ABSTRACT 

 As the amount of new fractured surfaces or ìdamaged rock 

layersî within roof rock increases, the stability of the rock mass 

decreases. While direct measurements of this phenomenon are not 

easily made, there is good circumstantial evidence to support this 

hypothesis. For example, it is common to observe increased cracks 

or fractures in the immediate mine roof rock before a roof fall. 

Likewise, roof drill holes placed in areas that later fail often reveal 

increased numbers and/or separations of fractures within the rock 

column through time. And finally, the frequency of microseismic 

activity, representative of rock fracturing, increases before a roof 

fall. 

 For this study, more than 700 microseismic emissions were 

collected from two underground limestone mine roof fall areas in 

southwestern Pennsylvania. Microseismic events were located and 

magnitudes determined using the moment magnitude technique. 

Moment magnitude is based on the event seismic moment, which is 

a measure of the seismic deformation. The amount of new fracture 

surface length was calculated based on the stored strain energy 

within the rock prior to fracture. In the two case studies presented, 

a significant amount of microseismic activity was observed as 

much as two days before the first signs of failure in the roof fall 

areas. Additionally, results from this analysis reveal much about 

the behavior of strata prone to failure and allows for the 

construction of hazard maps based on microseismic emissions. The 

potential use of this technique as a means of anticipating roof falls 

is analyzed and discussed. 

INTRODUCTION 

 For years, the international mining safety research community 

has attempted to develop technology that will provide warning of 

impending roof falls. Many different methods and techniques have 

been tried with varying degrees of success. A significant hindrance 

to applying roof fall warning techniques is the lack of knowledge 

concerning the behavior of pre-fall rock strata and their associated 

precursor characteristics. It is difficult to apply any technology if 

our basic understanding of the important characteristics of the 

phenomenon are unknown. Therefore, a key requirement is to 

determine the rock failure processes leading to a roof fall. It is also 

important to recognize that these pre-rock fall behaviors will 

depend on site specific conditions, such as geology, in situ stress, 

and mine layout. 

 The use of microseismic activity to warn of roof falls was first 

evaluated in the 1940's (Obert and Duvall, 1945a & 1945b) for 

monitoring general stability conditions. Later, Leighton and 

Stebley (1977) demonstrated that increased noise rates, from a 

system capturing events in the 36 to 44 kHz range, preceded the 

failure of a coal mine roof by as much as 15 minutes. They 

recorded ultrasonic signals because excessive amounts of cultural 

noise existed in the lower frequency ranges. Unfortunately, the 

ultrasonic frequencies attenuate rapidly, requiring geophones 

placement within 40 m of the roof fall events. These kinds of 

system requirements present numerous operational issues. Ideally, 

roof fall warning systems need to be capable of monitoring large 

areas of the mine with a minimal number of geophones. 

 Since this early work, much of the mining induced seismicity 

research has been focused on the rock burst, coal bump, and gas 

outburst problems. This focus on dynamic failures has produced 

analysis techniques that can be traced to earthquake seismology. 

Analytical procedures that rely on techniques developed for 

earthquake seismology to determine the magnitude and energy 

associated with rock failure events are ill equipped for the roof fall 

problem. Earthquakes are typically associated with fault planes 

with large rupture surfaces and comparatively small displacements. 

Events leading to roof falls are comprised of many small releases of 

energy that represent shear and tensile failure both through intact 

rock and along preexisting discontinuities in the rock. It is 

assumed that the rock stress drop is complete since the roof beams 

fail next to the mine opening, virtually eliminating any confining 

stress. Also, the rupture surfaces are typically much smaller than 

the width of the mine openings (<14 m). Therefore, roof fall 

analyses need to focus on smaller rock failure events with high 

source area stress drops and small rupture surfaces. 

 Gale (2003) proposed a technique to overcome this discrepancy 

by calculating the amount of new rock fracture length generated 

from each pre-roof fall seismic event. Defining roof stability based 

on this parameter has a well defined meaning. It also allowed for 

the summation of lengths so that the accumulation of damage can 

be tracked through time. Tracking the time and location of new 

rock fracture surfaces allows for the construction of isopach maps 

that display the intensity of new fractured ground within a wide 

mining area. These intensity data are then used to construct hazard 

maps. The effectiveness of these maps in warning of actual roof 

fall conditions at two different sites is assessed in this study. 

