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J. L. Ellenberger

National Institute for Occupational Safety and Health, Pittsburgh, Pennsylvania, USA

K. A. Heasley

National Institute for Occupational Safety and Health, Pittsburgh, Pennsylvania, USA

P. L. Swanson

National Institute for Occupational Safety and Health, Spokane, Washington, USA

J. Mercier

Willow Creek Mine, Helper, Utah, USA

ABSTRACT: The National Institute for Occupational Safety and Health (NIOSH) has continued the research role

of the former US Bureau of Mines (USBM) to engineer techniques that will reduce the hazards in the mining work

place associated with coal bumps. Recent research focused on a longwall coal mine in Utah with overburden

greater than 750 m (2,500 ft) containing several massive sandstone units. The primary field instrumentation at the

site was three-dimensional, full waveform, autonomous microseismic arrays placed underground and on the surface

in order to surround the active multi-panel longwall district. The purpose of these arrays was to help investigate

the strata mechanics associated with the redistribution of stress and the associated gob formation of the longwall.

Specifically, the seismic arrays were used to determine the timing and location of the failure in the strata

surrounding the active mining. Overall 13,000 seismic events were detected and located with on-site processing

during the five months the panel was being mined, including a magnitude (ML) 4.2 event. Of these, a smaller

subset of 5,000 well-located events was selected during post-processing to form a consistent data set for analysis

in this paper. From this data set, it was observed that the seismic events generally occurred in advance of the

longwall face, both above and below the panel, consistent with failure of the strata in the forward stress abutment

zone. Also, the occurrence of the ML 4.2 seismic event within 150-180 m (500-600 ft) of a deep cover longwall

face with no associated bump caused a re-evaluation of the nature of the connection between seismic activity and

coal bumps.

1 INTRODUCTION

Coal mine bumps, sometimes referred to as outbursts,

bursts, or bounces, have been recognized as a serious

problem in mining for more than 75 years. For the

purposes of this paper, a bump is defined as a sudden

release of strain energy that results in the expulsion of

coal from a rib, pillar, or floor in a catastrophic manner.

Beginning as early as the 1930's, the US Bureau of

Mines (USBM) conducted research investigating the

causes and potential mechanism of bumps in order to

avoid injuries in coal mines (Rice 1936). Research

conducted by the National Institute for Occupational

Safety and Health (NIOSH) continues to focus on the

reduction of bumps and bump hazards.

In the past, bumps have been acknowledged as

having a greater likelihood of occurrence at depths

greater than 300 m (1,000 ft), in the presence of a strong

stiff roof and/or floor, and when an unusually massive

unit exists in the main roof (Rice 1936; Iannacchione

and Zelanko 1995; Zelanko and Heasley 1995). Miners

working under one or more of these conditions need to

be constantly aware of the possibility of a coal bump.

Recently, the US coal industry experienced several

consecutive years with no fatal accidents resulting from

bumps, until November of 1996, when three fatalities

and five additional serious injuries occurred in a two

week period (MSHA 1996a,b).

Coal bumps are often associated with seismic events

that are large enough to be registered by regional

seismic networks. However, 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 seam level. In

reality, coal bumps are just one subset of mine-induced

seismicity, and like mine-induced non-bump events,

they can exhibit a wide range of magnitudes and energy

releases. Also, there are considerably more

documented mine-induced, non-bump, seismic events

than there are documented coal bumps. In fact, the

seismic network in Utah has found the most active

seismic area since 1962 to be in the vicinity of the

active coal mining with hundreds of mining-induced

events (M>2) recorded each year (Arabasz et al, 1996)

Recent bump research in NIOSH has centered on a

Utah longwall mine. At this site, the rugged topography
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Figure 2. Plan view showing mine layout, overburden,

geophone arrays, and the location of the ML

 4.2 event

Figure 1. Generalized stratigraphy.

combination of depth and strong geology indicated a

high likelihood of bumps. In order to investigate the

rock failure associated with the redistribution of stress

and the associated gob formation of the longwall in this

bump-prone situation, a microseismic system for

“listening” to the rock was chosen. The microseismic

system consisted of three-dimensional arrays placed

underground and on the surface surrounding the active

mining area. The initial analyses of the geologic,

geometric and mining parameters, and the associated

seismicity from this site are presented in this paper.

