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.Journal of Applied Geophysics 44 2000 369382www.elsevier.nlrlocaterjappgeo
The seismic velocity distribution in the vicinity of a mine tunnelat Thabazimbi, South Africa
C. Wright a,), E.J. Walls b, D. de J. Carneiro ca Bernard Price Institute for Geophysical Research, The Uniersity of the Witwatersrand, Johannesburg, Priate Bag 3,
Wits 2050, South Africab Department of Geophysics, The Uniersity of the Witwatersrand, Johannesburg, Priate Bag 3, Wits 2050, South Africa
c Formerly CSIR Mining Technology, PO Box 91230, Auckland Park 2006, South Africa
Received 4 June 1999; accepted 13 April 2000
Abstract
Analysis of the refracted arrivals on a seismic reflection profile recorded along the wall of a tunnel at an iron mine nearThabazimbi, South Africa, shows variations in Pwave velocity in dolomite away from the destressed zone that varybetween 4.4 and 7.2 kmrs, though values greater than 5.8 kmrs predominate along most of the profile. The seismicvelocities at the tunnel wall, however, vary between 4.2 and 5.2 kmrs. Timedepth terms are in the range from 0.1 to 0.9ms, and yield thicknesses of the zone disturbed by the tunnel excavations of between 2 and 9 m. The very low seismicvelocities away from the tunnel wall in two regions are associated with alcoves or cubbies involving offsets in the wall ofup to 10 m. The large variations in seismic velocity resolved over distances less than 15 m with signals of wavelengtharound 69 m are attributed to variations in the sizes and concentrations of fracture systems and cracks, and in the degree ofgroundwater saturation of the fracture systems. The results suggest that seismic velocity variations from reflection surveysmay also assist modelling studies of the stress regime in deep mines, particularly if both P and S wave velocity variationscan be determined. The seismic velocity variations inferred also show that application of refraction static corrections in theprocessing of inmine seismic reflection profiles is as important as in surface surveys, because of the higher frequencies ofthe seismic energy recorded in the deep mine environment. q 2000 Elsevier Science B.V. All rights reserved.
Keywords: Seismic reflection profile; Deep mine; Pwave velocity
1. Introduction
The use of highresolution seismic reflectionand refraction methods to assist mine development was pioneered more than 40 years ago .Schmidt, 1959 , and has become more wide
) Corresponding author. Fax: q27113397367.Email address: [email protected]
.C. Wright .
spread in recent years now that seismic dataacquisition equipment has become moreportable, and both computer software and highresolution data acquisition systems are relativelyinexpensive. Seismic reflection experiments applied in both mineral exploration and mine development often seek to image weakly reflecting and geometrically complex boundaries, andfor surveys conducted on the surface, accuratestatic corrections for both elevation changes and
09269851r00r$  see front matter q 2000 Elsevier Science B.V. All rights reserved. .PII: S09269851 00 000148

( )C. Wright et al.rJournal of Applied Geophysics 44 2000 369382370
the variable thickness of low velocity overburden and weathering are required to image weakly
reflecting structures Spencer et al., 1993;.Wright et al., 1994 . The extent to which static
corrections will be important in subsurface surveys, however, is still poorly known. Furthermore, to extract the maximum amount of geological information from reflection surveys, it ispreferable to use the refracted arrivals not onlyto provide static corrections, but to constraingeological interpretation between the survey
surface and the shallowest reflectors Wright et.al., 1994; Wright, 1996 .
The main objective of this paper is to providea detailed analysis of the refracted arrivalsrecorded during a highresolution seismic reflection survey in a mine tunnel at Thabazimbi,South Africa, to show that large variations inseismic velocity are created when a mine tunnelis excavated. These variations are probably dueto disturbance of the ambient stress field andconsequent destressing of the rock volume nearthe tunnel and small changes in temperature,resulting in the opening of cracks and fractures
within a few metres of the tunnel. The possibleuse of the seismic velocity variations inferredfrom such surveys to identify zones of weaknessand to assist in quantifying local stress inhomogeneities is also examined.
2. Geology of the Thabazimbi mine
Production of iron ore in the Thabazimbi areastarted in 1934, and the area was the leadingproducer of iron in South Africa for the nextquarter of a century. Ore is extracted from thebanded ironstone of the Penge Formation, whichoverlies a band of shale of thickness varyingfrom 2 to 10 m. Below the shale is the Frisco
Formation of the Malmani dolmites Figs. 1 and.2 . A marker band of chert occurs near the top
of the Frisco Formation with solution of theoverlying dolomite. This has caused collapse ofthe overlying Penge Formation resulting in
brecciation of the iron formations Van Deven.ter et al., 1986 .
..Fig. 1. Geological sketch map of the Thabazimbi mining area adapted from Van Deventer et al. 1986 .

