AN ANALYSIS OF ROCK FAILURE AROUND A DEEP

LONGWALL USING MICROSEISMICS

Keith A. Heasley, Section Chief, Geotechnical Engineering

John L. Ellenberger, Research Geophysicist

Paul W. Jeran, Research Geologist

National Institute for Occupational Safety and Health

Pittsburgh Research Laboratory

Pittsburgh, Pennsylvania USA

ABSTRACT

In this paper, a state-of-the-art, three-dimensional, full

waveform, microseismic system was used to analyze the rock failure

around a deep (> 750 m (2500 ft) of cover) bump-prone longwall

panel. The microseismic system consisted of both underground and

surface geophones coupled through radio telemetry and a fiber-optic

network to produce pseudo real-time detection and location of seismic

events surrounding the coal seam. This was the first microseismic

installation of this scope at a U.S. coal mine, and the system was

intended to help determine the exact strata mechanics associated with

the redistribution of stress and the associated gob formation at one of

the U.S.’s deepest longwall mines. Overall, 5,000 calibrated seismic

events were recorded during the mining of one panel, including a

Richter magnitude 4.2 event which occurred inside of the array.

Analysis of these events provides a number of notable insights into

the rock mass behavior. For instance, the longwall panel was

widened during the retreat process, and the mining-induced seismicity

shows a distinctly different behavior between the start of the panel,

the mining of the narrow part of the panel and the mining of the wider

part of the panel. Also, in itself, the acquisition of the ML 4.2 event

was a major milestone in geomechanical and bump research. This is

the first time that such an event has been recorded with this detail and

accuracy at a bump-prone coal mine in the U.S. The analysis of this

single event has set distinct new limits on the relationship between

mining seismicity and coal bumps.

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 (1-5). 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 (6). The results from these

measurements have been insights into longwall geomechanics that are

somewhat outside of previous strata mechanics understanding. 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 (2)).

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.

GEOLOGY

This research was conducted at a longwall coal mine in Utah,

situated in the area of the Book Cliffs and the eastern Wasatch Plateau

in the northwest corner of the Colorado Plateau. In this area, the coal

seams are located in the Blackhawk Formation of the Mesa Verde

Group (figure 1) (7). The study mine is primarily in the Castlegate

‘D’ Seam; however, over part of the study area, the underlying

Kenilworth Seam and the Castlegate ‘D’ Seam coalesce and the mine

operates in the joined seam. The resultant coalbed at the mine 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 geology immediately above and

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

deposited siltstones, mudstones, shales, sandstones, and coal. Above

the coal bearing portion of the Blackhawk Formation, approximately

150-180 m (500-600 ft) of braided stream deposits with numerous

lenticular sandstone channels occur (8). These braided and lenticular

deposits make up the immediate and main roofs of the mine.

Unconformably overlying the Blackhawk Formation is the

Castlegate Sandstone. The Castlegate is 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 (8). In the mine area, the Castlegate Sandstone lies

approximately 200 m (680 ft) above the “D” seam (8). Overlying the

Castlegate is the Price River Formation, consisting primarily of

sandstone with interbedded conglomerates and sandstones. The Price

River Formation is also about 180 m (600 ft) in thickness. The
Figure 1. Generalized stratigraphy

Figure 2. Plan view showing mine layout, overburden, geophone

arrays, and the location of the M 4.2 event

Digitizer

Fiber-Optic

Network

Spread Spectrum

Radio Telemetry

Underground Geophones

Underground

Data Acquisition

Data Acquisition

Surface

Main Data

Analysis

Surface Geophones

Digitizer

Digitizer

Digitizer

Figure 3. Schematic representation of the microseismic system

uppermost rocks exposed at the site are lacustrine deposits of the

North Horn Formation, consisting of interbedded claystones,

mudstones, limestones, siltstones, and sandstones. Overall, the

overburden at this mine reaches up to 900 m (3,000 ft) thick

(figure 2).

