19,700 E

20,200 N

19,800 £

20,000 E

15 neters|
cate.

4900 level

(c)

19,900

20,000 &

20,100 €

20,200 E

92

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19,800 N

‘“seste—

° 15 neters

5100 level

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Spod locations

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Table 2
Summary of model parameters

Date Mine site. Number of nodes, NN Dimensions* Ratio: NN/NP
July 1994 Homestake 859 9.62 x 8.02 x 3.67 3.87
October 1994 Homestake 6859 9.62 x 8.02 x 3.67 2.36
March 1993 Lucky Friday 10 648 6.40 x 5.20 x 4.30 441
November 1993 Lucky Friday 10 648 6.40 x 5.20 x 4.30 2.53

“length (m) x width (m) x height (m).

model updates were computed using the simulta-
neous iterative reconstruction technique, or SIRT
(Gilbert, 1972). The velocity distribution resulting
from five straight-ray iterations was used as the
starting point for the remaining 10 curved-ray
iterations. The total number of straight-ray itera-
tions was minimized due to the tendency to diverge
from a “best” linear solution (Friedel et al., 1993).
Ray tracing was performed by the bending method
of Um and Thurber (1987). This approach
involves perturbing a trial ray path until the travel
time is minimized.

In general, the results reflected a satisfactory
global minimum root-mean-square (RMS) travel
time residual (about 1.2 and 1.45ms for the
Homestake and Lucky Friday data sets, respec-
tively) and maximum velocity perturbation (RMS
difference between starting and updated velocity
mode!) following 15 iterations. RMS residuals
were approximately 7.4% (Homestake) and 8.6%
(Lucky Friday) of the RMS travel time, and were
approximately 30% of those associated with the
starting models.

4.5. Uncertainty

To evaluate uncertainty associated with the
tomographic inversion process, eight different uni-
form starting velocity models were used to produce
a set of reconstructions. These eight reconstruc-
tions were viewed as “random” selections from a
distribution of possible solutions to the under
determined set of equations. In this way, we could
use basic probability theory to calculate the likeli-
hood that the true velocity at any node exceeds a
threshold value. We selected a value of 6.0 km s~*
to represent the critical threshold velocity associ-
ated with highly stressed rock. Thus, a node whose
average velocity exceeds the threshold value by

one or two standard deviations has a probability
of 67 or 95% (Abramowitz and Stegun, 1979).

We recognize that the conditions for application
of the probability distribution are not rigorously
met in this application. Specifically, the eight veloc-
ity models on which the statistics are based,
although derived from different starting models,
are not truly independent estimates of the velocity
field, because they are all based on the same travel
time measurements. Consequently, the calculated
probabilities are overestimated and should not be
considered to be quantitative indicators. However,
they are useful qualitative guides with respect to
the prevailing anomalies.

The degree of uncertainty, for example, associ-
ated with the first set of Homestake reconstruc-
tions is visually apparent when comparing the 67,
90 and 95% iso-velocity surfaces shown in
Fig. 4(a-c). Those velocity anomalies appearing
at the 67% iso-velocity periphery are most likely
artifacts due to poor raypath coverage and sample
density. This assertion is substantiated by the fact
that these more uncertain anomalies do not appear
in the 90 and 95% iso-velocity figures. The lack of
change in velocity structure from 90 to 95% prob-
ability implies that either velocity surface may
prove sufficient for identifying anomalously high
stress regions. Recognizing that a low probability
surface incorporates too much tomographic uncer-
tainty, assigning too high a probability value, e.g.
99%, is also unreasonable as there is not enough
seismic information available to justify its use. For
these reasons, the remaining analyses are based on
using 95% iso-velocity surfaces.

4.6. Stress change due to ground failure

The computed 3-D iso-velocity surfaces for the
Homestake tomographic surveys conducted during


4. Tomographic imaging
4.1, Data acquisition

Seismic energy was introduced by impacting
rock bolts along mine drifts using an 8-Ib sledge
hammer. The first-arriving seismic energy was
simultaneously measured at 24 single component
100-Hz geophones. Each geophone was screwed
into a rock bolt at locations along the mine drift.
Fig. 3(a-d) gives a plan view of geophysical loca-
tions at two levels for both mine sites. An
instantaneous floating-point seismograph enabled
rapid, multichannel, data collection and storage
over a 120 dB. dynamic range. The signal character
was optimized by automatically stacking 20
records (sampled at 0.1-ms intervals), and applying
720 Hz highpass (or Jowcut) and 2 kHz anti-alias-
ing filters. Reciprocity checks were conducted to
test the repeatability of timing across and
between levels.