( )C. Wright et al.rJournal of Applied Geophysics 44 2000 369382 371
Fig. 2. Stratigraphy of the Proterozoic Chuniespoort Group,Western Transvaal Sequence adapted from Van Deventer
..et al. 1986 .
The seismic reflection profile was undertakenin a footwall drive through the dolomite of theFrisco Formation. Fig. 3 is a vertical sectionshowing the location of the tunnel in thedolomite, and the overlying and southerly dipping band of shale and Penge Formation. Animportant problem in development of the mineis the construction of a crosscut from the tunnelthrough the dolomite to the iron formation. Toconstruct such a crosscut, prior knowledge ofthe physical properties of the interveningdolomite is required. Wad exists within the
Fig. 3. Vertical section through the mine tunnel at Thabazimbi, showing the geological setting and location of targetsto be imaged by seismic reflection.
.dolomite Fig. 3 , which consists of soft, clayeymanganese oxide, where the dolomite has beendissolved and altered by percolating fluids. Ifwad were encountered during excavations, thetunnel would be weak and likely to collapse.The contact between wad and dolomite is sharp,so that wad cavities are expected to haveimpedance contrasts sufficient to generate observable reflections. No wad was encountered inthe walls of the mine tunnel.
3. The seismic experiment
The seismic survey was conducted duringApril 1997 by CSIR Mining Technology in thewall of a nearhorizontal tunnel excavated inrocks of the Frisco Formation at a depth of 910
m below the surface in an iron mine East Mine.of Fig. 1 owned by Iscor Mining. The detection
of zones of wad was the primary objective ofthe seismic study, so that a suitable position of acrosscut could be chosen. The geometry of the
.mine tunnel relative to the wad Fig. 3 is suchthat reflected or scattered energy from the
Table 1Parameters of Thabazimbi Seismic Survey, April 1997
No. of usable shots 217Shot type, size Explosive chargesand spacing of 50 g, 1 m apartDepth of shot holes 0.51.0 m
.Shotreceiver Offend Fig. 4configurationNo. of geophone 281locationsReceiver spread 48 channels,
takeouts 1 m apartGeophones Vertical component, one
100Hz geophone pertakeout mountedsideways in tunnel wall
Sampling rate 0.1 msRecord length 100 msData fold Variable average
of 24

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Fig. 4. Shotreceiver configuration used for the seismicreflection experiment.
dolomitewad and possibly the wadshaleboundary should be detectable. A Bison 48channel data acquisition system was used with100 Hz verticalcomponent geophones secured
horizontally into the vertical mine wall Table.1 . The geophones were placed 1m apart ap
proximately 1.3 m above the floor of the tunnel.Seismic sources were 50 g of explosive placedin holes 0.51.0m deep drilled horizontallyinto the wall of the tunnel at 1m intervals atroughly the same height as the geophones. Thesource locations were midway between the geophones.
The shotreceiver configuration is shown inFig. 4, and gives rise to 24fold coverage forCMP gathers at most geophone locations. Thisoffend arrangement with 12 shots per locationof the receiver spread was used because of thelogistic difficulties in working in a mine, andthe need to have sufficiently large shotreceiveroffsets for velocity analysis. Two hundred seventeen usable shot records were obtained withgeophone spreads covering a distance along the
.tunnel of 270 m Fig. 5 . The fieldwork wasconducted from east to west. In displaying theresults, west is always shown on the left of thediagrams. However, to illustrate methods of dataanalysis, the usual convention has been adoptedof plotting with the start and end of the profileon the left and right, respectively.
Firstbreak times were picked for all shots todetermine both static corrections and the seismic velocity structure adjacent to the mine tunnel. Diffraction effects due to geophones inpositions in which there is no straightline shot
receiver path within the dolomite due to the.alcoves present a problem in designing an ap
Fig. 5. Diagram showing shots and geophones in a horizontal plan.

