Detecting Strata Fracturing and Roof Failures from a Borehole Based 

Microseismic System 

T. S. Bajpayee, Lead Research Engineer 

A. T. Iannacchione, Principal Research Engineer 

NIOSH-Pittsburgh Research Laboratory 

Pittsburgh, PA
 

S. R. Schilling, UG Mine Engineer/Geologist
 

Pleasant Gap Mine 

Pleasant Gap, PA 

ABSTRACT 

Modern, highly productive underground mining operations 

need to assure the safety of their workforce by understanding 

where major strata fractures and roof failures could occur. Mines 

depend on a host of tools to assess ground instability. In-mine 

microseismic monitoring systems with sufficient number of 

sensors have also been used to provide this information. These inmine systems must be robust in design and moved regularly to 

keep up with the mining advancement. System performance 

issues depend on maintaining ever increasing cable runs, 

preventing component damage from mining activities (scaling, 

blasting, etc.) and avoiding signal degradation due to interference 

from mining equipment (fans, trucks, electrical equipment, etc.). 

A potential solution is to use a surface-based monitoring system 

having sensors placed in boreholes above and adjacent to the 

underground mine workings. These sensors must be capable of 

detecting and locating strata fracturing associated with rock failure 

events. A great advantage of a surface-based monitoring system is 

its independence from the underground mine infrastructure. There 

is less concern with cable maintenance problems, signal 

degradation by interference from mining activity, or loss of power 

from the mine’s power grid. The disadvantages include the cost of 

drilling boreholes and challenges associated with placing sensors 

in the boreholes, maintaining radio communication and solar 

power, and public interference (vandalism, etc.). 

This paper describes a case study where a surface-based 

microseismic system, using triaxial geophones in boreholes drilled 

from the surface, was deployed at a large limestone mine. The 

system was operational during a roof fall that occurred in October 

2007. It detected the first rock fracture event 17 minutes before 

the rock fall event. The geophone array was sensitive enough to 

identify all large rock fracture, impact, and blast events as well as 

medium-size rock fracture events occurring close to the geophone 

array. 

INTRODUCTION AND BACKGROUND 

Falls of ground represent one of the most significant hazards to 

our nation’s miners. Accidental deaths due to falls of ground 

account for 43% of all underground mining fatalities (NIOSH, 

2004). Reducing the number of ground fall injuries is a central 

goal of the National Institute for Occupational Safety and Health 

(NIOSH) mine safety research program. This study constitutes a 

part of the research goal of reducing injuries due to ground failure 

in the mining industry. A small but growing segment of the 

mining industry is underground stone. Generally, entries in stone 

mines have heights ranging from 6.1 m (20 ft) to 18.3 m (60 ft) 

that make it more difficult to quickly recognize deteriorating roof 

conditions. This study evaluates a surface-based microseismic 

system for monitoring rock fracturing and roof fall events at an 

underground stone mine in central Pennsylvania. 

Applications of microseismic systems to monitor roof stability 

have been examined by the underground mining sector. This 

technology has been used in many countries notably Australia, 

Canada, South Africa, United Kingdom, and the United States to 

monitor ground stability issues. An early evaluation of this 

technology by Obert and Duvall (1945a and 1945b) dates back to 

the 1940’s. When a roof rock fractures or moves along a slip 

plane, it typically emits microseismic emissions. Miners have 

often noticed the association of popping or cracking noises with 

fracturing of roof strata. Obert and Duvall (1967) have long 

recognized that for every audible noise, there most likely occurs 

an equivalent multitude of microseismic emissions. Each of these 

emissions signifies the formation of a new rupture surface or slip 

on an existing fracture surface. Development of new fractures can 

lower the overall rock mass strength (Hardy, 1975). Therefore, 

elevated levels of seismicity generally signal development of 

potentially unstable strata conditions (Brady and Haramy, 1994). 

