Relationship between radiated seismic energy and explosive pressure

for controlled methane and coal dust explosions in an underground mine

Michael M. Murphy a,⇑,

⇑ Corresponding author. Tel.: +1 412 386 4172; fax: +1 412 386 6891.

E-mail addresses: mmu5@cdc.gov (M.M. Murphy), ewestman@vt.edu (E.C. Westman),

ati2@pitt.edu (A. Iannacchione), thb0@cdc.gov (T.M. Barczak).

Thomas M. Barczak a

aOffice of Mine Safety and Health Research/Ground Control Branch, PO Box 18070, 626 Cochrans Mill Rd., Pittsburgh, PA 15236, United States

Erik C. Westman b,

bDepartment of Mining & Minerals Engineering, Virginia Polytechnic Institute & State University, 100 Holden Hall, Blacksburg, VA 24061-0239, United States

Anthony Iannacchione c,

cDepartment of Civil and Environmental Engineering, Swanson School of Engineering, University of Pittsburgh, 3700 O’Hara Street, 1130 Benedum Hall, Pittsburgh,

PA 15261, United States

abstract

Examination of seismic records during the time interval of the Sago Mine disaster in 2006 revealed a

small amplitude signal possibly associated with an event in the mine. Although the epicenter of the signature was located in the vicinity where the explosion occurred, it could not be unequivocally attributed

to the explosion. A greater understanding about the seismicity from mine explosions is required in order

to properly interpret critical seismic information. A seismic monitoring system located at NIOSH’s Lake

Lynn Experimental Mine has monitored 16 experimental methane and coal dust-based explosions. This

paper describes the research conducted to quantify a relationship between measured values of radiated

seismic energy and peak explosive pressure generated. The radiated seismic energy takes into account

seismic signature characteristics such as the frequency content, amplitude, and duration. On the other

hand, the size of the explosion is a function of the experimental design, dependent on factors such as

the presence of an explosion-containment structure, the mine geometry, and the amount of initial explosive fuel used during the explosion. The seismic signatures from methane and coal dust explosions were

analyzed using standard waveform analysis procedures. The procedures used to estimate the radiated

seismic energy were conducted using self-produced programs, which are explained in this paper. The

radiated seismic energy estimates were considered to be relative values for each experiment. A relationship was derived to correlate the relative radiated seismic energy to the size of the explosion, defined as

the peak pressure generated. It was also observed during this study that an explosion-containment structure can act as a major seismic source. Recommendations are made, based upon the findings of this study,

for improved collection of seismic data in the future.

1. Introduction

1.1. Statement of the problem

Seismic monitoring provides a powerful means for detection

and evaluation of events resulting from mining activity. Seismic

monitoring assessments in the mining community can be made

from a seismic network installed locally at the mine or records obtained from stations devoted to earthquake studies which are continuously recording, such as the US Geological Survey stations.

Seismic signature characteristics such as arrival times, amplitudes,

duration, and frequency content can help indicate the nature and

location of the source. In the past, most mining-related seismic

assessments have focused on events such as production blasts

from quarries, roof falls, and rock fractures (Iannacchione et al.,

2005a,b; Swanson et al., 2002). However, little or no effort has

been put towards examining the characteristics of a signature

emanating from a methane and coal dust explosion in an underground mine.

The Sago Mine disaster was a coal mine explosion that occurred

in 2006 in Sago, West Virginia (Gates et al., 2007). During the time

interval of the Sago Mine disaster, a small amplitude signal was

identified on records of the closest regional seismic network stations (Chapman, 2006). 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 disaster

provides an example of why these particular signatures should

be researched. This paper presents a portion of the findings from

a study aimed at examining seismicity from 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

(NIOSH) 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 (Cashdollar

et al., 2006; Sapko et al., 2000). This facility provided 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 methane and coal dust explosions.

The objective of the study was to analyze seismic signatures collected by the seismic monitoring system to begin to understand

their characteristics at different distances away from the source.