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Fig. 6. Method of projecting shot and geophone locationson to a reference line defined by the first shot location andlast geophone location of the profile.
propriate method of data analysis. In rare instances of extreme diffraction effects that wereevident in the firstbreak times for some geophone locations, data were eliminated from thecurrent analysis, which uses geometrical raytheory. For the traveltime analysis, the shot andgeophone locations were projected on to a reference line joining the first shot and the most
.distant receiver Fig. 6 . If the angle betweenthe line of length L joining the shot and receiver and the projected shotreceiver line SG isu , each measured distance L and time t arereplaced by corrected distances and times Lcosuand tcosu , respectively, for all subsequent analyses, except the estimates of seismic velocitiesclose to the tunnel wall, for which true shotreceiver distances were used.
The mine tunnel was in a location wheresources or receivers could not be placed inanother tunnel or on the surface, so that theintervening rock mass could not be imaged bytunneltotunnel or surfacetotunnel tomography. The present methodology is therefore advantageous in situations where such an approach cannot be used.
4. Method of data analysis
The seismic refraction methods used in mineral exploration have usually been the reciprocal
or plusminus method Hagedoorn, 1959;.Hawkins, 1961, Hawkins and Whiteley, 1981
or the generalized reciprocal method Palmer,.1981 , though an alternative approach involving
generalized linear inversion has been triedHampson and Russell, 1984; De Armorim et
.al., 1987; Leslie, 1995 . Recent modifications ofthe reciprocal and generalized reciprocal meth
.ods Palmer, 1990 have not considered theimportant problem of how to analyse real datasets in which errors in time measurements aresignificant, making interpretation very subjective. In this paper a new method of applyingnumerical and statistical methods to optimizethe resolution of lateral variations in seismicrefraction velocities has been used, in which thecomputations are faster than when generalized
.linear inversion or tomography are used .Wright, 1999a .
4.1. Shot delay times
While measuring firstbreak times for estimation of shot and receiver static corrections, itbecame clear that the measured travel times tothose geophones close to the shot point were
.often anomalously small Fig. 7 . The explanation is the presence of spurious and unpre
Fig. 7. Some specimen seismograms showing time delays .in triggering the recording system. Estimated minimum
. .delays for shots at locations 103 top and 105 bottomare 1.28 and 1.60 ms, respectively.

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dictable time delays in triggering the recordingsystem. Accurate estimates of these time delaysare essential for the successful processing of thereflection data, and must be undertaken prior tothe analysis of refracted arrivals to assist structural interpretation and the computation of thestatic corrections to be included in the reflectionprocessing. The two shot records shown in Fig.7 show that the recording system triggered afterthe earliest refracted arrivals had reached someof the geophones close to the shot point. Tofurther illustrate the shottiming problem, tworeceiver gathers are shown for locations 151 and
.158 Fig. 8 , which are 4 and 11 receiver locations, respectively, from the closest shot at location 147. Both sets of receiver gathers show
Fig. 8. Receiver gathers of locations 151 and 158, with the .nearest shot at location 147 left and the most distant at
.101 right . The early signal at location 122 corresponds tothe shot delay of 6.3 ms in Fig. 10. Note that there was no
.shot recorded at the location 121 of the zero trace.
Fig. 9. Method of determining relative shot delay times.
similar patterns of time variations in the firstbreaks due to two causes: shottiming errors,which tend to predominate, and the locations ofand conditions surrounding the shot points.
A leastsquares inversion procedure has beendevised for estimating these shot time delaysfrom the firstbreak picks. The procedure involves fitting a leastsquares line through thetimes for each shot gather that correspond torefracted paths in which energy has travelledbeyond the lowvelocity tunnel wall. Then, foreach shot, time residuals are estimated for allshot gathers that are recorded by two or moregeophones that lie within the distance rangeover which the leastsquares line was calculated. The time residuals are defined as thedifferences between the measured times and thetimes determined from the line fitted throughthe times for the reference shot at the same
.shotreceiver offset Fig. 9 . The residuals arethen averaged to give an estimate of the relativetime delays between the shots. The procedure isrepeated for all shots, and estimates of therelative time delays are optimised by a least
.squares inversion procedure Wright, 1999b .The assumption in this method is that lateral
variations in seismic velocities can be neglectedfor the region of the profile used in the calculations. To ensure that the assumption of lateralhomogeneity for the region covered by the inversion was approximately true, the seismicprofile of 217 shots was divided into 18 overlapping regions of 72 geophone locations and