During the past decade, microseismic monitoring techniques to 

characterize roof instability have been reported by Hayes (2000), 

Cai et al. (2001), Heasley et al. (2001), Iannacchione et al. (2004, 

2005), and Srinivasan et al. (2005). More recently, Ellenberger 

and Bajpayee (2007) studied the application of microseismic 

monitoring techniques for early detection of roof instability. 

A seismic event due to rock fracturing generates transient 

dynamic elastic waves that propagate through the surrounding 

rock mass. The p- and s-waves, also known as body waves, travel 

in a rock medium at characteristic velocities, Cp and Cs, given by 

the following equations (Persson et al., 1993), where Cp is the pwave velocity (m/s), Cs is the s-wave velocity (m/s), E is the 
 

 

 

 

modulus of elasticity (GPa), ν is the Poisson’s ratio, and ρ is the 

density (kg/m3

) of the medium. 

⎡ E(1−ν ) ⎤

1/ 2	 

Cp = ⎢ ⎥ ⎣(1+ν )(1− 2ν )ρ (1) ⎦

⎡ E ⎤

1/ 2 

Cs = ⎢ ⎥ ⎣2(1+ν )ρ (2) ⎦

Wave velocities at this study site were determined using several 

test blasts. The average value of the p-wave velocity was 6,039 

m/s (19,813 ft/s) and for the s-wave 3,017 m/s (9,899 ft/s). These 

values were used for computing source location. 

 Mining-induced microseismicity generally relates to shear and 

tensile fractures in a rock mass caused by normal mining 

operations. Five modes of failure examined by Gale et al. (2001) 

are: (1) shear fracture through intact rock material, (2) tensile 

fracture through intact rock material, (3) shear fracture of bedding 

planes, (4) tensile fracture of bedding, and (5) remobilization of 

pre-existing fractures. While the seismic waves due to shear 

fractures could literally propagate hundreds of meters, the seismic 

waves due to low-energy tensile failures rarely propagate beyond 

the first hundred meters. Low-energy tensile failures are difficult 

to locate unless the event is close to the seismic array. 

A borehole-based microseismic monitoring system was 

installed from the surface of a large underground stone mine in 

Center County, Pennsylvania with the objectives of: 

•	 increasing fundamental knowledge of roof behavior in a 

room-and-pillar stone mining operation, 

•	 evaluating a surface-based rock fracture and roof fall 

detection technology, and 

•	 examining the location and extent of roof instability prior 

to failure. 

This project is a cooperative effort between NIOSH and the 

operator of a large underground stone mine. NIOSH provided 

most of the data acquisition and monitoring instrumentation. The 

mine operator provided access to its site, necessary infrastructure, 

cost of drilling and maintaining boreholes, central data acquisition 

and processing computer, and support of other operational 

requirements. 

GEOLOGIC SETTING AND MINING CONDITIONS 

The study mine is extracting limestone by underground roomand-pillar mining in the Valentine member of the Linden Hall 

formation belonging to the Lower Middle Ordovician Limestones 

(Rones, 1969). The Valentine member is about 22.9 m (75 ft) 

thick in this area and comprises rocks of two closely related 

lithologic types. The upper 15.2 m (50 ft) comprises 

unlaminated, light-gray calcilutites ranging from 0.9 to 1.5 m (3 to 

5 ft) in thickness (Rones, 1969). The lower 6.1 m (20 ft) is well 

laminated with thickness ranging from 0.3 to 0.9 m (1 to 3 ft) with 

thin clay or fine bentonite partings. The Valentine Limestone 

comprising light-gray, chemically pure limestone lies above the 

dark-gray 12.8-m (42-ft)-thick Valley View member of the Linden 

Hall formation. About 9.1 m (30 ft) of dark-gray and thinlybedded section of Centre Hall member of the Nealmont formation 

lies above the Valentine Limestone. The Valentine Limestone 

generally dips at 15 degrees and the strike direction runs 

approximately at N60°E (Rones, 1969). 

The southward advance of mining is projected to extend below 

the Nittany mountain range to a depth of about 549 m (1800 ft). 

