Attenuation and duration of seismic signals generated from controlled

methane and coal dust explosions in an underground mine

Michael M. Murphy b,n,

n Corresponding author. Tel.: þ1 412 386 4172; fax: þ1 412 386 6891.
 

E-mail addresses: mmu5@cdc.gov (M.M. Murphy),
 

ewestman@vt.edu (E.C. Westman), thb0@cdc.gov (T.M. Barczak).
 

Thomas M. Barczak b

b NIOSH, Office of Mine Safety and Health Research, PO Box 18070, 626 Cochrans Mill Road, Pittsburgh, PA 15236, USA

Erik C. Westman a,

a Department of Mining & Minerals Engineering, Virginia Polytechnic Institute & State University, 100 Holden Hall, Blacksburg, VA 24061-0239, USA

1. Introduction

1.1. Statement of the problem

Seismic monitoring provides a useful means for detection and

evaluation of events resulting from mining activity. Seismic

signature characteristics such as arrival times, amplitudes, duration and frequency content can indicate the nature and location of

the source. In the past, most mining-related seismic measurements have focused on events such as rockbursts, production

blasts from quarries, roof falls and rock fractures [1,2,3,4]. However, little or no effort has been expended towards examining the

characteristics of a signature emanating from a methane and coal

dust explosion in an underground mine. The Sago Mine disaster in

2006 provides an example of why these particular signatures

should be researched. A small amplitude signal was identified on

records of the regional seismic network stations that were closest

to the mine [5]. The epicentral location of the small amplitude

signal was at the Sago Mine. However, it was unclear whether the

signature represented the explosion itself or another type of

mining-related seismicity such as a large roof fall. This paper

presents findings from a study aimed at examining seismicity

from methane and coal dust explosions by analyzing the attenuation and duration of seismic signatures collected from controlled

methane and coal dust explosions, with potential applications to

forensic studies of mine explosions such as the Sago Mine

disaster.

1.2. Proposed solution and objective

The Lake Lynn Experimental Mine (LLEM) is a facility associated with the National Institute for Occupational Safety and

Health’s (NIOSH’s) Office of Mine Safety and Health Research. The

research facility has conducted controlled methane and coal dust

explosions since 1983. These experiments provide insight into the

behavior and prevention of underground mine explosions [6,7].

This facility provides an ideal location and opportunity to monitor

the seismicity from controlled explosions in an underground

mine. A seismic monitoring system was installed at the Lake

Lynn Experimental Mine to collect seismic signatures emanating

from controlled methane and coal dust explosions. Seismic

signatures were analyzed to begin to understand their characteristics from the different explosions. These types of measurements

could not be found in other literature and characteristics found in

the signatures are hoped to provide better understanding of these

events when captured in the far-field. For this paper, the results

and conclusions discuss the attenuation and durations of the

seismic signatures. The attenuation of the seismic signatures was

studied by observing the maximum amplitude at different distances away from the source. The durations of the seismic

signatures were investigated to determine if the impact of the

varying experimental designs can be observed. The objective was

to determine if the attenuation and duration characteristics could

give insight into seismic signatures measured on a regional

seismic network station in the future.

Although the accessibility of the LLEM allowed for the unique

opportunity to monitor methane and coal dust explosions in an

underground environment, control of the experimental designs

was not made available. Therefore, the presence of explosioncontainment structures and the amount of initial explosive fuel

used during the experiments was based on research needs related

to mine seal design and was not controlled for the purposes of
this study. Ideally, experiments with replicated designs would be

monitored; however, this was not an option. Thus, this study was

considered to be an exploratory project where any finding or

observation made from the seismic signatures would be considered to be beneficial for further research.

2. Materials and methods

2.1. Lake Lynn Experimental Mine and instrument locations

The Lake Lynn Experimental Mine is a full-scale, underground

mining research facility on the site of a former limestone quarry.

The facility is located approximately 96 km (60 miles) southeast

of Pittsburgh, PA, and 16 km (10 miles) northeast of Morgantown,

WV. The west side of the facility, known as the old workings, was

mined when limestone was produced commercially from the site.

This area of the research facility resembles a layout typical of an

underground room and pillar stone mine. The dimensions of

entries in the old workings are 15.2 m (50 ft) wide by 9.1 m

(30 ft) high. The east side of the facility contains mine drifts that

are dimensioned to match configurations found in longwall coal

mines. The dimensions of these entries are 6.1 m (20 ft) wide by

2.0 m (6.5 ft) high. The size of the pillars in the simulated longwall gate roads is 24 x 12 m (80 x 40 ft). A-, B-, C- and D-Drifts are

approximately 480–495 m (1575–1630 ft) long. E-Drift, which

connects the entries at the inby end, is 155 m (510 ft) long.

