1

THE RELATIONSHIP OF ROOF MOVEMENT AND STRATA-INDUCED MICROSEISMIC EMISSIONS TO 

ROOF FALLS 

A.T. Iannacchione, P.R. Coyle, 

L.J. Prosser, and T.E. Marshall 

NIOSH 

Pittsburgh, PA 

J. Litsenberger 

Better Materials 

Connellsville, PA

ABSTRACT 

 For the first time in an underground stone mine, the 

relationship between roof movement and microseismic 

emissions was examined in conjunction with two distinct 

roof fall areas. As roof monitoring increases in 

acceptance and monitoring technology advances, the goal 

of providing reliable roof fall detection systems to 

enhance the safety of underground mine workers moves 

closer to reality. Instrumental to reaching that goal is the 

ability to interpret accurately and completely roof 

movement and microseismic emissions, which can serve 

as precursors to roof falls. This paper examines the 

capabilities of convergence and microseismic monitoring 

systems to better understand roof rock failure mechanics 

and to anticipate roof falls. An understanding of how 

these techniques are used and how they interact with each 

other is critical in developing the most effective ground 

control strategy for our nation’s mines. 

INTRODUCTION 

 Typically, when roof rock fractures it produces audible 

microseismic emissions of different magnitudes that are 

transmitted through rock as elastic waves with varying 

degrees of propagation. If sufficient fracturing occurs, the 

roof rock begins to deflect into the mine opening. Factors 

influencing the rate of roof rock movement have been 

discussed by several authors (Parker, 1973; Maleki and 

McVey, 1988; Altounyan, et al., 1997; and Iannacchione, 

et al., 2000). Generally, microfracturing begins to occur 

as strain levels approach 50% of the rock’s ultimate 

strength (Hardy, 1975). As individual layers of roof rock 

move closer to their ultimate strength, they begin to fail 

by rupturing or cracking in shear or tension (Obert and 

Duvall, 1967; Gale, et al., 2001). As more rock fractures 

occur within a defined volume of rock, the potential for 

the roof rock failing increases (Hardy, 1975; Brady and 

Haramy, 1994; Iannacchione, et al., 2001). 

 Both microseismic emissions and roof-to-floor 

convergences were measured by a NIOSH monitoring 

system installed within an underground limestone mine in 

southwestern Pennsylvania. The unstable conditions at 

this mine site provided a unique opportunity to examine 

the behavior of unstable roof rock and the relationship 

between roof rock fracturing and movement. 

 In the study area, the room-and-pillar technique is 

used to mine rooms 45 ft wide and 30 ft high within the 

Loyalhanna Limestone (Figure 1). The Loyalhanna 

Limestone is a very strong rock unit with unconfined 

compressive strengths as high as 30,000 psi. Locally, the 

Loyalhanna ranges from 50 to 70 ft thick with dips 

between 1 to 5 degrees. The rock mass characteristics of 

this unit consist of horizontal bedding planes spaced from 

1 to 5 ft apart, widely spaced vertical joints, and extensive 

crossbedding within the individual rock layers 

(Iannacchione and Coyle, 2002). Approximately 1,000 ft 

from the study area and under 400 ft of overburden, very 

high horizontal stresses were measured, reaching a 

maximum value of approximately 8,000 psi in the 

N 60° E orientation. The above factors are particularly 

important because the behavior of this roof rock is 

dominated by stiff, relatively thin rock layers and the 

levels of horizontal stresses contained within them. 

 As part of this research, four electro-mechanical roofto-floor convergence stations, consisting of a steel wire 

wound around a potentiometer, were attached to the roof 

and floor. As the roof and floor converge, a voltage 

change occurs and is transmitted to a data acquisition 

system. The data can then be used to produce a unit of 

distance by multiplying the voltage with a calibration 

factor. Currently within U.S. underground mines, 

variations of this technology are used to gather 

information concerning roof rock stability conditions, 

and, in some cases, are being used to warn of roof failure. 

However, this technology suffers from a practical 

constraint in that there is an inability to cover the 

underground mining environment with an adequate 

number of convergence stations. Therefore, significant 
2

issues exist concerning where and when to deploy 

convergence monitoring systems. 

 In addition to the convergence stations, 23 4.5-Hz 630-

ohm uniaxial geophones were connected to a DOS-based 

microseismic system. All but one of the geophones were 

mounted on the roof about 30 ft above the mine floor. 

One geophone was placed 69 ft 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 along the same vertical plane 

within the mine. Unfortunately, there is little research 

within this country over the last decade that supports the 

use of microseismic monitoring techniques to anticipate 

roof falls, and thus they are rarely used for this purpose. 

As commercial microseismic monitoring systems increase 

in reliability and become more robust, the potential use of 

this technology needs to be re-evaluated. 

