HIGH RESOLUTION SEISMIc REFRAcTION 

TOMOGRAPHY FOR DETERMINING DEPTH OF 

BLAST INDUcED DAMAGE IN A MINE WALL 

J. A. Singer1,

1

J. A. Singer, Chevron North American Exploration and Production 

c. A Link2, 

2

Address correspondence to: C. A. Link, Montana Tech, 1300 West Park St., Butte, MT, 59701, clink@mtech.edu 

& S. R. Iverson3 

3

S. R. Iverson, CDC (Centers for Disease Control)/NIOSH (National Institute for Occupational Safety and Health)

 SRL (Spokane Research Laboratory) 

ABSTRAcT 

High resolution seismic refraction tomography has proved to be a useful tool to 

effectively estimate depth of blast induced damage in a mine face. Excavation blast 

damage can be as shallow as 1 to 2m and requires resolution at a fraction of a meter 

for effective imaging. We used an accelerometer with flat frequency response to 

1000Hz to record data at spacings of approximately 0.25 to 0.5m along mine walls. 

First arrival times from the seismic data were used to produce P-wave velocity 

tomograms. The tomograms show transition from low velocity zones to velocity 

associated with competent rock. We interpret the low velocity zones to indicate 

residual fracturing from blasting. Experiments on a concrete test block and at 

two underground mine locations give results that are consistent with fractured 

rock transitioning to competent rock. Our method can be applied efficiently in an 

underground environment to provide continuous velocity information with depth into 

a mine wall over a span of approximately 10m. 
1. INTRODUcTION 

Blast induced rock mass Damage (BID) is a safety concern in underground 

mining because it contributes to a redistribution of stresses within rock such that 

rock mass strength weakens from resultant blasting fractures (Raina et al. 2000). 

Measurement of BID can thus be a useful tool to help design and refine blasting 

techniques for reduced rock fracturing. Knowledge of the amount of BID can also be 

used to maintain safe working conditions in pre-existing tunnels and support structures 

by locating zones where stress has weakened rock strength over time. 

High resolution seismic methods have the potential to assess the extent of 

BID by analyzing P-wave velocity variation with depth into the mine wall. P-waves 

are compressional waves observed in elastic media. The P-wave velocity generally 

increases with increasing consolidation of material and decreases with fracture 

density, thus, the fractured outer portion of a rock wall should show lower P-wave 

velocities than the inner undamaged rock (Kormendi et al. 1986). The method we 

have developed makes use of measurements on mine rock faces without the need for 

extra drill holes or borehole deployed equipment. 

Our approach is to determine P-wave seismic velocity as a function of depth 

into the mine face. In zones of mostly homogeneous rock, we associate zones of 

low P-wave velocity with increased fracture density. We construct P-wave velocity 

images using seismic traveltime tomography, an inversion method for estimating Pwave velocities using first arrival times from seismic recordings. Variations of the 

P-wave first arrival times are used to iteratively update a grid of velocities over the 

surveyed area. Low velocity zones related to BID can then be identified from the 

tomograms (Maxwell & Young 1993). 

Tomographic imaging using seismic data has been used frequently in the past 

few decades to study stress distribution and fracturing within rock masses. Typical 

seismic surveys have resolution on the order of 1m or greater and are usually large 

scale, covering cross-sectional areas of approximately 10,000m2

. Targets suitable for 

this scale are major cracks, faults, and depositional changes (Maxwell & Young 1993). 
Kormendi et al. (1986) studied the correlation of rock fracturing to P-wave 

velocity. They used piezoelectric transducers with frequency response from 1000Hz 

to 2000Hz as sensors and a hammer impulse source to collect seismic transmission 

data. An iterative algebraic reconstruction technique was used to generate velocity 

maps or tomograms of surveys with dimensions 100m by 150m. Their results showed 

that stress conditions for a large area can be effectively monitored by seismic velocity 

measurements. 

Maxwell and Young (1992) coupled cross-hole seismic tomography with 

microseismic monitoring to locate zones of anomalous stress and zones of failure 

in Canada’s Lockerby Mine. Hydrophones with flat frequency response from 1Hz 

to 15 kHz were used as sensors and blasting caps were used as the seismic sources. 

Damped least squares inversion was used to create tomograms of the 90m by 150m 

survey area. Low-velocity zones were identified in the tomograms and slated for 

further study. The cross-hole tomography study was continued using two imaging 

plane depths of 6m and 10m. This method was concluded to be a useful tool for blast 

damage assessment. 

Watanabe and Sassa (1996) used 24, single component, 40Hz natural frequency 

geophones to survey two sites in the Kamioka Mine in Japan. Explosives were used 

as impulse sources at each of the 24 geophone locations. Seismic tomography was 

used to analyze the first arrival data which produced images with a resolution of 

approximately 1m. The resulting velocity tomograms were able to clearly delineate 

faults. 

In another study, Friedel et al. (1997) conducted a series of 3-D seismic 

tomographic surveys in the Lucky Friday mine near Mullan, Idaho and the Homestake 

mine near Lead, South Dakota. Each survey used 24, 100Hz natural frequency 

geophones. First arrival times were then used to produce P-wave velocity tomograms. 

Seismic resolution for these surveys was calculated to be approximately 7m. Seismic 

surveys were conducted twice for both mines to show P-wave velocity changes over a 

period of several months. Differences in the tomograms were attributed to rock mass 

failure and stope advancement. 
Ashida (2001) used a tunnel boring machine as a passive source to seismically 

image an underground rock face. The seismic signal created by the tunnel boring 

machine was recorded by four three-component receivers with natural frequency of 

28Hz arranged on each side of the wall at a spacing of 6m. The x, y, z directions of 

the three-component geophones were orientated to the tunnel axis directions. Twelve 

single-component receivers with natural frequency of 40Hz were also arranged on 

each side of the wall every 3m. Both arrays produced images from P-wave amplitude 

data that correctly identified two faults 50m and 110m from the tunnel face. 

Our approach was to use available equipment (three-component accelerometer, 

seismic recording system) to make measurements along the mine face to be used 

with seismic refraction tomography to calculate P-wave velocity tomograms. We 

used a small hand hammer as a seismic source to tap hand placed stud bolts and were 

able to record seismic data with frequencies to 2000Hz. Using the first arrival times 

from these shot records we calculated P-wave velocity tomograms showing velocity 

variations 1 to 2m into the mine face that we interpreted to be associated with blast 

induced damage. 

2. EQUIPMENT AND REcORDING PARAMETERS 

High resolution seismic surveys have the potential for characterizing BID 

locally on very small scales. Our study uses an Applied Micro-Electro-Mechanical 

System (MEMS) SF3000L accelerometer as the seismic sensor. The accelerometer 

has a flat frequency response up to 1000Hz and is tri-axial. Output voltage for the 

accelerometer ranged from 0V for the horizontal axes to 1.2V for the vertical axis. 

The large output voltage associated with the vertical axis is due to the constant 

gravitational acceleration. This large output voltage prevented recording the vertical 

axis with our seismic recording system. 