For this paper, the relationship between the size of the explosion

and the amount of seismic energy estimated from the explosion

is examined. Prior to estimating the energy of the explosion, the

seismic monitoring system was calibrated. The purpose of the calibration is to determine specific constants that were unknown at

the time and were needed for the energy estimation. The results

from the calibrations lead to the development of a spreadsheet

which served as the program to calculate the seismic energy from

the data.

Throughout the world there are a great number of seismic monitoring stations that have the ability to capture the characteristic

ground vibrations caused by methane and coal dust explosions,

however finding them in seismic records is very difficult if the time

of the event is unknown. Analysis of these types of seismic records

could not be found in other literature, and results provided in this

study are hoped to take a first step in understanding these events

when captured in the far-field. The accessibility of the Lake Lynn

Experimental Mine allowed for the unique opportunity to monitor

methane and coal dust explosions in an underground environment; however, the control of the experimental designs was not

made available. The presence of explosion-containment structures

and the amount of initial methane and coal dust to fuel the explosion used during the experiments were based on research needs

related to mine seal design and was not controlled for the purposes

of the study. Ideally, experiments with replicated designs would be

monitored; however, since the facility was occupied with many

other research projects at the time of the study, having the experimental setups adjusted specifically for this study was not possible.

Thus, this study was considered to be an exploratory project where

any finding or observation made from the seismic signatures

would be considered beneficial.

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

and underground mine. The facility is located approximately

96 km (60 miles) southeast of Pittsburgh, Pennsylvania, and

16 km (10 miles) northeast of Morgantown, West Virginia. 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 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 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 three-quarters of the way down C- and D-Drifts.

These rooms are protected from the entry via blast-proof doors.

The mine layout, including both uniaxial and triaxial geophone

locations used for this experiment, is shown in Fig. 1.

The methane and coal dust explosions conducted at the Lake

Lynn Experimental Mine were ignited at the face of either A- or

C-Drift. The ignition chambers are highlighted in Fig. 1. The chosen

locations of the geophones were distributed in the mine around

the explosion test area at various distances. 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. Not all geophones

were present for each test monitored at LLEM. The geophone placement was 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.

Geologically, the mine is located in the Greenbrier limestone

formation (Triebsch and Sapko, 1990). 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 borehole

data were obtained from 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 roof to 4.3 m (14 ft)

and from 6.1 and 9.1 m (20 and 30 ft) is a fine-grained limestone.

A shale/claystone formation is located between the limestone

layers at 4.3 and 6.1 m (14 and 20 ft). The layers in the vertical

borehole geology can be seen in Fig. 2.

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 Cashdollar et al. (2007) and Gates et al.

(2007). Mine shot numbers 508–509 are referenced in Weiss and Harteis (2008a). Mine

shot numbers 513, 514, and 516–19 are referenced in Weiss et al. (2008b). Mine shot

numbers 523 and 524 are referenced in Millero (2008). Mine shot numbers 520–522 are

not yet published; however,the work was completed by Ken Cashdollar, EricWeiss, and

Sam Harteis as part of the mine explosion program for 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 ( 98% methane) injected into an ignition chamber at the face

of the drift. Fig. 3 is a photograph of one of the ignition chambers

at LLEM. A plastic diaphragm is draped across the entry to contain

the methane in the ignition chamber. A photograph of a partially

closed plastic diaphragm is shown in Fig. 4. 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 (Weiss

and Harteis, 2008a).

The flammable natural gas–air volume was ignited using a

triple-point ignition source. This ignition source consisted of three

sets of two 100-J electric matches that were equally spaced at midheight across the closed end of the drift and ignited at the same

time. 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 (Weiss and Harteis, 2008a). A photograph of explosive
coal dust and rock dust being suspended from the mine roof and

placed on the floor is shown in Fig. 5.

