collection. Jami Girard, mining engineer, Spokane Research Cen-
ter, and Charles Wideman, professor, Montana School of Mines
and Technology, provided fault-plane solutions. Jeff Whyatt,
mining engineer, Spokane Research Center, provided in situ stress
information and interpretation.

REFERENCES

Beus, M.J., and Chan, S.M., 1980. Shaft design in the Coeur
d'Alene Mining District, Idaho: Results of in situ stress and
physical property measurements. U.S. Bur. Mines RI 8435,
43 pp.

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

Friedel, M.J., Scott, D.F., Jackson, M.J., Williams, T.J., and
Killen, S.M., 1995b. 3D tomographic imaging of anomalous
conditions in a deep gold mine. In: Proc. 2nd Intern. Conf.
Mech. Jointed and Faulted Rock, pp. 689-695, Vienna,
Austria.

Jackson, M.J., and Tweeton, D.R., 1996. 3DTOM: three-
dimensional geophysical tomography. U.S. Bur. Mines RI
9617, 88 pp.

Maxwell, S.C., and Young, R.P., 1992. 3D seismic velocity
imaging using microseismic monitoring systems at Strathcona
Mine and Mines Gaspe. In: Proc., 94th AGM, Can. Instit.
Min.

Reasenberg, P., and Oppenheimer, D.H., 1986. FPFIP, FPPLOT,
and FPPAGE: Fortran computer programs for calculating and
displaying earthquake fault plane solutions. U.S. Geol. Surv.
OFR 85-0739, 109 pp.

Scott, D.F., Friedel, M.J., Jackson, M.J., and Williams, T.J., 1995.
Use of tomographic imaging as a tool to identify areas of high
stress in remnant ore pillars in deep underground mines. In:
U. S. Bur. Mines Spec. Publ. 01-95, pp. 323-333.

Um, J., and Thurber, C., 1987. A fast algorithm for two-point
seismic ray tracing. Bull. Seismol. Soc. Amer. 77:3, 972-986.

Wielandt, E., 1987. On the validity of the ray approximation for
interpreting delay times. In: Seismic Tomography, ed. by G.
Nolet. Reidel, Dordrecht, pp. 85-98.

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(B9):12,361-12,373.


KEY
+ Ditatation
O Compression
> Motion direction
T Least compressive stress

P Most compressive stress

Figure 5.—Fault-plane solution for fatal rock burst.

al. (1995a, 199Sb) and Young and Maxwell (1992) used seismic
tomography to evaluate mechanical conditions in metal/nonmetal
mines. Maxwell and Young (1992) were among the first to
initiate a simultaneous linear inversion of both source and
velocity structure to generate a three-dimensional analysis of a
mine.

Seismic tomography is a noninvasive geophysical technique in
which seismic waves are used to penetrate a rock mass and, based
on apparent velocities, generate stress gradient contours
(tomograms). Successive surveys can be used to compare
changes over time,

‘The methodology used at the Sunshine Mine was identical to
four earlier tomographic surveys conducted by USBM personnel,
two at the Lucky Friday Mine (March and November 1993) and
two at the Homestake Mine (July and October 1993). Results of
these surveys, along with a detailed description of the methods
used, are summarized in Friedel et al. (199Sa, 1995b) and Scott
et al. (1995).

The most important consideration when designing a seismic
tomography survey is to include the greatest number of crossing
ray paths at a variety of angles through the rock mass. Forty-
seven source receivers were installed on the 4600 level and 56
source receivers were installed on the 4400 level of the Sunshine
Mine. A total of 4,333 seismic ray paths were analyzed.

Seismic resolution is related to the wavelength of the propa-
gating energy. Higher frequencies (seismic energy with shorter
wavelengths) provide better resolution. ‘The computed wavelength
for this survey was 7.2 m (average velocity of 5.19 km/s divided
by a frequency of 720 Hz) and provides an estimate of the
smallest feature that can be recognized. Low-velocity areas will
have less resolution than high-velocity areas (Wielandt, 1987).
Installing geophones along the drifts enhances resolution of
fractured, low-velocity areas.

The nonuniqueness of transmission tomography is a concern.
Because viewing angles tend to be limited, starting models based
on velocities ranging from 4.6 to 6.0 were used. Although this
may be subjective, it is roughly equal to the random velocity
distribution.

Inversion of the data was completed with the software package
3DTOM (Jackson and Tweeton, 1996), which can be used on a
personal computer (PC). Inversion is a model-fitting process that
is based on travel-time residual error (calculated travel time minus
measured travel time), Ray tracing in the models begins with five
straight ray paths and finishes with 10 curved ray paths, The

curved ray method is described by Um and Thurber (1987) and
is used to perturb a trial ray path until travel time is minimized.