To achieve good coupling for data collection, the accelerometer was bolted to 

an aluminum wedge that was attached to the hand-installed stud anchor bolts. These 

stud anchor bolts held the accelerometer in place at survey sites and were also used 

as source points (Figure 1). We used a small hammer as the seismic source with an 

electronic trigger to send a signal to the system to begin recording when the source 
 

 

 

 

 

 

 

was initiated. This trigger was attached to the hammer and the stud bolt to complete 

a simple circuit when the bolt was struck. 

Figure 1. Photograph in the East Boulder mine showing accelerometer mounted on 

aluminum wedge attached to hand-installed stud bolt. The stud bolts were also used as 

the source points for the hammer hits. 

The recording equipment consisted of a Geode seismograph (Geometrics, 

Inc.) and Geometrics Seismodule Controller software to store the data on a laptop 

computer (Figure 2). Although both horizontal axes were recorded, we only used the 

axis perpendicular to the mine wall for analysis. The accelerometer output for the 

vertical axis containing gravitational acceleration produced a signal of approximately 

1.2V which was larger than the maximum allowable input voltage for the recording 

system. The recording sample interval for all of the surveys was 0.028033ms resulting 

in a Nyquist frequency of approximately 18kHz. A 60Hz notch filter was applied to 

all recordings to attenuate power line noise. Vertical stacking was also performed 

five times at each bolt station to improve the signal-to-noise ratio of the recordings by 

reducing random noise. Table I is a summary of the recording parameters used at all 

test sites. 

Figure 1. Photograph in the East Boulder mine showing accelerometer mounted on aluminum wedge 

attached to hand-installed stud bolt. The stud bolts were also used as the source points for the 

hammer hits. 

The recording equipment consisted of a Geode seismograph (Geometrics, Inc.) 

and Geometrics Seismodule Controller software to store the data on a laptop computer 

(Figure 2). Although both horizontal axes were recorded, we only used the axis 

perpendicular to the mine wall for analysis. The accelerometer output for the vertical 

axis containing gravitational acceleration produced a signal of approximately 1.2V which 

was larger than the maximum allowable input voltage for the recording system. The 

recording sample interval for all of the surveys was 0.028033ms resulting in a Nyquist 

frequency of approximately 18kHz. A 60Hz notch filter was applied to all recordings to 

attenuate power line noise. Vertical stacking was also performed five times at each bolt 

station to improve the signal-to-noise ratio of the recordings by reducing random noise. 

Table I is a summary of the recording parameters used at all test sites. 
 

 

 

 

 

 

 

 

 

 

 

 

Figure 2. Photograph at the Spokane Research Laboratory concrete block test site 

Seismograph 

Hammer w/switch 

Accelerometer cable to 

seismograph 

Seismograph 

Hammer w/switch 

Accelerometer cable to 

seismograph 

showing the arrangement of the recording system on a coaster wagon. 

 

 

 

 

 

 

 

 

 

 

 

 

arrangement of the recording system on a coaster wagon. Figure 2. Photograph at the Spokane Research Laboratory concrete block test site showing the 

3. DATA MANAGEMENT AND PROCESSING Table I. Recording parameters for seismic recordings at all test sites. 

3. DATA MANAGEMENT AND PROCESSING 

3.1. Data Formatting and Preliminary Analysis 

Table I. Recording parameters for seismic recordings at all test sites. 

Recording sample interval Nyquist frequency 28.033 Ps 17.836 kHz 

Nyquist frequency Notch filter 17.836 kHz 60 Hz 

Notch filter Vertical stacking at each source point 5 60 Hz 

Vertical stacking at each source point Channels recorded 5 2 

Channels recorded Receiver 2 3 comp. accelerometer 

Receiver Source 3 comp. accelerometer hand-held hammer 

Source hand-held hammer 

Table I. Recording parameters for seismic recordings at all test sites. 

3. DATA MANAGEMENT AND PROcESSING 3.1. Data Formatting and Preliminary Analysis 

3.1. Data Formatting and Preliminary Analysis 

We used a reciprocal recording approach with a single receiver and multiple 

shot locations instead of the conventional single source and multiple receiver 

locations. Thus the individual shot records with the same accelerometer location 

were merged to create a combined file with a trace for each source location resulting 

in a common receiver gather. We used the Seismic Processing Workshop® (SPW/ 

Parallel Geosciences) Input Output Utility to combine individual files into common 

receiver gathers. In addition to the notch filter applied in the field, a low cut filter was 

applied to the data to remove low frequency noise from 0 to 10Hz. 

We used a reciprocal recording approach with a single receiver and multiple shot 

We used a reciprocal recording approach with a single receiver and multiple shot locations instead of the conventional single source and multiple receiver locations. Thus

the individual shot records with the same accelerometer location were merged to create a locations instead of the conventional single source and multiple receiver locations. Thus combined file with a trace for each source location resulting in a common receiver gather. the individual shot records with the same accelerometer location were merged to create a We used the Seismic Processing Workshop® (SPW/Parallel Geosciences) Input Output combined file with a trace for each source location resulting in a common receiver gather. 

We used the Seismic Processing Workshop Utility to combine individual files into common receiver gathers. In addition to the notch ® (SPW/Parallel Geosciences) Input Output 

Utility to combine individual files into common receiver gathers. In addition to the notch filter applied in the field, a low cut filter was applied to the data to remove low frequency 

noise from 0 to 10Hz. filter applied in the field, a low cut filter was applied to the data to remove low frequency 

noise from 0 to 10Hz. 
The common receiver gather files were then reformatted for input into the 

seismic refraction tomography software. Figures 3a and 3b show common receiver 

gathers for a 20 source point survey. Figure 3a shows data prior to first arrival time 

picking illustrating a first arrival and the location of the accelerometer for that gather. 

The first arrival time picks for another gather are shown in Figure 3b and along with 

the picks from all gathers from the survey are then used in the refraction tomography 

software to calculate velocity tomograms. 

 

 

 

 

 

 

 

 

 

 

 

 

First Break 

Accelerometer 

Location First Break 

Accelerometer 

Location 

Figure 3(a) 

Figure 3(a) 

 

 

 

 

 

 

 

 

 

Figure 4 shows a frequency power spectrum for a typical shot record from the 

East Boulder mine in Montana. Usable frequencies extend to approximately 2000Hz. 

High frequencies are important for seismic resolution as the frequencies determine 

the wavelength of the seismic waves through the following relationship

v f O (1)

The common receiver gather files were then reformatted for input into the seismic 

refraction tomography software. Figures 3a and 3b show common receiver gathers for a 

20 source point survey. Figure 3a shows data prior to first arrival time picking 

illustrating a first arrival and the location of the accelerometer for that gather. The first 

refraction tomography software. Figures 3a and 3b show common receiver gathers for a 

The common receiver gather files were then reformatted for input into the seismic 

arrival time picks for another gather are shown in Figure 3b and along with the picks 

20 source point survey. Figure 3a shows data prior to first arrival time picking from all gathers from the survey are then used in the refraction tomography software to 

calculate velocity tomograms. illustrating a first arrival and the location of the accelerometer for that gather. The first 

arrival time picks for another gather are shown in Figure 3b and along with the picks 

from all gathers from the survey are then used in the refraction tomography software to Figure 4 shows a frequency power spectrum for a typical shot record from the 

calculate velocity tomograms. East Boulder mine in Montana. Usable frequencies extend to approximately 2000Hz. 