4.2. Seismic resolution

The resolution associated with seismic tomogra-
phy is related to the wavelength of the propaga-
ting energy. For transmission tomography, the
theoretical resolution is about one wavelength
(Williamson, 1991). Knowing the average rock
mass velocity (computed from the ensemble of
apparent velocities between individual source-re-
ciever pairs), and nominal survey frequency, an
estimate of the minimum resolvable limit is deter-
mined to be about 7m. Table 1 gives a summary
of the site properties. Thus, if seismic energy is
transmitted through high-velocity (high-stress)
regions then features larger than this limit should

Table 1
Summary of site survey parameters

be resolvable. The fact that first-arriving seismic
energy preferentially samples high-velocity regions
in a heterogeneous domain further enhances our
ability to image high-stress regions.

4.3. Grid and initial models

The minimum survey volume was defined as the
smallest rectangular parallelepiped enclosing all
source and receiver positions, with edges parallel
to the mine coordinates. To accommodate the
bending of rays outside the. minimum survey
volume, the computational domain was enlarged
by 20%. Calculations for both sites used a uniform
grid. A summary of model parameters is presented
in Table 2. Note that in both cases the dimensions
of individual voxels were comparable to the
average wavelength. Also, the number of grid
nodes for these surveys is about 2.44.4 times
greater than the number of travel times resulting
in an under-determined system of equations. To
reduce the indeterminacy, a global maximum
velocity constraint of 6.8 and 6.0 kms~! was
imposed during the Homestake and Lucky Friday
inversions, respectively. Based on the excellent
visual correlation between numerically derived
stress and tomographically derived velocity distri-
butions for the first Lucky Friday survey (Friedel
et al, 1995b), the remaining three grids were
thought to be adequate.

4.4, Inversion

The tomographic reconstructions were obtained
by combining iterative adjustment of the alternate
starting models, application of global constraints
and ray tracing (Jackson and Tweeton, 1996). The

Date Survey site Number of ‘Average velocity Nominal Wavelength Standard error
projections, NP (kms~') frequency, Hz — (m) (km s—1)

July 1994 Homestake 1770 5.06 720 7.02 0.120

October 1994 Homestake 2906 5.10 720 7.10 0.090

March 1993 Lucky Friday 2415 3.82 512 7.46 0.078

November 1993 Lucky Friday 4217 4.45 720 6.19 0.068



1000
800
600
400
200

NUMBER OF RAYPATHS.

VELOCITY, km/s

Fig. 7. Velocity histograms for Lucky Friday mine during first
week March and second week November, 1993.

This is particularly true in view of the abnormally
high horizontal to vertical stress ratio of 2. While
comparison of these average velocity values pro-
vides some qualitative information regarding
increasing stress in the survey region, it cannot
differentiate its spatial location.

To identify and monitor changing stress condi-
tions in the remnant ore pillar, seismic data from
the tomographic surveys were used to produce the
high (opaque) and low (translucent) iso-velocity
surfaces shown in Fig. 8(a,b). In addition to the
velocity surfaces, Fig. 8 also provides a 3-D render-
ing of the silver vein and corresponding mine
drifts. Like the Homestake mine, the Lucky Friday
mine also experienced a catastrophic release of
strain energy. This 3.5 Richter magnitude event
occurred 1 month prior to the first survey and
damaged select regions on both the 4900 and 5100
levels. The damaged regions are outlined in the
figure and appear as translucent, low velocity (low
stress), anomalies surrounding the drift.

It is interesting to note the disappearance of the
low-velocity anomaly in the region of ground
failure on the lower 5100 level. This apparent
increase in velocity further indicates an apparent
transfer of stress from below to the remnant struc-
ture during stope advancement. The long low
velocity anomaly, parallel to the crosscut drifts on
the lower level, is actually a drift that sustained
severe damage during the release of stored strain
energy. The ability to image this drift during the
second survey is due to increased tomographic
coverage. While the other drifts remain open, this
“ghost” drift was backfilled in an attempt to
mitigate future stability problems.

Another indication of apparent stress transfer
due to mining activity is the increase in the volume
of high velocity. For example, as mining progressed
downward (from March to November), the volume
of critically stressed rock appears to have expanded
by more than a factor of 2. The concentration of
stress along the hanging wall side of the veins
suggests that it may be exerting some degree of
structural control.

