81
The General Characteristics of Electromagnetic Radiation
During Coal Fracture and Its Application in Outburst Prediction
Xue-qiu He, En-yuan Wang and Zhentang Liu
China University of Mining and Technology
Xuzhou, Jiangsu 221008
P.R. China
ABSTRACT
Coal and methane outburst are catastrophic in coal mining, their prediction is difficult. In this paper, the electromagnetic
radiation (EMR) generated during coal or rock deformation and fracturing is measured and analyzed. The results show
that EMR truly exists during the fracture of coal or rock (with or without the presence of gas). It follows the Hurst
statistical rule, and it basically exhibits gradually enhancing tendency during the process. The EMR strength and
frequency are correlated to the coal or rock fracture process. Based on the experimental and theoretical studies, a new
method for coal and methane outburst prediction is proposed -the EMR method. This new method significantly
facilitates methane outburst prediction.
KEYWORDS
Electromagnetic Radiation (EMR), Acoustic Emission (AE), Coal Fracture, Coal and Methane Outburst, and Rock
Outburst.
INTRODUCTION
EXPERIMENT STUDY
Coal is a porous solid and consists of mainly carbon and
hydrogen. It also contains different liquids in its pores. In
coal layers the coal pores are often filled with gas. The
monitoring of the dynamic process of coal or rock
containing gas is a very important technology in modem
coal mining, especially in the prediction of coal and
methane outbursts, rock outbursts, roof falls and abutment
pressure displacements. However, the current prediction
technology was developed a long time ago, (such as
monitoring technology), most of these methods are complex
in application and the sensors must be coupled with coal or
rock wall during the process of monitoring.
Tests and theoretical analysis show that electromagnetic
radiation (El\1R) takes place during the deformation and
fracturing of coal or rock (He, 1994, 1995). This
phenomenon has also been verified by EMR researchers in
the research of earthquakes (Gress, 1987). EMR research
can be widely used to assess the stress condition of rock, to
reveal the mechanism of deformation and fracture, and to
predict the catastrophic phenomena of rock or coal. These
catastrophes are due to the stress change, which follows the
"Rheology Hypothesis" (He, 1992) and surely leads to some
characteristic change of EMR, which can be regarded as an
omen of rock destabilization. It is obvious that the research
and applications of EMR are important, especially in coal
mine safety.
In the experiment two types of coal were tested. The
first is a molded coal specimen, which is molded under
the pressure of 135Mpa in a steel model with coal
powder (the particle sizes are less than 0.4 mm); the
second is molded with original coal. The sizes of tested
coal specimen are <l>50x 1OOmm. The experiment
system consists of EMR sensors, AE sensors, signal
amplification system, load system, electromagnetic
wave shield system, coal specimen vacuuming system
and gas adsorption system. A total of 22 different coal
specimen were tested under the load of axial
compressive stress. Some of the original records are
shown in Figure 1. The sensor parameters are shown in
Table 1.
It can be proven that EMR is more sensitive than AE
during the fracture of coal. The frequency of EMR
signals is in a wide distribution, and some times it is not
synchronized with AE. According to the curves of stress
and strain plotted against time, the EMR signal number
is in some way direct proportional to the load.
82
PROCEEDINGS OF THE 8TH US MINE VENTILATION SYMPOSIUM
Table 1. The sensor parameters used in the test.
frequency
channel Signal type
AE(P)
1
2
AE(So)
3kHz
5
EMR
6
EMR
10kHz
81.6k
EMR
7
2.6MHz
8
EMR
EMR
814kHz
9
EMR
542kHz
10
6.9MHz
11
EMR
76kHz
12
EMR
EMR
6.9MHz
13
16
EMR
76kHz
1'1
--- ---- ---· -- · • · ·- - --.. --···-··- --· ·· - ·---, • • -· -- -- •- --- --·- -r- ----· • • •••· - --- -- -·
.z
-~ : ~~--------~------~\
43.3 -
::~~--------~------~~~~n
-i :~ ~~;.;;~~~:.:~.:~~~~~:;~~~~~~~~.._tiit~~-~:-...6-~-""--..· ~-~-~-..·-"-~-~-~-.--.-~-~
_g:~
~ 5~--------~----------~~------~---------4
11. .2.