MAPPING HAZARDS WITH MICROSEISMIC TECHNOLOGY TO ANTICIPATE ROOF 

FALLS - A CASE STUDY 

Anthony T. Iannacchione, Senior Scientist

Timothy Batchler, Mechanical Engineer 

Thomas E. Marshall, Engineering Technician

National Institute for Occupational Safety and Health 

Pittsburgh Research Laboratory 

Pittsburgh, Pennsylvania USA 
Study Site Conditions 

 Hazard mapping with microseismic activity was attempted at a 

room-and-pillar underground limestone mine with rooms 14 m 

wide and 9 m high (figure 1). The microseismic system had 

originally been installed at this site to evaluate different mine 

layouts used to control hazards associated with roof falls 

(Iannacchione et al., 2003). The unit mined is the Loyalhanna 

Limestone, a very strong rock unit with unconfined compressive 

strengths as high as 210 MPa. Locally, the Loyalhanna ranges 

from 15 to 21 m thick with dips between 1 to 7°. The rock mass 

characteristics of this unit consist of horizontal bedding planes 

spaced from 1 to 1.5 m apart, widely spaced vertical joints, and 

extensive cross bedding within the individual rock layers 

(Iannacchione and Coyle, 2002). The Loyalhanna is overlain by 

weak Mauch Chunk shales and siltstones. Approximately 300 m 

from the study area and under 120 m of overburden, high 

horizontal stresses were measured, reaching a maximum value of 

approximately 55 MPa in the N 60° E orientation (Iannacchione 

and Coyle, 2002). These factors are important because the 

behavior of this roof rock is dominated by stiff, relatively thin rock 

layers, subjected to excessive levels of horizontal stress. 

 As part of this research, 15 4.5-Hz 630-ohm uniaxial geophones 

were connected to a microseismic system. All but one of the 

geophones were mounted on the roof about 10 m above the mine 

floor. One geophone was placed 21 m beneath the mine floor 

within a borehole. The heights of the events above the mine roof 

could not be determined because almost all geophones were located 

within the same horizontal plane. Data collected included dates 

and locations of roof falls and face advances (figure 1). 

TWO DISTINCT ROOF FALL AREAS 

 Two major roof fall episodes occurred during this study. Each 

of these episodes was comprised of one to several individual roof 

falls that either fell suddenly as one mass or gradually as many 

smaller rock falls over distinct time intervals from a few minutes to 

many hours. The March 7 roof fall occurred in the southeast corner 

of the mine while April 21 roof fall occurred approximately 150 m 

to the west-northwest (figure 1). From November 1, 2001 to April 

21, 2002, 92 face cuts were advanced by blasting in the study area 

(figure 1). 

 The first sign of failure associated with the March 7 roof fall 

occurred when mine officials noticed a shear failure in the 

immediate roof around 3 am. By that afternoon, a small shallow 

fall extended approximately 0.6 m into the roof. Sometime during 

the evening of March 7 the roof fall cavity extended across the 

intersection and approximately 3 m into the roof, encountering the 

soft red shales of the Mauch Chunk Formation. From late March 

10 into the morning of March 11, the roof fall extended to the 

southeast where it was stopped by the unmined limestone 

perimeter. During the morning of March 12, the roof fall extended 

to the northwest, causing the roof bolts to fail in tension followed 

by the collapse of a large section of roof. Figure 1 shows the total 

area of the roof fall developed between March 7-12. Small rocks 

continued to dribble from the roof fall until March 14. The roof 

fall cavity created during this period extended over 38 m in length 

and up to 6 m in height (figure 2a). After the March 14 event, the 

area around the roof fall stabilized for a period of approximately 

two months. 

Figure 1. Mine map showing the location of the area 

monitored by the microseismic network. Also shown are the 

locations of the geophones, the faces mined from 

November 1, 2001 to April 21, 2002, and the March 7 and 

April 21 roof falls. 

Figure 2. Photographs of the March 7 (a) and April 21 (b) roof 

falls. 

March 7 Roof Fall March 7 Roof Fall

(a)

9 m

March 7 Roof Fall March 7 Roof Fall

(a)

April 21 Roof Fall April 21 Roof Fall

(b)
 The April 21 roof fall was initiated with a single roof slab fall 

around 8 am which eventually developed into a shallow roof fall 

that encompassed the entire intersection (figure 2b). This area had 

experienced some roof rock fracturing within the past year but had 

been relatively stable for several months. About two weeks after 

the April 21 roof fall, a series of instabilities occurred that resulted 

in several northerly roof fall extensions. 

