ANALYSIS OF SEISMIC SIGNATURES FROM GAS AND DUST BASED EXPLOSIONS 

AT THE LAKE LYNN EXPERIMENTAL MINE 

M. M. Murphy, NIOSH, Pittsburgh, PA 

A. T. Iannacchione, NIOSH, Pittsburgh, PA

E. C. Westman, Virginia Polytechnic and State Univ., Blacksburg, VA

M. C. Chapman, Virginia Polytechnic and State Univ., Blacksburg, VA 

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. More needs to be understood 

about the seismicity from mine explosions in order to properly interpret 

critical seismic information. A seismic monitoring system located at 

NIOSH’s Lake Lynn Experimental Mine has monitored experimental 

gas and dust based explosions and impact tests. The seismic 

signatures from these different events were analyzed using standard 

waveform analysis procedures. The magnitude of seismic energy 

during a gas and dust explosion was found to be more dependent on 

mine entry geometry and seal behavior rather than on the amount of 

explosive fuel present. The results suggest a large explosion that’s not 

restricted can generate potentially the same magnitude as a small 

explosion that is restricted. 

Introduction 

Purpose of the Study 

Seismic monitoring provides a powerful means for detection and 

evaluation of seismic events resulting from mining activity. Seismic 

signature characteristics such as P- and S-wave arrival times, 

amplitudes, signal duration and frequency content can give indication 

to the nature and location of the seismic source. Seismic data have 

been utilized in the past to analyze mining related events such as 

production blasts from quarries, roof falls and microseismic emissions 

[1-3]. However, little research has been conducted on seismicity from 

gas and dust explosions. The Sago Mine disaster in 2006 [4] provides 

an example of why the seismicity from explosions should be studied. 

A small amplitude signal was seen on records of regional seismic 

network in the time interval of the disaster [5]. The epicentral location 

of the signal was at the Sago Mine, however it is unclear whether the 

seismicity represents the explosion itself. This preliminary study will 

begin the process to understand the seismicity of mine explosions by 

studying the waveforms generated by these events at a research mine. 

In order to conduct the study, a microseismic monitoring system 

was installed at the National Institute of Occupational Safety and 

Health’s (NIOSH) Lake Lynn Experimental Mine (LLEM) located 

approximately 96 km (60 miles) southeast of Pittsburgh, PA. NIOSH’s 

interest on the design and behavior of seals during mine explosions 

provided the opportunity for this research [6]. Necessary calibration 

measurements had to be conducted on the monitoring system in order 

to obtain essential variables and relationships for moment magnitude 

and radiated seismic energy calculations. Three gas and dust 

explosions at LLEM were monitored, two of which included a seal in 

the path of the explosive wave. The three explosions were analyzed in 

terms of moment magnitude and radiated seismic energy. Impact 

tests, in the form of dropping different sized material to the floor, were 

conducted to obtain physical characteristics at LLEM necessary for 

technical analysis. The impact tests were also analyzed in terms of 

moment magnitude and radiated seismic energy. 

Lake Lynn Experimental Mine Overview 

The Lake Lynn Experimental Mine is a full-scale mine research 

facility on the site of a former limestone quarry and underground mine. 

Research conducted at the site historically and currently has included 

studies on mine fires, explosions, performance of roof support products 

and seal design. Geologically, the mine is located in the Greenbrier 

limestone formation [7]. 

The mine layout is seen in figure 1. The west side of the facility, 

known as the old workings, was mined commercially in the 1960s. 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 which 

were 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. A-, B-, C- and E-Drifts are approximately 480 – 495 m (1,575 – 

1,630 ft) long. E-Drift is 155 m (510 ft) long. The size of the pillars in 

the entry section is 24 x 12 m (80 x 40 ft). 