5. Discussion and conclusions

The velocity structure we have imaged can be
interpreted qualitatively in terms of stress anoma-
lies, in a way that is broadly consistent with
measured in situ stresses. Reconstructed regions
of low velocity correlate with locations of stress-
relieved zones surrounding drifts and sites that
experienced catastrophic ground failure. Iso-
probability maps show robust high-velocity ano-
malies and provide the best estimate of the loca-
tions of anomalously high-stress regions.

Various factors should be borne in mind when
interpreting the relationships between p-wave
velocity, stress, and in attempts to evaluate ground
stability hazards using such observations. First,
we have used the term “stress” quite generally,
without distinguishing between hydrostatic and
differential stresses or their orientations. A related
limitation lies in our treatment of velocity as
isotropic, which may produce artifacts in the
reconstructions if anisotropy is significant.

Despite these limitations, the final velocity
reconstructions are consistent with all available
“ground truth” information; even difficult-to-
image low-velocity features such as regions that
experienced ground failure are accurately repre-
sented. We therefore consider the final model as a
limited but nevertheless essential tool for ground
stability hazard evaluation. In particular, large
releases of strain energy affect rock volumes with
dimensions many times larger than the 7-m domi-
nant wavelength of this study, and anomalous
stress variations on such a scale are more amenable
to accurate tomographic reconstruction.


Temporal imaging of mine-induced stress change using
seismic tomography

M.J. Friedel**, D.F. Scott®, T.J. Williams”

* University of Minnesota, Department of Geological Engineering, Minneapolis, MN, USA
> US Department of the Interior, Bureau of Mines, Spokane Research Center, Spokane, WA, USA

Received 9 April 1996; accepted 9 September 1996

Abstract

In an effort aimed at monitoring mining-induced stress changes that may influence ground failure, the US Bureau
of Mines conducted a series of active 3-D seismic tomographic surveys at the Homestake (Lead, South Dakota) and
Lucky Friday (Mullan, Idaho) mines. Existing rock bolts were used to mount geophones and as strike points for
introducing seismic energy using a 3.6 kg sledge hammer. Travel time measurements were recorded spanning up to
160 m between drifts at several levels. The tomographic uncertainty was minimized by computing iso-velocity surfaces
of constant probability based on statistics from multiple reconstructions and a threshold criteria. Periodic imaging
using tomographic velocity surfaces demonstrated that temporal and spatial changes in the local stress field occurred
in response to ground failure and stope advancement at a gold and silver mine, respectively.

Keywords: Temporal imaging; Mine-induced stress; Seismic tomography; Geophysics

1. Introduction
1.1. Problem

Mining activity in the western US and elsewhere
reaches depths of more than 2 km. The lithostatic
stress at this depth is significant, on the order of
50 MPa, and mining activity causes substantial
perturbation of the ambient stress field. Removal
of ore and fracturing of adjacent material by
blasting results in the transfer of stress to mine
pillars and other intact rock volumes.
Concentration of stress to levels exceeding the
strength of rock and/or geological discontinuities

* Corresponding author.

often results in a catastrophic and generally unpre-
dictable release of strain energy, e.g., Richter mag-
nitudes of 2.2 and 3.5 in the Homestake and Coeur
d@Alene mining districts, posing grave danger to
miners and increasing production costs due to
clean up, lost time and the need for additional
rock support.

Although a variety of countermeasures are in
use in these rupture-prone mines, no method has
proven entirely reliable. Conventional countermea-
sures can be passive, e.g., observation of
temporal/spatial sequences of rock mass failure
using microseismic monitoring, or active, e.g.,
excavation sequencing, application of counter
stresses through bolt installation, and/or destress-
ing through a drill and blast sequence. Despite the


Acknowledgment

The authors thank the management of
Homestake and Hecla mining companies for pro-
viding underground access and information related
to ground failure and seismic activity.

References

Abramowitz, M. and Stegun, I.A., 1979. Handbook of Mathe-
matical Functions. Dover, New York.

Blake, W. and Cuvelier, D., 1990. Developing reinforcement
requirements for rock burst conditions at Hecla’s Lucky
Friday mine. Proc. 2nd Int. Symp. Rock Bursts and Seismi-
city in Mines °88, Balkema, Rotterdam, pp. 407-409.