ROOF FALL CHARACTERISTICS 

 Rock failures in bedded formations with excessive levels of 

horizontal stress can be initiated when the stiffest and thinnest beds 

in the roof strata begin to buckle from the horizontal loading 

(figure 3a). When these layers buckle, shear and tensile rupture 

between layers and low-angle shears through the intact rock layers 

can occur (figure 3b). Eventually the beam begins to cantilever, 

initiating a tensile failure along the fixed contact area at the edge of 

the roof fall (figure 3c). One-by-one the individual roof beams are 

strained to failure, shedding their load to adjacent layers which are 

also strained to failure. In the end, the shape of the roof cavity is 

defined by the sub-vertical tensile failures (figure 3d). Quasi-stable 

conditions are achieved as the roof takes on the more stable arched 

shape. 

 Sometimes these falls remain active at their axial ends where 

the stress concentrations are the highest. There also seems to be a 

slow, methodical de-stabilization of roof beams in the very highest 

sections of the roof fall. Experience has shown that roof falls can 

slowly expand upward over the course of many years. 

 Roof falls at the study site, are typically 14 m wide by 8 m high 

and extend up to 150 m in length. A single roof fall could easily be 

associated with several hundred shear and tensile seismic events 

that are detectable with a microseismic system similar to the one 

used in this study. The amount of seismic energy released by these 

rock fracture events is dependent upon the size of the fracture, the 

characteristics of the in situ rock unit, the pre-failure state of stress, 

and the mode of failure (shear vs. tension). Because the latter three 

characteristics are unknown, it is assumed that the size of the event 

is largely dependent on the size of the new fracture surface. 

 The smaller fractures require more geophones at locations 

closer to the source events. In the study area, 15 geophones had 

been deployed in an asymmetric pattern. The geophones closest to 

the two roof falls were more than 75 m from these areas. Many of 

the smallest seismic emissions associated with the rock fractures 

were below the detection threshold of the system. Therefore the 

source of the activity recorded at the study site primarily represents 

only the larger events emanating from the roof fall areas. 

SIZE OF MICROSEISMIC EVENTS 

 Analysis capabilities are limited when the event magnitude is 

unknown. Characterization of event magnitude allows for 

interpretation of subtle relationships that are not possible with data 

limited to time and location. Moment magnitude (M) is a 

technique used by many international researchers and technicians 

to determine the size of microseismic events. The moment 

magnitude is based on its seismic moment (Mo). 

 Moment magnitude (Hanks and Kanamori, 1979) is based on 

the static seismic moment by the relationship: 

 M = (2/3) · log10(Mo) - 6.0, (1) 

where Mo is expressed in SI units (N-m). The seismic moment 

(Aki, 1968) is determined from the observed Fourier displacement 

amplitude spectrum of body waves, the density of the rock at the 

source, the P wave velocity, the distance between source and 

receiver, and P wave radiation pattern coefficient for source and 

receiver. Seismic moment is a measure of deformation. Seismic 

moments and moment magnitudes were determined for 715 

microseismic events reported on in this study. 

 The radiated seismic energy, Es (Boatwright and Fletcher, 

1984), another measure of the seismic source, is determined from 

the radiation pattern coefficient for P or S waves, the integral of the 

squared velocity for P or S wave motion, the density of the rock at 

the source, the S wave velocity, and the distance between source 

and receiver. Seismic energy values were determined with the 

Engineering Seismology Group Canada, Inc. (ESG) microseismic 

Figure 3. Generalized sequence in which individual roof beams fail and develop into large roof falls. 

Tensile

Shear Tensile

d) Quasi-stable arch-shaped roof

b)

Tensile failure through intact layer

Seismic 

emissions

Initial buckling with shear and tensile failure 

along bedding layers

Beam cantilevers

Shear failure through intact layer 

c)

a)

Tensile

Shear Tensile

d) Quasi-stable arch-shaped roof

b)

Tensile failure through intact layer

Seismic 

emissions

Initial buckling with shear and tensile failure 

along bedding layers

Beam cantilevers

Shear failure through intact layer 

c)

a)
analysis software program (Anon, 2002). From November 1, 2001 

until April 21, 2002 the seismic moment ranged from 3.73E+5 N-m 

to 1.04E+9 N-m while the seismic energy ranged from 0.01 to 174 

Joules. The seismic moment and seismic energy typically are 

linearly related (figure 4). Variations in the data are attributed to 

the seismic moment using only the P wave portion of the signature, 

while seismic energy calculations uses the entire signature. 

