.
The down/up hole S wave velocity log is derived using a single-or 3-pod well-locking
geophone array. Each pod consists of 3 orthogonal 14 Hz geophones which can be
locked against the side of the borehole with a motor-driven bow spring. The
orientation of the single-or multi-pod array can be done from ground surface down to
a maximum depth of 100 m. commonly the array is moved vertically in increments of
1 meter. The seismic source is placed close to the borehole on ground surface;
commonly a steel I-beam or wooden plank loaded by the front wheels of a light truck
is struck horizontally to obtain polarized shear wave energy.
Dr. P. Anbazhagan
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The first arrival data from all three components is examined using commercial
picking and display routines. Least squares fits of the data are computed and plotted;
commonly a 3-pt fit is displayed.
Shear wave velocities can be used to indicate the presence or absence of soft soils and
resonant boundaries for earthquake hazards assessment and can be used to estimate
liquefaction potential of non-cohesive soils. The values can also be used to estimate
the ultimate strength of cohesive soils, and to identify the presence of stress
anisotropy associated with natural or man-made slopes. Empirical relationships
between shear wave velocity and soil porosity have been developed.
4.12 Seismic Cross-Hole Test:
The seismic cross-hole test provides dynamical soil parameters down to depths of
100 or 150 m based on the determination of wave velocities. These test methods
are limited to the determination of horizontally traveling compression (P) and shear
(S) seismic waves at test sites consisting primarily of soil materials. The focus of
the cross-hole test is on the determination of the shear wave velocity (S-wave).
This data may be used as input into static/dynamic analyses, as a means for
computing shear modulus, Young‘s modulus, and Poisson‘s ratio, or simply for the
determination of anomalies that might exist between boreholes. In addition
material damping can also be determined from cross-hole test.
Seismic cross-hole tests use two or more boreholes to measure wave propagation
velocities along horizontal paths. The simplest configuration consists of two
boreholes, one of which contains an impulse energy source and the other receiver
is shown in Figure 4.16. A borehole geophone is installed in the receiver hole and
tightly coupled to the borehole walls by means of a pneumatic packer. A good
coupling is needed since s-waves cannot be transmitted by the groundwater. Source
and receiver are always installed at same depth and moved parallel along the
boreholes to achieve a velocity profile with depth. When possible, use of more than
two boreholes is desirable to minimize possible inaccuracies resulting from trigger
time measurement, casing and backfill effects, and site anisotropy. From the
borehole spacing and difference in arrival time at adjacent pair of boreholes, the
velocity of seismic wave is computed.
Since the impulse sources must be located in the borehole, variation of P-wave/Swave content is more difficult than other methods in which impulse source kept at
the surface. When explosives are used, the wave content is shifted toward higher pwave content (Woods, 1978). The best results are generally obtained when the
polarity of the impulse source is reversible, hence preference for mechanical
sources over explosive sources. Mechanical impulse source includes the vertical
impact loading of rods connected to borehole packers, torsional impact loading of a
torque foot at bottom of the borehole (Stokoe and Hoar, 1978), and other
techniques.
Dr. P. Anbazhagan
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Figure 4.16: Seismic cross-hole test
4.12.1 Features:
1) Cross-hole test often allows individual soil layers to be tested since layer boundaries
are nearly horizontal.
2) It also detects hidden layers which are often invisible by seismic refraction survey.
3) Real time wave form display while testing.
4) This method gives inappropriate velocities when higher velocity layer exist nearby.
5) Cross-hole test can yield reliable velocity data to depths of 30 to 60 m using
mechanical impulse source, and to greater depth with explosive sources.
6) An amplitude attenuation measurement has been used to compute material damping
ratio of soils.
Dr. P. Anbazhagan
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