( )C. Wright et al.rJournal of Applied Geophysics 44 2000 369382 375
Fig. 10. Relative time delays for shots with minimumvalue set to zero.
3036 shots with about twothirds overlap between adjacent data sets, and a separate inversion for relative delay times was undertaken foreach region. In instances when a shot had manymissing firstbreak times due to large delays intriggering of the recording system, a damped
.leastsquares procedure Lines and Treitel, 1984was required with several iterations to ensureconvergence to a solution with low estimatedvariances. The results of the separate inversionswere pieced together by a weighted averagingscheme. The overall solution is nonunique because an arbitrary constant can be added to allrelative time delays without affecting the validity of the solution. The smallest time delay wasset to zero, which corresponds to triggering ofthe recording system at or close to the initiationtime of the seismic energy. This should give theminimum time delays required to align the shotgathers properly for further processing, thoughit is possible to overcorrect the data if sometime picks were made systematically early. Wethus assume that there is just one shot for whichthere is no delay or a very small delay intriggering.
Fig. 10 shows the relative shot delays plottedas a function of shot number. About twothirds
of the time delays lie between 0.7 and 1.8 ms 7.and 18 samples , and there are several outliers
corresponding to delays of between 4 and 10ms. Because of the high frequencies of the
recorded seismic data significant energy at 1.kHz , static corrections need to be determined
with errors of no more than 0.3 ms. The procedure for estimating relative shot time delaysallows such accuracy to be achieved.
4.2. Timedepth terms, nearsurface elocitiesand thickness of the destressed layer
.The reciprocal method Hawkins, 1961 wasused to estimate the timedepth terms that allow estimates of the thicknesses of the de
.stressed layer Fig. 11 . Suppose a shot at A isrecorded by geophones placed at B and C, and ashot at C is recorded by geophones at A and B;A, B and C lie in the same vertical plane. If thevertical velocity gradients in the lower mediumare small, the timedepth is given by
2 t s t q t y1r2 t q t , 1 . .D AB CB AC CAwhere the ts denote times. If reversed shotreceiver paths are not available so that one of the
. .terms of Eq. 1 is missing t say , we canCA .rewrite Eq. 1 as
2 t s t q t y t . 2 .D AB CB ACIn this situation, the shot at A may be well
off the end of the recording spread, so that caremust be taken to ensure that velocity gradientsorthogonal to the survey surface do not result in
Fig. 11. Principle of the reciprocal method, when all shotsare on one side of the recording spread locations. t is thevtime taken to travel the path BZ.

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systematic differences in t as the offset ACDbecomes larger. Since such velocity gradientsoccur in most survey areas, this situation isgenerally unavoidable. In the present experiment, however, all shots were on one side of therecording spread, so that equivalence of timedepth terms at adjacent shot and receiver locations must be assumed for the reciprocal methodto be applicable.
Seismic velocity increases gradually awayfrom a mine tunnel. A linear increase in seismicvelocity with increasing distance away from thetunnel wall was assumed and used to derive arelationship between t and the nearverticalDtravel time from the layer to the boundary, t . IfvV and V are the Pwave velocities at the1 2tunnel wall and at the base of the destressed
zone the region affected by the tunnel excava.tion , the nearvertical travel time, t , to depthv .z Fig. 12 is given by0
t sKy1ln V rV , 3 . .v 2 1where V sV qKz , where K is a constant, so2 1 0that
z s V yV rK . 4 . .0 2 1 . .Eliminating K between Eqs. 3 and 4
z s V yV t rln V rV . 5 . . .0 2 1 v 2 1In Fig. 12,
t s t y t . 6 .D D1 D2From the expressions for travel times and
distances for rays propagating in a medium inwhich velocity varies linearly with depth Kleyn,
.1983, pp. 5758 , it is shown that1r2y1 2 2t sK ln V r V y V yV 7 . . 5D1 1 2 2 1 /
and, if XsAC in Fig. 12,
1r2y1 2 2t sXrV sK V yV rV . 8 . .D2 2 2 1 2 . . .Using Eqs. 3 , 7 and 8 ,
t s t y t sC t , 9 .D D1 D2 f v
where1r22 2C s ln V r V y V yV . 5f 1 2 2 1 /
1r22 2y V yV rV rln V rV . 10 . . .2 1 2 2 1Then,
z s V yV t rC ln V rV . .0 2 1 D f 2 11r22 2s V yV t r ln V r V y V yV . . 52 1 D 1 2 2 1 /
1r22 2y V yV rV . 11 . .2 1 2We are not sure that the measured times are
absolute; the times corrected for late triggeringof the recording system are not necessarily closeto the instant of initiation of the seismic energy.The nearsurface velocities must therefore bedetermined by numerical differentiation of thetimes at short distances. All times at true distances less than 10 m, corrected for relativeshottiming errors, were used in the analysis.For each location of the receiver spread, datawere arranged in increasing order of shotreceiver distance, and velocities for different distance windows were estimated for windowlengths of about 5 m. In instances when therewas a decrease in velocity with increasing off
Fig. 12. Diagram showing linear variation of velocity withdepth and way of estimating timedepth terms. t is thevtime taken for seismic energy to travel path BC. t andD1t are the times taken to traverse the ray path AB and theD2nearhorizontal path AC, respectively. The timedepth tDs t y t .D1 D2