The uniaxial compressive strength of Valentine Limestone ranges 

from 100 to 145 MPa (14,500 to 21,000 psi) and the maximum 

horizontal stress from 14.9 to 29.6 MPa (2,170 to 4,300 psi) in the 

N80°E direction (Esterhuizen and Iannacchione, 2004). The 

elastic modulus of Valentine Limestone is about 50 GPa (7.25x106

psi). 

The mine management adopted a novel stress control mine 

design to alleviate the effects of high horizontal stress and 

improve ground stability. Pillars are rectangular having their long 

axis oriented parallel to the maximum horizontal stress direction. 

Pillar size is approximately 46 x 20 m (150 x 65 ft). The 

percentage of extraction is roughly 60% with 40% left in pillars to 

support the superincumbent strata. Crosscuts are staggered to 

develop three-way intersections. During development, rooms are 

driven to 15.2 m (50 ft) wide by 7 m (23 ft) high using 4.4-m 

(14-ft) deep wedge pattern blast holes. Subsequently, an 

additional 11.3 m (37 ft) of rock is extracted by floor-benching. 

The overburden thickness ranges from outcrop to approximately 

274.3 m (900 ft). Figure 1 shows the benched and development 

areas in relation to boreholes A, B, C, and D where seismic 

sensors are installed. 

The dark-gray, thin-bedded Centre Hall Limestone 

immediately above the Valentine is laminated and not strong 

enough to constitute the immediate roof beam. On the other hand, 

Valentine Limestone is massive and strong to form a stable roof 

beam. In areas where the mining depth exceeds 274.3 m (900 ft), 

a layer of Valentine Limestone of thickness 1.8 m (6 ft) or more is 

left at the top of the entry to act as an immediate roof beam. In 

addition, a stable roof profile is maintained, where possible, by 

implementing controlled blasting using emulsion explosives with 

non-electric delay detonators. A significant outcome is enhanced 

ground stability, reduction of guttering, and diminution of roof 

fractures and roof failures. Good ground control measures are 

used at this mine to ensure long-term stability of entries, 

particularly entries related to haulage, ventilation, and escape 

ways. Implementation of a safety culture, since the operations 

began in June 1998, has contributed to prevention of fatal groundfall injuries. Additionally, no lost-day injuries were attributed to 

roof fall events since 2001. 

MICROSEISMIC SYSTEM DESCRIPTION 

A surface-based microseismic monitoring system developed 

and marketed by Engineering Seismology Group1

 (ESG), Canada 

was installed at the study mine. During the initial drilling phase, 

two boreholes collapsed prior to installing any geophones. The 

presence of a Bentonite layer in the overburden most likely caused 

the borehole collapse. New holes were drilled in different 

locations and geophones were successfully installed. Eight 

triaxial geophones were installed in four deep boreholes drilled 

from the surface. Table 1 lists the depth of the Valentine roof 

level below the sensor location. This depth ranged from 9.2 m 

(30.2 ft) to 175.4 m (575.3 ft). Borehole B extends about 297.3 m 

(975.3 ft) below the surface level. 

1 Mention of any company or product does not constitute endorsement by 

the National Institute for Occupational Safety and Health.. 

 

The microseismic system consists of a central site located at 

the mine office and four borehole sites distributed over the current 

and projected mining progression areas. At each borehole site, a 
 local data acquisition station (figure 2) is set up with its processor 

unit, power supply, radio communication, GPS timing, and 

geophones. Two geophones and associated data transfer cables 

were carefully handled and placed in the borehole (figure 3). The 

cable ends were then connected to the local data acquisition 

station. 

Roof fall Roof Fall Roof Fall 

Location C Location B Location A 

0 Scale, ft 

Scale, 

500 

15 

Benched Area 

Development Area 

. Figure 1. Roof fall areas and borehole locations on a mine map. 

Table 1. Depth of Valentine Limestone below seismic

sensors (triaxial geophones). 

