1

MICROSEISMIC ACTIVITY ASSOCIATED WITH A DEEP LONGWALL COAL MINE

K.A. Heasley, J.L. Ellenberger, and P.W. Jeran

NIOSH

Pittsburgh, PA

ABSTRACT

A deep longwall coal mine was instrumented with a threedimensional microseismic system in order to help determine the

exact strata mechanics associated with the rock failure,

redistribution of stress and the associated gob formation from

the longwall. Overall, 5,000 well calibrated seismic events were

recorded during the mining of the panel. Analysis of these

events showed a close correlation between the seismic activity

and advance rate, and that the majority of the recorded seismic

activity occurs in the immediate area of the advancing longwall

face.

INTRODUCTION

In recent years, microseismic systems have been used in

coal mines in Australia and the United States to gain a better

understanding of the ground failures and rock mechanics

involved with longwall mining (Ellenberger et al., 2001; Gale et

al., 2001; Kelly et al., 1998; Luo et al., 1998; Swanson, 2001;

Westman et al., 2001). These microseismic systems "listen" to

the rock and determine the timing and location of the failure of

the rock strata surrounding the longwall panel. The recent

hardware and software advances in microseismic systems have

allowed this geophysical monitoring technique to provide

practical geomechanical measurements at operating mines

(Swanson, 2001). The results from these measurements have

been insights into longwall geomechanics that are somewhat

outside of previous strata mechanics understanding. For

instance, the microseismic events and associated rock failure

have mostly been recorded from well in front of the longwall

face, with a noticeable lack of seismic activity coming from the

gob area. The seismic events have been distributed fairly

evenly above and below the seam and the predominent fracture

mechanism has been shear failure (as determined from focal

analysis and numerical modeling (Gale et al., 2001)).

The primary objective of the field work presented in this

paper was to examine the strata failure behavior of a deep,

bump-prone longwall mine using a three-dimensional seismic

monitoring system. By analyzing the observed rock failure, we

hope to increase our knowledge of the processes governing

caving of the massive main roof, the compaction and load

acquisition of the gob, the failure of the floor, and the stress

redistribution in the coalbed and surrounding strata. The

application of this knowledge will enable better mine designs in

the future in order to mitigate dangerous bump occurrences and

unexpected failures of the massive overburden.

The study mine primarily operates in the Castlegate ‘D’

Seam of the Blackhawk Formation. The coalbed ranges from

2.4 to 6.0 m (8 to 20 ft) in thickness with an extraction thickness

of 2.4 to 3.0 m (8 to 10 ft). The immediate and main roofs of the

mine consist of braided and lenticular Blackhawk shales,

siltstones and sandstone deposits for some 120 to 180 m (400

to 600 ft). Overlying these deposits is the Castlegate

Sandstone, a massive, cliff forming sandstone that is 120 to 180

m (400 to 600 ft) thick with the lower 90 m (300 ft) being more

compact and massive than the upper portion of the unit (Barron

et al., 1994). On top of the Castlegate, another 540 m (1,800 ft)

of rugged sedimentary deposits bring the maximum overburden

up to 900 m (3,000 ft) (figure 1). The geology immediately

below the seam consists of thinner (<3 m (< 10 ft)) layers of

siltstones, mudstones, shales,and coal, with the Kenilworth and

Aberdeen sandstones being near seam massive units.

The microseismic monitoring array installed at the mine

consisted of 23 geophones, 14 geophones in the mine entries

and 9 geophones on the surface above the mine. The array had

lateral and vertical extents of 2.2 and 0.8 km (1.4 by 0.5 mi),

respectively, and essentially surrounded the first two longwall

panels (figure 1). The signals from the geophone array were

ultimately collected by the main data analysis computer at the

mine office. Here the signals were automatically analyzed in

order to calculate the event locations. These locations were

then automatically displayed on a computer generated mine

map for real time use by mine personnel. Over 13,000 seismic

events were automatically detected and located during the

mining of panel 2.

In order to obtain a consistent, high-quality, data set of

event locations for the final analysis, the raw waveform data

from the field were reprocessed in the laboratory using an

improved layered seismic velocity model which best fit a

number of known events. Also, the quality of the seismic data

was upgraded by including only events with a minimum of 8

stations (with at least 3 surface stations and 3 underground

stations) reporting good first-arrival picks. This post-processing

procedure winnowed the original 13,000 events down to a good

quality data set consisting of 5,024 events from panel 2

(Heasley et al., 2001).

EVENT TIMING

One of the first aspects of the seismic data that was

investigated in detail was the number and timing of the events.

During the active mining of the panel, 5,024 good quality events

were recorded. This is an average of 29 events per day with a

minimum of 0 and a maximum of 136 events per day. However,

these events were not evenly distributed over the panel. The

data shows three distinct periods of seismic activity (figure 2).