High frequencies are important for seismic resolution as the frequencies determine the 

wavelength of the seismic waves through the following relationship Figure 4 shows a frequency power spectrum for a typical shot record from the 

East Boulder mine in Montana. Usable frequencies extend to approximately 2000Hz. 

High frequencies are important for seismic resolution as the frequencies determine the (1) 

wavelength of the seismic waves through the following relationship 

where v is the seismic velocity, f is the frequency of the seismic wave and O is the 

where v is the seismic velocity, f is the frequency of the seismic wave and λ is the 

wavelength of the wave. The resulting resolution of the tomographic images is closely 

related to the wavelength of the seismic wave. Using a seismic velocity of 4000m/s 

and frequency of 2000Hz, the resulting seismic wavelength is 2m. Thus the resulting 

velocity images will have sub-meter resolution.

wavelength of the wave. The resulting resolution of the tomographic images is closely v f O (1) 

related to the wavelength of the seismic wave. Using a seismic velocity of 4000m/s and 

where frequency of 2000Hz, the resulting seismic wavelength is 2m. Thus the resulting velocity v is the seismic velocity, f is the frequency of the seismic wave and O is the 

images will have sub-meter resolution. 

related to the wavelength of the seismic wave. Using a seismic velocity of 4000m/s and 

wavelength of the wave. The resulting resolution of the tomographic images is closely 

 

 

Figure 3(a) 
first breaks 

Figure 3(b) 

 

 

 

Figure 3(a) Common receiver gather created from 20 individual source point 

recordings illustrating first arrival and accelerometer location. Figure 3(b) Another 

common receiver gather for a 20 station survey. The accelerometer position is at 

station 4. First arrival time picks and modeled first arrival times calculated from the 

resulting velocity model are shown as colored X’s. 

Periodogram Power Spectral Density Estimate 

100 

80 

60 

40 

20 

0 

-20 

-40 

0 2 4 6 8 10 12 14 16 

Frequency (kHz) 

Power/frequency (dB/Hz) 

Figure 4. Frequency power spectrum for typical shot from the 

East Boulder mine, Montana. 

Figure 3(a) Common receiver gather created from 20 individual source point 

recordings illustrating first arrival and accelerometer location. Figure 3(b) Another 

common receiver gather for a 20 station survey. The accelerometer position is at station 

4. First arrival time picks and modeled first arrival times calculated from the resulting 

velocity model are shown as colored X’s. 

Figure 4. Frequency power spectrum for typical shot from the East Boulder mine, Montana. 

The range of usable frequencies extends up to 2000Hz. These higher frequencies 

are necessary for resolving velocity changes at sub-meter precision. 
The range of usable frequencies extends up to 2000Hz. These higher frequencies 

are necessary for resolving velocity changes at sub-meter precision. 

3.2. Seismic Refraction Tomography 

Seismic refraction tomography is an alternative to conventional layered seismic 

refraction methods. Refraction tomography performs well in many situations where 

conventional methods fail, for example, where there are significant lateral or vertical 

gradients. 

There are several seismic refraction tomography software packages available. 

The one we used is called Rayfract® from Intelligent Resources Inc. Rayfract® images 

subsurface velocity using seismic first arrival energy propagation modeling. First 

the seismic data are imported and the 2-D profile geometry is defined, then the first 

arrival times are picked for all gathers (Intelligent Resources Inc. 2009). These times 

are used to iteratively update an initial 1-D velocity model using wave path modeling 

and inversion of the difference between predicted and actual travel-times. 

Rayfract® uses a smooth inversion method to image velocity structure. The 

method is suitable for extreme topography and strong lateral velocity variation. No 

assignment of traces to refractors is required as in conventional layered refraction 

analysis. The initial 1-D velocity gradient model is determined from the first arrival 

travel times. This initial model is then iteratively refined using 2-D Waveform Eikonal 

Travel-time (WET) (Schuster and Quintus-Bosz 1993) tomographic inversion. The 

Fresnel volume approach is used in the inversion algorithm (Watanabe et al. 1999). 

This method differs from other inversion algorithms because it incorporates the fastest 

waveform along with waveform arrivals up to a half period slower. 

Conventional ray tracing is limited to the modeling of one ray per first arrival; 

The WET approach models multiple signal propagation paths contributing to one 

first arrival. The Eikonal solver used for travel-time field computation also explicitly 

models diffraction in addition to refraction and transmission of seismic waves 

(Intelligent Resources Inc. 2009). Statistics such as the mean residual and maximum 

error of the fit of the modeled travel times to the measured travel times are used to 

quantify the goodness of fit and identify outlier time picks. Also, 2-D plots of the 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Travel-time (WET) (Schuster and Quintus-Bosz 1993) tomographic inversion. The 

wave coverage as a function of depth along the profile are calculated to help identify 

problematic zones or areas with low data coverage. Quality control of velocity models 

is also performed by direct graphical comparison of the picked first arrival times to 

those calculated from the model solution.

When looking at wave coverage plots, it is important to be aware that zones 

of low coverage, by their very nature are zones of low velocity as seismic energy 

follows the path of shortest time ala Fermat’s principle. Conversely for high velocity 

zones where seismic wave paths will tend to concentrate. Thus, the coverage plots 

are useful for quality control purposes as well as reflecting the nature of the material 

through with the waves travel. 

To fully take advantage of the capabilities of tomography inversion, data 

coverage should be high, preferably at least 7 to 8 shots per array of 24 geophones. 

Furthermore, the different geophone arrays should overlap by several geophone 

stations so as to avoid low data coverage at the boundaries (Mattsson 2007). We 

collected our data, albeit in the form of receiver gathers, with up to 20 shots per 

receiver location. Table I summarizes the number of source locations and receiver 

locations for each survey site.

Fresnel volume approach is used in the inversion algorithm (Watanabe et al. 1999). This 

method differs from other inversion algorithms because it incorporates the fastest 

waveform along with waveform arrivals up to a half period slower. 

Conventional ray tracing is limited to the modeling of one ray per first arrival; 

The WET approach models multiple signal propagation paths contributing to one first 

arrival. The Eikonal solver used for travel-time field computation also explicitly models
 

diffraction in addition to refraction and transmission of seismic waves (Intelligent
 

Resources Inc. 2009). Statistics such as the mean residual and maximum error of the fit 

of the modeled travel times to the measured travel times are used to quantify the 

goodness of fit and identify outlier time picks. Also, 2-D plots of the wave coverage as a 

function of depth along the profile are calculated to help identify problematic zones or 

areas with low data coverage. Quality control of velocity models is also performed by 

direct graphical comparison of the picked first arrival times to those calculated from the 

model solution. 

