SPECTRAL FEATURES OF ELASTIC VIBRATIONS CLOSE TO TECTONIC DISLOCATIONS IN COAL SEAMS V. E~ Krupin and T. D. Murzadilov

Unforeseen tectonic dislocations in coal seams create difficulties and danger in the conduct of mining. Advanced detection is aided by a seismic method which is currently used in the coal basins of the country. However, the complexity of the results obtained makes it necessary to draw on more advanced methods of analysis for improving the reliability of seismic prediction of dislocations. Isolation of tectonic dislocations in coal seams by the seismic method directly from mine workings requires detailed analysis of the features of useful waves and their frequency spectrum. It is well known that in many cases, close to both dislocations with a break in continuity and pllcative dislocations there are zones of highly fractured and deformed coal. These zones are characterized in the first instance by reduced elastic vibration propagation velocity compared with undislocated sections. In addition, from seismic results obtained previously by the authors at a Kuzbass mine it was established that close to a dislocation there is a change in the spectral composition and the shift occurs in the direction of higher frequencies. There is reference in [i] to the fact that reflected waves are richer in high frequency components than waves of other types arriving simultaneously with reflected waves. With the aim of checking this established fact the authors carried out an evaluation of changes in the spectrum of the arbitrary pulse of a seam wave. The method of evaluation was similar to that used for the reflection coefficient from a thin flat layer [2, 3]. The onedimensional problem is considered. In it a number of assumptions are made; it is considered that the wave is plane; no consideration is given to the effect of rock scarps which occur during disruption of seam continuity. In the representative model boundaries between the coal seam and surrounding rock are rectilinear, and between the dislocated sections there is clear and perpendicular stratification. In addition, since for seam waves there is relatively little attenuation this factor was not considered due to complexity of the solution. Evaluation of the effect of attenuation is a subsequent stage in the study. These assumptions do not upset the general statement of the problem. The problem was considered separately for dislocated and nondislocated zones of the coal seam. Expressions were obtained for both cases describing the change in pulse spectrum, similar to the reflection coefficient from thin layers. The pulse spectrum in the nondlslocated part of the coal seam before tectonic dlslocation according to the scheme in Fig. la was considered in the form of the sum of expressions:

g' (co) = g (o~) e %r, go (co) = Vxg(o~)e mtC'~~

V~ (o) e~l('-'0-O +'i''-',

gl (o) = (WxW../V~)

(i) "W W "V "V~ "r e%( '''o-')+4m'l 9

9

9

9

..

9

*

9

9

9

9

9

9

*

9

.

.

.

.

g,. (~) = (wxwdvD v ~ g (co)d'~(~"o-~')+~'~e, where g(~) is the pulse spectrum for a seam wave created by the source; kt and k2 are wave vectors in nondislocated and dislocated parts of the seam whose imaginary parts characterize attenuation; r is distance from the origin; ro is distance from origin to the boundary ~f the first dislocated zone; Z width of the dislocated zone; Wt, W2, and Vt, V2 are transmission

VNIIGIS, Oktyabr'skii. Translated from Fiziko-Tekhnicheskie Problemy Razabotki Poleznykh Iskopaemykh, No. 1, pp. 24-28, January-February, 1983. Original article submitted June 18, 1981.

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0038-5581/83/1901-0020507.50

9 1983 Plenum Publishing Corporation

~ # i l l l l l l # l t l / l l l

llllllllllllll~llllll

/ i l l /

,;, ..........

9

..

ro

Fig. i. Scheme for evaluating the s p e c t r u m of a pulse close to a tectonic dislocation; in nondislocated (a) and dislocated (b) parts of the coal seam.

o,e~/-N

o.,5/ ~

a~

f

I

9

I

1

{,H~

o I~

/

br

\

o t"

v

,

t

~

IDP 2pP .1 ,2 ^3 ^~ ~ ~ [/" Detonation'point1 Detonationpoint3 / (~s-~ rn), cso-~Om). Detoriat~'on i S a n d s t o t i r . . . . ., . . ' . ' : , ' " " " . . - . De~orme~ c o a t c ~ points. .... ...,....". . . . . . . . . . . . . . . . . . . . . . . . . . ......... 40