Friedel, M.J., Tweeton, D.R., Jackson, M.J., Jessop, J.A. and
Billington, S., 1992. Mining applications of seismic tomogra-
phy. Expanded Abstracts, Ann. Int. Mtg, Soc. Expl. Geo-
phys., New Orleans, LA, October 25-28, pp. 58-62.

Friedel, M.J., Jackson, M.J., Tweeton, D.R. and Olson, M.S.,
1993. Application of seismic tomography for assessing yield
pillar stress conditions. Proc. Twelfth Int. Conf. Ground
Control Min., Morgantown, WV, August, pp. 292-300.

Friedel, M.J., Jackson, M.J. and Williams, E.M., 199Sa. Tomo-
graphic imaging of coal pillar behavior: observations and
implications. Int. J. Rock Mech. Min. Sci., 33(1), 1995.

Friedel, M.J., Jackson, M.J., Scott, D.F., Williams, T.J. and
Olson, M.S., 1995b. 3D tomographic imaging of anomalous
conditions in a deep silver mine. J. Appl. Geophys., 34(1),
1-21,

Friedel, M.J., Scott, D.F., Jackson, MJ., Williams, TJ. and
Killen, S.M. 1996. 3D tomographic imaging of anomalous
stress conditions in a gold mine. J. Appl. Geophys., (in
press).

Gibowicz, S.J. and Kijko, A., 1994. An Introduction to Mining
Seismology. Academic Press, San Diego, 399 pp.

Gilbert, P., 1972. Iterative methods for the three-dimensional

reconstruction of an object from projections. J. Theoret.
Biol., 36, 105-117.

Jackson, M.J. and Tweeton, D.R., 1996. 3DTOM — Three
Dimensional Geophysical Tomography, BuMines RI 9617,
84 pp.

Johnson, J.C., Pariseau, W.G., Scott, D.F. and Jenkins, F.M.,
1993, In situ stress measurements near the Ross shaft pillar.
BuMines RI 9446, 17 pp.

Maxwell, S.C, and Young, R.P., 1992. 3D seismic velocity
imaging using microseismic monitoring systems at Strath-
cona mine and mines gaspe, Proc. 94th AGM, Canadian
Institute of Mining.

McCreary, R., McGaughey, J., Potvin, Y., Ecobichon, D.,
Hudyma, M.. Kanduth, H. and Coulombe, A., 1992. Results
from microseismic monitoring, conventional instrumenta-
tion, and tomography surveys in the creation and thinning
of a burst-prone still pillar. PAGEOPH, 139, 349-373.

Radcliffe, K.S., Thill, R.E. and Jessop, J.A., 1986. Use of acous-
tic velocity for predicting stress under uniaxial compression.
115th Ann. AIME Mtg. Reprint 86-147, 11 pp.

Scott. D.F., 1993. Geologic investigation near an underhand
cut-and-fill stope, Lucky Friday mine, Mullan, ID, BuMines
RI, 9470, 25 pp.

Thill, R.E., 1982. Coal and rock properties for premine plan-
ning and mine design. BuMines IC 8973, pp. 15-35.

Um, J. and Thurber, C., 1987. A fast algorithm for two-point
seismic ray tracing. Bull. Seismol. Soc. Am., 77, 972-986.
Westmann, E.C., Foss, M.M. and Williams, E.M., 1994, Com-
parison of stress distribution images generated with P, Sh
and SV velocity tomograms. Expanded Abstracts, Ann. Int.
Mtg, Soc. Expl. Geophys., Los Angeles, CA, October 23-28,

Williamson, P.R., 1991. A guide to the limits of resolution
imposed by scattering in ray tomography. Geophys.,
56(2), 202-207.

Young, R.P., (Editor), 1993. Rock Bursts and Seismicity in
Mines '93, A.A. Balkema, Rotterdam.

Young, R.P. and Hill, J.J., 1985. Seismic characterization of
tock masses before and after mine blasting. Proc. 26th US
Symp. Rock Mech., Rapid City (SD), pp. 1151-1158.

Young, R.P. and Maxwell, S.C., 1992. Seismic characterization
of a highly stressed rock mass using tomographic imaging
and induced seismicity. J. Geophys. Res., 97, 12361-12373.