Instrument rooms in the mine are located approximately threefourths of the way down C- and D-Drifts. These rooms are

protected from the entry via blastproof doors. A 70-ton bulkhead

door with a steel framework is located between C- and E-Drift and

is closed during all of the explosions. The mine layout is shown in

Fig. 1, including both uniaxial and triaxial geophone locations

used for this experiment.

2 1

3 

5 

6 

4 

9 

8 

10 

11 

12 

Uniaxial geophones 

Triaxial geophones 

Instrument rooms 

(containing geophones 

#1, #2 and #3) 

(61 m) 

Fig. 1. Layout of the Lake Lynn Experimental Mine with geophone location and

instrument room locations highlighted.

Geologically, the mine is located in the Greenbrier limestone

formation [8]. A borehole drilled into the roof approximately

9.1 m (30 ft) high in the old workings of the mine showed the

different formations at the test site. The data from the borehole

were obtained from the NIOSH engineers who conducted the

borehole logging. The geology observed from the borehole logs in

the old workings was confirmed to be approximately the same

geology in the area where the explosions were ignited and most

of the measurements were taken. From the surface of the roof to

4.3 m (14 ft) above the mine is a fine-grained limestone. A shale/

claystone formation is located between 4.3 and 6.1 m (14 and

20 ft). Between 6.1 and 9.1 m (20 and 30 ft), which is at the end of

the observation hole, is a fine-grained limestone.

The methane and coal dust explosions conducted at LLEM

were ignited at the inby end of A- or C-Drift. The chosen locations

of the geophones were distributed around the explosion at

various distances. The locations of both uniaxial and triaxial

geophones are indicated on the mine map in Fig. 1. Over the

time interval of the experiment, nine triaxial and three uniaxial

geophones were utilized. Geophone 7 is not listed on the mine

map because it is a ‘‘dummy’’ geophone used for testing purposes.

Geophone 13 was installed on a limestone outcrop outside of the

mine to collect data away from the source. Not all geophones

were present for each test monitored at LLEM. The geophone

placement became more proactive as the study progressed; i.e.,

additional geophones were added to the existing array in specific

locations as a response to new findings in the data. In some cases

geophones had to be removed due to their locations interfering

with other experiments at LLEM. In addition to the seismic system

at the mine location, the nationwide US Geological Survey (USGS)

stations were studied at the time of the explosions. The closest

USGS station to LLEM is located at Mt. Chateau, WV, which is

approximately 16 km (10 miles) south of the mine.

2.2. Methane and coal dust explosions

The Lake Lynn Experimental Mine can accommodate a variety

of explosion configurations. The explosions1

1 Mine shot numbers 506–524 were conducted for purposes other than this

study. Mine shot numbers 506–507 are referenced in Refs. [9] and [10]. Mine shot

numbers 508–509 are referenced in Ref. [11]. Mine shot numbers 513, 514, and

516–19 are referenced in Ref. [12]. Mine shot numbers 523 and 524 are referenced

in Ref. [13]. Mine shot numbers 520–522 are not yet published; however, the work

was completed by Ken Cashdollar, Eric Weiss, and Sam Harteis as part of the mine

explosion program for the NIOSH Office of Mine Safety and Health Research’s then

Disaster Prevention and Response Branch.

can be located in A-,

B-, C- or D-Drifts, and a typical explosion consists of natural gas

(E98% methane) injected into an ignition chamber at the face of

the drift. A mine ventilation brattice is draped across the entry to

contain the methane in the ignition chamber. An electric fan with

an explosion-proof motor housing mixes the natural gas with the

air to result in an approximate 9.5% methane-air concentration.

The flammable natural gas-air volume is ignited using a triplepoint ignition source. This ignition source consists of three sets of

two 100 J electric matches that are equally spaced at mid-height

across the closed end of the drift and ignited at the same time.

Five barrels filled with water, located near the outby end of the

ignition chamber, act as turbulence generators to achieve a

projected pressure pulse. To increase the explosion pressure,

either the ignition chamber was lengthened or pulverized coal

dust was suspended on shelves from the mine roof starting just

outside of the ignition chamber.

For this study, the size of the explosion was defined as the

peak pressure generated in the drifts of the mine. Pressure

measurements were taken in the C-Drift or A-Drift ribs, depending on whether the explosion took place in C-Drift or A-Drift.

The data were sampled at 1500 samples per second and were

obtained from the LLEM staff. The pressure measurements

were not on the same time scale as the seismic measurements.

An example of a pressure–time curve is plotted in Fig. 2. In the

figure, the multiple curves represent the different locations where

the pressure measurement was taken along the drift. The peak

pressure generated for this example is around 0.38 MPa (55 psi),

which would be considered the size of that particular explosion.

A total of 19 explosions were analyzed for this paper, summarized
in Table 1. The table indicates the size of the explosion, whether

the explosion took place in A- or C-Drift, and the containment

geometry.