When looking at wave coverage plots, it is important to be aware that zones of 

low coverage, by their very nature are zones of low velocity as seismic energy follows
 

the path of shortest time ala Fermat’s principle. Conversely for high velocity zones
 

where seismic wave paths will tend to concentrate. Thus, the coverage plots are useful 

for quality control purposes as well as reflecting the nature of the material through with 

the waves travel. 

To fully take advantage of the capabilities of tomography inversion, data coverage 

should be high, preferably at least 7 to 8 shots per array of 24 geophones. Furthermore, 

the different geophone arrays should overlap by several geophone stations so as to avoid 

low data coverage at the boundaries (Mattsson 2007). We collected our data, albeit in the 

form of receiver gathers, with up to 20 shots per receiver location. Table I summarizes 

the number of source locations and receiver locations for each survey site. 

Table I. Summary of the number of source locations and the number of accelerometer locations for Table I. Summary of the number of source locations and the number of accelerometer each of the seismic surveys.
 

locations for each of the seismic surveys. 

One drawback of the WET inversion approach is the risk of velocity artifacts
 

when using a low degree of smoothing in combination with low data coverage. This is the 

Site No. 

source 

No. 

accelerometer 

Avg. source 

location 

locations locations spacing (m) 

Lucky Friday 16 5 0.4 

East Boulder 1 19 6 0.5 

East Boulder 2 10 5 0.6 

Missile site 20 11 0.15 

pre-blast 

Missile site 11 6 0.15 

post-blast 
One drawback of the WET inversion approach is the risk of velocity artifacts 

when using a low degree of smoothing in combination with low data coverage. This 

is the case especially if there are significant topography variations. We encountered 

some significant topography variations on the mine walls in the East Boulder mine in 

Montana. Another drawback is that the gradient approach used by the software may 

indicate increasing velocity with depth, even though the bedrock is almost homogenous 

rock with presumably more or less constant P-wave velocity (Mattsson 2007). 

Advantages with the WET-technique are that it is fast, automated and all arrival 

time data are used in the creation of the velocity model. Quality control is based on 

statistics calculated from travel-time misfits and ray coverage plots. In the traditional 

refraction approach, the final velocity model is the result of a combination of manual 

interpolation and personal judgments made by the interpreter. This makes the 

traditional refraction approach less general compared with the tomographic inversion 

model (Mattsson 2007). 

4. FOUR TEST cASES AND RESULTS 

We applied our refraction tomography approach at four test sites. The first test 

took place at the Spokane Research Laboratory’s missile test site. A concrete block 

had been constructed and instrumented to study the effects of a controlled blast on 

the material. We did seismic measurements both pre- and post-blast to determine 

the extent of fracturing introduced after blasting. Seismic P-wave velocity for the 

competent concrete in the block was approximately 3000m/s. 

The second and third test sites were in the Stillwater Mining Company’s East 

Boulder mine near Big Timber, Montana at the 6700 level. The bedrock consisted 

of gabbro and norite and the P-wave velocity for undamaged rock at both sites was 

approximately 6500m/s. 

The fourth test took place in Hecla Mining Company’s Lucky Friday mine at 

Mullan, Idaho at a depth from the surface of about 2km at the 4900 level. The rock at 

the test site was a phyllitic argillite with a P-wave velocity of approximately 5000m/s. 

Refer back to Table I for a summary of the recording geometry at each location. 
 

 

 

 

 

4.1.1. Spokane Research Laboratory Concrete Block 

4.1.1.1. Experimental setup 

The first experiment involved a two-stage survey. A concrete test block was 

constructed by the NIOSH Spokane Research Laboratory (SRL) to be tested as part 

of the blasting studies in the miner safety program. The dimensions of the base of 

the concrete block were 3.0m by 3.0m with height 1.5m (Figure 5(a)). The block was 

instrumented with strain gauges to measure blast wave decay and an accelerometer to 

measure the acceleration of the burden as it broke free from the block. We performed 

our seismic refraction surveys before and after the blasting to image any changes in 

P-wave velocity caused by the blasting. 

Survey geometry comprised 20 stations with a spacing of 0.15m. Stud anchor 

bolts were placed at these locations by inserting them into pre-drilled holes filled 

with epoxy. The bolts were placed at a height of 0.8m from the ground. Data were 

recorded from the accelerometer at each of the even numbered stations in the pre 

blasting survey. Figure 5(b) shows the survey in progress on the block. The postblasting survey was to be a duplicate of the original survey; however, the portion of 

the block holding bolts 1 through 9 was destroyed (Figure 5(c)). Post-blast data were 

recorded with the accelerometer at even stations 10 through 20. 

refraction surveys before and after the blasting to image any changes in P-wave velocity 

caused by the blasting. 

Survey geometry comprised 20 stations with a spacing of 0.15m. Stud anchor 

bolts were placed at these locations by inserting them into pre-drilled holes filled with 

epoxy. The bolts were placed at a height of 0.8m from the ground. Data were recorded 

from the accelerometer at each of the even numbered stations in the pre blasting survey. 

Figure 5(b) shows the survey in progress on the block. The post-blasting survey was to 

be a duplicate of the original survey; however, the portion of the block holding bolts 1 

through 9 was destroyed (Figure 5(c)). Post-blast data were recorded with the 

accelerometer at even stations 10 through 20. 

Figure 5(a) 
 

 

 

Figure 5(a) Dimensions and some of the instrumentation on the concrete block 

test. The single blast hole is horizontal and was lengthened to 2.4m from 1.2m shown 

in the drawing. The hole was filled with Dyno AP up to 38cm from the end. Figure 

5 (b) Pre-blast view of concrete test block used for blasting test. Note accelerometer 

and anchor bolts used for both attaching the accelerometer and as seismic source 

locations. Figure 5(c) Concrete test block after blasting. Rough edges and cracking 

are visible on the damaged end. We observed a velocity decrease of 500m/s to a depth 

of 0.15m between stations 10 and 12. 

Figure 5(b) Figure 5(c)
 

Figure 5(a) 

Figure 5(a) Dimensions and some of the instrumentation on the concrete block 

test. The single blast hole is horizontal and was lengthened to 2.4m from 1.2m shown in 

the drawing. The hole was filled with Dyno AP up to 38cm from the end. Figure 5 (b) 

Pre-blast view of concrete test block used for blasting test. Note accelerometer and 

anchor bolts used for both attaching the accelerometer and as seismic source locations. 

4.1.1.1. Results 

As described in detail in section 3.2, we used Rayfract® refraction tomography 

software (Intelligent Resources Inc.) to generate the velocity tomograms. Rayfract® 

produces three outputs. The first is a 1-D velocity gradient starting model for the nonlinear 

inversion process, the second shows the wave coverage within the survey, and the third 

image is the final P-wave velocity tomogram. In the results that we show for the four test 

cases, we present the final velocity tomograms and the wave coverage plots. 