2 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) (Barron et al. 1994). The subject mine

is primarily in the Castlegate ‘D’ Seam; however, over

part of the area studied where the seams coalesce, the

mine operates in the joined Kenilworth Seam and the

Castlegate ‘D’ Seam. Normally, the Kenilworth Seam

lies below the Castlegate ‘D’ and above the Castlegate

‘C’ in a cyclic sequence of coal bearing rocks (Barron,

1994). In the Blackhawk Formation the primary

interburden consists of a series of regressively deposited

sandstones which were reworked. The 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 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 channel

sandstones occur. These lenticular deposits make up

the immediate and main roof over the mine.

Unconformably overlying theBlackhawk Formation

is the Castlegate Sandstone. The Castlegate is a

massive, cliff forming sandstone that is 120 to 180 m

(400 to 600 ft) thick and approximately 200 m (680 ft)

above the “D” seam in the mine area, with the basal 90

m (300 ft) being more compact and massive than the

upper portion of the unit. 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 uppermost rocks

exposed at the site are lacustrian deposits of the North

Horn Formation, consisting of interbedded claystones,

mudstones, limestones, siltstones, and sandstones.

 

3 MICROSEISMIC FIELD SITE

The primary objective in this field effort was to

instrument a deep, bump-prone longwall mine with a

three-dimensional seismic monitoring system in order

to examine the behavior of the main roof, gob, and

floor. This microseismic system "listened" to the rock

and determined the timing and location of the failure of

the rock strata surrounding the longwall. By analyzing
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Figure 3. Schematic representation of the microseismic

system.

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 order to mitigate dangerous bump

occurrences.

The monitoring array consisted of 23 geophones

deployed both underground and on the surface. 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 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, another 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 (Swanson 2001), over 13,000 seismic

event were detected and located during the mining of

panel 2.

In order to obtain a consistent data set of event

locations for this preliminary 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

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 postprocessing procedure winnowed the original 13,000

events down to a good quality data set consisting of

approximately 5,000 events from panel 2.

Taking an overall look at the results of the postprocessing procedure, a shift of the events by as much

as 75 m (250 ft) toward the headgate and away from the

tailgate is exhibited. This shift is thought to result from

deviations of the actual heterogenous, anisotropic,

seismic velocity structure from the assumed

homogenous, isotropic, layered velocity model. For

instance, 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 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.

4 SEISMIC EVENTS

The final set of 5000 events from panel 2 were analyzed

in two groups since the face width increased from 165

to 245 m (550 to 820 ft) when the barrier pillar

separating panels 1 and 2 ended at approximately three

quarters of the length of the panel (Figure 2). A review

of the data revealed that the number of events were

somewhat evenly divided between the two groups with

2,447 events occurring in the initial “narrow” three

quarters of the panel, and 2,577 events occurring in the

final “wide” one quarter of the panel. To initially

visualize the recorded seismicity, the locations of the

events were normalized to the advancing longwall 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
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Figure 4. Plan view of the normalized event locations

for the first three quarters of panel 2.

Figure 6. Vertical view parallel to face advance

showing the normalized event locations for first the

three quarters of Panel 2.

Figure 8. Vertical view parallel to the longwall face

showing the normalized event locations for the first

three quarters of Panel 2.

Figure 9. Vertical view parallel to the longwall face

showing the normalized event locations for the last

quarter of Panel 2.

Figure 5. Plan view of the normalized event locations

for the last quarter of panel 2.

Figure 7. Vertical view parallel to face advance showing

the normalized event locations for the last quarter of

Panel 2.

visualization process are shown in plan view in Figures

4 and 5, in a vertical view parallel to the advance

direction in Figures 6 and 7 and in a vertical view

parallel to the longwall face in Figures 8 and 9.