2 1

3

5

6

4

9

8

10

11

12

Uniaxial Geophones

Triaxial Geophones

Instrument 

Rooms (contains 

Geophones #1, #2 

and #3)

Bulkhead 

Door

Drop Site

2 1

3

5

6

4

9

8

10

11

12

Uniaxial Geophones

Triaxial Geophones

Instrument 

Rooms (contains 

Geophones #1, #2 

and #3)

Bulkhead 

Door

Drop Site

OLD 

WORKINGS 

Figure 1. Lake Lynn Experimental Mine map with geophone locations 

indicated. Geophone #7 was used as a test geophone and is not 

included in geophone monitoring array. Triaxial geophones were 

installed in the drift area while uniaxial geophones were installed in the 

old workings. The drop sites used for the impact studies are located in 

the old workings. 

A bulkhead door is shown in figure 1 at the intersection of E-Drift 

and C-Drift. This 70-ton door, constructed in place with steel 

framework and reinforced concrete, was closed during the explosion 

tests. 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 submarine-type doors. The instrument rooms 

provide a pre-wired network so the digitizers can be connected to a 

central PC outside of the mine. 
 

Microseismic Monitoring System Components 

The basic components of the microseismic monitoring system 

include the geophone, digitizer, modem rack, serial hub and data 

acquisition computer, as outlined in figure 2. The triaxial geophones 

used are the Sercel L-281

, which have a natural frequency of 4.5 Hz. 

Two different types of digitizers, manufactured by ISS International are 

used: Microseismic (MS) boxes and Quake Seismometer (QS) boxes. 

Both types of digitizers have a sampling rate of 2,000 samples per 

second and contain six channels for input. The modem rack allows up 

to sixteen digitizers online with the system via DB-9 connector ports. 

The serial hub takes the sixteen ports from the modem rack and 

converts them into a single 100/10 LAN cable connected to the data 

acquisition computer. The digitizers send data to the computer when 

the geophones are triggered based upon pre-set threshold values. 

Figure 2. Components of the microseismic monitoring system.

Experimental Procedure 

Digitizer Calibration 

Before the seismically recorded magnitude and energy can be 

calculated for events monitored in this study, the number of digital 

counts for a one volt input signal as a function of frequency needed to 

be determined. The phase response of the digitizers is not needed. 

The number of counts per volt for each digitizer was measured by 

using a waveform generator to input a sinusoidal wave of a specific 

frequency. The response to each calibration frequency, between 50-

1,000 Hz at 50 Hz intervals, was digitized and plotted in figure 3. The 

QS box has a constant response at both low and high frequencies. 

The sudden change in high frequency response of the curve is due to 

the anti-alias filter near the Nyquist frequency of 1,000 Hz. 

 

1

Mention of specific products or manufacturers does not imply 

endorsement by the National Institute of Occupational Safety and 

Health.

1.00E-03

1.00E-02

1.00E-01

10 100 1000

Frequency

Digital Counts per Volt

QS Box Curve

MS Box Curve

Constant at both low 

and high frequencies

Constant at low frequencies 

and has a change in 

response at high frequencies

C = 2.70E-02 digital counts / volt

Drop off due 

to Nyquist

frequency

1.00E-03

1.00E-02

1.00E-01

10 100 1000

Frequency

Digital Counts per Volt

QS Box Curve

MS Box Curve

Constant at both low 

and high frequencies

Constant at low frequencies 

and has a change in 

response at high frequencies

C = 2.70E-02 digital counts / volt

Drop off due 

to Nyquist

frequency

Figure 3. Digitizer maximum amplitude output from sinusoidal test 

calibration. The measured points are represented by the dots in each 

of the curves. 

The results show that the MS box response is constant at low 

frequencies up to approximately 100 Hz. The loss in amplitude at 

higher frequencies is due to a low-pass filter built into the MS box. The 

amplitude response of the low-pass filter G(ω) was derived. 

1 G( ) ω = ⎛ ⎞3 (1) ω 1+ ⎜ ⎟ ⎜ ⎟ ⎝ωc ⎠

where Ω = 2πf, f = frequency in Hz, 

ωc = 2πfcorner, fcorner = frequency in Hz. 