Fig. 1. Layout of the Lake Lynn Experimental Mine with geophone location, instrument room locations and ignition chamber locations highlighted.

End of hole 29”9’ 

25 ft 

20 ft 

15 ft 

10 ft 

Fracture – 6’3”, open 1” 

5 ft 

0 ft 

Mine roof 

Fig. 2. Vertical borehole geology of the old workings at the Lake Lynn Experimental

Mine. The layers detected in the old workings were considered to be consistent with

the geology of the drift area.

Fracture/Joint – 26’2” 

LIMESTONE 

Fracture 24’ 

(around hole) 

SHALE/CLAYSTONE 

14 20 ft 14– – . 

Vertical fracture 

LIMESTONE 

Fig. 3. Ignition chamber located at the end of C-Drift. (Figs. 3–5 were provided by

Eric Weiss of NIOSH, Office of Mine Safety and Health Research’s then Disaster

Prevention and Response Branch.)

Fig. 4. Plastic diaphragm used to contain methane within the ignition chamber.

For this study, the size of the explosion is defined as the peak

pressure generated during the experiment. Pressure measurements

were taken from the drift area in the mine. The data were sampled

at 1500 samples per second and were obtained from the staff at the

Lake Lynn Experimental Mine. The pressure measurements were

not on the same time scale as the seismic measurements. An exam­
ple of a pressure–time curve is plotted in Fig. 6. The multiple curves

represent the different instruments located throughout the drift

area in the mine. The peak pressure generated for the example in

Fig. 6 is around 55 psi (0.38 MPa), which would be considered as

the size of that particular explosion. A total of 16 explosions were

analyzed for this paper. In some cases, an explosion containment

seal was in the propagation path of the explosion. Fig. 1 indicates

the location of the explosion containment seal 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.

Fig. 5. Example of coal and rock dust being suspended from the mine roof and

placed on the mine floor.

Pressure (psi) 55 

50 

45 

40 

35 

30 

25 

20 

15 

10 

5 

0 

-5 

Peak pressure generated 

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 

0.38 

0.33 

0.28 

0.23 

0.18 

0.13 

0.08 

0.03 

-0.02 

-0.07 

Pressure (MPa) 

Time (sec) 

Fig. 6. Example of pressure–time curves used to obtain the size of the explosion, as defined by the peak pressure generated.

2.3. Seismic transducers

Geophones were chosen as the seismic transducer to record seismic waves emanating from the methane and coal dust explosions

and were placed in locations to surround the area of these explosions. All geophones were located on the mine roof except for

one, which was located in the C-Drift instrument room floor (geophone #1 in Fig. 1). 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 the response was constant but that

spurious responses could occur.

Surface measurements 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 on the surface above the A-Drift face shortly before mine shot number 523.

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.

2.4. Seismic monitoring system

The seismic monitoring system was installed within the existing structure of the Lake Lynn Experimental Mine. 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. Throughout the project, the seismic system utilized four MS boxes and four

QS boxes. Due to licensing agreements with the manufacturer, a

maximum of 10 triaxial or 30 uniaxial geophones, or a combination

of both, could be connected to the seismic system. 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.

2.5. System calibration

In order to estimate the radiated seismic energy, the seismic system needed to be calibrated to determine the total system response

to ground velocity. In some instances this step would be unnecessary—for example if the manufacturer’s commercial software were

used—however, the more manually based ‘‘step-by-step’’ analysis

of the signatures was necessary for the study. With the manufacturer’s existing commercial software, certain assumptions made

during the seismic analysis in background processes would have

been unknown, and it was determined that better observations

and conclusions would result if all the assumptions made during

the calculations were identified. Therefore, spreadsheets using

visual basic code were created by the authors and other NIOSH

engineers were used to analyze all the seismic signatures.