-2175
€
x -2200/
z
oj -2225
Oo
SOUTHING
@
—N
-2175 Fe .
£ —
x -2200 act Z L
i . e -
o 2205)
a
88esea 662 8 6 8
sop ee BBB RES
THIN
(b) Sm EASTING, m

Fig. 4, Iso-velocity surfaces for first tomographic survey at
Homestake mine: (a) 67% probability; (b) 90% probability; (c)
95% probability. -

the second week in July and third week of October
are presented in Fig. 5. The variation in spatial
high-velocity anomalies (where strain energy is
stored) with time is attributed to local changes in
the ambient stress field following a 2.2 Richter
magnitude seismic event on September 5, 1993. In
reviewing the seismic activity (events exceeding
0.5 Richter magnitude) shown in Fig. 6, the first
survey was conducted during a period of low but
constant rate of seismicity (four events per month).
Following another 2-month period, the seismicity
increased with continued stope mining to 11 events
per month when a 2.2 Richter magnitude seismic
event occurred. Between the time of this event and
second survey, the seismicity increased from three
to 19 events per month. This increased rate of
seismicity is attributed to a redistribution in the
stress field further supporting the tomographic
interpretation.

4.7. Stress change due to stope advancement

Velocity histograms for the two Lucky Friday
mine surveys conducted in the first week of March

-2175) ~
— .
x -2200 .-
z= .
8.2205
o ray
UT HII
(a) NG, m EASTING, m
1
a
-2175 — -
E ">
xz -2200 Nein
E sn ‘
Ww -2225)
aa
sone ® ® 8 35 &
UTHIN
) Sm EASTING, m

Fig. 5. Three-dimensional iso-velocity surfaces (95% prob-
ability) of Homestake mine: (a) tomographic survey 1 (second
week July 1993); (b) tomographic survey 2 (third week
October 1993).

20

SEISMICITY, Events/Month

123.4 5 6 7 8 9 10 11 12
MONTH, 1993

Fig. 6. Seismic activity (Richter magnitude exceeding 0.5) at
Homestake mine.

and second week of November 1993 are shown in
Fig. 7. In contrast to the Homestake mine, there
is a statistically significant increase in average site
velocity (from 3.82 to 4.45 kms~'). The increase
in apparent velocity is consistent with a model of
stress transfer from the lower mined-out portions
of veins to the upper intact remnant ore structure.


1494
_ (4900 Level)

Silver Vein
a : 1555m
(5100 Level)
0 25
Scale, m

Fig. 2. Three-dimensional northeasterly view of Homestake mine study site.

underhand cut and fill was engaged at a depth of low modulus cement-sand. Fig.2 provides a

1540 m (5150 level). By the second survey, mining 3-dimensional northeasterly view of the 4900 and
had advanced downward by about 10 m to a depth 5100 level drifts and crosscuts, backfilled stopes
of 1540 m. These stopes are also backfilled with a and approximate extent of remaining silver ore.
600 E Ww
no
°
=
©
2
es at
s i=]
ry
a
2
fa) 450E
( Geophone location
N<—
(1 Homestake formation
030
sae [3 ettison formation

EB Poorman formation

Fig. 3. Plan view of geophone stations and mine drifts: (a) 7100 level (2164 m) Homestake mine; (b) 7250 level (2210 m) Homestake
mine; (c) 4900 level (1494 m) Lucky Friday mine: (d) 5100 level (1555 m) Lucky Friday mine.


vary in size depending on the ore body size. Ore
extraction generally proceeds in an upward direc-
tion in lifts that are about 3 m thick. Following
extraction, the stopes are backfilled with a low-
modulus mix of sand and waste rock in an attempt
to mitigate ground stability problems. Fig. 1 pro-
vides a 3-dimensional southeasterly view of various
mine features including the 7100 and 7250 level
drifts, backfilled stopes, and approximate extent
of ore reserves.

3. Lucky Friday mine
3.1. Geology

The Lucky Friday mine, owned and operated
by Hecla Mining Company, is located in the Coeur
d’Alene Mining District near Mullan, Idaho, The
two primary rock formations at the mine are the
argillitic St. Regis, and the Revette, composed
primarily of vitreous quartzite with thinly (1 cm
to tens of cm) interbedded sericitic quartzite and
argillite (Scott, 1993). The steeply southward-dip-
ping (about 80°) east-west trending, Lucky Friday
vein is composed of silver—lead—zinc that continues

to depths of more than 1.62 km. In the region of
study, the Lucky Friday vein bifurcates into what
are known locally as the hanging and footwall
veins. Unconfined laboratory sonic measurements
conducted on intact samples of vitreous quartzite
revealed that p-wave velocities average about
4.45 km s~!.