( )C. Wright et al.rJournal of Applied Geophysics 44 2000 369382 377
Fig. 13. Pwave velocity profile in the undisturbed zone away from the tunnel wall with 95% confidence limits on thesolution, and the less accurate velocity profile for the region near the surface of the tunnel wall.
set, due to inadequate resolution, the windowlength was increased until the velocities increased monotonically with increasing distance.This method tends to give an upper bound to the
surface velocities, and may therefore overestimate the shallow velocities by a small amount.The scattered results were then smoothed, first
by the method of summary values Jeffreys,
Fig. 14. Timedepth terms as a function of receiver location.

( )C. Wright et al.rJournal of Applied Geophysics 44 2000 369382378
Fig. 15. Method of adjusting shot times to correspond to asingle shot placed on the surface of an underlying refractor.
.1937; Bolt, 1978 , then by a cubic spline, andthen further smoothed using a 21point slidingaverage to give the velocity profile shown inFig. 13, in which the variation in velocity isfrom 4.2 to 5.1 kmrs.
After correcting all measured times for errorsin shot timing, the data were sorted into common receiver gathers. This enables timedepth
.terms Fig. 12 to be derived using the reciprocity principle in which interchangeability ofshots and receivers is assumed. A search overall shot and receiver gathers for which the shotreceiver offset exceeded 4.0 m was used todetermine the timedepth terms at locationsaway from the ends of the profile, assuming thatshots and receivers at the same location numberhave the same timedepth term. These valuesvary between 0.1 and 0.9 ms and are plotted inFig. 14, and the number of timedepth terms ateach location varies between 14 and 660.
4.3. Seismic elocity ariations away from tunnel wall
Seismic velocity variations away from thetunnel wall were determined using the method
.of Wright 1999a . The timedepth terms forthe shot and receiver were subtracted from eachmeasured time to project the shots and receiversdown below the destressed zone. A leastsquares line was then fitted through the times
for each shot gather. Average residuals relativeto all other shot gathers that have times measured for at least two of the same geophones as
the reference shot were then calculated Fig..15 . The set of relative corrections was opti
mized in a leastsquares sense and applied toeach shot gather to give a single travel timeversusdistance relationship corresponding to a
hypothetical shot at one end of the profile Fig..16 . To ensure convergence, data were divided
into several overlapping ranges, and the separate inversions were pieced together. In mostinstances, a damped leastsquares procedure wasrequired, and convergence was slower whenshots with many times missing were used. Theconstruction of a single traveltime curve allowsflexibility in the choice of distance windows forestimating variations in seismic velocity, and
thus ensures effective use of the data Wright,.1999a .
The method of summary values Jeffreys,.1937; Bolt, 1978 was used to estimate seismic
..velocities V of Eq. 11 in distance windows2of between 14 and 26 m, with 200600 pointsin each window. These windows were chosen togive errors in velocity of 0.73.3%, with smallerand larger errors corresponding to regions ofrelatively homogeneous and widely scattereddata respectively. A cubic spline involving asmall amount of smoothing was then fittedthrough the velocities to give values at intervals
Fig. 16. Corrected travel times plotted as a function ofoffset.