The calibration of the seismic system helped to measure digital

counts for a 1-V input signal as a function of frequency. This measurement, called the digitizer constant C, was used to determine
total system response to ground velocity. In turn, the total system

response to ground velocity helped determine the radiated seismic

energy. To obtain the digitizer constant, the peak-to-peak amplitude of a 2-V signal was generated by a signal generator and measured by the seismic system at different frequencies. The signal

generator was checked by an oscilloscope to maintain a constant

2-V signal and a multimeter was used to validate each frequency.

The response to each calibration frequency, between 50 and

1000 Hz at 50-Hz intervals, was digitized by both the MS and QS

boxes and then plotted. The results and calibration points of each

specific frequency are shown in Fig. 7.

1.00E+07 

CQS 

CMS 

1.00E+06 

Digital Counts per Volt 1.00E+05 

QS Box Curve MS Box Curve 

1.00E+04 

10 100 1000 

Frequency 

Fig. 7. Calibration results for both MS and QS boxes.

The digitizer constant is where each curve intersects the y-axis.

The values of the digitizer constant were 1.54e106 counts/V for the

MS box and 3.85e106 counts/V for the QS box. The responses of the

anti-aliasing filters for each digitizer are also observed in Fig. 7.

The QS box had a constant response at both low and high frequencies. The sudden change in high-frequency response of the curve is

due to the anti-aliasing filter near the Nyquist frequency of

1000 Hz. This response was to be expected since the A/D converter

collects 2000 samples per second.

The results for the MS box show a constant response at low frequencies up to approximately 100 Hz. The loss in amplitude at

higher frequencies is due to an anti-aliasing low-pass filter built

into the MS box. The amplitude response at higher frequencies

can be modeled based upon the calibration response. The amplitude response of the low-pass filter, defined as G(x), was derived

and is shown in the following equation:

1 GðxÞ 1⁄4 3 x

ð1Þ

1 þ xc

( )

where x = 2pf, f is the frequency in Hz, and xc = 2pfcorner, fcorner is

the frequency in Hz.

The corner frequency of the filter was found to be around

250 Hz. The resulting filter response is plotted in Fig. 8.

0.01 

0.1 

1 

Response Value

10 100 1000 

Frequency 

Fig. 8. Filter response, G(x), for MS box.

The filter

response is constant up to approximately 100 Hz, then changes

as a result of the anti-aliasing filter inside the MS box.

Once the number of digital counts for a 1-V signal had been

identified for each digitizer, the system amplitude response to

velocity was derived based upon the previous calibration results

and specification sheets of the geophones provided by the manufacturer. The instrument response of the geophone was derived

using a single degree of freedom damped spring-mass system

model (Aki and Richards, 2002) and the Sercel geophone product

sheet (Sercel, 2005). The system amplitude response to ground

velocity, defined as v(x), is shown in Eq. (2) and is plotted in Fig. 9.

v(ω) - Response to Velocity

(Digital Counts / m/s) 1.00E+09 

QS Box MS Box 

1.00E+07 

1.00E+05 

0.1 1 10 100 1000 

Frequency (Hz) 

Fig. 9. Complete system amplitude response to ground velocity, V(x). The natural

frequency of the geophone is highlighted by the dotted line at 4.5 Hz.

0 ( ) 1

ECx2

R

R

t

s

B C vðxÞ 1⁄4 @qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiA

GðxÞ ð2Þ

 2 x2

o x2 þ ð2hxoxÞ

2 

where E is the geophone electrodynamic constant (V/m/s), C the

digitizer constant, determined from a sinusoidal input test (digital

counts/V), x = 2pf (frequency in Hz), Rs the shunt resistance for a given damping (X), Rt the total circuit damping (X), xo = 2pfnatural

(natural frequency in Hz), and h is the geophone damping.

The asymptotic slope below the natural frequency has a value of

2 and a value of 0 after the natural frequency, agreeing with the

expected amplitude frequency response to velocity for a geophone

(Havskov and Alguacil, 2006). For the MS box, the response

changes near 100 Hz due to the anti-aliasing filter.