The digitizer constant, needed to calculate the system amplitude 

response to both ground displacement and velocity, was also 

determined from the calibration. The constant, C, is 0.027 counts/volt 

for the MS box and 0.034 counts/volt for the QS box. 

Determination of System Amplitude Response to Ground 

Displacement and Velocity 

The system amplitude responses to ground displacement and 

velocity are needed for moment magnitude and radiated seismic 

energy calculations. System amplitude responses to ground 

displacement and velocity can be derived using a single degree of 

freedom damped spring-mass system model [8], the Sercel geophone 

product sheet [9] and the results of the digitizer calibration test 

described previously. The system amplitude response to ground 

displacement, D(ω), is shown below and plotted in figure 4. 

⎛ 3 ⎛ R ⎞ ⎜ ECω ⎜ s ⎞

⎟ ⎟ ⎜ ⎟ ⎝ R ⎟ (2)

( ) ⎜ D ω = t ⎠ ⎜ G( ) ω

⎜ ( ) ⎟ ω ω 2 2 − 2

ο + ( ) 2hωοω 2

⎟ ⎜ ⎟ ⎝ ⎠

where: E = geophone electrodynamic constant (volts/m/s), 

C = digitizer constant, determined from sinusoidal 

input test (digital counts/volt), 

Ω = 2πf (frequency in Hz), 

Rs = shunt resistance for a given damping (ohms), 

Rt = total circuit damping (ohms), 

ωo = 2πfnatural (natural frequency in Hz), and 

 H = geophone damping. 
Figure 4 shows that above the geophone natural frequency of 4.5 

Hz, the slope of the response line is equal to one. Below the natural 

frequency of the geophone, the slope is equal to three. This response 

behavior is expected for an amplitude response to ground 

displacement using a velocity transducer [8]. At frequencies above 

100 Hz for the MS box, the response changes because of the low pass 

filter. The system amplitude response to ground velocity, V(ω), is 

shown below and plotted in figure 5. 

0.001

0.01

0.1

1

10

100

1000

10000

0.1 1 10 100 1000

Frequency (Hz)

D(? ) - Response to Displacement 

(Digital Counts / m)

QS Box MS Box

Effect of low pass 

filter in MS box

f

natural = 4.5 Hz 0.001

0.01

0.1

1

10

100

1000

10000

0.1 1 10 100 1000

Frequency (Hz)

D(? ) - Response to Displacement 

(Digital Counts / m)

QS Box MS Box

Effect of low pass 

filter in MS box

f

natural = 4.5 Hz

Figure 4. System amplitude response to displacement. 

⎛

ω2 ⎛ R ⎞ ⎞ ⎜ EC ⎜ s ⎟ ⎟ ⎜ ⎟

( ) ⎜ ⎝ R ⎠ ⎟ (3)

V ω = t

⎜ ( ) ⎟G( ) ω

⎜ ω h ⎟ ⎜ ο −ω 2 2 2 + ( ) 2 ωοω 2

⎟ ⎝ ⎠

0.0001

0.001

0.01

0.1

1

10

0.1 1 10 100 1000

Frequency (Hz)

V(? ) - Response to Velocity 

(Digital Counts / m/s)

QS Box MS Box

f

natural = 4.5 Hz

Effect of low pass 

filter in MS box

0.0001

0.001

0.01

0.1

1

10

0.1 1 10 100 1000

Frequency (Hz)

V(? ) - Response to Velocity 

(Digital Counts / m/s)

QS Box MS Box

f

natural = 4.5 Hz

Effect of low pass 

filter in MS box

Figure 5. System amplitude response to velocity. 