From the plan view in Figures 4 and 5, it can be seen

that most of the seismic activity generally lines up with

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.) In comparing Figure 4 to 5,

the events appear more dispersed (in relation to the

face) in the first part of the panel and generally behind
1325

pgg

above and behind the face as the gob developed to the

full sustained height carried throughout the rest of the

panel. Or, the relative dispersion could be affected by

the reduced constraint on the locations at the beginning

of the panel which is at the edge of the array. Also, it is

interesting to note that the events in the final wider

quarter of the panel are very concentrated in front of the

face and occur more frequently (about 3 times more

often per given advance). We believe that the linearity

of the events in the final quarter of the panel is due to

the well established gob and that the increased

frequency is a response to the wider panel; however, the

affect of spatial variation in event detection sensitivity

may also be a factor, but its role has not yet been

analyzed in detail.

Figures 4 through 9 indicate that the seismic activity

is mostly located in the face area and generally in front

of the face. This agrees with seismic data from other

coal mining sites (Luo et al. 1998) and has been

interpreted to include 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 side view in Figures 6 and 7, it can

be seen that the vast majority of the seismic activity is

occurring in the face abutment zone, but that there is a

notable absence of recorded seismic activity coming

from the gob area. Also in Figures 6 and 7, it can be

seen that the seismicity is originating both above and

below the seam level. This response also coincides well

the response observed at other field sites (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 7, 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.

Figures 8 and 9 display the distribution of events in

a vertical view parallel to the face. In Figure 9, it is

important to note that the center of the face has moved

40 m (135 ft) further away from the headgate as the face

width was increased on the tailgate side. In Figures 8

and 9, it can again be noted that the events appear more

dispersed in relation to the seam in the first part of the

panel, and generally more located above the seam. This

is consistent with the hypothesis expressed earlier that

the higher events were a result of the initial gob

formation during the first part of the panel where

gp(

distribution may be affected by the spatial constraint on

event location due to array geometry).

5 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. There was very little indication of 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 US coal mine. This event

caused rock slides from critical slopes on the nearby

highway and 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

(Figures 2, 6 and 8). 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 (Swanson & 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.
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From examining the seismicity at the site, several

general observations can be made. First, the events in

the first three quarters of the panel appear to be more

dispersed both vertically and horizontally than the

events in the last (wider) quarter of the panel. Not only

are the events relatively concentrated in front of the face

in the last quarter of the panel; but also, the event rate is

about three times as high as that at the beginning of the

panel (2,447 events for 1,000 m (3,000 ft) versus 2,577

for 300 m (1,000 ft)). We hypothesize that this is a

result of the initial gob formation during the first part of

the panel versus a well established gob and wider face

in the last quarter of the panel. Also, from examining

the seismic events, it can be observed that the events

generally occur in advance of the longwall face and

were approximately evenly distributed above and below

the panel. This observation is consistent with the

interpretation that the seismic events coming from

failure of the strata in the forward stress abutment zone

that is vertically symmetric about the coal seam. 

A magnitude 4.2 seismic event occurred within an

active longwall panel and was recorded by the minewide 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 (Ellenberger & 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 was never determined. Using the

mine-wide seismic system the ML

 4.2 seismic event was

relatively accurately located within 150-180 m (500-

600 ft) of a deep cover longwall face with no associated

bump. This documents a rather dramatic example of

how large seismic events do not necessarily result in

face damage, and 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 to

the working face. 

7 REFERENCES

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Seismicity in the Wasatch Plateau and Book Cliffs

Coal Mining Districts, Utah, USA-Overview and

Barron, L.R., DeMarco, M.J. & Kneisley, R.O. 1994.

Longwall Gate Road Stability in Four Deep Western

U. S. Coal Mines. USBM IC 9406.

MSHA. 1996a. C-2 Mine - I.D. No. 15-07201 Harlan

Cumberland Coal Company, Dione, Harlan County,

Kentucky. US Department of Labor, Mining Safety

and Health Administration Report of Investigation,

November 20.

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95.

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Earthquake. Personal communication, May 18.

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