Borehole 

ID Sensor ID

Depth of Valentine 

roof below sensor 

location, m (ft) 

Depth of sensor 

below the surface 

level, m (ft) 

A Sensor 1 99.8 (327.4) 76.5 (251.0) 

A Sensor 2 130.6 (428.4) 45.7 (150.0) 

B Sensor 1 144.9 (475.3) 152.4 (500.0) 

B Sensor 2 175.4 (575.3) 121.9 (400.0) 

C Sensor 1 9.2 (30.2) 110.9 (364.0) 

C Sensor 2 39.7 (130.2) 80.5 (264.0) 

D Sensor 1 85.0 (279.0) 152.4 500.0) 

D Sensor 2 115.5 (379.0) 121.9 (400.0) 

Figure 2. Microseismic data acquisition station at a 

borehole site. 

Figure 3. Cables leading from the geophones to the data 

acquisition processor. 

 The computer at the central site is running a network data 

acquisition system for real time data acquisition and processing by 

interfacing with the four borehole stations using TCP/IP 

communications. The essential system components at the central 

site are shown in figure 4. The central site controls each borehole 

site and continuously monitors all 24-channels of the seismic array. 

Event triggering is controlled by setting limits for individual 

channels. 
 

 

 

 

 

 

 

 

 

 

 

 

Each borehole data acquisition station is powered by six, 12­

volt, rechargeable batteries. A twin-unit solar panel recharges the 

battery pack. The solar panel is mounted on the roof of the 

instrument housing (figure 2). Each borehole station has 2 GB 

internal storage capacity and is a web-enabled microseismic 

recorder/server with a sampling rate from 50 Hz to 10 kHz. The 

sampling rate during this study was 4 kHz. The essential system 

components of a borehole site are shown in figure 5. Each 

borehole data acquisition and processor station communicates 

with two attached triaxial geophones and the GPS receiver-timing 

unit. Each timing unit receives a time signal from the GPS and 

transfers the data to the system so that all four borehole stations 

are synchronized and record seismic signals on a common time 

base. Eight triaxial geophones, each capable of measuring seismic 

wave in three perpendicular directions, are installed in four 

boreholes (figure 1). 

Figure 4. Essential system components at the central site. 

Figure 5. Essential system components at a borehole site. 

OCTOBER 2007 ROOF FALL 

A roof fall occurred during the afternoon of October 22, 2007, 

in an abandoned part of the mine. During a routine afternoon 

inspection, an unusually high concentration of air-borne dust near 

the southeast end of the mine was observed. As the inspection 

progressed, it was observed that several brattices in the area were 

knocked down. These two observations indicated that an air-blast 

had been produced due to a roof fall. Upon further examination, it 

was observed that an entire intersection of roof at location A 

(figure 1) had fallen. The intersection area from the bottom of the 

benched floor to the top of the roof-line was filled with fallen roof 

debris. It is estimated that the roof cavity extended about 18 m 

(60 ft) or more above the normal roof-line. An estimated 31,000 

tons of roof rock had fallen in this intersection. 

It was further noticed that another intersection, location B 

(figure 1), had also collapsed and the debris completely filled the 

entire height from the bottom of the benched floor to the visible 

roof-line. The condition of this intersection was similar to the 

intersection at location A. The pillars around these two fall areas 

were intact and did not show any signs of degradation due to the 

impact of falling roof debris or stress redistribution pursuant to the 

roof falls. These 4-way intersections were drifted in November 

2001 and benched in June 2006. About 12-ft of immediate roof 

layer further to the northeast of these falls, location C, had also 

collapsed (figure 1). This fall was about 29 m (95 ft) long and 

covered the entire width of the entry. Roof fall areas in locations 

A, B, and C were abandoned and bermed off in July 2006. 

Microseismicity Associated with the 

October 2007 Roof Fall 

The background seismicity rate prior to the October 22 roof 

fall was about two events per day. Figure 6 shows a plot of thirty 

five rock fracture events recorded during this roof fall episode. 

The first rock fracture event was recorded at 1:37 PM on October 

22 and within a period of 17 minutes another eight rock fracture 

events were recorded. Thereafter, the event rate slowed down. It 

resurged at 1:33 PM on October 23 and fifteen events were 

recorded within a period of about 1 hour and 11 minutes. Later, 

the microseismicity rate returned to the background level. 