 Observation data were collected which consisted of 

the date and location of roof falls and face advances 

(Figure 1 and Figure 2). From March to June 2002, roofto-floor convergence was measured during several periods 

of unstable roof rock conditions. In addition, about 2,400 

microseismic emissions were recorded that represent the 

development of new rock fracture surfaces. Knowledge 

was gained on roof rock failure patterns and the 

capabilities of the roof monitoring systems. The goal of 

this research is to develop a more complete understanding 

of the mechanisms that cause roof failures and to improve 

warning techniques for unstable roof rock conditions. 

These advancements would lead to safer and less 

hazardous working environments for our nation’s miners. 

TWO DISTINCT ROOF FALL AREAS 

Figure 1. Location of geophones, roof-to-floor 

convergence stations, and production 

faces advanced between March and 

June 2002. 

500 ft 

distance

Production 

faces

Roof Fall A

250 ft 

distance

Roof Fall B

Convergence 

CS4 station

Geophone

CS1

CS3

CS2

500 ft 

distance

Production 

faces

Roof Fall A

250 ft 

distance

Roof Fall B

Convergence 

CS4 station

Geophone

CS1

CS3

CS2

Figure 2. Location of Roof Fall A and B areas. Also shown are the locations and dates of the 

distinct unstable roof rock periods. Note: Periods A-2, B-1, and B-3 are classified as 

semi-stable roof rock and are discussed later in the paper.

A- 4 Unstable 

Period, May 18 

– Sept. 16

Roof Fall B

A- 3 Unstable 

Period, May 4-

17 A- 1 Unstable 

Period, March 

4-14

B- 4 Unstable 

Period, May 13-22

B- 5 Unstable 

Period, May 

23 - June 3

B- 2 Unstable 

Period, April 

18-23

Roof Fall A

A- 4 Unstable 

Period, May 18 

– Sept. 16

Roof Fall B

A- 3 Unstable 

Period, May 4-

17 A- 1 Unstable 

Period, March 

4-14

B- 4 Unstable 

Period, May 13-22

B- 5 Unstable 

Period, May 

23 - June 3

B- 2 Unstable 

Period, April 

18-23

Roof Fall A
3

 Two major roof falls occurred during this study. Each 

of these falls comprised numerous individual roof falls 

that fell either as one mass or in distinct segments during 

different periods of time. Roof Fall “A” occurred in the 

southeast corner of the mine while Roof Fall “B” 

occurred approximately 500 ft to the west-northwest of 

Roof Fall A (Figure 2). From March to June 2002, 48 

face cuts were advanced by blasting in the study area 

(Figure 1). The circles surrounding the two roof falls 

(Figure 2) represent the approximate area where most of 

the microseismic events occurred. 

 Roof Fall A developed from three unstable roof rock 

periods. The first period occurred from March 4 to 14 

and can be described as failing in several distinct stages 

(Figure 2, period A-1). The initial observed stage of 

unstable roof rock period A-1 occurred when a shear 

failure was noticed by mine officials around 3 am on 

March 7. By that afternoon, a small shallow fall extended 

approximately 2 ft into the roof. Sometime during the 

evening of March 7 the roof fall cavity extended across 

the intersection and approximately 10 ft 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. Small 

rocks dribbled from the roof fall until March 14. The roof 

fall cavity created during this period extended over 125 ft 

in length and up to 20 ft in height (Figure 3a). After the 

March 14 event, the area around the roof fall seemed to 

stabilize. 

 A second unstable period occurred in the Roof Fall A 

area starting on May 4. The period was defined by a 

series of small roof falls and ended on May 17 with a 

fairly large roof fall extension (Figure 3b). At this time 

the roof fall cavity occupied the area shown in Figure 2, 

period A-3. On May 18 roof rock to the northwest of A-3 

began to slowly fail over a four-month period until 

September 16 when a large roof fall occurred (Figure 2, 

period A-4). 

 Roof Fall B was located some 500 ft away from Roof 

Fall A and included three distinct periods of unstable roof 

rock conditions. The first unstable period occurred from 

April 18 to the 23, when a shallow roof fall encompassed 

an entire intersection (Figure 2, period B-2). This area 

had experienced some roof rock fracturing in the past and 

a period of relative stability followed. About two weeks 

later a series of instabilities occurred that resulted in 

several major roof fall extensions. First, a large shear 

Figure 3. Photographs showing the size and character of the roof falls within the 

study area. Note: Dates on photo refer to the date when the picture was taken. 
4

failure propagated rapidly across an entry on May 6. By 

May 22 the entire intersection had fallen (Figure 2, period 

B-4 and Figure 3c). Two weeks later on June 6, this roof 

fall extended both to the south and the northwest (Figure 

2, period B-5). By mid-June, Roof Fall A extended 

almost 250 ft in length, while Roof Fall B was closer to 

300 ft in length. 