The pre-blasting survey tomogram shows P-wave velocity quickly transitioning 

from low velocity at the block surface to greater than 3000m/s, the approximate velocity 

of competent concrete, at 1m depth (Figure 6). A velocity contour at 3000m/s is 

included on the tomogram for reference. The 2.85m survey imaged P-wave velocities 

approximately 1m into the block. The test block showed near surface oxidation from 

weather exposure to approximately 3cm depth. Oxidation or inconsistent curing may 

be the cause of the lower velocity observed from stations 912 to 920. 
the third ima e is the final Pwave velocit tomo ram. In the results that we show for the

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

g y g 

four test cases, we present the final velocity tomograms and the wave coverage plots. 

The pre-blasting survey tomogram shows P-wave velocity quickly transitioning 

from low velocity at the block surface to greater than 3000m/s, the approximate velocity 

of competent concrete, at 1m depth (Figure 6). A velocity contour at 3000m/s is included 

on the tomogram for reference. The 2.85m survey imaged P-wave velocities 

approximately 1m into the block. The test block showed near surface oxidation from 

weather exposure to approximately 3cm depth. Oxidation or inconsistent curing may be 

the cause of the lower velocity observed from stations 912 to 920. 


 

Figure 6 (b)

Figure 6 P-wave velocity tomogram from the concrete block pre-blasting P-wave velocity tomogram from the concrete block presurvey showing P-wave velocity quickly transitioning from a lower velocity at the block 

surface to greater than 3000 m/s within the block. Horizontal axis is profile distance in
 

meters; vertical axis is depth into the block behind accelerometer positions. 9xx numbers
 

blasting survey showing P-wave velocity quickly transitioning from a lower velocity 

at the block surface to greater than 3000 m/s within the block. Horizontal axis is 

along top represent station positions spaced 0.15m (0.5ft). Figure 6 (b) Wave coverage 

plot for the pre-blast survey showing even coverage through most of the imaged area.
 

Plot dimensions are the same as for Figure 6 (a) and the color scale indicates the number
 

profile distance in meters; vertical axis is depth into the block behind accelerometer 

positions. 9xx numbers along top represent station positions spaced 0.15m (0.5ft). 

of wave paths through each velocity cell or pixel. Figure 6 (b) Wave coverage plot for the pre-blast survey showing even coverage 

through most of the imaged area. Plot dimensions are the same as for Figure 6 (a) The post-blasting survey spanned only 1.5m and P-wave velocities were imaged

0.3m into the block (Figure 7). The decrease in imaging depth is related to the decreased and the color scale indicates the number of wave paths through each velocity cell or horizontal extent of the recording stations. Station 10 (910 in Figure 7) was located on 

the blasting side where velocities were expected to decrease due to the blast damage. The 

previous survey showed velocities between 2800 and 3000 m/s at 0.2m into the block
 

(Figure 6). The post-blast survey shows a decrease in velocity between stations 10 and
 

12 at 0.2m depth. At a depth of 0.2m, station 12 shows a velocity of 2600m/s compared 

pixel. 

Figure 6 (a) (a) Final Final 

 

Depth into face (m) 

4600
 

4400
 

4200
 

stations 1 to 20
 4000
 

902 904 906 910 912 914 916 918 920
 3800
 0 

3000 m/s contour: 

competent cement 

extent of 

post-blast survey 

3600
 

3400
 

3200
 

3000
 

2800
 

-0.5 2600
 

2400
 

2200
 

2000
 

1800
 

1600
 

1400
 

1200
 

0 0.5 1 1.5 2 2.5 3
 

Distance along block (m) 1000
 

800
 m/s 600

Figure 6(a) 

0 0.5 1 1.5 2 2.5 3 

-0.5 

0 902 904 906 910 912 914 916 918 920 

0 

5 

10 

15 

20 

25 

30 

35 

40 

45 

50 

55 

60 

65 

70 

75 

Depth into block face (m) 

Distance along block (m) 

hits/cell 

stations 1 to 20 

Figure 6 (b) 
The post-blasting survey spanned only 1.5m and P-wave velocities were 

imaged 0.3m into the block (Figure 7). The decrease in imaging depth is related to 

the decreased horizontal extent of the recording stations. Station 10 (910 in Figure 7) 

was located on the blasting side where velocities were expected to decrease due to the 

blast damage. The previous survey showed velocities between 2800 and 3000 m/s at 

0.2m into the block (Figure 6). The post-blast survey shows a decrease in velocity 

between stations 10 and 12 at 0.2m depth. At a depth of 0.2m, station 12 shows a 

velocity of 2600m/s compared to 3100m/s in the pre-blast survey. 

 

 

 

 

 

 

 

-0.2 

-0.1 

600 

1100 

1600 

2100 

2600 

3100 

3600 

m/s 600 

1100 

1600 

2100 

2600 

3100 

3600 

stations 10 to 20 

( 1 to 9 destroyed) 

910 912 914 916 918 920
 

Depth into block face (m) 

0 

910 912 914 916 918 920 

( 1 to 9 destroyed) 

3000 m/s contour: 

competent cement 0 0.5 1 1.5 

-0.2 

-0.1 

0 

3000 m/s contour: 

competent cement 

Distance along block (m) 

Figure 7 (a) 

 

 

 

 

 

 

Figure 7 (a) stations 10 to 20 30

stations 10 to 20

Depth into block face (m) Depth into block face (m) 

0 0.5 1 1.5

m/s 

Figure 7 (a) Distance along block (m)

910 912
 914 916 918 920
 

Distance along block (m)

4
 

hits/cell 

2
 

0 

Figure 7 (b) 

Figure 7 (a) Final P-wave velocity tomogram from the post blast survey showing

P-wave velocity within the remaining portion of the concrete test block. Horizontal axis 

is profile distance in meters; vertical axis is depth into the block behind accelerometer Figure 7 (a) Final P-wave velocity tomogram from the post blast survey showing

P-wave velocity within the remaining portion of the concrete test block. Horizontal axis positions. A decrease in velocity can be seen at station 910 in the post-blast survey 

is profile istanc in meter ; vertical axis is depth into th block behind accelerometer comparedd to the epre-blast ssurvey. Figure 7 (b) Wave coveerage plot showing mostly even 

positions. A decrease in velocity can be seen at station 910 in the post-blast survey wave coverage for the portion of the block surveyed. 9xx numbers along top represent 

 

 

-0.1 

0 

2 

4 

6 

8 

10 

12 

14 

16 

18 

20 

22 

24 

26 

28 

30 

6 

8 

10 

12 

14 

16 

18 

20 

22 

24 

26 

28 ( 1 to 9 destroyed) 

Depth into block face (m) 

stations 10 to 20

0 

910 912 914 916 918 920 

( 1 to 9 destroyed) 

0 0.5 1 1.5 

-0.2 

-0.1 

0 

-0.2 

0 0.5 1 1.5

Distance along block (m) 

hits/cell 

Figure 7 (b) 
Figure 7 (a) Final P-wave velocity tomogram from the post blast survey 

showing P-wave velocity within the remaining portion of the concrete test block. 

Horizontal axis is profile distance in meters; vertical axis is depth into the block 

behind accelerometer positions. A decrease in velocity can be seen at station 910 in 

the post-blast survey compared to the pre-blast survey. Figure 7 (b) Wave coverage 

plot showing mostly even wave coverage for the portion of the block surveyed. 9xx 

numbers along top represent station positions spaced 0.15m (0.5ft). 