A two-dimensional survey was completed on the 4400 and
4600 levels, The third dimension of the survey was between
levels, where the geophones were located on one level and signals
were generated on the other (figure 2).

Figure 3 is a set of tomograms based on travel-time velocities
through the pillar. Figure 34 and 3D show good correlation
between development (haulageways and crosscuts) and lower
velocities, indicating fractured rock in the skin around these
openings. The expected loading of the pillar from mining activity
correlates with the higher velocities.

7 CONCLUSIONS

Seismic activity concentrated 15 to 30 m west of and about 15 m
above the rock-burst location indicate a possible destressed area,
a result that correlates well with the low velocities determined by
the tomographic survey. The seismic activity did not appear to
be associated with the high-velocity areas shown by the
tomograms, but rather with areas where there was a velocity
gradient. Incteased seismic activity in the area of 73,750E and
80,100 to 80,200S (figure 34) could be a result of the rock mass
trying to reach equilibrium, Bedding lies against the vein and
may be causing bending of softer beds and the release of energy.
The rock in the pillar is a very hard, sericitic-to-vitreous quartzite
not commonly found in this mine. This rock can store large
amounts of energy, which can be released catastrophically. The
hard vitreous quartzite strata are parallel to the east-west maxi-
mum horizontal stress, which may provide a simple mechanical
explanation of the stress rotation and concentration in the strata
(Whyatt, personal communication, 1995). Similar stress anoma-
lies have been reported in association with localized rock bursting
throughout the world.

An in situ stress analysis of the pillar and the surrounding rock
mass shows a probable stress rotation from northwest-southeast
to nearly east-west. This analysis correlates well with the tomo-
grams and a right-lateral, strike-slip interpretation for the fault
solution (oriented N 78° E). The northeast-southwest stress con-
centration detected by the tomographic survey and the right-
lateral, strike-slip (N 78° E) solution indicated by first-motion
studies suggest that the stress rotation extends through the meas-
urement site. Core disking, concentrated seismic activity, break-
outs in the raisebore, and tomographic and in situ data provide
support to the hypothesis that there was a concentration of high
stress and a rotated stress field near and at the rock-burst site.
These factors, including the presence of a very hard and brittle
tock and the buildup of stress on the east side of the vein with a
destressed area on the west side of the vein, combined to cause
the rock burst. .

This investigation is exciting for several reasons. First, it
provides a reliable case study that directly confirms the value of
using seismic tomography to identify relative stress. Second, it:
suggests that stress anomalies may be associated with localized
rock bursting throughout the district. Finally, this investigation
confirms that a more comprehensive analysis of rock-burst haz~
ards is possible by combining seismicity, geology, in situ stress
measurements, fault plane solutions, and seismic tomography.

8 ACKNOWLEDGMENTS

The authors appreciate the logistic support and help of Sunshine
Mining Co. staff, including Harry Cougher, senior vice-president,
chief operating officer-Mining; Curt Johnson, chief engineer, and
Tom Valley, drafter. Terry McMahon, mining engineer (retired),
U.S, Bureau of Mines, Spokane Research Center and Jerry Hollis,

” technician, Sunshine Mining Co., assisted in field work and data


Investigation of a rock-burst site, Sunshine Mine, Kellogg, Idaho

D.EScott & T.J.Williams
Spokane Research Center, National Institute for Occupational Safety and Health, Wash., USA

M.J.Friedel
Lakeville, Minn., USA

ABSTRACT: Rock masses in deep-level mines are subject to high stresses, which can result in unexpected failure of rock into mined-out
‘openings. Historically, various independent methods have been used to discem the causes of rock bursts and to evaluate stress conditions
in rock masses susceptible to violent failure. Personnel from two research centers of the former U.S. Bureau of Mines, Spokane and Twin
Cities, investigated a rock burst that occurred in May of 1994 in a rock mass between the 4400 and the 4600 levels of the Sunshine Mine,
Kellogg, ID. The team evaluated the current state of stress in the pillar and the most likely cause of the rock burst by studying the
seismic history of the pillar, examining the gcology, analyzing the available in situ stress data, calculating possible fault-plane solutions
for the burst, and performing a seismic velocity tomographic survey. The results of the study showed that post-rock-burst seismicity was
concentrated about 15 to 30 m west of and about 15 m above the rock-burst location. The pillar is composed of a very hard sericitic-to-
vitreous quartzite of a type rarely found in the mine, but that is often found in other district mines having a history of seismic activity.
In situ stress analysis of the pillar and the surrounding rock mass showed a probable stress rotation from northwest-southeast to east-west.
Of four fault plane solutions, the most probable was either a left-lateral, strike-slip movement with a compressive principal stress oriented
nearly north-south, or a right-lateral, strike-slip movement oriented east-west. Finally, tomographic images showed low-velocity areas
(interpreted as areas of low stress) associated with the crosscuts and haulageways, and high-velocity areas (interpreted as areas of high
stress) associated with the rock-burst location. High velocities were also found in a planar area oriented in an elongate northeast-
southwest direction.