Geophone Location and Placement 

A geophone location map with identification numbers can be seen 

in figure 1. Triaxial geophones are located in the drift area and uniaxial 

geophones are located in the old workings. The uniaxial geophones 

were oriented vertically and were mounted on the ground. All triaxial 

geophones were mounted on the excavation surface about 3 m (10 ft) 

from the floor, except for Geophone #1 which was on the floor and 

Geophone #4 located on a concrete block attached to the roof. The 

horizontal components of the triaxial geophones were similarly oriented 

with components parallel and perpendicular of A-D drifts, 

approximately 50 degrees east of true north. A metal bar with holes on 

each end holds the geophone in place while pieces of all-thread rods 

are bored into the roof to tighten the bar and secure the geophone. 

The horizontal components were leveled and the vertical components 

were checked for verticality. No epoxy was used to mount the 

geophones. 

Explosion Tests 

Natural gas (~98% methane) is injected into a 14 m (47 ft) long ignition 

chamber at the face of C-Drift, and an electric fan with an explosionproof motor housing mixes the natural gas with the air to result in an 

approximately 9.5% methane-air concentration. The flammable natural 

gas-air volume was ignited using a triple-point ignition source. This 

ignition source consists of three sets of two 100-J electric matches that 

are equally spaced at midheight across the closed end 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 is lengthened or pulverized coal dust is 

suspended on shelves from the mine roof starting just outside of the 

ignition chamber. Pressure transducers at the location of the heading 

seal (or the seal construction across C-drift approximately 98 m (320 ft) 

from the face) were used to measure the peak pressure of the 

explosion. When the explosion pulse reaches the seal across C-drift, it 

is reflected, and the resulting total reflected pressure can be about 

twice the incoming pressure pulse value. The three explosion tests 

conducted at Lake Lynn Experimental Mine were named LLEM Shots 

#507 through #509. Figure 6 is a schematic of the initial amount of 

explosive fuel, seal locations and measurement of explosion pulse 

pressure for each test. 

(a) Mine Shot #507

20-22 psi

(b) Mine Shot #508

57- 61 psi 

(c) Mine Shot #509

120 - 132 psi 

Figure 6. Setup of the three explosion tests during the experiment. 

The pressure value is the peak generated pressure of the explosive 

wave measured at the location of the seal in the heading. 

Shot #507 was conducted on November 1, 2006. For this 

experiment, there was no seal located across C-Drift entry. The test 

was designed to generate 20-22 psi transverse pressure against the 

crosscut seals. The initial amount of explosive fuel was 18.7 m3

 (661 

ft3

) of natural gas and 3.6 kg (8 lbs) of coal dust. Shot #508 was 

conducted on April 10, 2007 and Shot #509 was conducted on April 

12, 2007. The purpose of these shots was to test a heading seal 

across C-Drift. The same seal was tested during both shots. The 

pressure against the heading seal was approximately 57-61 psi for 

Shot #508 and 120-132 psi for Shot #509. The pressure transducer 

was located near the center of the seal in a horizontal position facing 

the oncoming explosion. The initial amount of explosive fuel was 18.7 

m3

 (661 ft3) of natural gas and 3.6 kg (8 lbs) of coal dust for Shot #508. 

For Shot #509, the amount of initial explosive fuel was 35.8 m3

 (1,265 

ft3

) of natural gas and 14.5 kg (32 lbs) of coal dust. The seal failed 

during Shot #509. For Shot #507, the uniaxial geophones in the old 

workings were not part of the monitoring system. For Shot #508, 

Geophones #1 and #2 in C-Drift were unable to monitor. 

Drop Tests 

Drop tests were conducted to help obtain P- and S-wave 

velocities necessary for analysis. Two locations inside of the mine 

were chosen for drop sites (figure 1). At both sites the limestone floor 

was exposed and was free of debris. Rocks of various sizes were 

dropped from a height of 3.2 m (10.6 ft). Also dropped during the 
experiment were a 725 kg (1,600 lb) concrete block and a 1,429 kg 

3,150 lb) block of steel. Seven drop tests are included for this study. 

uring the drop tests, only Geophones #10, #11 and #12 close to the 

rop sites in the old workings registered the events. 