To more easily identify zones of velocity change from the pre-blast to the postblast survey, we generated a difference plot of the two tomograms (Figure 8). 

 

 

 

 

Stations 11 to 20 common to pre- and post-blast 

Zone of maximum blast damage 

Figure 8 

 

 

 

 

 

Both 

tomograms are grids of velocity values, so the pre-blasting survey was regridded 

and shortened to match the geometry of the post-blasting survey and subtracted 

from the post-blasting grid to form a difference tomogram to highlight any velocity 

differences. The difference tomogram shows a prominent low velocity zone that 

occurs between stations 10 and 12 beginning at a depth of 0.15m. The velocity in this 

area is approximately 500m/s lower after blasting occurred. This result was expected 

because the lower portion of the survey near station 10 is closest to the detonation 

and should have experienced the most BID. The positive velocity difference near the 

bottom of the difference tomogram is most likely an artifact caused by poor velocity 

resolution near the bottom edges of the two component tomograms resulting from the 

different geometries for the two surveys. 

tomograms are grids of velocity values, so the pre-blasting survey was regridded and 

shortened to match the geometry of the post-blasting survey and subtracted from the postblasting grid to form a difference tomogram to highlight any velocity differences. The 

difference tomogram shows a prominent low velocity zone that occurs between stations 

10 and 12 beginning at a depth of 0.15m. The velocity in this area is approximately 

500m/s lower after blasting occurred. This result was expected because the lower portion 

of the survey near station 10 is closest to the detonation and should have experienced the 

most BID. The positive velocity difference near the bottom of the difference tomogram 

is most likely an artifact caused by poor velocity resolution near the bottom edges of the 

two component tomograms resulting from the different geometries for the two surveys. 

Figure 8 

Figure 8. Difference tomogram of velocity images from the pre- and post-blasting 

surveys on the SRL concrete block. A low velocity zone at 0.2m depth is evident on the 

left edge of the survey and is interpreted to be a zone damaged from the blast. Horizontal 

axis is profile distance in meters; vertical axis is depth into the block behind 

accelerometer positions. 7xx numbers along top represent station positions spaced 0.15m 

(05ft)
Figure 8. Difference tomogram of velocity images from the pre- and postblasting surveys on the SRL concrete block. A low velocity zone at 0.2m depth is 

evident on the left edge of the survey and is interpreted to be a zone damaged from the 

blast. Horizontal axis is profile distance in meters; vertical axis is depth into the block 

behind accelerometer positions. 7xx numbers along top represent station positions 

spaced 0.15m (0.5ft). 

4.1.2. Stillwater Mining Co. East Boulder Mine, Big Timber, Montana 

4.1.2.1. Experimental setup 

Two sites at Stillwater Mining Company’s East Boulder Mine were surveyed to 

measure BID. The East Boulder Mine is located near Big Timber, Montana. Platinum 

group elements are mined underground from an ore deposit known as the J-M Reef. 

We chose sites free of support bolts and mesh for the surveys. Background 

noise was avoided as much as possible and we tried to locate rock wall surfaces that 

had reasonably smooth faces for attaching the accelerometer with its recording axis 

perpendicular to the wall. 

The first site was located on the 6700 footwall. This wall is parallel to the 

longitudinal orientation of the J-M Reef and was far removed from machinery noise 

(Figure 9). The survey length was 9.2m and 19 anchor bolt stations were installed at 

an approximate spacing of 0.5m. The survey area comprised gabbro between stations 

1 through 8 and norite between stations 8 through 19. The uneven surface of the wall 

limited suitable locations for attaching the accelerometer to stations 1, 3, 14, and 16. 

Figure 10 is a photograph of the setup at the survey site. 
 

 

 

 

Area of study 

10m along 

north rib 

Gabbro 

(green) 

Norite 

(brown) 

Figure 9. Schematic showing the location of the first test site at the East Boulder mine 

6700 level. The study area is the blue line covering the transition from gabbro to norite. 

 

 

 

 

 

 

reasonably smooth faces for attaching the accelerometer with its recording axis 

perpendicular to the wall. 

The first site was located on the 6700 footwall. This wall is parallel to the 

longitudinal orientation of the J-M Reef and was far removed from machinery noise 

(Figure 9). The survey length was 9.2m and 19 anchor bolt stations were installed at an 

approximate spacing of 0.5m. The survey area comprised gabbro between stations 1 

through 8 and norite between stations 8 through 19. The uneven surface of the wall 

limited suitable locations for attaching the accelerometer to stations 1, 3, 14, and 16. 

Figure 10 is a photograph of the setup at the survey site. 

Figure 9. Schematic showing the location of the first test site at the East Boulder mine 6700 level.
 

The study area is the blue line covering the transition from gabbro to norite.

Figure 10 Figure 10 

Figure 10. Photograph of the first East Boulder survey site 1. The rough mine 

wall limited useable recording locations for the accelerometer. The bolt spacing was 

approximately 0.5m. Station numbers are spray painted on the mine wall above the stud 

bolts. 

The second test site was on the 695 decline crosscut wall. This site was chosen 

for its perpendicular orientation to the J-M Reef longitudinal axis. Rock in this wall is 

composed of norite and the surface was more uneven than the first site. Figure 11 is a 

schematic showing the location of the survey line. 

Figure 10. Photograph of the first East Boulder survey site 1. The rough mine 

wall limited useable recording locations for the accelerometer. The bolt spacing was 

approximately 0.5m. Station numbers are spray painted on the mine wall above the 

stud bolts. 
The second test site was on the 695 decline crosscut wall. This site was chosen 

for its perpendicular orientation to the J-M Reef longitudinal axis. Rock in this wall 

is composed of norite and the surface was more uneven than the first site. Figure 11 

is a schematic showing the location of the survey line. 

Area of study 

5m along 

east rib of 

crosscut 

Norite 

(brown) 

Figure 11 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 11. Schematic showing the location of the survey line for the second test 

in the East Boulder mine. The rock surveyed was norite and the surface was very 

uneven with mesh compounding the data acquisition. 

The survey geometry for the second site consisted of 10 stations with an 

approximate spacing of 0.6m and the total survey length was 6.05m. The bolt positions 

for both sites were surveyed using a laser level and tape by referencing bolt positions 

on the wall to a laser line datum to give inverse wall topography. Recordings were 

made with the accelerometer at stations 1, 6, 7, and 9 as these were the only locations 

we could install the aluminum wedge to the wall for attaching the accelerometer. This 

site was an active working area and we tried to make our recordings during lulls in 

background noise caused by haul trucks. 

Figure 11 

Figure 11. Schematic showing the location of the survey line for the second test in 

the East Boulder mine. The rock surveyed was norite and the surface was very uneven 

with mesh compounding the data acquisition. 