0

2

4

6

8

Cumulative Event Count

45 

30 

15 

0 

19-Oct 21-Oct 23-Oct 25-Oct 

First Rock 

Fracture 

First Roof Fall 

Impact 

Background Seismicity 

Level 

Elevated 

Seismicity 

Level 

Figure 6. Cumulative plot of rock fracture events. 

Roof fall impact signatures are different from rock fracture 

signatures. Generally, rock fracture signatures display sharp pand s-arrivals and the wave amplitude generally decays rapidly 

within a fraction of a second. The rock fracture event shown in 

figure 7 lasted for about 275 ms and the maximum particle 

velocity recorded was 0.05 mm/s. A spectral analysis of the rock 

fracture waveform using the ESG software yielded a corner 

frequency of 75 Hz. The amplitude diminished rapidly for all 

frequencies over 75 Hz. Roof fall impact signatures are long 

emergent waves of generally more than a second in duration. The 

roof fall impact event shown in figure 8 essentially represents a s­
 

 

 

 

 

 

 

 

wave signature. It is characterized by a duration of about 3 sec, 

maximum particle velocity of 0.2 mm/s, and a corner frequency of 

22 Hz. Exact location and source parameters of impact events are 

difficult to calculate due to the emergent nature of the wave. The 

first roof fall impact signature was observed at 1:54 PM on 

October 22. Two more roof fall impacts quickly followed the first 

impact. Another three roof fall impacts were observed much later 

on October 22. Currently, plans are being finalized to install 

additional sensors to extend seismic coverage area and enhance 

array sensitivity. 

P 

S 

275 ms 

Figure 7. Seismic signature of a rock fracture event showing 

p-and s-wave arrivals. 

3 sec

SUMMARY AND CONCLUSIONS 

3 sec 

Figure 8. Seismic signature of an impact event. 

Falls of ground represent a great hazard for the miners in 

underground situations, particularly in stone mines where entry 

heights range from 6.1 m (20 ft) to 18.3 m (60 ft). Microseismic 

monitoring is another tool to anticipate roof fall hazards by 

assessing stability conditions in underground stone mines. Inmine seismic monitoring has been used to warn miners about 

potential roof instability in other types of mining, and the goal 

here is to apply this technology to the underground stone mining 

sector. 

A surface-based system comprising eight triaxial geophones is 

installed at a large stone mining operation conducting 

room-and-pillar mining. The system is self-contained and does not 

depend on an underground power grid. The monitoring system 

detected elevated level of microseismicity during a large roof fall 

that occurred in an abandoned part of the mine. The system 

detected the first elevated microseismic event 17 minutes before 

the first rock fall. The monitoring system displayed a marked 

increase in the seismicity level from the background rate (2 events 

per day) during the roof fall episode. Plans are underway to install 

additional geophones to extend the seismic coverage area and 

improve array sensitivity. NIOSH plans to continue this study to 

further evaluate the relationship between microseismicity and roof 

instability. 

DISCLAIMER 

The findings and conclusions in this paper 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. 

REFERENCES 

1. Brady, B.T. and Haramy, K.Y., (1994). High Amplitude 

Stress Wave Generation and Damage Induced by Roof 

Caving in Mines. Proceedings of the First North American 

Rock Mechanics Symposium, Austin, TX, pp. 1033-1040. 

2. Cai, M., Kaiser, P.K. and Martin, C.D., (2001). 

Quantification of Rock Mass Damage in Underground 

Excavations from Microseismic Event Monitoring. 

International Journal of Rock Mechanics and Mining 

Sciences 38:1135-1145. 

3. Ellenberger, J.L. and Bajpayee, T.S., (2007). An 

Evaluation of Microseismic Activity Associated with Major 

Roof Falls in a Limestone Mine: A Case Study. 2007 SME 

Annual Meeting and Exhibit, Denver, Colorado, preprint 07­

103, pp. 1-5. 