MICROSEISMIC SYSTEM

The microseismic monitoring array consisted of 23 geophones

deployed both underground in the mine entries and 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

two longwall panels (figure 2). The underground seismic array

consisted of 14 geophones in the mains and bleeders around the

longwall panels. The underground geophones were cabled to a

central underground computer where the signals were collected and

transmitted via fiber-optic network to a main data analysis computer

in the mine office (figure 3). On the surface above the mine, 9

geophones were distributed over the panels (figure 2). The signals

from the surface network were digitized and transmitted by radio to

a digital data acquisition system residing on the same network as the

main data analysis computer at the mine office (figure 3). In the data

analysis computer, the microseismic event signals were automatically

analyzed in order to calculate the event locations, which were then

displayed on a computer generated mine map for use by mine

personnel. Using this automatic field location process (6), over

13,000 seismic events were detected and located during the mining of

panel 2 between December 1999 and May 2000.

MICROSEISMIC DATA PROCESSING

In order to obtain a consistent data set of event locations for this

analysis, the raw waveform data were reprocessed in the laboratory.

The lab processing utilized an improved seismic velocity model.

Seismic velocity data from sonic logs obtained in nearby boreholes

were used to create a starting model. Then a number of calibration

blasts and other control events with known locations (which were

largely at the accessible periphery of the array) were used to further

constrain the layered velocity model. This process culminated in a

layered seismic velocity model which best fit the available data,

although there still appears to be notable spatial variations in the

seismic velocity structure that are not considered in the simple

uniform layered model. Finally, only the events with a minimum of

8 stations (with at least 3 surface stations and 3 underground stations)

reporting good first-arrival picks were kept in the database. This

post-processing procedure winnowed the original 13,000 events down

to a good quality data set consisting of approximately 5,000 events

from panel 2.
20

40

60

80

100

120

140

12/2/199

9

1/1/2000 1/31/200

0

3/1/2000 3/31/200

0

4/30/200

0

5/30/200

0

6/29/200

0

Date

Events/day

0.0

5.0

10.0

15.0

20.0

25.0

Advance Rate - m/day

Events

Advance Rate

Figure 4. A comparison of the number of events per day with

the mining advance rate

Figure 5. A three-dimensional view of the seismic events with the event size scaled by magnitude

 (the ML 4.2 event removed for clarity)

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 six

month period that the panel was actively being mined, 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, the temporal distribution of these events was not very even,

as seen in figure 4 and table 1. In the graph in Figure 4, one can see

three distinct periods of seismic activity. From the start of the panel

on December 2, 1999 until January 20, 2000, when the face had

advanced about 300 m (1,000 ft), the seismic activity was fairly low,

averaging around 5 events per day. (Also, in general, the events in

this time period were relatively smaller (figure 5) and more scattered

about the advancing face than in subsequent periods (1)). Then, from

January 21 until April 8th, there was a period of intermediate seismic

intensity. This period corresponds to the face moving from 300 to

900 m (1,000 to 3,000 ft) of advance at which point the face was

stopped and widened from 165 to 245 m (550 to 820 ft) (figure 2).

Within this time period, the activity was relatively higher at the

beginning and at the end as the face approached the pillars connecting

the adjacent headgate. Also, at several times during this time period

the face was stopped and the seismic activity likewise went to zero

(figure 4). In the final time period, April 16th to May 24th, when the

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

activity was quite high averaging 64 events per day.

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

Total

Panel 0 - 1200 12/02/99 05/25/00 4,930 29

Early

Panel 0 - 300 12/02/99 01/20/00 212 5

Mid

Panel 300 - 900 01/21/00 04/08/00 2,232 28

Final

Panel 900 - 1200 04/16/00 05/24/00 2,486 64

The next analysis 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 (9). To perform this analysis, the

meters of advance of the longwall face per shift were correlated
y = 8.3378x + 7.2533

R2

 = 0.5827

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12

Advance per Shift (m)

Number of Events per Shift Figure 6. A graph of the correlation between the advance rate

and the number of events per shift for the final part of the panel

-200.0

-100.0

0.0

100.0

200.0

300.0

-400.0 -300.0 -200.0 -100.0 0.0 100.0 200.0 300.0 400.0

Distance in Front of Face (m)

Distance Across Panel (m)

Gob Panel 2 

Headgate

Tailgate

\

Figure 7. Plan view of the normalized event locations for the last part of panel 2

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). All of the results of this analysis are shown in table 2 and an

example of the linear correlation for the final part of the panel is

shown in figure 6. First, looking at the entire panel in table 2, 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 r2

 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.