The containment geometry is a reference to whether a mine

seal was in the propagation path of an explosion down the length

of the entry and whether seals were within the crosscuts. Fig. 1

indicates the location of the mine seal in the propagation path of

the explosion in the A- or C-Drift entry and the location of mine

seals in the crosscuts, which are all represented by small thick

black lines. The seals in the propagation path of the explosions

were approximately 115 m (375 ft) away from the face of the

drifts. In most scenarios during the study, seals were also

constructed in the crosscuts between A- and B-Drifts and B- and

C-Drifts. Table 1 explains the locations of crosscut seals in more

detail. For example, for mine shot number 503, Table 1 indicates

that the explosion was contained within a sealed area and that

the ignition location was at the C-Drift face. Therefore, the

explosion was contained within the area created by the first

three sealed crosscuts in B- and C-Drifts, a mine seal in the C-Drift

entry, and a bulkhead door between C- and E-Drifts (however, the

explosion did not destroy any of the seals). In this study, five

experiments were conducted where an explosion was fully

contained within a sealed area either in A- or C-Drifts. Four

experiments were conducted where an explosion was ignited

within a sealed area in C-Drift and the pressure wave from the

explosion destroyed the seal.

Peak pressure generated 

-0.07 

-0.02 

0.03 

0.08 

0.13 

0.18 

0.23 

0.28 

0.33 

0.38 

-5 

0 

5 

10 

15 

20 

25 

30 

35 

40 

45 

50 

55 

Pressure (MPa)

Pressure (psi) 

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 

Time (s) 

Table 1

Summary of explosions analyzed during the study.

Fig. 2. Example of pressure–time curves used to obtain the size of the explosion, as defined by the peak pressure generated.

Size of explosion (MPa) Size of explosion (psi) Ignition location Containment geometry

Mine shot #503 1.20E-01 17.4 C-Drift face Explosion contained within sealed area

Mine shot #504 1.37E-01 19.9 C-Drift face Explosion contained within sealed area

Mine shot #505 3.80E-01 55.1 C-Drift face Explosion ignited in sealed area, seal in entry destroyed

Mine shot #506 6.17E-01 89.5 C-Drift face Explosion ignited in sealed area, seal in entry destroyed

Mine shot #507 1.47E-01 21.3 C-Drift face First three crosscuts sealed, no seal in path of explosion

Mine shot #508 3.46E-01 50.1 C-Drift face Explosion contained within sealed area

Mine shot #509 6.31E-01 91.5 C-Drift face Explosion ignited in sealed area, seal in entry destroyed

Mine shot #510 1.01E-01 14.6 A-Drift face All crosscuts sealed, no seal in path of explosion

Mine shot #513 9.56E-02 13.9 A-Drift face All crosscuts sealed, no seal in path of explosion

Mine shot #514 1.03E-01 15.0 A-Drift Face All crosscuts sealed, no seal in path of explosion

Mine shot #516 8.98E-02 13.0 A-Drift face All crosscuts sealed, no seal in path of explosion

Mine shot #517 9.27E-02 13.5 A-Drift face All crosscuts sealed, no seal in path of explosion

Mine shot #518 9.16E-02 13.3 A-Drift face All crosscuts sealed, no seal in path of explosion

Mine shot #519 1.72E-01 24.9 A-Drift face All crosscuts sealed, no seal in path of explosion,

however last two seals in the crosscuts failed

Mine shot #520 1.07E-01 15.6 A-Drift face First five crosscuts sealed, no seal in path of explosion

Mine shot #521 1.10E-02 1.6 A-Drift face All crosscuts sealed, no seal in path of explosion

Mine shot #522 1.09E-01 15.8 A-Drift face First five crosscuts sealed, no seal in path of explosion

Mine shot #523 3.09E-01 44.8 A-Drift face Explosion contained within sealed area

Mine shot #524 5.52E-01 80.0 A-Drift face Explosion contained within sealed area

2.3. Seismic monitoring system

Geophones were chosen as the seismic transducer to record

seismic waves emanating from the methane and coal dust

explosions. The geophones surrounded the area of the methane

and coal dust explosions at the Lake Lynn Experimental Mine.

Geophones 1, 10, 11, 12 and 13 were anchored onto the mine

floor while the rest of the geophones were anchored to the mine

roof. No grout was used while anchoring the geophones to the

roof or floor. Both the triaxial and uniaxial geophones had a

natural frequency of 4.5 Hz. The output chart provided by the

manufacturer only includes responses up to 90 Hz; nevertheless,

for this study, it was assumed that the response was constant but

that spurious responses could occur. Surface measurements

approximately 60 m (200 ft) above the mine were taken near

the end of the study using a three-component digital output

seismometer. The instrument was installed at a depth of one

meter below the ground surface above the A-Drift face. The

instrument was set to sample at 500 samples per second and
was oriented in the same manner as the geophones installed

inside of the mine. Data from this seismic instrument were only

available for mine shot numbers 523–524. The seismic monitoring system was installed within the existing structure of LLEM.