4. Esterhuizen, G.S. and Iannacchione, A.T., (2004). 

Investigation of Pillar-Roof Contact Failure in Northern 

Appalachian Stone Mine Workings. Proceedings of the 23rd 

International Conference on Ground Control in Mining, 

Morgantown, WV, pp. 320-326. 

5. Gale, W.J., Heasley, K.A., Iannacchione, A.T., Swanson, 

P.L., Heatherly, P. and King, A., (2001). Rock Damage 

Characterization from Microseismic Monitoring. 

Proceedings of the 38th U.S. Rock Mechanics Symposium, 

Washington, D.C. 

6. Hardy, R., (1975). Emergence of AE/MA as a Tool in 

Geomechanics. Proceedings of the First Conference on 

Acoustic Emission-Microseismic Activity in Geologic 

Structures and Materials, University Park, PA, June 9-11, 

Series on Rock and Soil Mechanics 2(3):13-31. 

7. Hayes, P., (2000). Moonee Colliery: Renewing the 

Economic Viability of a Mine Using Microseismic And 

Hydraulic Fracturing Techniques. Proceedings of the 19th 

International Conference on Ground Control in Mining, 

Morgantown, WV, pp. 38-44. 

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

(2001). ,Microseismics. Proceedings of the 20th 

International Conference on Ground Control in Mining, 

Morgantown, WV, pp. 280-286. 

9. Iannacchione, A.T., Coyle, P.R., Prosser, L.J., Marshall, 

T.E. and Listenberger, J., (2004). The Relationship of Roof 

Movement and Strata-Induced Microseismic Emissions to 

Roof Falls. 2004 SME Annual Meeting and Exhibit, Denver, 

Colorado, preprint 04-58, Littleton, CO, Society for Mining, 

Metallurgy, and Exploration, Inc., pp 1-9. 
 

 

 

10 Iannacchione, A.T., Esterhuizen, G.S., Bajpayee, T.S., 

Swanson, P.L. and Chapman, P.C., (2005). Characteristics 

of Mining-Induced Seismicity Associated With Roof Falls 

and Roof Caving Events. Proceedings of the 40th U.S. Rock 

Mechanics Symposium, Anchorage, AK, June 25-29 and 

American Rock Mechanics Association, Alexandria, VA, 

June 1-10. 

11. National Institute for Occupational Safety and Health 

(NIOSH) (2004). Worker Health Chartbook. 2004 DHHS 

(NIOSH) Publication No. 2004-146, Washington, DC: US 

Department of Health and Human Services, Public Health 

Services, Centers for Disease Control and Prevention, 

National Institute for Occupational Safety and Health. 

12. Obert, L. and Duvall, W.I., (1945a). The Microseismic 

Method of Predicting Rock Failures in Underground Mining, 

Part 1, General Method. U. S. Bureau of Mines, RI 3797. 

13. Obert, L. and Duvall, W.I., (1945b). The Microseismic 

Method of Predicting Rock Failures in Underground Mining, 

Part 2, Laboratory Experiments. U. S. Bureau of Mines, RI 

3803. 

14. 

15. 

Obert, L. and Duvall, W.I., (1967). Rock Mechanics and 

the Design of Structures in Rock. John Wiley and Sons, Inc., 

New York, NY, pp. 113-188. 

Persson, P., Holmberg, R. and Lee, J., (1993). Rock 

Blasting and Explosives Engineering. CRC Press, New York, 

NY, pp.342. 

16. 

17. 

Rones, M., (1969). A Lithostratigraphic, Petrographic and 

Chemical Investigation of the Lower Middle Ordovician 

Carbonate Rocks in Central Pennsylvania. General Geology 

Report G 53, Bureau of Topographic and Geologic Survey, 

Commonwealth of Pennsylvania. 

Srinivasan, C., Sivakumar, C., Gupta, R.N., (2005). 

Source Parameters of Seismic Events in a Coal Mine in India. 

27th Seismic Research Review: Ground Based Nuclear 

Explosion Monitoring Technologies, Rancho Mirage, 

California, September, https://www.nemre.nnsa.doe.gov/ 

review2005.