~ O] v

0

v

;'00

200 ~ Hz

Fig. 2. Results of frequency analysis for the record of seam wave oscillations close to a tectonic dislocation: a) spectral curves (5(1) and 6(1) are before the dislocation; 7(1) and 7(3) are in the dislocation: 3(3) and 3(1) are after the dislocation); b) graphs for change in frequency for the maximum values and at a level of 0.7 of the basic extremum of spectral curves in relation to distance up to to the dislocation; c) geological section of the seam and layout of the detonation and reception points. In the curve numbering the first figure corresponds to the number of the reception point, and the second to the detonation point. factors (transmittance) through interface 1 in the positive and negative directions and reflection factors from the interfaces 1 and 2, respectively; g'(~) is the forward wave "spectrum; go(u) and gi(~) are spectra for waves reflected from the first and second boundaries for the dislocation zone, respectively; gn(~) is the wave spectrum taking account of repeated reflection from boundaries within the dislocated zone. Summation of expressions

(i) for n from 0 to = gives the resultant pulse equation

. " i~l(O_ro--r) gsum = g (co) I e~'1, -+- F 1 ((WlW,/l,'~) - - V,) --a@ "~- -.tk.l i --

,

(2)

V~e 2ik~-t "

As can be seen from (2), the function describing the deformed modulus for the original spectrum has the form

, (k,,ko, ro, l, y,, V,, ~V1,W..) = le~,

v, +((W,W~/F,)-I -- F~e2~z' t Y,) V~e~'i'"e~t'(,O_r)

.

(3)

This expression is simplified close to the dislocation boundary with r = ro

/~ = V (a + b= + ~

cos 2kd)l(i + v : - 2v; co, 2~,z),

(4) 21

TABLE 1 Frequency ratio

~ace of" observation

S. M. Kirov rn/ne Breevskll seam. wall 9'I Maierovskii seam. wall 3 Dyagllewkl/seam, wall B Bere~-o~l~aya-1 mine Seam XX~ wall 51 Seam XH0 wall 368 Taezhnaya mine Andreev~/l seam, wall 43 Koksovyl seam, wall 31 Volkov mlne Kamerovskll seam, wall 13

1,9 1,6 I

1,5 1,4 0,9 1,4 1,4 1,2 1,2

Butovskaya mine Konglomeratovyi seam

1

1,2

T

!

!

I

F i g . 3. The ~ t u r e of change in the pulse spectr~ according to the calculation equation; I) original pulse s p e c t r ~ ; 2, 3, 4) spectral curves at different distances from the dislocation with Z = i0 m; 5, 6,'7) with Z = 2 m. Curves 4 and 7 were obtained at the boundary with the dislocation.

where a = i + Va; b = V t W I - VxV~ ~ V~2. The latter was obtained assuming that the imaginary part "k", characterizing attenuation, is small.

22

It follows from (4) that close to the dislocation there will be a boost in certain components of the seam wave spectrum. With b ~ 0 the frequency close to which there is a boost in the spectrum will be determined by the equation 2k21 = 1 with 2kZ = 2wn, then

1. = "Ch,/2L where Ck2 is channeled wave velocity in the dislocated l is width of the dislocated zone.

(5) part of the layer; n = I, 2, 3, 4...;

With b < 0 the condition under which there is a boost in frequency expression cos 2kzl ---i with 2k~ = ~n, then f'n = nC~ [41. The scheme for evaluating

the spectrum for a dislocated

is determined

zone is given in Fig. lb.

The type of function characterizing the change in wave spectrum in a dislocated obtained in the same way as in the previous case

s, = [e

+ v$

-

by the

area was

(6)

yves').

As can be seen from (6) with r = I the modulus of the function leads to an increase the spectral component for the same frequencies A =

where Ck2 is seam wave velocity

=

in the dislocated

Expressions (5) and (7) have an identical parts of the layer.

in

(7)

zone.