The geophone cables used during the calibration process were

not the specific cables used in the mining environment during

the study. However, the cables used during the calibration and in

the mine were of the same type. The length of geophone cable used

in the mine was well under the limits of the manufacturer’s

recommendation, so it is assumed that the cables in the mining

environment had no adverse effect on the seismic signatures.

2.6. Calculation of radiated seismic energy

The radiated seismic energy, Es, is expressed in units of joules

and is defined in Eq. (3) (Boatwright and Fletcher, 1984).

E 1⁄4 4pqcR2 s ðI1 þ I2 þ I3Þ ð3Þ

where q is the density of the rock at the source (kg/m3), R the distance from source to receiver (m), c the P- or S-wave velocity (m/s),

and I is the integral of squared velocity for a geophone orientation

(m2

/s2/Hz).

The radiated seismic energy relationship used in this study is a

simplified version from the equation derived in Boatwright and

Fletcher (1984). Rock drop tests were conducted in the mine to
obtain values for the P- and S-wave velocities. The P- and S-wave

velocities were calculated to be 4511 m/s (14,800 ft/s) and

2604 m/s (8545 ft/s). These values are in the same velocity range

as determined for another limestone formation in Pennsylvania

(Iannacchione et al., 2005a).

For this study, I was calculated using the Fourier amplitude

spectrum of ground velocity, which is a similar method from

Iannacchione et al. (2005a). The Fourier transformation of a seismic

signature was defined as S(x). This dataset can be divided by the

instrument correction v(x), defined earlier in Eq. (2). The result

is V(x), which is the Fourier amplitude spectrum of ground

velocity in units of m/s/Hz, as shown in the following equation:

ð Þ

SðxÞ VðxÞ 1⁄4 ð Þ 4

v x

An example of the Fourier amplitude spectrum of ground velocity is shown in Fig. 10.

 

 

 

Velocity (m/s/hz) 1.00E-01 

1.00E-03 

1.00E-05 

1.00E-07 

0.1 1 10 100 1000

Frequency 

Fig. 10. Example of the Fourier amplitude spectrum of ground velocity, V(x). The

blue line represents the signature from the event and the pink line represents the

background noise.

The blue2

2 For interpretation of color in Figs. 7, 9, 10, the reader is referred to the web

version of this article.

line represents the amplitude

spectrum of ground velocity for the explosion. The pink line represents the amplitude spectrum of velocity for the background noise.

This was calculated from a portion of the seismic signature prior

to the explosion. On the left side of Fig. 10, the two curves are

approximately parallel and of similar value. Between approximately

10 and 300 Hz, they are separated, and on the right side of the figure

they are completely merged. The separated area between signal and

noise curves is squared and represents the integral of squared velocity needed for calculating the radiated seismic energy. When the signals are merged, it means that the signal-to-noise ratio is very low

and thus the energy from that specific frequency has approximately

the same amount of energy in both the signal and background noise.

A signal-to-noise ratio criterion of at least 3 was taken into consideration when calculating the integral of squared velocity, with a similar criterion used in Iannacchione et al. (2005a). A program was

coded to calculate the integral of squared velocity I. The program

squared the value of V(x) for both the signal and noise curves, then

substracted V(x) of the noise from the signal at each frequency. The

results were summed up and the summation represented I. This

summation also represents the area found between the signal and

noise curves in Fig. 10. The parameters for I1, I2, and I3 represent

the three axis on the triaxial geophone.

3. Results

The calibration of the seismic system helped develop the formulas and methods used for calculating the radiated seismic energy

estimates for the mine shots. For this study, a total of 16 explosions

were monitored. Mine shot numbers 506–509 were conducted in

the C-Drift entry (Fig. 1). Mine shot numbers 510–524 were conducted in the A-Drift entry (Fig. 1). Five tests were conducted

where an explosion-containment seal was in the path of the explosive wave: mine shot numbers 506, 508, 509, 523 and 524. For two

of these five tests—mine shot numbers 506 and 509—the explosion

caused the seal to fail. A tabulated summary of the size of the

explosion, the initial explosive energy, the radiated seismic energy,

and the percentage of the initial energy for mine shot numbers

506–524 can be viewed in table 1.