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.Fig. 17. Horizontal plan of mine tunnel showing approximate boundary of destressed zone top , and thicknesses of the .destressed region adjacent to the tunnel wall, as determined from Eq. 11 .
.of 2 m Fig. 13 . At each geophone location, anestimate of the thickness of the destressed zone
.was calculated using Eq. 11 to give the resultsof Fig. 17.
5. Seismic results and their interpretation
Fig. 13 shows the smoothed seismic velocities for both the surface of the mine tunnel andthe region away from the tunnel excavation.There is a rough parallelism between the twocurves, except that the faster seismic velocityprofile for undisturbed rock exhibits greater resolution of lateral variations, because of the large
.number of separate time measurements 9512 .There are three regions between distances ofy175 and y185 m, y280 and y310 m, andfrom y340 to y360 m, where the seismicvelocities in undisturbed rock are low comparedwith the estimated values on the tunnel surface.The first small local minimum is for a region
.behind an alcove Fig. 17 , where the termbehind is used to signify the direction in whichthe survey was recorded, with all shots to theeast of the geophone spread. The two largeminima in seismic velocity between y280 andy310 m, and y340 and y360 m, cover theregion both in front of and behind an alcove,making it unlikely that the velocity anomaly is aconsequence of not having properly accountedfor diffraction effects around the alcove walls.The very low seismic velocities suggest thatthese two regions are zones of weakness or high
.concentrations of dry fractures see Section 6 ,or involve voids or potholes not far from thetunnel wall.
The velocities of Fig. 13 were used to determine the thickness of the disturbed zone using
.Eq. 11 . The three gaps in the data are due toeither absence of one or both velocities for input
.into Eq. 11 , or very small differences betweenV and V . If the difference between V and V1 2 1 2is small, it seems likely that fracturing or voids

( )C. Wright et al.rJournal of Applied Geophysics 44 2000 369382380
in the dolomite persist to considerable distancesfrom the tunnel wall. On the other hand, substi
.tution into Eq. 11 will yield very large thicknesses for the destressed zone for moderate
.sized timedepth terms. Clearly, use of Eq. 11is only warranted if there is a velocity increaseacross the destressed zone of at least 0.8 kmrs.
While estimates of the thickness of the disturbed zone are widely scattered, there is a trendfrom values of 78 m around the y110 mposition towards lower values, reaching minimum values close to 4 m between y230 andy290 m, followed by a slight upward trend toabout 5 m when the Xcoordinate is near y340m.
6. Discussion
Rocks within the uppermost few kilometresof the earths crust contain fractures that may berandomly distributed, or oriented in particularplanes due to local or regional stress fields,resulting in seismic anisotropy. These crackscause the seismic velocities to be lower than fora crackfree material, the amount of loweringdepending on both the density of cracks andwhether the cracks are dry or contain water.Overburden pressure will close cracks, resultingin an increase in seismic velocity for a particular rock type with increasing depth. A deepmine tunnel results in a change in the localstress regime, with lowering of the seismic velocities close to the tunnel walls, due to bothopening of existing cracks, and the formation ofnew cracks during the tunnel excavation process.
The theories for seismic wave propagation inisotropic and anisotropic distributions of cracks
.in rocks have been reviewed by Hudson 1981 .and Crampin 1981 , respectively. It seems rea
sonable to use variations in seismic velocity in aparticular rock type to infer crack properties. InFig. 13, the maximum measured velocity of 7.2kmrs is assumed, for illustrative purposes, toapproximate the conditions of a crackfree
dolomitic rock mass. This value was used tocalculate the seismic velocities and Poissonsratio as a function of crack density parameter,for both dry cracks and cracks filled with aviscous fluid, assuming an intrinsic Poissonsratio of 0.25. The results shown in Fig. 18 werederived using the isotropic theory of OConnell
.and Budiansky 1977 with the modifications .suggested by Henyey and Pomphrey 1982 .
The predicted velocity variations, comparedwith those in Fig. 13, suggest that the measuredvariations in Pwave velocity can only be explained if the cracks in some areas are dry orpartially saturated, because they are too large tobe explained in terms of watersaturated cracksat moderate crack densities. If shearwave velocities were also measured, better constraintson the fluid content of zones of fractures andfracture density would be provided. However,the present experiment did not provide shearwave arrivals of sufficient clarity to enable suchan analysis to be undertaken. The significance
.of the crack density parameter Fig. 18 is that itcan be related to the actual distribution of cracksif independent information on the spectrum ofcrack sizes or areas can be obtained. Sinceseismic velocities and the distribution of cracksare also related to the stress regime, future
Fig. 18. Seismic velocity variations and Poissons ratio asa function of crack density parameter for dry and saturatedcracks. The results were computed using the selfconsistent
.theory of OConnell and Budiansky 1977 , with the modi .fications proposed by Henyey and Pomphrey 1982 .