O.El
- 1LZ
26.1
t~:~ ~.~~~~~~~~~~~~~~~~~~~~~~~
-13 . ~~--------~----~IL--~~~~~~~~--~-4
Z9 . 3
1: :~ ~~~~~~~~~~~~~~~~~~~~~~
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_:__-==-=L..-.::_.:...___=+------~~
b .Ol~~;;;;;;;~~~;~~N;;ii~Z~~~~~~~~-~~~;~~~~~~~M~~
3 .4
8 .0
1i:~ ~--------:_
____~_:____.:..._~--~~~------:_~
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-~ : ~~--------:_----~~--.:.._~.:.._~~~~I---~~
~ : ~~~~~~~~~~~~~kM~~~~~~~~~
~~ : ~~--------~----------~--------~----~--~
2~ : :~--------~-----~----~~-.~~~--~--~~
-zi:i~d~~~~~~~·~~~~~;;i~~~;;;~j~~~~;~~~~~Mw~~~~;~
0 .8
3~ :~ ~--------~~--~--~~~~~~~------~--4
~~ : ~~~~~~~~~~~~~~~~~~~~~~~
-17. 1 L_________~----~~~L-------~~------~~
1
;!]I .·---,-,--:-..~~~~~~~~~~ ~~ w~:-~·~ ~ ~ ~ ~
a-4 t(59)=61478.369ms
11 . .,
~ : ~------------~------~~~~~. ~~~~,.~~~~--
1~:~~--------~----------rlL~--------~---------~ : ~~--~~~~~.-_.~~~~~~~~~~~--~~
-~a :. a~~~~~~~~~~;~~~;;;~~~;~•~;;~~~~;;~~;;~~;;~;~u;~
1.5 Ill
-
~~: ~ 1 ==-=-~ :~~-=-~~-:=~~~~;..:---=~~~-
___?-!~~-~r--- ------ ----- -~~!?-1
- 11 . 2~----2----+~----~--~-----L----~----~---­
-2:7 . 3
l.Z.S . t.J
..~--,----+-----------
6Z .S ~---------+--------~
---··----
-~i ~ i
-~i ~ ~
~:~:!
- 2 ~"...1
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o.e
- -~ - -- · · t-··-
--- - ---
ULf>
~ : ~ ~~~~~~~~~~~~~~~~~~~~~~~
z~:~~----~---+---------l~--------~~~~~~~
~~ : ~ ~~~~~~~~~~~~~~~~~~~~~~~
---- ---·-----7---------- .
-11.2L_
__
1
1-···-- ·-·-- ·---
------~~~------~--~~~~~------~~
--+ ··--·-·--- -····---,---·,
. a.t. -- - - -
:
:
··· -----iaz.;.'-- ··· - - -····-z. .-i:i:ti
a-5 t(67)=65999.99ms
70
60
50
i:
:l)
l! I
IQ
a-2 t(26)=42576.676ms
v
_)_/U _ /VY~lc J
Q
0
7.3.---------,-~-----.-.---------,---------.
~:~~~~~--~--_.--~~~~~~)~~~~~~
f
I
/
70
50
II)
t~)
~~:~ ~--------~------~-+~------~---------4
b
~:~ ~~------~----~~~~~~~~--------~
-'"J . 5
12.5.0
0.6
- ~~:~ ~_i------4-----~--~----~---+~------~
o.s
6~ : ~~~------~--------~--------~--------~
1.2 ."/
El . O
-~~0.0
: ~ ~~~~~~~~~~~~~~~~~~~--~
~~:~~~~~--~--~--~-+~--~~~~~--~-4
..
·~:~ ~~~~~~~~~~~~~~~~~~~~~
- 1:1 . 7.1 _. ,
0. 0 ~--~--~~------~~------~~~------~~
: : ~ ~.--~~~~~~--~~~~
~~~--~~--~
lZ~::~~------~--------~----------+---------~
~~ : ~~-T------~--------~----------+---------­
- 6~:~~_i----T<-h.---~--~----------+----------
~ : ~H+~~~~~*H~~~~~~~~~~~~~~~
~~ : ~~--------~----~--~----------~--~-----
z~:~ ~-4. .~--~--------~~~~. .~~--------~
- z~o :.e~ w;~~;~~~~~~~~~~;~~;;~~~;;~~~;~~;~;~;~;~~;~;i
;~:~ ~--~~--~--~----~------~--+---------~
1
~ :~ ~~~~~Uk~~~~~~~~Hf~~~~~~~
-1-:J . '\L---------~-------=~'--------=-="---------=-~
a-3 t( 48)=55086.945ms
~
I).-I
g
1).3
/
0.:!
/
0.1
0
v
-
-
v
/
...--'
0
50
10
c
70
THE GENERAL CHARACTERISTICS OF ELECTROMAGNETIC RADIATION DURING COAL FRACTURE 83
AND ITS APPLICATION IN OUTBURST PREDICTION
~v
~
~
1~I
~
I
~
/
0
0
-
/
10
/
I
0
so
20
20000
c~fu\~~ I
40000
60000
t(ms)
80000
100000
70
l(s)
d
a-1 ~a-5 Some original events of AE and EMR during the
deformation of coal specimen under the axis compressive
stress. In every figure, the ordinate is breadth of AE or
EMR; the abscissa is time, t(x) is the time to start sampling.
This experiment is under the touching off AE signal. In
every figure, the first two record lines are AE signals; from
the third to the last record lines are EMR signals measured
with different frequency sensors as shown in Table 1.
b is the record of total number of AE during the fracture
(time/sec).
c is the curve ofstrain to time.
d is the curve ofstress to time.
Figure I. The AE and EMR records of original coal
specimen (from Xuzhou Coal Mining Bureau, Jiangsu
province, China) during the process offracture.