Data Analysis Procedure 

Data collected during the experiment were analyzed in terms of 

tatic seismic moment [10], moment magnitude [11] and radiated 

eismic energy [12]. 

etermination of Moment Magnitude 

The static seismic moment, Mo, is derived from the Fourier 

isplacement amplitude spectrum. 

4πρ R Ω c3

ο = ο (4)

Fc

here ρ = density of the rock at the source (kg/m3

), 

R = distance from source to receiver (m), 

Ωo = low frequency spectral plateau seen in the Fourier 

displacement amplitude spectrum (m/Hz), 

C = P- or S-wave velocity (m/s), and 

Fc = P- or S-wave radiation pattern coefficient. 

(

D

d

s

s

D

d

M

w

For the static seismic moment calculations, it is assumed the 

events produce an isotropic wave radiation pattern. The static seismic 

moment is expressed in units of N•m. The low frequency spectral 

plateau, Ωo, can be found from the Fourier amplitude spectrum of 

ground displacement. The raw signal collected by the geophone is in 

units of digital counts. The Fourier amplitude spectrum of that raw 

signal, in units of digital counts per Hz, can be represented by S( ). 

Thus, the Fourier amplitude spectrum of ground displacement, in units 

of meters per Hz, is determined from the Fourier amplitude spectrum 

from the raw signal and the system amplitude response to ground 

displacement described earlier. 

( ) ( ) (5) 

An example of the Fourier amplitude spectrum of ground 

displacement can be seen in figure 7. The noise curve represents the 

pre-signal noise. A dotted line shows the overall trend for the signal 

displacement curve. The low frequency spectral plateau is where the 

curve flattens out above low frequencies, as indicated in figure 7. 

Although three low frequency asymptotes were found for each triaxial 

geophone (one per component), only one value of moment magnitude 

is reported, based on the arithmetic mean of the three asymptotes for 

the three components. The moment magnitude, M, is determined from 

the static seismic moment by the relationship: 

2 M = log( ) M − 6 (6)

3 ο

Determination of Radiated Seismic Energy 

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

The radiated seismic energy for an isotropic source is given by: 

E = 4πρ 2( ) I ( ) s cR 7 1 + I 2 + I 3

where ρ = density of the rock at the source (kg/m3

), 

R = distance from source to receiver (m), 

c = P- or S-wave velocity (m/s), and 

integral of squared velocity for a geophone I = orientation (m2

/s2/Hz). 

( ) ω

ω

D

S fD =

1.E-09

1.E-08

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

0.1 1 10 100 1000

Frequency (Hz)

Displacement (m/Hz)

Signal Noise

Oo = 4.05E-05 m/Hz

Low Frequency Spectral Plateau (Oo)

1.E-09

1.E-08

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

0.1 1 10 100 1000

Frequency (Hz)

Displacement (m/Hz)

Signal Noise

Oo = 4.05E-05 m/Hz

Low Frequency Spectral Plateau (Oo)

Figure 7. Example of the Fourier amplitude spectrum of ground 

displacement. The noise curve is the amplitude spectrum of the 

background noise and the signal curve is the amplitude spectrum of 

the event being characterized.

The integral of squared velocity, I, is determined from the Fourier 

amplitude spectrum of ground velocity. The Fourier amplitude 

spectrum of ground velocity, in units of m/s per Hz, is determined from 

the Fourier amplitude spectrum of the raw signal and the system 

amplitude response to ground velocity. 

( ) ( ) S ω V f = ( ) (8) V ω

An example of the Fourier spectrum of ground velocity can be 

seen in figure 8. The integral of squared velocity is the area between 

the signal and noise curves where they separate and merge together 

between two frequency values. The two curves showed similar trends 

up to 7 Hz, so this would be the frequency where the energy area 

begins. Where the two curves merged back together is the frequency 

where the energy area ends. For the uniaxial geophones, equal 

amplitude is assumed on all three components. The single component 

estimate for I is multiplied by a factor of three. 