3.2. Mining activity

The study region, known as the “remnant ore
pillar,” represents a block of ground that is verti-
cally defined by previous mining activity delimited
by the 1493 (4900 level) and 1555 m (5100 level)
depths. The width of this remnant structure is
defined by the lateral extent of crosscuts on the
two levels (e.g., 4900, 87, 93 and 95; and 5100:
haulage, 88, 92), while its length is defined by the
distance spanning the outer most crosscuts
(Fig. 2). In this region, bedding strikes east-west,
essentially parallel to the vein, and dips south—
southwest at approximately 60°. The N40°W
trending South Control fault terminates the west-
ern edge of the Lucky Friday vein. At the time of
the first tomographic survey, mining activity using

7400 level drift
(2164 m depth)

7250 level drift
(2210 m depth)

0

Scale, m

50

Fig. 1. Three-dimensional southeasterly view of Homestake mine study site.


use of these countermeasures, catastrophic ground
failure continues, often with no microseismic pre-
cursors (Blake and Cuvelier, 1990). As mining
proceeds to greater depths, the unabated failure of
ground into underground mine openings will
remain a persistent safety hazard and an impedi-
ment to production. Presently, there remains the
need to develop a simple, rapid, and cost-effective
tool for periodic imaging of mine structures under
the full range of stress conditions. One such prom-
ising “high resolution” geophysical technology is
active 3-dimensional seismic tomography.

1.2. Approach

The rationale for using seismic tomography to
aid in mapping mine-induced, stress-related phen-
omena was based initially on laboratory observa-
tions of the interrelationship between velocity and
stress (e.g., Thill, 1982; Radcliffe et al., 1986).
Recently, Friedel et al. (1995b) demonstrated that
a good visual correlation existed between a simple
mechanical model and velocity distribution derived
from a 3-D underground tomographic survey. In
general, both laboratory and field observations
suggest that high- and low-velocity regions in a
homogeneous rock mass may be indicative of
elevated and diminished stress, respectively. These
findings are further supported by a numerical study
in which structural heterogeneities, e.g., fractures
were found to enhance the sensitivity of velocity
to relative changes in magnitude of in situ stresses.
In the event that normal stress across a geological
discontinuity is high, the in situ velocity may
approach that for intact rock.

To date, a number of researchers have applied
seismic tomography to investigate mine-induced
stress. Early studies focused on coal measure rocks:
a comprehensive review is provided by Gibowicz
and Kijko (1994). While encouraging, these studies
were limited by the application of 2-D linearized
solutions, insufficient raypath coverage (sampling
and viewing angles) and inability to impose explicit
constraints. Recent studies have improved on these
early efforts by incorporating curved ray tracing,
various constraints, and extension to 3-D (Young
and Hill, 1985; Young, 1993; Friedel et al., 1992,

1993, 1995a,b, 1996; McCreary et al., 1992;
Maxwell and Young, 1992; Young and Maxwell,
1992; Westmann et al., 1994; Jackson and Tweeton,
1996). This paper describes the use of 3-D seismic
tomography for monitoring mining-induced stress
changes at the Homestake gold mine and Lucky
Friday silver mine.

2. Homestake mine
2.1. Geology

The Homestake gold mine, owned and operated
by Homestake Mining Company, is located in
Lead, South Dakota. The geology is typified by
altered Precambrian metamorphic rock occurring
within a Tertiary east-west trending arch. The host
tock is the Homestake formation, a green-gray
chlorite schist. Accessory rock types include
quartzites and phyllites of the Ellison and Poorman
formations, respectively. All three formations con-
tain about 15% disseminated sulfides, such as
pyrrhotite and arsenopyrite.

Unconfined mechanical and sonic measurements
conducted on samples of these rock types in the
laboratory indicate a weak lithological variation
in elastic properties (Johnson et al., 1993). In
particular, the compressional wave velocity varied
less than 10% between rock types. In this case,
those tomographically-reconstructed _ velocities
that deviate by 1 kmsTM* (+2 S.D.) from the
average 4.34 kms~? laboratory p-wave velocity
may be attributed to mining-induced, stress-related
phenomena.

2.2. Mining activity

Currently, mining activity at the Homestake
occurs to a depth of 2438 m (8000 level). The
tomographic study site occupies a block of ground
between 2164 (7100 level ) and 2210 m (7250 level)
depths (the level designation is equivalent to depth
in feet from the surface). Mining of the Homestake
formation occurs within the study region in the
form of mechanized cut-and-fill and vertical crater
retreat. Ramps provide access to the stopes which