Table 2. The results of correlating the seismic activity to the

advance rate.

Advance

(m)

Linear Regression (y = Ax + B)

Events

per m

(A)

Fixed

events per

shift (B)

Correlation

coefficient2

(r2)

Total

Panel

0 - 1200 2.81 3.87 0.2498

Early

Panel

0 - 300 0.70 0.04 0.6681

Mid

Panel

300 - 900 3.10 1.92 0.4886

Final

Panel

900 - 1200 8.34 7.25 0.5827
-200.0

-100.0

0.0

100.0

200.0

-300.0 -200.0 -100.0 0.0 100.0 200.0 300.0

Distance in Front of Face (m)

Distance Above Panel (m)

Gob Panel 2

Figure 8. Vertical view parallel to face advance showing the normalized event locations for the last part of Panel 2

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 5). 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.

Third, it appears that there is a higher density of smaller events and

an increased frequency of larger events in the last part of the panel as

compared to the relatively high percentage of medium size events in

the middle part of the panel. Finally, the event locations appear to be

horizontally shifted by as much as 75 m (250 ft) toward the headgate

and away from the tailgate. This shift is thought to result from

deviations of the actual heterogenous, anisotropic, seismic velocity

structure from the assumed homogenous, isotropic, layered velocity

model. In particular, we know that the geology is highly variable

horizontally, and that the previous gob on one side of the panel

influences the seismic velocity. So in analyzing the seismic locations,

it must be remembered that the overall event locations can be shifted

a little horizontally or vertically by a change in the velocity model.

However, the final velocity model applied to the data is the best

compromise that minimizes the mismatch between calculated event

locations and known source locations for a certain number of events.

In addition, the relative location and magnitudes of the events are

fairly consistent and can be used confidently in making inferences.

Development and use of a spatially heterogeneous, time-dependent,

three-dimensional seismic velocity model for event locations is

beyond the scope of the present work.

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 beginning of the panel provide similar

information (1). The results of normalizing the location of the events

from the final part of the panel to the face position are shown in plan

view in figure 7, in a vertical view parallel to the advance direction in

figure 8 and in a vertical view parallel to the longwall face in figure 9.

Figures 7 through 9 indicate that the seismic activity is mostly

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

This agrees with seismic data from other coal mining sites (5) and has

been interpreted to represent the failure of the strata in the forward

stress abutment zone. 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. From the plan view in

figure 7, 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 above.)
-200.0

-100.0

0.0

100.0

200.0

-300.0 -200.0 -100.0 0.0 100.0 200.0 300.0

Distance Across Face (m)

Distance Above Panel (m)

Tailgate

Headgate

Panel 1 Panel 3

Figure 9. Vertical view parallel to the longwall face showing the normalized event locations for the last part of Panel 2

from the side view in figure 8, it can be seen that the vast majority of

the seismic activity is occurring in the face abutment zone, and that

there is a notable absence of recorded seismic activity coming from

the gob area. Also in Figure 8, 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 (5) and

is consistent with a front abutment stress field that is vertically

symmetric about the coal seam. In fact, in figure 8, a majority of the

seismic activity appears to be 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.

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 2). 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 north-northwest (10). 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 approximately evenly distributed above and

below the panel (1). 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 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 mine-induced, 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 (11). 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 no

noticeable coal discharge, this instance documents a rather dramatic

example of how large local seismic events do not necessarily result in

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.

In summary, the unique results of this microseismic research

project have increased our understanding of the longwall strata failure

processes and promise to improve the science of future mine designs.

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