Table 1

Tabulated summary showing the size of the methane and coal dust explosion, initial

explosive energy of the methane and coal dust explosion, radiated seismic energy

calculated from the data, and the percentage of the original energy found in the

seismic energy.

Pressure

(MPa)

Initial

energy (MJ)

Radiated seismic

energy (MJ)

% of initial

energy

Mine shot

#506

0.617 799 26.60 3.33

Mine shot

#507

0.147 710 14.41 2.03

Mine shot

#508

0.346 710 9.59 1.35

Mine shot

#509

0.631 1525 24.58 1.61

Mine shot

#510

1.101 511 0.29 0.06

Mine shot

#513

0.096 511 0.12 0.02

Mine shot

#514

0.103 511 0.68 0.13

Mine shot

#516

0.090 511 0.17 0.03

Mine shot

#517

0.093 511 1.00 0.19

Mine shot

#518

0.092 511 0.07 0.01

Mine shot

#519

0.172 1085 2.17 0.20

Mine shot

#520

0.107 511 1.72 0.34

Mine shot

#521

0.011 511 0.0029 0.00057

Mine shot

#522

0.109 511 1.72 0.34

Mine shot

#523

0.309 638 11.54 1.81

Mine shot

#524

0.552 1085 18.86 1.74

The method to estimate the size of the explosion in relation to

pressure is explained in Section 2.2. The estimates of the initial

explosive energy are based upon the amount of coal dust and

methane gas reported from the mine shot summaries obtained

from the Lake Lynn Experimental Mine staff. The combustion properties of methane were needed to provide a factor to relate the volume of methane to an energy value. For methane, the heat of

combustion was found to be 50 MJ/kg (Gexcon, 2006). Consultation with experienced engineers at NIOSH, who have extensively

studied methane explosions, verified that the same factor could

be used to estimate the amount of energy contained within the

coal dust. For some of the mine shots, the coal dust was mixed with

rock dust. In these instances, the amount of coal dust was not considered in the initial explosive energy estimation because the rock

dust concentration considerably weakened the explosion.

The size of the explosion was not exclusively based on the

amount of initial methane and coal dust. Containment within a

sealed area or the placement of electrical matches in relation to

the mine face in the ignition zone could cause larger peak pressures. The radiated seismic energy estimations are considered to

be relative values between the different experiments. It is possible
that due to the proximity of the geophones to the explosion, the

radiated seismic energy values were calculated from P-waves, Swaves, and surface waves, since these waves had little time to separate in the waveform. If the seismic energy had been measured

from a borehole further away from the explosion, a more absolute

value of energy could have been calculated.

4. Discussion

4.1. Estimated energy as a percentage of the original energy

The amount of seismic energy from an explosion is typically less

than 1% of the original energy of the source (Perret, 1968). The final

column in Table 1 shows the percentage of the initial seismic energy of the methane and coal dust explosions from the initial

explosive energy. In all cases where the percentage of initial explosive energy was less than 1%, the explosion did not have an

obstruction in its path as it propagated down the drift. The degree

of decoupling of the methane explosion to the limestone is believed to be a significant factor in the transfer of seismic energy

into the rock. The methane explosion is ignited in an open space

and the transfer of the energy occurs as the expanding pressure

wave interacts 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 rockon-rock contact. Because the pressure wave is not coupled well

with the surrounding rock, the transfer of seismic energy from

methane explosions into the rock is significantly less than other

mining-related mechanisms that create seismic energy.