From the start of the panel through the first about 300 m (1,000

ft), the seismic activity was fairly low, averaging around 5

events per day (table 1). Generally, these events were

relatively smaller and more scattered about the advancing face

than in subsequent periods (Ellenberger et al., 2001). A higher

level of seismic activity (28 events per day) was noted as the

face advanced from 300 to 900 m (1,000 to 3,000 ft) at which

point the face was stopped and widened from 165 to 245 m (550

to 820 ft) (figure 1). In the final time period, as the longwall

advanced 300 m (1,000 ft) with the wider face, the seismic

activity was quite high averaging 64 events per day.
2 

Figure 2. A comparison of the number of events per day with the mining advance rate

Figure 1. Plan view showing mine layout, overburden,

geophone arrays, and the location of the M 4.2 event
3

Figure 3. A three-dimensional view of the seismic events with the event size scaled by magnitude he

ML 4.2 event removed for clarity)

Table 1. A comparison of the seismic activity between different parts of the panel

Advance (m) Starting Date Ending date Total # of

events

Average # of

events per day

Correlated Events

per meter

Correlation

coefficient2

Total Panel 0 - 1200 12/2/1999 5/25/2000 4,930 29 2.81 0.2498

Early Panel 0 - 300 12/2/1999 1/20/2000 212 5 0.70 0.6681

Mid Panel 300 - 900 1/21/2000 4/8/2000 2,232 28 3.10 0.4886

Final Panel 900 - 1200 4/16/2000 5/24/2000 2,486 64 8.34 0.5827

The next investigation was to correlate the advance rate

of the longwall face to the amount of associated seismic

activity. Previous research has shown a direct correlation

between the mining advance rate and the induced seismicity

(Arabasz et al., 1997). To perform this analysis, the meters of

advance of the longwall face per shift were correlated against

the number of good quality seismic events that were detected

during that shift (and within 4 hours after the face stopped on an

idle shift). The results of this analysis are shown in the right

side of table 1. First, looking at the entire panel in table 1, a

linear correlation of 2.81 events per meter of advance is

determined, but the r

2

value only 25%. However, if the panel is

again broken into the same three distinct periods, or section, as

above, a much stronger correlation is obtained. In fact, the

average r

2

value for the panel divided into the three sections is

58%. This squared value of the correlation coefficient is fairly

significant for this type of mining data and signifies that 58% of

the fluctuation in the shift-based number of events can be

explained by the corresponding fluctuation in the shift advance

rate. Mechanistically, this implies that a majority of the seismic

events are a direct result of the advancing face. The same

relative intensity of the seismic activity as noted before in the

three sections of the panel is, of course, evident in the

correlation with advance rate. The first part of the panel

averages 0.7 events per meter of advance versus 3.1 events

per meter of advance for the middle part of the panel and 8.3

events per meter for the final part of the panel.

EVENT LOCATION

Once the optimized velocity model was determined and

the final set of good quality seismic events was produced, the

location and magnitude of the events were investigated. One of

the best means that we found to visualize this “four”

dimensional data was to plot the events in three-dimensional

space as spheres which are scaled by magnitude (figure 3).

Taking an overall look at this figure, a number of observations

can be made. First, the lack of events in the first 300 m (1,000

ft) of the panel is evident as is the high density of events in the

last 300 m (1,000 ft) of the panel. Second, the events appear

to be fairly evenly distributed above and below the panel.

In order to visualize the location of the seismic events in

relation to the advancing longwall face, the locations of the

events were normalized to the face position and plotted on three

orthogonal planes such that the center, or zero point, of the

normalized coordinate system corresponds to the center of the

longwall face at seam level. In this paper, only the events from

the last part of the panel will be specifically presented and

analyzed using the relative face position since the events at the

other sections of the panel provide similar information

(Ellenberger et al., 2001). The results of normalizing the

location of the events from the final part of the panel to the face

position are shown in a plan view in figure 4 and in a vertical

view parallel to the advance direction in figure 5.
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Figure 4. Plan view of the normalized event locations for the

last part of panel 2

Figure 5. Vertical view parallel to face advance showing the

normalized event locations for the last part of panel 2

Figures 4 and 5 indicate that the seismic activity is mostly

located in the face area and generally in front of the face. Also,

there is a notable absence of recorded seismic activity coming

from the gob area. The location of the seismic data in front of

the face agrees with seismic information from other coal mining

sites (Gale et al., 2001, Luo et al., 1998) and has been

interpreted to represent the failure of the strata in the forward

stress abutment. It is thought that the rock failures that occur

in the confined high stress area in front of the face are well

recorded by the seismic system due to the high energy release

and good transmission characteristics; however, the low energy,

unconfined tension failures of the immediate roof in the gob

behind the face are not well recorded because of the low energy

release and the high attenuation in this generally broken rock

area. Also in figure 5, it can be seen that the seismicity is

originating both above and below the seam level. This response

also coincides well with the response observed at other field

sites (Gale et al., 2001, Luo et al., 1998) and is consistent with

a front abutment stress field that is vertically symmetric about

the coal seam. In fact, in figure 5, the majority of the seismic

activity is coming from the floor. This response may be due to

the presence of more competent floor strata or to a shift of the

event locations due to inaccuracies in the assumed velocity

model, and the lack of an extended geophone array below the

coal (Ellenberger et al., 2001; Heasley et al., 2001). From the

plan view in figure 4, it can be seen that most of the seismic

activity generally lines up in front of the advancing face. There

is a little skewness to the event data, with the seismic activity

occurring further in front of the headgate than the tailgate.