MOMENT MAGNITUDE VARIATIONS 

 Moment magnitude values of events from November, 2001 

until April 21, 2002 are shown in figure 5. The events range from 

approximately -2.29 to 0.01 with an average value of -1.18. The 

data shows definitive event clustering in association with the 

March 7 and April 21 roof falls. It is also evident that a relatively 

steady rate of background seismicity occurs between these two roof 

fall events. This data indicate that increased microseismic activity 

only started two days prior to the March 7 roof fall. Additionally, 

there is some evidence that the overall magnitude of events drops 

slightly after the March 7 roof fall episode. 

 The locations of the microseismic events were entirely within 

the area shown in figure 1. Most of the events were within 100 m 

of the center of either the March 7 and April 21 roof fall episodes. 

Often the largest events were within 30 m of the roof falls. In 

contrast, many of the microseismic events occurring away from the 

fall areas were comparatively small. 

TECHNIQUE FOR DETERMINING NEW ROCK FRACTURE 

LENGTH 

 In this study, the seismic energy for each event has been 

measured and the corresponding new fracture length calculated. 

This methodology is utilized to provide the mine operator and 

worker with a more useable and effective means of interpreting 

microseismic data. Miners can relate to the amount of new 

fractured rock developed in the roof above their heads. Gale 

(2003) determined an energy release method based on the stored 

strain energy within the rock before fracture. According to 

Hatherly, et al. (2003) this method is in line with current rock 

mechanics principles. Gale derived equation 2 for determining 

energy release from shear through intact rock: 

Energy(shear failure) = Lf 3 * ∆τxy2

 / (2*G) [units of N·m] (2) 

where: 

∆τxy = Shear stress drop ~ Unconfined compressive strength/2 

 Lf = Length of fracture, m 

 G = Shear modulus = (S wave velocity)2

 x density 

Because shear through intact rock produces the most energy, 

equation 2 probably represents the source mechanism associated 

with the majority of the events detected by the NIOSH 

microseismic system. The shape of the shear rupture surface is 

assumed to be square. 

 Core was recovered from a nearby mine in the Loyalhanna 

Limestone and tested at NIOSHís Pittsburgh Research Laboratory. 

Eleven NX size cores produced an average uniaxial unconfined 

compression strength of 136.4 MPa. This yielded an estimated 

∆τxy value of 68.2 MPa. While Gale suggested using the shear 

modulus calculated from Youngís modulus and Poissonís ratio, this 

study used the shear modulus from available S wave velocity data. 

The shear modulus of 1.65 x 104

 MPa is calculated from a shear 

wave velocity of 2,574 m/s and a density of 2,500 kg-m/s. 

 Through a derivation of the above formulas, the length of new 

fractures formed as a result of both shear and tensile failure can be 

calculated: 

 (3) 

 The energy used in this equation comes from seismic energy 

calculations discussed earlier. McGarr (1999) has indicated that 

the maximum seismic efficiency for earthquakes is less than 6%. 

Event energy was calculated by dividing the seismic energy by a 

seismic efficiency of 4%. In this way, equation 3 was used to 

estimate the length of new fracture lengths produced by each 

microseismic event. 

NEW FRACTURE LENGTH ESTIMATES AND 

PRECURSOR ROOF FALL PATTERNS 

 Calculations of the new fracture lengths produced between 

November 1, 2001 and April 21, 2002 in the study area are shown 

in figure 6. Approximately 41 m of new fractures were developed 

during this time. Almost 15 m of these new fracture lengths were 

developed between March 6 and 12 and approximately 12 m 

between April 19 and 21. After the initial March 7-12 roof fall, the 

rate of new fracture development was reduced to pre-March 7 roof 

fall values. 

Lf (shear failure) = Energy 2 G xy 3 2 ( ∗ ( ∗ )) / ∆τ

y = 2E+07x0.6074

R2 = 0.5424

1.E+05

1.E+06

1.E+07

1.E+08

1.E+09

1.E+10

1.E-03 1.E-02 1.E-01 1.E+00 1.E+01 1.E+02 1.E+03

Seismic Energy, Joules

Seismic Moment, N-m

Figure 4. Relationship between the seismic moment and 

seismic energy for each of the 715 events used in this study. 