ANALYSIS
Based on test results, a general concept of EMR during the
fracture of coal is built up, and it can be found that the
frequency is spread across a very wide range. Every microfracture or displacement in the coal emits a different
frequency signal of EMR. And sometimes it is coupled with
the AE of coal fracture, not even homologous to the AE.
The general characteristics of EMR and AE during the
whole process of coal fracture are shown in Figure 2.
a Time sequence ofAE and EMR
~ ~~ fziAZJzzm
~
Q
~
•
•
I
m
@
kr•
b The curve ofstress to time
Figure 2. The general characteristics of AE and EMR
during the fracture of original coal specimen (from
Xuzhou Coal Mining Bureau, Jiangsu, China).
Finally, by statistical analysis, EMR signals during
the deformation and fracture of coal or rock conforms
quite well to the statistical rule, with Hurst index H
between 0.5-1.0. The analysis results prove that
prediction of coal or rock fracture is possible by
monitoring the EMR since it is positively correlated
with the stress and deformation, i.e., EMR enhances
with the increase of deformation.
The Hurst index H of EMR of each frequency band
is different. The leading frequency band of EMR during
the fracture process is changing. So, it is not suitable to
predict outburst by monitoring a spot frequency of
EMR.
APPLICATION
i ·;1 '- - -+1--+
- J-+- ~tfiH-tl- t ~- -t t-i~ Ji H- - l'I
0
20000
40000
60000
t(ms)
80000
100000
Based on laboratory experiment and subsequent
analysis, an EMR monitoring equipment with wide
frequency band is developed. It has been used in the
prediction of rock outburst or coal and methane outburst
in 8th Coal Mine, Pindingsan Coal Mining Bureau,
Henan Province, with some of the results shown in
Figure 3.
100
t(ma)
~~I
0
I
20000
~A~~~~~r 1 1
40000
60000
t(ms)
20
80
80000
100000
u..
15
60
10 <(
40
5
20
1
22 43 64 85106127148169190211232253
Fpds3af 01
Ti rre
a
84
PROCEEDINGS OF THE 8TH US MINE VENTILATION SYMPOSIUM
AKNOWLOGEMENT
1 15 29 43 Sl 71 85 00 113127141155169163197211Z25239
Fpds3ct 10
li 119
b
I would like to express thanks to the "China National
Science Foundation Committee", "The Foundation
Committee of Span Century Person with Ability of
China" and "Foundation Committee of Doctoral Subject
Research". Both committees have provided support for
the author's research.
REFERENCES
1
7
13 19 25 31 37 43 49 55 61 67
Fpd s 3a f 11
n ""
c
In Figure 3, A and F mean separately the amplitude and
frequency of EMR measured on site.
a--- EMR monitoring results before the explosion in the coal
head of a downhill roadway.
b--- EMR, 23 minutes after the explosion in the coal head of
a downhill roadway.
c--- EMR 41 minutes after the explosion in the coal head of
a downhill roadway.
Figure 3. Site EMR monitoring results before and after an
explosion in the outburst prone coal head of a downhill
roadway, 81h Coal Mine, Pingdingsan Coal Mining Bureau.
Site experiment results, show that both the amplitude and
the pulse number ofEMR before and after explosion change
greatly. The number of EMR pulses oversteps the limit of
the equipment just 23 minutes after the explosion, as shown
in Figure 3b. And then they both decline with time. Some
parallel experiments between outburst prone and nonoutburst prone heads are carried out, and in the same time
other outburst prediction methods are also used. Finally, it is
proved that the higher the EMR pulse number and the EMR
amplitude are, the larger chance for the outburst.
CONCLUSION
Both the laboratory and on site experiment it can be found
and proved that the signal of EMR is more sensitive than
that of AE during the damage or fracture of coal or rock. By
EMR monitoring during coal or rock fracture, much fracture
information can be gathered. And this technology can be
used in the prediction of coal, methane, and rock outbursts.
Even so, there is much research work to be done in
future, especially to produce an outburst prediction guide
line through site experiment, to ascertain the orientation of
EMR.
Cress, GO., and Brady, B.T., eta/., 1987, "Sources of
Electromagnetic Radiation from Fracture of Rock
Samples in Laboratory", Geophys. Res. Lett., Vol.
14, pp. 331-334.
He, X.Q., and Zhou, S.N., 1992, "The Rheological
Fracture Properties and Outburst Mechanism of
Coal Containing Gas," Proc. ll 1h Inti. Conference
on Ground Control in Mining, Aziz, ed.,
Wollongong, Australia, pp. 575-579.
He, X.Q., and Liu, M.J., 1994, "Laboratory Research on
Electromagnetic Emission from Fracture of Coal
and Rock Containing Porous Gas," Proc. Inti. Symp.
on New Development in Rock Mechanics and
Engineering, X. H. Xu, ed., Shengyang, China, pp.
567-572.
He, X.Q., 1995, "Fracture Electromagnetic Dynamics of
Coal or Rock Containing Gas," China University of
Mining & Technology Press, Xuzhou, China.