(Also, it can be seen that the events appear shifted towards the

headgate. This may be a manifestation of the deviation of the

actual velocity structure from that assumed in the model as

discussed elsewhere.(Ellenberger et al., 2001; Heasley et al.,

2001))

MAGNITUDE 4.2 EVENT

On March 6, 2000, at 7:16 pm, MST, a magnitude (ML) 4.2

“earthquake” occurred in the overburden above the mine and

within the confines of the active mine-wide seismic array. The

event vibrations were strongly felt by the miners, but there was

very little damage from the event on the working longwall face,

and only a few rib spalls were evident in the development

entries. This is the first time that such an event has been

recorded with this detail and accuracy at a U.S. coal mine. This

event caused rock slides from critical slopes on the nearby

highway, which damaged automobiles. The train tracks

adjacent to the highway at that point were also temporarily

blocked. Underground, multiple roof falls occurred in the

bleeder entries to the west of the first panel and several seals

were cracked around the previously abandoned panel. Also, a

significant amount of methane was rapidly liberated resulting in

a temporary evacuation of the mine. Fortunately, there were no

injuries.

Using the optimized velocity model for the site, this event

was located 90 m (300 ft) in front of the active face, 170 m (560

ft) above the coal seam and 10 m (35 ft) in from the edge of the

60 m (200 ft) wide barrier pillar between the active and the

previous panel (figure 1). This location puts the event near the

top of the Blackhawk Formation and the base of the massive

Castlegate Sandstone. The event occurred when the active

face was approximately 30 m (100 ft) from aligning with the

recovery room of the previous panel.

Using P-wave first motion data from the mine wide

seismic monitoring system, three temporary University of Utah

stations located near the mine and the University of Utah

regional seismic network, a well constrained focal mechanism,

which fits all of the available P-wave data, was determined. The

preferred focal mechanism indicates oblique reverse faulting on

a plane dipping steeply to the south or shallowly to the northnorthwest (Swanson and Pechmann, 2000). The focal

mechanism of the event is consistent with the roof strata failing

and the Castlegate formation falling into the gob. The location

and size of the event and the relative locations of the previous

and active longwall faces suggest that the ML 4.2 event was a

failure of the main roof essentially over both panels in the

vicinity of the base of the Castlegate. Whether a functional

failure of the intervening barrier pillar to fully support the

overburden may have preceded and helped initiate the major

failure of the main roof is not clear at this time.

CONCLUSIONS

From examining the seismicity at the site, several general

observations can be made: 

The event rate is low at the beginning of the panel, about

six times higher in the middle of the panel, and twice again as

high at the wider end of the panel (table 1). We hypothesize

that this is a result of the initial gob formation during the

beginning part of the panel versus a well established gob

through the middle of the panel and then a wider face in the last

part of the panel. In fact, when the panel width increased by

50%, we see the seismic activity increase by more than 100%.

Also, the correlation between the face advance rate and the

seismic activity implies that the majority of the seismicity is a

direct and fairly immediate response to removal of the coal and

the associated stress redistribution. Looking at the location of

the seismic events, it can be observed that the events generally

occur in advance of the longwall face and are approximately

evenly distributed above and below the panel (Ellenberger et al.,

2001). This observation is consistent with the interpretation that

the observed seismic events come from failure of the strata in

the forward stress abutment zone and is consistent with

observation at other sites where the predominant recorded

failure mechanism was shear fracture in front of the face as

opposed to tensile failure in the gob.

A magnitude 4.2 seismic event occurred within the active

longwall panel and was recorded by the mine-wide seismic
 5

system giving a unique opportunity to characterize important

overburden deformation processes. It has long been

acknowledged that not every potentially hazardous bump

generates a regional seismic event, nor does every mineinduced, regional seismic event manifest itself as a coal

outburst at the seam level. Numerous larger (> M 2.0) seismic

events have been located near active mines by regional seismic

systems (Ellenberger and Heasley, 2000). Some of these

seismic events were associated with coal bumps underground,

but many of the larger seismic events caused no observable

underground damage. Given the location accuracy of the

regional seismic systems, the exact proximity of the seismic

event to the coal seam and bump location could not have been

determined. Using the mine-wide seismic system in this study,

the ML 4.2 seismic event was relatively accurately located some

150-180 m (500-600 ft) above the longwall face. So at least in

this one case, we know that the large seismic event was

associated with overburden failure and not with pillar or panel

failure. Also, since this very large event was within 180 m of a

highly stressed longwall face and there was little coal discharge

on the longwall face, this instance documents a rather dramatic

example of how large local seismic events do not necessarily

result in serious face damage. Therefore, this one occurrence

suggests that, in order to control coal bumps, mine designers

and safety personnel generally need to be more concerned with

the seismic events, stress and geologic anomalies that are

relatively close (within 30 m (100 ft)) to the working face. Also,

the location of this large overburden failure above the

intervening “barrier” pillar and the relative closeness of the two

longwall faces at the time of the event, suggest that the two

longwall gobs were functionally combined at the location of the

failure.

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