-2.50

-2.00

-1.50

-1.00

-0.50

0.00

0.50

1-Nov 1-Dec 31-Dec 30-Jan 1-Mar 31-Mar 30-Apr

Time

Moment magnitutde

Individual events March 7 roof fall April 21 roof fall

Figure 5. Variation in moment magnitudes for the 715 events 

from November 1, 2001 to April 21, 2002. 
 What is of most interest to this study is the character of the prefall microseismic activity. Figure 7a shows activity before and 

after 3 am on March 7 when the first visible sign of failure occurred 

with a shear crack and a small rock fall. By the afternoon of the 

same day, a small section of roof, approximately 0.6 m deep, 

extended into the roof. Figure 7a shows a moderately intense 

period of microseismic activity starting around 2 am and gradually 

declining until 8 am. More importantly, another moderately intense 

period of microseismic activity occurred at 5 pm on March 6, some 

10 hours before the first recognition of the unstable roof rock 

conditions. It is also interesting to note that the period between 

6:30 pm on March 6 and 2 am on March 7, prior to the roof fall, 

was marked by very low microseismic activity. 

 Figure 7b shows activity before and after 8 am on April 21 

when a slab of rock fell from the roof, signaling the beginning of 

that roof fall. An intense spike lasting approximately 2 hours in 

duration occurred 41 hours before the start of the roof fall. This 

was followed by approximately 8 hours of moderate activity which 

was in-turn followed by approximately 10 hours of quiet. Activity 

increased again around noon on April 20 and continued at a fairly 

constant rate through the initiation of the roof fall some 20 hours 

later. 

CONSTRUCTING HAZARD MAPS 

 The data presented indicate that an increased level of 

microseismic activity occurred before the first recognition of roof 

failure at 3 am on March 7 and 8 am on April 21. How does the 

location of this activity relate to the roof fall events? If all events 

were of equal magnitude, a simple two-dimensional plot of event 

locations would suffice. However, as previously demonstrated, 

rock fracturing events have significant variation in magnitude. 

Typically, the event size would be represented by a symbol scaled 

to the moment magnitude of the microseismic event. From 

November 1, 2001 to March 5, 2002 at 3 am, some 48 hours before 

the first signs of failure in association with the March 7 roof fall, 

166 microseismic events occurred within the study area. During 

this time microseismic events in the study area appear to be 

occurring in a fairly random fashion. There was no evidence of the 

impending roof fall from the microseismic information. 

 The trend changes dramatically at approximately 12 noon on 

March 5. Sixty-two microseismic events occurred in the 48 hour 

period before the first sign of failure in association with the 

March 7 roof fall. These events tend to cluster around the March 7 

roof fall producing approximately 3.9 m of new fracture lengths. 

 The study area was divided into 30.5 m squares and all the new 

fracture lengths found in each square were added together. The 

sum of all new fracture lengths for each zone was used to construct 

an isopach map for the March 7 roof fall (figure 8). In general, the 

zones surrounding the March 7 roof fall tend to have total fracture 

lengths of greater than 0.2 m with many greater than 0.4 m. 

 A similar analysis was performed for the April 21 roof fall 

episode. Figure 9 shows the combined totals of new fractured rock 

within 30.5 m squares from 8 am on April 19 until 8 am April 21. 

This 48 hour interval produced almost 11 m of new fracture length 

from 217 microseismic events. There is a strong correlation 

between the intensity of new rock fractures and the location of the 

April 21 roof fall. The majority of the microseismic events that 

represented rock fracture development were within 50 m of the roof 

fall. 

0

5

10

15

20

25

30

35

40

45

1-Nov 1-Dec 31-Dec 30-Jan 1-Mar 31-Mar 30-Apr

Time

Cumulative fracture length, m

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

Individual fracture length, m

Cumulative Individual

Initial 8 am roof 

failure on April 21

Initial 3 am roof 

failure on March 7

0

5

10

15

20

25

30

35

40

45

1-Nov 1-Dec 31-Dec 30-Jan 1-Mar 31-Mar 30-Apr

Time

Cumulative fracture length, m

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

Individual fracture length, m

Cumulative Individual

Initial 8 am roof 

failure on April 21

Initial 3 am roof 

failure on March 7

Figure 6. Graph displays both the individual and cumulative 

new fracture lengths generated from November 1, 2001 to 

April 21, 2002. 