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

0.1 1 10 100 1000

Frequency (Hz)

Velocity (m/s / Hz

Signal Noise

Signal and noise start to 

have different trends

Area between curves used 

for energy calculation Signal and noise 

merge back together

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

0.1 1 10 100 1000

Frequency (Hz)

Velocity (m/s / Hz

Signal Noise

Signal and noise start to 

have different trends

Area between curves used 

for energy calculation Signal and noise 

merge back together

Figure 8. Example of the Fourier amplitude spectrum of ground 

velocity. The noise curve is the amplitude spectrum of the background 

noise and the signal curve is the amplitude spectrum of the event 

being characterized. Energy below the natural frequency of the 

geophone was included in some cases because the amplitude 

response formulas were derived for all frequencies. 

Determination of P- and S-Wave Velocity 

The results from the seven drop tests can be seen in Table 1. An 

earlier study showed that P- and S-waves are generated during drop 

tests [1]. The P- and S-wave arrival time difference was minimal for 

each drop, so the S-wave had to be determined from the P-wave 

arrival time. The S-wave velocity was calculated by dividing the Pwave velocity by 3 . The P- and S- wave velocities used for technical 

analysis were 4,511 m/s (14,800 ft/sec) and 2,604 m/s (8,545 ft/sec), 

respectively. These values are in the same velocity range as another 

limestone formation in Pennsylvania [1]. 
 

Table 1. P- and S-wave velocities (m/s) of the drop tests conducted for 

the study. 

Test P-Wave S-Wave

1 3,677 2,123

2 4,760 2,748

3 4,252 2,455

4 4,511 2,604

5 4,111 2,373

6 6,380 3,683

7 4,577 2,643

Mean 4,610 2,661

Standard deviation 8,58 495 

Median 4,511 2,6048

Discussion of Results 

Discussion of Explosion Tests 

While it is usually advantageous to have instruments close to the 

events, in some cases it can present a problem. At close range, a 

variety of phases such as acoustic waves and seismic surface waves 

may be present in the waveforms, in addition to the direct seismic P 

and S waves of interest. The moment magnitude and radiated seismic 

energy equations as defined here should be estimated from the direct 

P or S wave arrivals. Thus, values for magnitude and energy would be 

misrepresented if the analysis of the waveform included more than just 

a direct body wave arrival. 

It is believed that recordings from the closest instruments, 

Geophones #1, #2, #4, #6, #8 and #9 recorded multiple phase arrivals. 

In order to investigate this theory, the waveform from Geophones #10 

and #12 for Shot #509 was analyzed. There appears to be two distinct 

phases in this waveform, as shown in figure 9. The two phases are 

believed to be the seismic energy due to the explosion and the 

acoustic air blast, respectively. Knowing the P-wave velocity 

determined from the drop tests and difference in arrival times between 

the two phase arrivals, a velocity can be found for the second arrival. 

The velocity of the second phase arrival was found to be approximately 

320 m/s (1,050 ft/sec), consistent with the speed of sound of air. The 

latter is a function of temperature, which for the mine (at 55° F) would 

be approximately 340 m/s (1,100 ft/sec), sufficiently close to validate 

the second phase as the acoustic air blast. Much closer to the ignition 

of the explosion in C-Drift, the waveforms can be expected to 

represent a combination of the acoustic and seismic energy; however, 

the clear distinction as shown in figure 9 won’t be seen. 

-4.00E-04

-3.00E-04

-2.00E-04

-1.00E-04

0.00E+00

1.00E-04

2.00E-04

3.00E-04

4.00E-04

00:00:00.000 00:00:05.000

Time (Seconds)

Amplitude (Digital Counts)

First phase arrival – Seismic energy

Second phase arrival – Acoustic air blast

-4.00E-04

-3.00E-04

-2.00E-04

-1.00E-04

0.00E+00

1.00E-04

2.00E-04

3.00E-04

4.00E-04

00:00:00.000 00:00:05.000

Time (Seconds)

Amplitude (Digital Counts)

First phase arrival – Seismic energy

Second phase arrival – Acoustic air blast

Figure 9 . Geophone #10 waveform from Shot #509.