The two A/D converters used in the study, named MS boxes and

QS boxes, both had a sampling rate of 2000 samples per second

and contained six channels for input. The MS boxes had 22-bit

resolution (14 bit A/D with 8 bit gain) and the QS boxes had a 24

bit resolution. The seismic monitoring system was a trigger-based

system, in contrast to a real-time continuous monitoring system.

This system only recorded and saved data based upon two or

more geophones being triggered by an event.

2.4. Method for determining amplitude

The maximum amplitudes of the seismic signatures were

studied in order to calculate the attenuation factor. The maximum

seismic wave amplitudes were converted into units of m/s from

the original digital counts. The maximum ground velocity, or

amplitude, is a reference to the peak particle velocity, not the

velocity of the seismic wave traveling through the medium.

2.5. Method for determining attenuation

The attenuation of the seismic signatures was studied to

determine how much the amplitude decays as the seismic wave

travels through the limestone. The attenuation was calculated in

terms of decibels, which compares the ratios of amplitudes

observed at different locations within the mine. The formula used

to calculate attenuation is

( ) V d 1 B 1⁄4 20log V2

ð1Þ 

where V1 is the measured input (volts), and V2 is the reference

input (volts). Since amplitude is linearly related to the voltage

from the geophone, a ratio of measured and reference amplitude

values are used in the decibel calculation instead of volts. The

measured input is the measured amplitude at different distances

away from the source. The reference input would be related to an

initial amplitude observed at zero distance away from the source

of the explosion. However, since a geophone was never present at

the location of the source, this parameter had to be estimated

from the seismic data for each explosion. An example is shown for

mine shot number 506 in Fig. 3.

Amplitude (m/s) 

1.00E+00 

1.00E-01 

1.00E-02 

y = 0.053e-0.036x 

R2 = 0.9307 

1.00E-03 

1.00E-04 

1.00E-05 

0 50 100 150 200 250 

Distance (m) 

Fig. 3. Plot of amplitude versus distance for mine shot #506.

For this example, the x-axis is on

a logarithmic scale. The amplitude of ground velocity was

observed to decrease exponentially with distance. The reference

input, V1, can be estimated at 0.053 m/s based upon where the

regression line crosses the y-axis

Once the reference points were calculated for each mine shot,

the attenuation for each mine shot was calculated and plotted.

An example demonstrating the attenuation of amplitude for mine

shot number 506 is plotted in Fig. 4. For this example, the x-axis is

on a logarithmic scale.

Decibels (dB) 

0 

-10 

-20 

-30 y = -0.3109x 

R2 -40 = 0.9334

-50 

-60 

-70
 

10 100
 

Distance (m) 

1000 

Fig. 4. Plot of attenuation for mine shot #506.

2.6. Method for determining duration

The overall duration of the mine shots helps give insight into

how the experimental design may affect the seismic signatures

observed at different locations. For example, when a mine seal is

in the path of the explosion, if a signature is significantly longer in

duration than when the mine seal is not there under similar

conditions, then it can be inferred that the seal had a significant

effect on the signature. The time frame of the seismic signature

was defined by the window between the first arrival and when

the signature was believed to drop back into the background

noise. Due to the complexity of the seismic signatures, the

method of analysis was conducted in a qualitative manner. In

some cases, this was difficult to observe so bandstop filters were

applied to the noisy waveforms in an attempt to evaluate the

signature further. In addition, in some cases reflections of the

pressure wave hitting the mine face or mine structures were

observed. However, these events were not included in the duration calculation, with only the seismic signature from the explosion being of interest. In cases where the explosions and

reflections were merged together and inseparable, the full duration was reported.

3. Results and discussion

Table 2 shows a summary of the attenuation factors for the

mine shots analyzed. The attenuation factors shown in Table 2

were calculated by the same method described earlier for mine

shot number 506. The maximum pressure from each explosion is

also given. No correlation could be made between the size of the

explosion or experimental design and the attenuation factors

observed, as seen in the scatter plot in Fig. 5. In this scatter plot,

the confined explosions are represented by a diamond shape and

the unconfined explosions are represented by a square shape. The

lowest attenuation factor was observed for mine shot number

509, the largest explosion based on the maximum pressure

generated. The highest attenuation factor was observed for mine

shot number 505, one of the largest explosions. The average

attenuation factor for the fourteen mine shots studied was
-0.20 dB/m with a standard deviation of 0.080 dB/m. For each

explosion, it appears that the amplitude of ground velocity

attenuates very quickly.

Table 2

Attenuation factors and size of the explosions for each mine shot during the study.