The survey geometry for the second site consisted of 10 stations with an 

approximate spacing of 0.6m and the total survey length was 6.05m. The bolt positions 

for both sites were surveyed using a laser level and tape by referencing bolt positions on 

the wall to a laser line datum to give inverse wall topography. Recordings were made 

with the accelerometer at stations 1, 6, 7, and 9 as these were the only locations we could 

install the aluminum wedge to the wall for attaching the accelerometer. This site was an 

active working area and we tried to make our recordings during lulls in background noise 

caused by haul trucks. 

4.2.2. Results 

Velocity tomograms from the East Boulder Mine surveys include the wall 

topography of the tunnel faces. The geometry of the rock faces and the limited 

accelerometer recording locations for both surveys led to gaps in wave coverage. Wave 

coverage is an important indicator of how much confidence can be placed on calculated 

velocities for an area. We also increased the wave-path widths for the WET tomography 

in Rayfract® to improve wave coverage and provide a more robust result. For both data 

sets, the wave-path width was set to 0.6 % of one wave period from the default of 0.2 %. 

Undamaged rock at both test sites was expected to have a P-wave velocity of 

approximately 6600m/s (personal communication, Kathryn Dehn). Survey site 1 results 
 

 

 

4.1.2.2. Results 

Velocity tomograms from the East Boulder Mine surveys include the wall 

topography of the tunnel faces. The geometry of the rock faces and the limited 

accelerometer recording locations for both surveys led to gaps in wave coverage. 

Wave coverage is an important indicator of how much confidence can be placed on 

calculated velocities for an area. We also increased the wave-path widths for the 

WET tomography in Rayfract® to improve wave coverage and provide a more robust 

result. For both data sets, the wave-path width was set to 0.6 % of one wave period 

from the default of 0.2 %. 

Undamaged rock at both test sites was expected to have a P-wave velocity 

of approximately 6600m/s (personal communication, Kathryn Dehn). Survey site 

1 results (Figure (12a)) show P-wave velocity nearing 6000m/s at the bottom of the 

tomogram which is interpreted to be undamaged rock. BID depth is estimated to 

extend 1.0m into the rock face. Wave coverage was consistent over the extent of 

the survey (Figure (12b)), except the shallow depths in the center where the velocity 

falsely appears to be less than 500m/s. We interpret the anomalously low velocity 

areas near the surface to be caused by lack of P-wave coverage.

(Figure (12a)) show P-wave velocity nearing 6000m/s at the bottom of the tomogram
 

which is interpreted to be undamaged rock. BID depth is estimated to extend 1.0m into 

the rock face. Wave coverage was consistent over the extent of the survey (Figure 

(12b)), except the shallow depths in the center where the velocity falsely appears to be 

less than 500m/s. We interpret the anomalously low velocity areas near the surface to be 

caused by lack of P-wave coverage.
 

Figure 12 (a)

 

 

 

 

Stations 1 to 19 

no wave coverage zone 

6600m/s – competent rock 

gabbro zone (1-8) 

norite zone (9-19) 

profile parallel to J-M reef 

Figure 12 (a) 
 

 

 

 

 

 

 

 

 

 

6600m/s – competent rock 

norite zone (9-19) 

Figure 12(a) P-wave velocity tomogram from East Boulder site 1. Undamaged 

rock velocities begin at 6600 m/s. BID was estimated to a depth of 1.0m (3.3ft).

Horizontal axis is profile distance in meters; vertical axis is depth into the mine wall 

behind accelerometer positions. 5xx numbers along top represent station positions

spaced approximately 0.5m (1.6ft). Figure 12(b) Wave coverage plot for East Boulder 

site 1 illustrating the poor coverage in the near surface caused by the highly irregular 

wall topography. 

Figure 12 (b) 

The velocity tomogram for the survey at site 2 is shown in Figure 13(a). The 

tomogram for survey site 2 shows a maximum P-wave velocity of 4300m/s. The area

between stations 1 and 3 has low wave coverage (Figure 13(b)) so velocity values 

there have lower confidence but are still reasonable. Velocities near station 6 are high 

at the surface, possibly due to the topography of the wall face or to variation in rock

competence. The depth for BID was estimated to be greater than 1.5m, the maximum 

depth into the wall for the tomogram. 

 

 

Stations 1 to 10 

profile perpendicular to J-M reef 

norite 

6600m/s– competent rock 

Figure 13(a) 

Figure 12 (b)

Figure 12(a) P-wave velocity tomogram from East Boulder site 1. Undamaged 

rock velocities begin at 6600 m/s. BID was estimated to a depth of 1.0m (3.3ft). 

Horizontal axis is profile distance in meters; vertical axis is depth into the mine wall 

behind accelerometer positions. 5xx numbers along top represent station positions 

spaced approximately 0.5m (1.6ft). Figure 12(b) Wave coverage plot for East Boulder 

site 1 illustrating the poor coverage in the near surface caused by the highly irregular wall 

topography.

The velocity tomogram for the survey at site 2 is shown in Figure 13(a). The 

tomogram for survey site 2 shows a maximum P-wave velocity of 4300m/s. The area 

between stations 1 and 3 has low wave coverage (Figure 13(b)) so velocity values there 

have lower confidence but are still reasonable. Velocities near station 6 are high at the 

possibly due the topography of the wall face to or to variation in rock 

competence. The depth for BID was estimated to be greater than 1.5m, the maximum 

depth into the wall for the tomogram. 
 

 

 

 

 

 

6600m/s– competent rock 

norite 

Figure 13(a) 

Figure 13(b)
 

Figure 13(a) P-wave Figure 13(a) P-wave tomogram from East Boulder survey site 2 shows velocity velocity tomogram from East Boulder survey site 2 

variable velocity layering possibly associated with rough tunnel wall or to less damaged 

zones of rock. Undamaged rock velocity at this site was 6600 m/s. BID was estimated to 

be greater than 1.5m, the maximum depth into the wall for the tomogram. Horizontal 

shows variable velocity layering possibly associated with rough tunnel wall or to less 

damaged zones of rock. Undamaged rock velocity at this site was 6600 m/s. BID was 

axis is profile distance in meters; vertical axis is depth into the mine wall behind estimated to be greater than 1.5m, the maximum depth into the wall for the tomogram. accelerometer positions. 6xx numbers along top represent station positions with 

approximate spacing 0.6m (2.0ft). Figure 13(b) Wave coverage plot for East Boulder site 

behind accelerometer positions. 6xx numbers along top represent station positions 

with approximate spacing 0.6m (2.0ft). Figure 13(b) Wave coverage plot for East 

Boulder site 2 shows reasonable coverage for stations 603 to 608. The low velocity 

zone from station 601 to 603 has minimal wave path coverage. 

Horizontal axis is profile distance in meters; vertical axis is depth into the mine wall 

Figure 13(b)
 

4.1.3. Hecla Lucky Friday Mine, Mullan, Idaho 

4.1.3.1. Experimental setup 

The fourth seismic test was conducted at Hecla’s Lucky Friday Mine in Mullan, 

Idaho. This mine is located in the Coeur d’Alene District and is famous for its silver 

rich deposits. Ore veins are located almost 2km below the surface. 