In five of the six cases studied where the percentage of initial

explosive energy was greater than 1%, the explosion was either

contained within a sealed area or the explosion caused a seal failure. This suggests that an explosion contained within a sealed area

or the destruction of a seal by an explosion can result in a large

transfer of seismic energy. For example, mine shot numbers 519

and 524 each contained the same amount of initial explosive energy; however, the containment of the explosion within a sealed

area for mine shot number 524 shows a much larger peak pressure

generated and a greater radiated seismic energy estimate. The

source of additional seismic energy is believed to originate from

the explosive wave coming into contact with the explosion-containment structure. The explosion-containment structures are

keyed directly into the ribs of the mine entry, which explains the

source of the additional seismic energy.

In the sixth case, involving mine shot number 507, there was no

seal in the path of the explosion; however, the initial energy was

over 1%. For this particular shot, a geophone close to the source

could have been affected by the air blast. Geophone #1, on the cement floor in the C-Drift instrument room, recorded eight times

more energy than geophone #2, which was in the same instrument

room but on the roof further away from the explosion. Geophone

#1 was approximately 1.2 m (4 ft) away from the acoustic air blast,

while geophone #2 was approximately 4.6 m (15 ft) away from the

air blast. The data from geophone #1 were most likely affected by

the air blast passing by, and this caused a large energy estimate for

mine shot number 507.

4.2. Relationship between the radiated seismic energy and size of the

explosion

Different methods of analyzing seismic data include studying

the amplitude, duration, and frequency content of a particular seismic signature. The radiated seismic energy, as calculated by Eq. (3),

takes these parameters into account by considering the integral of

squared velocity. The radiated seismic energy equation is also

derived from physical characteristics of the environment such as

the medium density and the seismic wave velocity. Since this single

equation takes into account the significant parameters observed

during this study, a relationship between radiated seismic energy

and size of the explosion was derived. The size of the explosion, defined as the peak pressure generated, is characterized by experimental setup parameters such as the mine geometry, presence of

an explosion-containment seal, and the amount of explosive methane and coal dust used. Previous seismic studies have found a

power relationship between the size of underground blasts and

seismic parameters (Willis and Wilson, 1960; Jalinoos and White,

1986). Therefore, for this study a power relationship between the

radiated seismic energy and peak pressure generated was assumed.

Fig. 11 shows the correlation between the radiated seismic energy and size of the explosion. The correlation coefficient for this

trend is 0.90. In order to estimate the size of the explosion based

upon an estimated radiated seismic energy, the following relationship in Eq. (5) was derived from the trend in Fig. 11.

y = 96.346x2.3284 

0 

5 

10 

15 

20 

25 

30 

35 

Radiated Seismic Energy (MJ) 

R2 = 0.898 

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 

Peak Pressure Generated from the Explosions (MPa) 

Fig. 11. Power relationship of the radiated seismic energy versus size of the

explosion for all the mine shots observed.

 E 1

2:33

P 1⁄4 s ð5 96:35 Þ

where P is the peak pressure of the explosion (MPa), and Es is the

radiated seismic energy (MJ).

A more detailed relationship of the estimated energy and the

size of the explosion, which integrates both Eqs. (3) and (5), is reported in the following equation:

P 1⁄4 4pqcR2

ðI1 þ I2 þ I3Þ

96:35 ! 1

2:33

ð6Þ

where P is the size of the explosion (MPa), q the density of the rock

at the source (kg/m3

), R the distance from source to receiver (m), c

the P- or S-wave velocity (m/s), and I is the integral of squared

velocity for a geophone orientation (m2

/s2/Hz).

Two important considerations to take into account about Eq. (6)

are that the relationship is based upon relative radiated seismic energy estimations (as opposed to absolute values) and the scaling

differences between the Lake Lynn Experimental Mine and an actual active mine. The radiated seismic energy estimations are relative because there are some unknowns in the data that are due to

the placement of the geophones. For example some geophones

were so close that a full distinction between P-, S- and surface

waves could not be made because they had not had enough time

to separate. Also, because of all the crosscuts in the drift area of

LLEM, the seismic waveforms might have included reflections of
the pressure wave in the crosscuts. Therefore, while the radiated

seismic energy estimations might not be the exact value, they

can be related to one another based upon the size of the explosion.