Size of

explosion

(MPa)

Size of

explosion

(psi)

Attenuation

factor

Mine shot #503 1.20E-01 17.4 -0.152

Mine shot #504 1.37E-01 19.9 -0.239

Mine shot #505 3.80E-01 55.1 -0.381

Mine shot #506 6.17E-01 89.5 -0.311

Mine shot #508 3.46E-01 50.1 -0.209

Mine shot #509 6.31E-01 91.5 -0.084

Mine shot #510 1.01E-01 14.6 -0.193

Mine shot #513 9.56E-02 13.9 -0.168

Mine shot #514 1.03E-01 15.0 -0.211

Mine shot #516 8.98E-02 13.0 -0.167

Mine shot #517 9.27E-02 13.5 -0.137

Mine shot #518 9.16E-02 13.3 -0.114

Mine shot #519 1.72E-01 24.9 -0.161

Mine shot #520 1.07E-01 15.6 -0.273

Average -0.200

Standard deviation 0.080

-0.4500 -0.4000 -0.3500 -0.3000 -0.2500 -0.2000 -0.1500 -0.1000 -0.0500 

0.0000 

Size of Explosion (psi) 

100.0 

90.0 

80.0 

70.0 

60.0 

50.0 

40.0 

30.0 

20.0 

10.0 

0.0 

Confined Explosions Unconfined Explosions 0.7 

0.6 

0.5 

0.4 

0.3 

0.2 

0.1 

0 

Size of Explosion (MPa) 

Attenuation Factor 

Fig. 5. Scatter plot showing no correlation between the attenuation factors and size of the explosion. The confined explosions and unconfined explosions are distinguished

from one another by different symbols.

One goal for the study was to collect the signatures on a

regional scale using the USGS seismic networks; however, signatures could not be identified for any of the explosions at regional

distances. The attenuation factors explain why the shots were not

observed on the Mt. Chateau regional seismic network, 10 miles

south of the mine. Geophone 13 was installed outside of the mine

to collect data for mine shot numbers 510–520; however, explosion vibrations were not sufficient to trigger the sensor. This

geophone was over 500 m away when the explosions were

ignited inside of A-Drift.

Although attempts were made to monitor the explosions from

greater distances, the signal attenuated into the background

noise. The geophones in the old workings were connected to the

system after mine shot number 506 and were over 500 m away

from the source. They were triggered on mine shot numbers 508

and 509, each of which generated pressures on the order of

0.35 MPa (50 psi). Although the largest explosions were able to

destroy mine seals, they were unable to be seismically observed

at the regional network due to the seismic wave attenuation.

Another fact to keep in mind is that these explosions are

largely decoupled from the limestone. The explosion was ignited

in an open space and the transfer of the energy occurred as the

expanding pressure wave interacted with the surrounding rock.

This is very different from other types of seismic sources such as

an earthquake or a roof fracturing in an underground mine where

the source is direct rock-on-rock contact. Even with a quarry

blast, the explosive fuel is packed tightly into the rock. Therefore,

the transfer of seismic energy from methane explosions into the

rock is significantly less than other mining-related mechanisms

that create seismic signatures. For a mine explosion to be

observed on a regional network, a large amount of seismic energy

(stemming from the explosion itself or subsequent roof falls/seal

destruction) would have to be transferred into the rock so that the

seismic signature does not attenuate into the background noise.

Based on the attenuation measurements from this study, for the

Lake Lynn Experimental Mine or similar circumstances where an

explosion is ignited in an open space inside of a volume similar to

the ignition chamber, an explosion significantly larger than

0.62 MPa (90 psi) would be required to generate enough seismic

energy for the signature to be observed at a regional seismic

network station 16 km (10 miles) away.

Duration trends were better observed for the seismic signatures when groups of geophones between 0 and 150 m and over

were analyzed separately. These trends are likely because at close

distances, the P-, S- and body waves are not as fully separated as

they are at further distances. Also, the complexity of the seismic

signatures provided another degree of difficulty in determining

the signal duration. To demonstrate, the LLEM supervisor indicated that it is possible that the pressure wave from the explosion

moved into each crosscut as it traveled down the drift, reflecting

within the crosscuts. It is possible that these reflections were

detected by the closest geophones to the explosion and not by the

geophones at a further distance.