1 INTRODUCTION 4400 and 4600 levels, the only other opening was the 18-m-high,

2.7-m-wide, and 4.6-m-long where the rock burst occurred.

(On May 13, 1994, one miner was killed and another injured in a
rock fall resulting from a rock burst at the Sunshine Mine,
Kellogg, ID (figure 1). The Sunshine Mining Co. and the former
U.S. Bureau of Mines (USBM) agreed to conduct research to
determine the cause and then-current state of stress of the rock
mass above and around the rock-burst site.

The rock burst occurred in a raise 18.3 m above track level on
the 4600 level in the Chance vein footwall. The area studied was
a pillar (CFO?) between the 4400 and 4600 levels (-1344 and -
1392 m below the collar of the Jewel shaft, respectively) and
between 73500E and 74500E, and 7960S and 8030S (figure 2).
The pillar was approximately 52 m high, 183 m long, and 152 m
wide. Above the pillar, a 6-m-high, 91-m-long, and 2.4-m-wide
stope had been excavated. Intact rock surrounded the remaining
portions of the pillar. Other than development openings on the

e
024 No uray
uu

Scale, km

Kellogg

Burke
= Qsum Osburn
Sunshine & —TM@ = __F
Mine Galena’
Mine

Figure 1.—Location of Sunshine Mine, Kellogg, ID

This study involved combining several types of assessments,
including seismicity, geologic, in situ stress measurements, fault
plane solutions, and seismic tomography results to determine
relative stress at the rock-burst site.

2 SEISMICITY

For the purposes of this paper, a seismic event is defined as the
initial appearance of seismic energy on a seismic record. Seismic
events include microseismic events, rock bursts, and earthquakes.
Only microseismic events and rock bursts associated with mining
will be discussed here. Microseismic events are defined as events
that usually displace less than 1 to 2 m? of material into a mine
opening, are less than 0.5 Richter magnitude, or result in less than
30 mm of displacement on a seismograph. Accuracy of locating
a microseismic event is confined to a radius of about 8 m,
depending on its magnitude. Determining the location of larger
events is less accurate because a larger volume of rock is
involved; conversely, the location of smaller events is more
accurate. Microseismic events are detected with underground
geophones, but are normally not measured on a surface
seismograph because of the small amount of energy released.
They generally occur near drifts, haulageways, or stopes and
cause popping, spitting, and spalling of the surrounding rock
mass.

In this study, a rock burst is defined as the sudden and
sometimes violent release of accumulated energy expressed when
a volume of rock is strained beyond its elastic limit. Rock bursis
can be classified as strain, crush, or slip. Strain bursts are small
and localized, while crush and slip bursts can cause extensive
damage to drifts and stopes. Events exceeding 0.5 Richter
magnitude or greater than 30 mm displacement on a seismograph
are classified as rock bursts,

The May 13 rock burst was located by a macroseismic system
and was calculated to have originated at about 73775E, 80021S,



LEGEND

1 Core samples 1-3, 8 — 79,700
2 Core samples 4-5, 10, 11
3 Core samples 16-7, 9, 12

4400 level — 79,800

jource/receiver — 79,900

location
12 shaft

— £0,000

— 80,100

— 80,200

— 80,300

73,500 79,750 74,000 74,250

Figure 2—Location of fatal rock burst and source and receiver sites for seismic tomography survey

79,700

79,200
73,900
—20,000
20,100
—20,200
80,300
73,500 73.750 74,000 74,250 73,500 73,750 74,000 74280
D
LEGEND
VELOCITY, knvs
(>60 [455.0 © Seismic event
Elsseo [4045

Hisoss Mi<s0

Sop gy
> RAI
2? rome

DEPTH, m below
shaft collar

73,500 73,750 74,000 74,250

80,300 80,100 79,900

Figure 3—Velocity tomograms showing locations of seismic events, A, Depth = -1330 to -1360 m; B, depth = - 1360 to -1373 m;
C, depth = -1373 to -1403 m; D, cross section 73,759E.


and -1373 m below the collar of the Jewel shaft (figure 2). The
macroseismic system is designed to record digitized waveforms
sensed by triaxial geophones and to target mining-induced events
large enough to cause damage to mine structures. These events
range from Richter magnitude 0.5 to 2.5 and-have frequencies
between | and 500 Hz. Accuracy is about plus or minus a 30-m
radius.