The two geophones in D-Drift, Geophones #3 and #5, are 

assumed to record pure seismic energy, because they were not in the 

path of the air blast and were relatively far away. The magnitudes and 

energies, as calculated from geophones in D-Drift, are shown in figure 

10 and 11, respectively. The magnitude and energy values were 

calculated with the assumption that the explosive source is in the far 

field. Based upon these two charts, it can be seen that Shot #508 has 

a higher magnitude and energy when compared to Shot #507. As 

stated previously, these two tests contained the same initial amount of 

explosive fuel and the only difference was Shot #508 contained a 

heading seal (as seen in figure 6). The results show that the addition 

of a seal in the path of an explosive wave generates more seismic 

energy. The additional seismic energy can be a result of the seal 

flexing and causing the roof, floor and rib to vibrate. Between these 

two shots it can be concluded that the magnitude of seismic energy is 

more dependant on mine entry geometry and seal behavior than on 

the amount of fuel present. Therefore, the results suggest a large 

explosion that’s not restricted can generate potentially the same 

magnitude as a small explosion that is restricted. For Shot #509, the 

magnitude and energy is higher than the previous two shots. 

However, this shot started with a higher amount of initial explosive fuel 

and the seal fails, which causes impacts of seal material debris against 

the rib and floor. The additional seismic energy recorded for this 

explosion is likely due to these factors. 

Moment Magnitude

1.0

1.2

1.4

1.6

1.8

2.0

Test without heading seal

(shot #507)

Test with heading seal

(shot #508)

Test with heading seal

failure (shot #509)

Figure 10. Moment magnitude values of the three mine shots. Note 

that Shots #507 and #508 had the same initial explosive fuel

1

10

100

1000

10000

100000

Test without heading seal

(shot #507)

Test with heading seal

(shot #508)

Test with heading seal

failure (shot #509)

Radiated Seismic Energy (MJ)

Figure 11. Radiated seismic energy values of the three mine shots. 

Discussion of Drop Tests 

A plot of magnitude versus mass of material and energy versus 

mass of material for the drop tests can be seen in figures 12 and 13, 

respectively. If a regression line is fit through the whole dataset in 

figure 11, the value of R2

, the square of the sample correlation 

coefficient, is 0.39 which is very poor. Looking at figure 12, the 

regression line through the whole dataset has an R2

 value of 0.78. The 

data scatter and poor fit appear to be related to how the material 

impacted the ground. When the concrete and steel blocks hit the 

limestone floor, the amount of surface area that made contact was very 

high. In contrast, most of the limestone blocks were bulky and 

irregular in shape. When they hit the floor, little of the limestone 

surface made direct contact and a lot of energy was consumed by 

breaking the limestone rock into multiple pieces. Thus, the amount of 

seismic energy effectively transferred into the ground may be highly 

variable. Based on observations during the tests, the concrete block, 

the steel block and the 4- and 12-ton limestone rocks hit the floor with 

maximum surface contact. Regression lines through those data points 

are shown in figures 12 and 13, with R2

 values of 0.86 and 0.99, 
respectively. These high R2

 values suggest that the degree of surface 

contact upon impact is an important contributor to seismic efficiency 

and high magnitude estimates. 

R2 = 0.86

R2 = 0.39

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

0 2 4 6 8 10 12 14

Mass of Material (tons)

Moment Magnitude

Limestone Blocks

Concrete Block

Steel Block

R2 = 0.86

R2 = 0.39

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

0 2 4 6 8 10 12 14

Mass of Material (tons)

Moment Magnitude

Limestone Blocks

Concrete Block

Steel Block

Figure 12. Moment magnitude versus mass of material dropped from a 

height of 3.2 m (10.6 ft). The solid regression line is for all the data 

points. The dashed regression line is for the circled four data points, 

which indicate the dropped material that impacted the floor with 

maximum surface contact. 