March 5-7, 2002

-2.00

-1.50

-1.00

-0.50

0.00

0.50

3/5/02 12:00

AM

3/5/02 12:00

PM

3/6/02 12:00

AM

3/6/02 12:00

PM

3/7/02 12:00

AM

3/7/02 12:00

PM

Time

Moment magnitude

0

0.05

0.1

0.15

0.2

0.25

0.3

Individual fracture length, m

Moment Magnitude March 7 roof fall Individual fracture

April 19-21, 2002

-2

-1.5

-1

-0.5

0

0.5

4/19/02 12:00

AM

4/19/02 12:00

PM

4/20/02 12:00

AM

4/20/02 12:00

PM

4/21/02 12:00

AM

4/21/02 12:00

PM

Time

Moment magnitude

0

0.05

0.1

0.15

0.2

0.25

0.3

Individual fracture length, m

March 7 roof fall April 21 roof fall

Moment Magnitude Individual fracture

Precursor 

activity

Precursor 

activity

a)

b)

March 5-7, 2002

-2.00

-1.50

-1.00

-0.50

0.00

0.50

3/5/02 12:00

AM

3/5/02 12:00

PM

3/6/02 12:00

AM

3/6/02 12:00

PM

3/7/02 12:00

AM

3/7/02 12:00

PM

Time

Moment magnitude

0

0.05

0.1

0.15

0.2

0.25

0.3

Individual fracture length, m

Moment Magnitude March 7 roof fall Individual fracture

April 19-21, 2002

-2

-1.5

-1

-0.5

0

0.5

4/19/02 12:00

AM

4/19/02 12:00

PM

4/20/02 12:00

AM

4/20/02 12:00

PM

4/21/02 12:00

AM

4/21/02 12:00

PM

Time

Moment magnitude

0

0.05

0.1

0.15

0.2

0.25

0.3

Individual fracture length, m

March 7 roof fall April 21 roof fall

Moment Magnitude Individual fracture

Precursor 

activity

Precursor 

activity

a)

b)

Figure 7. Moment magnitudes and new fracture lengths for 

microseismic events that occurred 2 days prior to the March 7 

(a) and April 21 (b) roof falls. 
Figure 8. A hazard map containing the total new fracture lengths within 30.5 m square grids within the 

study area over the 48 hours prior to the March 7 roof fall. 

Figure 9. A hazard map containing the total new fracture lengths within 30.5 m square grids within 

the study area over the 48 hours prior to the April 21 roof fall. 
SUMMARY AND CONCLUSIONS 

 In this study, 715 microseismic signatures were collected and 

analyzed from an array of 15 geophones to examine failure patterns 

associated with two large roof falls. In particular, the study 

determined the appropriateness of constructing hazard maps from 

microseismic data to warn of impending roof falls. Each event was 

manually located and its moment magnitude and seismic energy 

determined. The magnitude of these events ranged between -2.29 

and 0.01 with an average of -1.18, while the seismic energies 

varied between 0.01 and 174 Joules. The lengths of new fracture 

surfaces were calculated for each event, producing an estimated 

total of 41 m of new fracture lengths. Most of the new fracture 

lengths were produced in conjunction with the two roof falls. Of 

most interest to this study is that 3.9 m of new fracture lengths were 

produced in the 48 hours prior to the first recognition of failure in 

association with the March 7 roof fall. Furthermore, the majority 

of the new fracture surfaces occurred within 100 m of the March 7 

roof fall. The April 21 roof fall produced 11 m of new fracture 

surfaces in the 48 hour period prior to the first signs of roof failure. 

The majority of these microseismic events were within 50 m of the 

roof fall. 

 Because these new fracture lengths are an integral part of roof 

falls, their detection prior to a roof fall event is crucial for 

enhancing worker safety. This study shows that for this set of data 

and under these geologic and mining characteristics, increased 

levels of microseismic activity occurred within 48 hours of the 

initial observable rock fall. This microseismic activity is directly 

related to the failure of individual rock layers which comprise the 

strata within the zone of roof that will eventually define the roof 

fall. 