( )C. Wright et al.rJournal of Applied Geophysics 44 2000 369382 381
modelling studies of the stresses in mine velocity variations.
The setting up of a seismic survey in a tunnelinvolves drilling into the wall to install geophones and shot locations. Both the drilling andthe detonation of explosives will result in theformation of new cracks. However, the newcracks should extend only for distances of ametre or so from the geophones or shot points.The effects of both drilling and the explosivecharges used in the experiment on local fracturing would be only a small addition to thosealready produced in excavating the tunnel. Wetherefore assume, to a first approximation, thatthe influence of the seismic experiment on thecrack distribution is of secondary importance,although additional experiments would be required to confirm this.
The seismic signals have dominant frequencies of around 750 Hz, indicating wavelengthsin the range 69 m, so that velocity variationsover a distance range of 34 m should bedetectable with highquality data. Knowledge ofthese velocity variations is required to enablereliable static corrections to be derived for reflection processing, especially the contributionfrom topographic variations associated with
the cubbies or alcoves up to 2.3ms differ.ences between nearby shot or receiver locations .
However, allowing for variations in seismic velocity both along and orthogonal to the tunnelwall is equally important, since they produceshortwavelength variations of up to 0.7 ms.Any reflected signals will have periods of about1.3 ms, so that accurate static corrections mustbe applied if any reflected energy is to beenhanced through stacking.
The method of correcting for shottiming errors will also tend to remove differences intimedepth terms between shots. Adverse consequences arise from the need to use receivergathers to define timedepth terms, and therefore to assume that adjacent shots and receivershave the same timedepth terms. The result willbe some smoothing of the differences betweentimedepth terms. However, if relative shot
timing errors had not been estimated, the datawould have been uninterpretable. We note thatthis problem does not arise for splitspreadrecording or if shots are detonated at or nearboth ends of the recording spread.
7. Conclusions
Recorded seismic signals from small explosive charges in a mine tunnel about 900 mbelow the surface have dominant frequencies ofaround 750 Hz. These frequencies are very highcompared with signals recorded on the surfacefrom similar sources in tamped holes at theEarths surface, which usually have dominantfrequencies of less than 200 Hz. Better resolution of reflectors and scatterers of seismic energy away from the mine tunnel is thereforeanticipated compared with surveys on the surface, but accurate refraction static correctionsobtained through careful analysis of refractedarrivals must be applied during processing.
Pwave velocities in dolomite away from thedestressed zone vary between 4.4 and 7.2 kmrs,though values greater than 5.8 kmrs predominate along most of the profile. The seismicvelocities at the tunnel wall, however, varybetween 4.2 and 5.2 kmrs. Estimated thicknesses of the zone disturbed by the tunnel excavations vary between 2 and 9 m. The very lowseismic velocities away from the tunnel wall intwo regions are associated with alcoves or cubbies involving offsets in the wall of up to 10 m.The large variations in seismic velocity resolvedover distances less than 15 m with signals ofwavelength around 69 m are attributed largelyto variations in the sizes and concentrations offracture systems and cracks, though variationsin the composition of the dolomite and thedegree of water saturation in cracks and voidsmay also be important contributors to the velocity variations. The results also suggest that seismic velocity variations from reflection surveysmay assist the interpretation of modelling studies of the stress regime in deep mines.

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Acknowledgements
w .xWe thank Iscor Mining Iscor Pty for allowing us to publish results derived from seismic data recorded in one of their mines, andstaff of both CSIR Mining Technology andIscor Mining for providing the facilities for dataacquisition and for assistance with field operations. We also thank Fred Stephenson of CSIRMining Technology for critically reading themanuscript.
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