Four mine shots conducted in C-Drift were selected to compare the durations of seismic signatures of methane and coal dust
explosions with and without the destruction of mine seals. These

experiments included two explosions in which a mine seal in the

path of the explosion failed (mine shot numbers 506 and 509),

one in which a mine seal was in the path of the explosion but it

did not fail (mine shot number 508), and one in which no

obstruction was in the path of the explosion (mine shot number

507). For geophones 1 and 2, located less than 150 m away from

the source in the C-Drift instrument room, data were not available

for mine shot number 508. The average durations are reported in

Table 3. The averages were taken using data from two geophones

on all three components, thus giving a total of six samples. Table 3

also indicates whether a seal was in the propagation path of the

pressure wave and if it was destroyed. As the table shows, when a

mine seal is in the path of a pressure wave, a longer duration is

recorded and the structure has a direct effect on the seismic

duration. The longer duration was attributed to the destruction of

the mine seal and impacts of the seal fragments against the mine

roof, rib and floor. The standard deviations indicate that the

differences in the average durations are statistically significant.

Representative signatures from the data reported in Table 3 are

plotted in Figs. 6 and 7.

Table 3

Average durations from geophones 1 and 2 for mine shot numbers 506–509.

Mine seal Average

duration (s)

Standard

deviation (s)

Number

of

samples

Mine shot

#506

Yes-destroyed 3.10 0.21 6

Mine shot

#507

No 1.86 0.25 6

Mine shot

#509

Yes-destroyed 3.95 0.23 6

8.00E+04 

Digital Counts

4.00E+04 

0.00E+00 

-4.00E+04 

-8.00E+04 

00:00:00.000 00:00:05.000 

Time (s) 

Fig. 6. Representative waveform from Geophone 2 for mine shot number 507. The

two lines indicate the interval which determined the duration. For this experiment, the explosion was able to move freely down C-Drift.

1.50E+05 

7.50E+04 

Digital Counts

0.00E+00 

-7.50E+04 

-1.50E+05 

00:00:00.000 00:00:05.000 

Time (s) 

Fig. 7. Representative waveform from geophone 2 for mine shot number 509. The

two lines indicate the interval which determined the duration. For this experiment, the explosion destroyed a seal in the C-Drift entry.

The signatures are for mine shot numbers

507 and 509 and are from geophone 2 located on the C-Drift roof.

The average durations for geophone numbers 3 and 5, located

over 150 m away from the source in D-Drift, are reported in

Table 4.

Table 4

Average durations from geophones 3 and 5 for mine shot numbers 505–509.

Mine seal Average

duration (s)

Standard

deviation (s)

Number

of samples

Mine shot

#506

Yesdestroyed3.56 0.13 6

Mine shot

#507

No 2.26 0.14 6

Mine shot

#508

Yes 4.22 0.14 6

Mine shot

#509

Yesdestroyed3.50 0.08 6

Similar observations as before can be made where a

longer duration is observed when a mine seal is in the path of the

explosion. The duration is observed to be longest when the

explosion is confined within the sealed area without destroying

the seal. In this scenario, the pressure pulse is reflected repeatedly

between the structure and mine face. Not enough data were

available from E-Drift (geophones 4 and 6) to calculate significant

averages.

For mine shot numbers 510–521, three geophones were

located within 150 m of the explosions, which were the closest

geophones to the source. These instruments were geophones 2,

4 and 6. Table 5 shows the average durations observed in the

seismic signatures from these geophones and their standard

deviation.

Table 5

Average durations from the closer geophones during mine shot numbers 510–519.

Mine

seal

Average

duration (s)

Standard

deviation (s)

Number of

samples

Mine shot

#510

No 1.92 0.17 3

Mine shot

#513

No 2.08 0.07 3

Mine shot

#514

No 1.48 0.39 9

Mine shot

#516

No 2.09 0.04 3

Mine shot

#517

No 1.89 0.23 9

Mine shot

#518

No 1.76 0.19 3

Mine shot

#519

No 3.83 0.95 6

The only outlier appears to be mine shot number 519,

which showed a duration approximately twice as long as the

other mine shots. This particular explosion was almost twice as

large as the other explosions in Table 5. Also, during this mine

shot number 519, the seals located in the final two crosscuts

between A- and B-Drifts failed (Table 1), which most likely was

captured in the seismic signatures. The seal in crosscut six was

cracked and some blocks were displaced, while the seal in crosscut seven was completely destroyed and some of the blocks were

blown into C-Drift. The displaced blocks were a potential source

of seismic energy as they contacted the mine roof, rib and floor,

likely causing a longer duration in the seismic signatures for mine

shot number 519.

Geophones located further away, specifically geophones 3 and

5 located in D-Drift, showed durations which were in the same
range, as reported in Table 6.

Table 6

Average durations from the far-away geophones during mine shot numbers 510–
 

519 and 521.

Mine seal Average

duration (s)

Standard

deviation (s)

Number

of samples

Mine shot #510 No 2.20 0.09 3

Mine shot #513 No 1.93 0.05 6

Mine shot #514 No 2.25 0.05 3

Mine shot #516 No 2.06 0.08 6

Mine shot #517 No 2.09 0.10 6

Mine shot #518 No 1.85 0.10 3

Mine shot #519 No 2.29 0.10 6

Mine shot #521 No 0.61 0.01 5

These geophones, which are further

beyond the near-field environment, are not affected by factors

such as reflections or crosscut seal interactions. Mine shot

number 521 showed the shortest duration of any methane and

coal dust explosion, with an average duration of 0.61 s. This

explosion was very weak in comparison to the others, as indicated

by its maximum pressure value of approximately 0.014 MPa

(2 psi) (Table 1).