Because this area of the mine had not been fully developed, it
was not adequately monitored by a seismic array at the time of
the fatal rock burst; however, the array has subsequently been
expanded to include the area of the May fatality. Records of
seismic events in the CF02 pillar were collected and evaluated for
the months of October through December 1994 and correlated
with the results of the three-dimensional seismic tomographic
survey conducted in December 1994. Thirty-one events were
recorded by the system for the 3-month period. These events had
magnitudes ranging from 0 to 10 mm as measured by
displacement by the surface seismograph needle. Figure 3 is a set
of tomograms that shows seismic activity between the 4400 and
4600 levels. Table 1 shows the number of seismic events
associated with each depth range.

Table 1—Seismic activity

Elevation range, m No. of events Figure
~506 to -536 ... 8 3A
-536 to -549 .. 10 3B
-549 to -547... 13 3c

In plan view, most of the events occurred just west of the
CFO2 crosscut (figure 34-C). In cross section, looking west
(figure 3D), the events form an X with the vein. This indicates
that the seismic events were probably related to the geometry of
the vein as most events occurred in the hanging wall above the
top of the raise.

3 GEOLOGY

Hard, vitreous, white-to-tan quartzite of the lower part of the
upper Revett Formation constitutes most of the pillar rock mass.

4400 level

Figure 4.—Geologic cross section looking west and showing
relationship of bedding to vein and mine workings

In the Coeur d’Alene Mining District, this formation has a history
of increased rock-burst activity relative to other stratigraphic
units, Soft argillite is interlayered randomly throughout the quart-
zite. The average strike of the Revett in the pillar is nearly east
and average dip of bedding is about 77° to the south. The vein,
which consists of tetrahedrite, galena, and gangue, strikes about
N 80° W and dips about 55° to the southwest. On the 4400 level,
6 m cast of the top of the projected raise from the 4600

level, a right-lateral, strike-slip fault strikes nearly north and dips
35° to the east, offsetting the vein by 1.2 m. Figure 4 is a cross
section looking west and shows the relationship of bedding to the
vein and workings.

4 IN SITU STRESS

Several lines of evidence point to significant levels of
maximum horizontal stress, At the Sunshine Mine, a stress
measurement obtained using an experimental U.S. Geological
Survey (USGS) solid inclusion gage (Beus and Chan, 1980)
indicated that the orientation of maximum horizontal stress was
N 80° E. A raisebore at the site corroborated this direction,
indicating a similar orientation of the maximum principal stress.
Beus and Chan reported core disking in holes oriented perpen-
dicular to maximum horizontal stress and to bedding. The num-
ber of breakouts produced during boring of a 1.2-m-diam raise at
the measurement site also suggested high stress levels (Beus,
1995, personal communication) and confirmed an east-west orien-
tation.

Development openings in the vicinity of this site experienced
unusually concentrated rock burst and other seismic activity and
were probably associated with the elevated stress levels. The in
situ stress field in the vicinity of the Sunshine Mine is most likely
variable. However, the raisebore breakout observed by Beus and
Chan near the site of the tomographic survey offers evidence that
the stress field has been rotated to almost east-west.

§ FAULT-PLANE SOLUTIONS

Fault-plane solutions (also known as source mechanisms, focal
mechanisms, or first-motion studies) are a visual representation of
possible failure planes that correspond to the shear movement
responsible for a given seismic event. When a seismic event
‘occurs, shock waves radiate out in a spherical pattern from the
source. In general, the first wave to reach the recording geo-
phones is a primary, or P, wave followed by a secondary, or S,
wave, By studying the relative upward or downward motion of
the P-wave at each geophone (i.e., whether the first motion was
compressional or dilatational), fault-plane solutions for the event
can be computed.

‘The solution presented is one of four possible solutions and
was generated by FPFIT/FPPLOT software (Reasenberg and
Oppenheimer, 1986) developed by USGS. Since the event
occurred outside the geophone array and at approximately the
same level as some of the geophones, the solutions are not well
constrained, The surviving miner's account of rock motion during,
the event indicates that a shock wave was generated from the
north and struck him in the back as he was facing south. Based
‘on the miner's account of the motion and knowledge of typical
regional stress distributions, a left-lateral, strike-slip movement
oriented N 15° W, or a right-lateral, strike-slip movement oriented.
N 78° E seems to be a probable fault-plane solution (figure 5).

6 TOMOGRAPHIC STUDY

Several researchers have investigated the use of seismic
tomography to evaluate stress in underground mines. Friedel et.