There is a vast volumetric difference between the drifts at LLEM

versus a sealed off area in a mine. A 0.14-MPa (20-psi) explosion

occurring at an underground mine would contain more energy

compared to a 0.14-MPa (20-psi) explosion occurring at the

test facility where the measurements were made. To generate a

0.14-MPa (20-psi) explosion within a larger volumetric area, more

energy needs to be involved in creating that event.

5. Conclusions and recommendations

Seismic signatures emanating from controlled methane and

coal dust explosions at the Lake Lynn Experimental Mine were

studied. The objective was to analyze these signatures and to

determine if the size of the explosion could be estimated from

the radiated seismic energy recorded by the geophones. For this

paper, a total of 16 explosions were analyzed. The explosions were

experimentally different from each other in terms of the placement

of explosion-containment seals and blast pressures generated by

the methane and coal dust creating the explosion. These differences resulted in explosions contained within sealed areas, explosions freely propagating down a mine drift, and varying amounts of

coal and/or rock dust used to fuel or weaken the explosions. A seismic system was installed in the mine and geophones were

mounted on the mine roof or floor. Calibration of the seismic system was performed to estimate the radiated seismic energy from

the explosions through a spreadsheet using visual basic code.

When the explosive wave was allowed to freely move down the

mine drift, less than a percent of the original energy was transferred as seismic energy into the rock. The methane explosion

was ignited in an open space and the transfer of the energy occurred as the expanding pressure wave interacted with the surrounding rock. Therefore, the transfer of seismic energy from

methane explosions into the rock was significantly less than other

mining-related mechanisms that create seismic energy. When

there was an explosion-containment seal included in the experimental design, the transfer of seismic energy increased significantly. The seals were keyed directly into the ribs of the mine

entry and were found to efficiently transfer seismic energy. The

radiated seismic energy estimates calculated for each experiment

were considered to be relative values. A power relationship was

determined which related the radiated seismic energy to the size

of the explosion, as defined by the peak pressure generated. The

radiated seismic energy estimate took into account the seismic signature characteristics for each experiment while the size of the

explosion was a function of the experimental design. The factors

that most affected the seismic signatures were the mine geometry,

the size of the methane and coal dust explosion, and the location of

the explosion-containment structure.

During this study, the geophones were only anchored onto the

surface of the mine roof or floor. A future consideration is having

the geophones properly grouted in a borehole. This would allow

for more accurate radiated seismic energy assessments since a better observation of P- and S-waves could be made. Another consideration involves having more control over the experimental variables.

It is recommended for future research to simplify the experimental

design so that unobstructed methane explosions of different sizes

could be examined, independently of seal structures and crosscuts.

Once this effort would be completed, a series of controlled explosions at varying sizes, using a controlled explosion-containment

structure, could be evaluated. Following this, additional studies

could be completed with explosion-containment structures that

are structurally different and/or crosscuts could be introduced into

the experimental design to examine the effect in the seismic

signatures.

The research conducted in this study simply presents the first

step in collecting seismic signatures from underground explosions,

and the preliminary results identify the potential for seismic data

to be used in forensic studies of mine explosions such as the Sago

Mine disaster. It is believed that this research shows there is a relationship between the size of an underground explosion and the

radiated seismic energy. If more controlled experiments are conducted and a scaling factor between the Lake Lynn Experimental

Mine and an operating coal mine is found, it is possible that seismic records from seismic monitoring stations can estimate the

pressure generated by an underground coal mine explosion.

Acknowledgements

The methane and coal dust explosions were designed and conducted by K.L. Cashdollar, E.S. Weiss, and S.P. Harteis as part of the

mine explosion program for the NIOSH Pittsburgh Research Laboratory’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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