Mine shot numbers 523 and 524 were the only experiments

conducted in A-Drift where the explosion was contained within a

sealed area. During these two experiments, seals were placed in

the crosscuts and a seal blocked the propagation path of the

explosion. In both cases, the seals were able to withstand the

pressure wave. Seismic data from inside of the mine were only

available from the geophones located far away from the source in

D-Drift. Compared to the durations observed for tests without

seals, the effect of the seal placement in the entry appears to have

a direct effect on the duration of the seismic signature (Table 7).

Table 7

Average durations for the closer geophones during mine shot numbers 523

and 524.

Mine

seal

Average

duration (s)

Standard

deviation (s)

Number of

samples

Mine shot

#520

No 3.72 0.25 9

Mine shot

#522

No 5.41 0.32 6

When Table 7 is compared with Table 6, the explosions are up to

and over one second longer in duration when contained within a

sealed area versus freely moving down the entry. This difference

in duration was observed for the same geophones when the tests

were conducted in C-Drift (Table 4). Again, it is believed that

when the explosion is contained within a sealed area, the

pressure pulse is reflected repeatedly between the seal structure

and mine face.

In order to validate the pressure pulse reflection between the

structure and the mine face, a seismometer was placed on the

ground surface above the A-Drift face for mine shot numbers 523

and 524. Again, during this shot, the explosion was ignited at the

A-Drift face and a seal was placed in the propagation path of the

explosion (however, the seal was not destroyed). The pressure

pulse reflection could be observed extremely well when a lowpass filter set at 15 Hz was applied to the signature from the

surface seismometer. The lowpass filtered waveform from the

surface seismometer for mine shot number 523 is plotted in

Fig. 8.

250000 

200000
 

150000 

100000 

Counts 

50000 

l Digita

0 

-50000

00:00:00.00000:00:05.00000:00:10.00000:00:15.00000:00:20.00000:00:25.00000:00:30.00000:00:35.000 

Time (s) 

-100000 

-150000 

-200000 

Fig. 8. Filtered waveform from the surface seismometer during mine shot

number 523.

During mine shot number 524, the same period could be

observed on the surface seismometer and could also be seen on

the signatures collected from inside the mine.

Fig. 8 shows that after the initial explosion, the signature

contains a repeatable period at 0.62 s on average and lasting over

15 s. This observation can be attributed to the pressure wave

being confined within the sealed area. As an example, Zipf et al.

considered the scenario of an explosion in a mine entry with both

ends closed and filled with an explosive methane–air concentration [14]. Initially, the slow deflagration stage involves a laminar

flame speed of approximately 3 m/s (10 ft/s). As the deflagration

accelerates, the turbulent flame speed increases to approximately

305 m/s (1000 ft/s). The pressure in the burned gas behind the

flame increases and an acoustic wave propagates to the speed of

sound. For this study, it is assumed that the pressure wave

traveling down the mine entry has a velocity comparable to the

speed of sound in air. The speed of sound in air is a function of

temperature, which for the mine, at 286 K (551 F), would be

approximately 335 m/s (1100 ft/s). The mine seal is located

approximately 115 m (375 ft) away from the source location

where the explosion ignites. If the pressure wave travels 230 m

(750 ft) (twice the distance of the mine face to the mine seal) at

335 m/s (1100 ft/s), it should come in contact with either the

mine seal or mine face approximately every 0.68 s. This value is

approximately the same as the period observed in Fig. 8, which is

on an average 0.62 s. This implies that the pressure wave was

bouncing back and forth within the sealed area, creating a seismic

signal either when the wave came into contact with the mine seal

or the mine face. However, it cannot be determined which of the

wave reflections is causing the seismic signature.

The final major observation was for mine shot numbers 520

and 522, where the explosion was allowed to enter different parts

of the mine due to open crosscuts between A- and B-Drifts. This

allowed the explosive pressure wave to possibly enter into B- and

C-Drifts. An example of the possible path of the explosion is

shown in Fig. 9, where the closest geophones are indicated by

green dots. An example of a signature from the closer geophones

from this type of geometry is shown in Fig. 10. The experimental

design causes a more complex scenario than the previous experiments. The signatures observed from the closer geophones,

located in the C-Drift instrument room and E-Drift, appeared to

be affected by interactions of the pressure wave entering into the

B- and C-Drifts. Table 8 indicates the average duration of the

signatures observed from these geophones for the two mine
shots. These durations are significantly longer than those that

were previously observed. The seismic signature of the methane

and coal dust explosion could not be differentiated from the

seismicity of the pressure wave interacting with the ribs and mine

structures as it entered into B- and C-Drifts, as seen in the

example of the signature complexity in Fig. 10. Not enough data

are available to compile significant findings for the geophones

further down D-Drift.