Seismic recordings were made on a freshly blasted face in a 4900 level Gold 

Hunter vein drift. The face in this survey was considerably smoother than the excavation 

faces in the East Boulder Mine surveys (Figure 14). Exposed rock in the excavation 

was composed of phyllitic argillite with a P-wave velocity of approximately 5200m/s 

(personal communication, Ted Williams). The survey had 16 stations with an average 

spacing of 0.4m. The total length of the survey along the face was 6.6m. Recordings 
 

 

 

 

 

 

 

 

 

 

 

 

The fourth seismic test was conducted at Hecla’s Lucky Friday Mine in Mullan, 

Idaho. This mine is located in the Coeur d’Alene District and is famous for its silver rich 

deposits. Ore veins are located almost 2km below the surface. 

Seismic recordings were made on a freshly blasted face in a 4900 level Gold 

Hunter vein drift. The face in this survey was considerably smoother than the excavation 

faces in the East Boulder Mine surveys (Figure 14). Exposed rock in the excavation was 

composed of phyllitic argillite with a P-wave velocity of approximately 5200m/s 

(personal communication, Ted Williams). The survey had 16 stations with an average 

were taken at accelerometer stations 3, 5, 9, 13, and 15 where good coupling could be 

obtained to mount the aluminum wedge for the accelerometer. 

spacing of 0.4m. The total length of the survey along the face was 6.6m. Recordings 

were taken at accelerometer stations 3, 5, 9, 13, and 15 where good coupling could be 

obtained to mount the aluminum wedge for the accelerometer. 

Figure 14. Photograph showing portion of rock face in the Lucky Friday mine. Spray painted 

numbers are above locations of epoxy mounted stud bolts. 

4.3.2. Results 

Phyllitic argillite 

with competent 

P-wave velocity 

approximately 

5200m/s 

Station nos. in 

red paint 

(~0.4m) 

Figure 14. Photograph showing portion of rock face in the Lucky Friday mine. Spray 

painted numbers are above locations of epoxy mounted stud bolts. 

4.1.3.2. Results 

P-wave first arrivals were picked on the accelerometer recordings from stations 3, 5, 

13, and 15. Station 9 recordings had high noise levels and were unusable. Lack of 

data at station 9 left gaps in wave coverage in the center of the survey at depths less 

than 0.7m (Figure 15(b)). Wave-path width in the software was again increased to 0.6 

% of one wave period to smooth gaps caused by low wave coverage. 

The P-wave velocity tomogram (Figure 15(a)) reaches 5200m/s, the velocity of 

competent rock, at approximately 1.5m into the tunnel face. The interior of the wall 

displays a more uniform velocity distribution than the East Boulder surveys. This 

could be related to the smoother mine face for the Lucky Friday survey or, conversely, 

the variable velocity distribution in the East Boulder surveys could be representative 

of the rock competence mirrored by the rough topography on the mine wall. The 

distribution of wave coverage (Figure 15(b)) also indicates the smoother velocity 

variation at the Lucky Friday site. 

P-wave first arrivals were picked on the accelerometer recordings from stations 3, 

5, 13, and 15. Station 9 recordings had high noise levels and were unusable. Lack of 
 

 

 

 

Figure 15(b) 

 

 

 

Figure 15 (a) P-wave velocity tomogram from the Lucky Friday survey shows 

good rock quality beginning at about 1.5m depth. Horizontal axis is profile distance in 

meters; vertical axis is depth into the mine wall behind accelerometer positions. 1xx 

numbers along top represent station positions with approximate spacing 0.4m. (b) 

Wave-path coverage for the Lucky Friday survey is more even which is indicative of 

the smoother velocity distribution in the wall. This could be a combination of a more 

homogeneous nature of the rock as well as the smoother mine wall topography. 

 

Figure 15(a) 

 coverage (Figure 15(b)) also indicates the smoother velocity variation at the L

y site. 

Stations 1 to 16 

5200m/s– competent rock 

Stations 1 to 16 

5200m/s– competent rock 

Figure 15(a) 

Figure 15(a) 

one wave period to smooth gaps caused by low wave coverage.
 

data at station 9 left gaps in wave coverage in the center of the survey at depths less than The P-wave velocity tomogram (Figure 15(a)) reaches 5200m/s, the velocity 0.7m (Figure 15(b)). Wave-path width in the software was again increased to 0.6 % 

competent rock, at approximately 1.5m into the tunnel face. The interior of the wall one wave period to smooth gaps caused by low wave coverage.

displays a more uniform velocity distribution than the East Boulder surveys. This could 

be related to the smoother mine face for the Lucky Friday survey or, conversely, the The P-wave velocity tomogram (Figure 15(a)) reaches 5200m/s, the velocity of variable velocity distribution in the East Boulder surveys could be representative of the competent rock, at approximately 1.5m into the tunnel face. The interior of the wall 

rock competence mirrored by the rough topography on the mine wall. The distribution of displays a more uniform velocity distribution than the East Boulder surveys. This could 

of of
5. cONcLUSIONS 

We have found that high resolution refraction tomography using a high frequency 

receiver can be an effective method for determining BID depth in a mine wall. A 

difference in P-wave velocity between concrete test block pre and post-blasting 

surveys was apparent on the blasted edge, with a decrease in P-wave velocity of 

approximately 500m/s. This velocity difference within the survey was located in 

the zone where blast damage was expected to have the greatest impact. The target 

velocity for good rock quality of 6000m/s in the East Boulder mine were found at 

an average depth of 1.0m at survey site 1 and 1.5m at survey site 2. A survey at the 

Lucky Friday mine showed good rock quality velocity of 5200m/s at approximately 

1.5m depth into the wall. 

A few site criteria should be considered to maximize data quality. Smoother rock 

faces help to improve sensor coupling and orientation for accelerometer use. Rock 

faces should also be mesh free if possible. Sensor recording stations should be as 

closely spaced as possible throughout the survey to keep wave coverage uniform at 

shallow depths. Survey sites should also be removed from mining activity to decrease 

noise in the recordings. 

Although we demonstrated our approach by making surface measurements along 

mine wall faces using refraction tomography, we think that a better approach would 

be to use a borehole deployed system into the mine face. This would require purchase 

(or rental) of a borehole source and receiver along with recording system to record 

high frequencies (>5000Hz) to allow increase resolution. The advantage of the 

borehole system would be the ability to penetrate the boreholes to the estimated depth 

of competent rock and thus evenly image the space between the boreholes at high 

resolution. With the refraction tomography approach, the nature of the process is a 

decrease of wave coverage with depth. Our work was accomplished with in-house 

equipment and demonstrated the feasibility of the refraction approach. A higher 

level of resolution and accuracy could be obtained with a borehole system and a 

transmission tomography approach. 
AckNOWLEDGEMENTS 

This study was made possible by the financial support of the National Institute of 

Occupational Safety and Health Order No. 212-2006-M-18282 under project director 

Stephen Iverson. Special thanks are extended to Ted Williams, Kathryn Dehn, Jeff 

Johnson, and William Hustrulid for their assistance in the seismic data collection. 

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