 Data presented was manually processed, removing all cultural 

and mining related seismic signatures. This produced event 

locations and magnitudes that had a high degree of confidence so 

that accurate new fracture length data could be produced. Realtime application of this technology in the mining industry will 

require a comprehensive approach to develop systems that 

accurately and consistently produce fracture length data. There is 

also a need to calibrate the estimated fracture lengths with 

laboratory and/or field observations. 

 The trends established from this case study are dependent on 

itsí unique geology, the state of the mineís stress conditions, and 

the properties and layout of the microseismic system deployed. It 

is possible that additional data clarity may have been achieved with 

a geophone array that completely surrounded the roof fall areas. 

Unfortunately, limited underground access will make that condition 

difficult to achieve. It is also possible that the amount of pre-roof 

fall rock failures may change in intensity and timing with different 

geologic and stress conditions. 

 In general, this study demonstrates the potential for this 

technology to locate future roof fall events in space with, in this 

case, a high degree of reliability. Determining the exact timing of 

these events is much more difficult and may prove to be beyond the 

capabilities of the technology. The data presented, along with 

recommendations from another NIOSH study (Iannacchione et al., 

2004), outline a potential strategy for using monitoring 

technologies to anticipate, and plan for, roof falls in the 

underground environment. 

REFERENCE 

Aki, K. (1968). Seismic Displacements Near a Fault. Journal of 

Geophysical Research, 73, pp. 5359-5376. 

Anon, (2002). Hyperion Seismic System Manual, Engineering 

Seismology Group, Canada, Inc., p. H-2. 

Boatwright, J. and Fletcher. J.B (1984). The Partition of Radiated 

Seismic Energy Between P and S Waves. Bull. Seism. Soc. Am., 

74, pp. 361-376. 

Gale, W. (2003). Fracture Mode and Size Related to Microseismic 

Measurements. In 3D Stress Effects, Rock Damage and Longwall 

Caving as revealed by Microseismic Monitoring, ed. by Hatherly, 

Gale, Medhurst, King, Craig, Poulsen and Luo. Report to the 

Australian Coal Association Research Program, ACARP Project 

C9012, March, pp. 40-72. 

Hanks, T. and Kanamori, H. (1979). A Moment Magnitude Scale. 

Journal of Geophysical Research, 84, pp. 2348-2350. 

Hatherly, P., Gale, W., Medhurst T., King, A., Craig, S., Poulsen, 

B. and Luo, X. (2003). 3D Stress Effects, Rock Damage and 

Longwall Caving as Revealed by Microseismic Monitoring. 

ACARP Project C9021, Exploration and Mining Report 1008F, 

March, 178 pp. 

Iannacchione, A.T. and Coyle, P. (2002). An Examination of the 

Loyalhanna Limestoneís Structural Features and Their Impact on 

Mining and Ground Control Practices. Proceedings, 21st 

International Conference on Ground Control in Mining, 

Morgantown, WV, Aug. 6-8, pp. 218-227. 

Iannacchione, A.T., Marshall, T.E., Burke, L.M., Melville, R., and 

Litsenberger, J. (2003). Safer Mine Layouts for Underground 

Stone Mines Subjected to Excessive Levels of Horizontal Stress. 

Mining Engineering, April, pp. 25-31. 

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

Litsenberger, J. (2004). The Relationship of Roof Movement and 

Strata-Induced Microseismic Emissions to Roof Falls. SME 

Preprint 04-58, SME Annual Meeting, Denver, CO, Feb. 23-25, 

9 pp. 

Leighton, F. and Steblay. B.J. (1977). Application of 

Microseismics in Coal Mines. Proceedings, First Conference on 

Acoustic Emissions/Microseismic Activity in Geologic Structures 

and Materials, ed. by Hardy and Leighton, Pennsylvania State 

University, June 9-11, 1975, pp. 205-229. 

McGarr, A. (1999). On Relating Apparent Stress to the Stress 

Causing Earthquake Fault Slip. Journal of Geophysical Research, 

B, Solid Earth and Planets, Vol. 104, No.2, Feb. 10, pp. 3003-3011. 

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

Predicting Rock Failure in Underground Mining, Part 1, General 

Method. U.S. Bureau of Mines, Report of Investigation 3797, 7 pp. 

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

Predicting Rock Failure in Underground Mining, Part 2, Laboratory 

Experiments. U.S. Bureau of Mines, Report of Investigation 3803, 

14 pp.