Fig. 9. Schematic of the experimental design for mine shot numbers 520 and 522. The design indicates that the propagation path of the pressure wave was allowed to

enter into B- and C-Drifts. The green dots indicate the geophone locations within the bounds of the figure. (For interpretation of the references to color in this figure legend,

the reader is referred to the web version of this article.) Amplitude (Digital Counts) 

1.00E+04 

8.00E+03 

6.00E+03 

4.00E+03 

2.00E+03 

0.00E+00 

-2.00E+03 

-4.00E+03 

-6.00E+03 

-8.00E+03 

-1.00E+04 

-1.20E+04 

00:0000:010 0:020 0:030 0:040 0:050 0:060 0:070 0:08 

Time (s) 

Fig. 10. Example of a seismic signature collected during mine shot number 522 by

geophone 2. The signature appears to contain multiple reflections as a result of the

pressure wave from the methane and dust explosion entering into B- and C-Drifts.

Table 8

Average durations for the closer geophones during mine shot numbers 520

and 522.

Mine seal Average

duration (s)

Standard

deviation (s)

Number

of samples

Mine shot

#523

Yes 3.62 0.26 6

Mine shot

#524

Yes 3.08 0.03 6

4. Conclusions

Seismic signatures emanating from controlled methane and

coal dust explosions at the Lake Lynn Experimental Mine were

studied. The objective was to determine if the attenuation and

duration measurements could give insight into future seismic

signatures measured on a regional seismic network station.

A total of nineteen explosions were monitored between the

different stages of this study. No correlation could be made

between the size of the explosions and attenuation factors for

the different mine shots. The average attenuation factor for all the

mine shots was -0.20 dB/m. The explosions were monitored by

seismic instruments locally at distances of 500 m (1640 ft) and

through a regional seismic network station 16 km (10 miles)

away, but no signatures could be observed at those distances. The

measured attenuation factor explained why the signatures were

not observed at far-away distances. The methane and coal dust

explosions were ignited in an open space and the transfer of the

energy occurred as the expanding pressure wave interacted with

the surrounding rock. This situation is very different from other

types of seismic sources such as an earthquake or a roof fracturing

in an underground mine, where the source is direct rock-on-rock

contact. Therefore, the transfer of energy from methane explosions into the rock is significantly less than other mining-related

mechanisms that create seismic energy. For the Lake Lynn

Experimental Mine, an explosion significantly larger than

0.62 MPa (90 psi) would have to be conducted in order to observe

the seismic signature on a regional scale.

The duration of the seismic signatures was shortest for explosions with no seals in the path of the pressure wave and longest

for those where a seal blocked the propagation path of the

explosive pressure wave. For the experiments where the explosion was confined within the sealed area, a geophone on the

surface of the mine observed signal durations for up to 15 s. The

containment geometry also played a role in the duration of the

seismic signatures. When the explosion was contained within a

sealed area, the seismic signatures oscillated with a period

corresponding to the pressure wave reflecting between the mine

seal and mine face. The signatures were also observed to be very

complex when crosscuts were opened and the explosive pressure

wave entered into different areas of the mine, causing reflections

between drifts and crosscuts.

The preliminary results identify the potential of seismic data

to be useful in forensic studies of mine explosions such as the

Sago Mine disaster. The results show that the introduction of a

mine seal had a direct effect on the seismic signature, causing a

longer duration, potentially due to the destruction of the seal.

Also, there were different characteristics observed in the seismic

signatures when comparing experiments where the seal was

destroyed and when the seal contained the explosion in a

tunnel-like environment. Finally, the signature was most complex

when there were no seals and the explosion moved freely

between the crosscuts and drifts.
Currently, it is very difficult to separate the seismic signature

into different events such as the explosion itself, reflections of the

pressure wave within the crosscuts, and the destruction of the

seal. Further research, including the use of more geophones on

the surface above the mine or placing geophones in a borehole,

could minimize the complexity within the signatures. This would

also give more opportunity to characterize the effect of a mine

explosion or destruction of a mine seal on a seismic signature.

Acknowledgments

The methane and coal dust explosions were designed and

conducted by K.L. Cashdollar, E.S. Weiss, and/or S.P. Harteis as

part of the mine explosion program for the NIOSH Office of Mine

Safety and Health’s then Disaster Prevention and Response

Branch. The authors would also like to thank Dr. Martin Chapman

of the Virginia Tech Department of Geosciences and Dr. Peter

Swanson of NIOSH for their contributions during the study.

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