R2 = 0.99

R2 = 0.78

0

50

100

150

200

250

300

350

400

450

500

0 2 4 6 8 10 12 14

Mass of Material (tons)

Radiated Seismic Energy (MJ)

Limestone Blocks

Concrete Block

Steel Block

R2 = 0.99

R2 = 0.78

0

50

100

150

200

250

300

350

400

450

500

0 2 4 6 8 10 12 14

Mass of Material (tons)

Radiated Seismic Energy (MJ)

Limestone Blocks

Concrete Block

Steel Block

Figure 13. Seismic energy versus mass of material dropped from a 

height of 3.2 m (10.6 ft). The solid line is for all the data points. The 

dashed regression line is for the circled four data points, which indicate 

the dropped material that impacted the floor with maximum surface 

contact.

Conclusions 

The following conclusions are based upon the measurements of 

moment magnitude and radiated seismic energy calculations from the 

three mine shots and the seven drop tests: 

• During the explosion tests for this study, some of the 

geophones were believed to have captured a variety of 

waveform phases, such as acoustic waves and seismic 

surface waves. With these phases included in the waveform, 

the measured values for moment magnitudes and radiated 

are inaccurate because the recorded waveforms are not from 

a pure seismic source. 

• The unwanted phases would not ordinarily be present in the 

waveforms from an explosive event if the geophones were 

further away or in a borehole. However, in the case of this 

study, the geophone array and the inability at the time to put 

geophones in a borehole allowed these phases to be 

recorded. 

• Geophones #3 and #5 were assumed to capture only seismic 

energy because these geophones were not in the path of the 

explosive wave and relatively far away. 

• The magnitude of seismic energy during a gas and dust 

explosion was found to be more dependent on mine entry 

geometry and seal behavior than on the amount of explosive 

fuel present. The results suggest a large explosion that’s not 

restricted can generate potentially the same magnitude as a 

small explosion that is restricted. 

Disclaimer 

The findings and conclusions in this report have not been formally 

disseminated by the National Institute for Occupational Safety and 

Health and should not be construed to represent any agency 

determination or policy. 

Future Research 

An analytical method is being developed to separate the 

unwanted phases from the waveforms. By separating out these 

phases, more data is provided for the explosive test results which will 

provide additional understanding of the seismic signatures. The 

second phase arrival, as seen in figure 9, was shown to be the 

acoustic air blast. This provides a starting point to successfully remove 

that phase from the waveforms. A similar method is being developed 

to separate out the seal failure and subsequent impacts from the 

waveforms captured during Shot #509, to see how much the additional 

explosive fuel had an effect on the seismic energy. More explosive 

tests at the Lake Lynn Laboratory Experimental Mine are planned for 

the future and will be monitored. 

References 

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"Characterizing Roof Fall Signatures From Underground Mines," 

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Rockburst and Seismicity in Mines, Perth, Australia, March 9-11, 

Australian Centre for Geomechanics. p. 619-629. 

2. Iannacchione, A. T., G. S. Esterhuizen, T. S. Bajpayee, P. L. 

Swanson and M. C. Chapman (2005), "Characteristics of MiningInduced Seismicity Associated with Roof Falls and Roof Caving 

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http://www.msha.gov/Fatals/2006/Sago/sagoreport.asp 

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Virginia Polytechnic Institute and State University: Blacksburg, 

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 http://www.msha.gov/Fatals/2006/Sago/sagoreport.asp 

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8. Aki, K. and P.G. Richards (2002), "Quantitative Seismology," 

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Books. 
9. Sercel (2005), "L-28 Digital Grade Geophone," 

ftp://ftp.sercel.com/pdf/brochures/GeophonesHydrophones.pdf. 

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