form for the dislocated

and nondlslocated

Analytical evaluation in accordance with the expression obtained was compared with the results of frequency analysis for waves emitted close to a dislocation during mine seismic observations at one of the Kuzbass mines (the Volkovskii seam at the Volkov mine). The coal seam 4.5 m thick in the test section was faulted by tectonic dislocation with an amplitude of 1.5 m (Fig. 2c). The working intersected the dislocation at a right angle which made it possible to track wave propagation before and after the dislocation. Spectral curves 5(1), and 6(1), 7(1) and 7(3) in Fig. 2a were obtained from experimental data for a seam wave whose propagation rate was 1560 m/sec. In the numbering of Fig. 2 the first figure gives the number of the reception point and the second gives the number of the detonation point; the record at the first instrument installation was obtained from the first detonation point and that at the second was obtained from the third detonation point. From the curves presented it can be seen that as the dislocation is approached the typical increase in spectral component of the descending branch of the basic extremum increase even more, and in the area of the dislocation it is transformed into a separate maximum at a frequency with a multiple base. In the region beyond the dislocation high-frequency oscillations are attenuated as the distance from the dislocation increases (curves 3(3) and i(3)), and this may be explained by the effect of the dislocated Jointed zone of the coal. Graphs were plotted for the change in frequency for the maximum basic extremum and at a level of 0.7 of it in relation to distance from the dislocation (Fig. 2b) from spectral curve data in Fig. 2a. On approaching the dislocation there is an increase in the proportion of high frequencies and then a gradual decrease. It should be noted that it was not possible to track the seam wave after the dislocation; an analysis was made of wave oscillations whose confinement was established hypothetically, and the very fact of an increase in the proportion of high frequencies is undoubted. On the basis of equations obtained consideration was given to the nature of change in the seam wave spectrum for an undlslocated section. Curve 5(1) in Fig. 2a (i in Fig. 3) was taken as a starting point. Calculations were carried out in a computer. As a result of transformations the spectra presented in Fig. 3 were obtained. Curves 2, 3, and 4 were obtained with the conditions C, - 1500 m/sec, Z = i0 m with different distances from the dislocation (the last mentioned curve is located in the region of the dislocation). There are

23

no marked changes in the spectrum. A similar calculation was carried oun for conditions when the width of the dislocated zone was 5 m. In the spectral curve corresponding to a point at the boundary an additional extremum in the frequency occurred at about 140 Hz. More typical chanses in the spectrum were obtained with a dislocated zone width of 2 m (curves 5, 6, and 7, and the latter corresponds to a point at the boundary wlth the dislocation). The second group of curves have changes comparable with those which were obtained for the extreme data (see Fig. 2); in the spectrum an additional extremum appeared at a frequency of 180 Hz and the frequency of the basic extremum changed on approaching the dislocation. From this it may be concluded that depending on the width of the dislocated zone higher frequency oscillations may appear in the spectrum of a channeled wave. The orisin of extremums in multiple basic frequencies is of considerable interest. As a consequence of this it follows that waves reflected from the dislocation may exhibit frequencies of both the basic extremum and much hisher values. An increase in the high frequency component in the spectrum of a reflected wave is partly confirmed by the results for chanses in the frequencies observed for forward channeled and reflected waves according to observations obtained in carrying out work at a number of Kuzbass mines (see Table 1). As can be seen from Table i, in certain cases the frequency of reflected waves does not change in comparison with the incident wave. It is possible that this is connected not only with reflections at a frequency of the incident wave, but also with the fact that the frequency of the first resonance, caused by the dislocated zone, coincides with the frequency of the incident wave. The established occurrence of higher frequencies in the spectrum of reflected waves is important in isolating disjunctive dislocations. Further study of this phenomenon will make it possible to increase the amount of information provided by the seismic method. LITERATURE CITED 1. 2. 3.

G . A . Gamburtsev, Fundamentals of Seismic Prospecting [in Russian], Gostoptekhizdat, Moscow (1959). L . M . Brekhovskikh, Waves in Layered Media, Academic Press (1960). I . S . Berzon, A. M. Eplnat'eva, G. N. Pariiskaya, and S. P. Starodubrovskaya, Dynamic Characteristics of Seismic Waves in Actual Materials [in Russian], Izd. Akad. Nauk SSSR, Moscow (1962).

COMBINED ESTIMATE OF THE MECHANICAL PROPERTIES OF A ROCK MASS AT GREAT DEPTH AT THE DEPOSIT-SURVEYING STAGE G. D. Manev

Knowledge of the mechanical properties of rocks is necessary for the solution of all problems in rock geomechanics which are associated with the working of deposits of mineral resources and at all stages in the planning, construction, and use of mining facilities. Determining the mechanical properties of the rock is of great interest in the initial stages of deposit surveying, when mining operations have not yet taken place. In this case, a unique source of information on the state of the rock mass is core drilling and geophysical survey methods. The entire complex of the rock's physical and mechanical properties is determined under laboratory conditions on the cored material. In this work means are considered for determining the calculated characteristics of a rock mass, based on the data from laboratory investigations, break-up fracturing of the cores, and seismoacoustic studies in drillholes.

Scientific-Research Institute for Mineral Resources, Sofia. Translated from FizikoTekhnlcheskie Problemy Eazrabotkl Poleznykh Iskopaemykh, No. i, pp. 28-35, January-February, 1983. Original article submitted February 5, 1982.

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0038-5581/83/1901-0024507.50

9 1983 Plenum Publishing Corporation