 As a result of these observations and in combination 

with data from convergence and microseismic monitoring 

systems, it is possible to establish periods of semi-stable 

and unstable roof rock conditions. Six periods of unstable 

roof rock conditions were identified, three in each of the 

two roof fall areas. In addition, three periods of relative 

stability, herein referred to as semi-stable, were observed. 

ROOF-TO-FLOOR CONVERGENCE FROM 

MARCH TO JUNE 

 The first roof-to-floor convergence station, CS1, was 

installed on March 14 (Figure 4). At this point in time, 

station CS1 was approximately 75 ft to the northwest of 

the March 4 to 14 roof fall (period A-1). Immediately 

after the installation, the convergence rate decreased. By 

March 20, convergence reached a steady rate of 

approximately 0.03 inches/day and remained so until May 

3. At this time, approximately 1.71 inches of cumulative 

convergence had occurred. From May 4 to May 17, the 

second unstable period during Roof Fall A produced an 

additional convergence of 2.82 inches (Figure 4, period 

A-3). Station CS1 operated until May 16 at which time it 

was destroyed by a rock fall. 

 On March 29, convergence station CS2 was installed 

approximately 50 ft west of CS1. Because of the greater 

distance from the roof fall, less than 0.11 inches of 

cumulative convergence occurred until May 18. On this 

day, the roof around station CS1 failed and the roof rock 

to the northwest entered a long period of instability that 

lasted from May 18 until September 16 (Figure 4, period 

A-4). Because of conditions at this site, mine personnel 

relocated the convergence stations on June 1 to ensure 

mineworker safety. 

 Convergence station CS4 was installed on May 22 

approximately 100 ft away from Roof Fall B (Figure 4, 

period B-4). The convergence rate remained steady at 

0.26 in/day until May 28 at which time it dropped to 

approximately 0.07 in/day. This station was in operation 

until June 13 when it was relocated farther away from the 

fall area. 

MICROSEISMIC ACTIVITY FROM MARCH TO 

JUNE 

Figure 4. Cumulative convergence and microseismic emissions from Roof Fall A and B areas with 

associated stability periods. 

0

1

2

3

4

5

1-Mar 22-Mar 12-Apr 3-May 24-May 14-Jun

T im e

Cumulative Convergence, inches

0

200

400

600

800

1000

1200

1400

1600

Cumulative Microseismic Events

CS1 CS2 CS4 R o o f F a ll A E ve n ts R o o f F a ll B E ven ts

U n stab le p e riod s

B -3

R o of F a ll B

S e m i-stab le p e riod s

R o of F a ll A B -1 B -2 B -4 B -5

A -1 A -2 A -3 A -4

0

1

2

3

4

5

1-Mar 22-Mar 12-Apr 3-May 24-May 14-Jun

T im e

Cumulative Convergence, inches

0

200

400

600

800

1000

1200

1400

1600

Cumulative Microseismic Events

CS1 CS2 CS4 R o o f F a ll A E ve n ts R o o f F a ll B E ven ts

U n stab le p e riod s

B -3

R o of F a ll B

S e m i-stab le p e riod s

R o of F a ll A B -1 B -2 B -4 B -5

A -1 A -2 A -3 A -4
5

 Almost 2,400 microseismic events were located in the 

two roof fall areas. Approximately 800 occurred in Roof 

Fall A area and another 1,600 in Roof Fall B area 

(Figure 4). In Roof Fall A area, 473 events produced a 

rate of 43 events/day in association with the March 4 to 

14 roof fall (period A-1). During the semi-stable roof 

rock period from March 15 to May 3, microseismic 

activity was reduced to an average of 1.2 events/day 

(period A-2). On May 4 and again on May 15 and 16, 

activity increased in conjunction with local roof 

instabilities (period A-3). After this time, microseismic 

activity assumed a higher rate of 10.8 events/day. The 

roof in the area surrounding station CS2 also experienced 

increased rates of convergence (Figure 4). 

 Microseismic activity did not occur in the Roof Fall B 

area until immediately after the activity associated with 

the Roof Fall A March 4 to 14 episode diminished (period 

B-1). A semi-stable rate of 1.9 events/day was 

maintained from March 12 until April 17. Over the next 

six days, 373 events were recorded, producing an average 

rate of 62.2 events/day (period B-2). Again, a period of 

decreased activity, averaging 3.9 events/day, followed 

until May 13 (period B-3). This time span included the 

initial shear roof failure that occurred on May 6. From 

May 13 to 22, 375 events were associated with a large 

roof collapse (period B-4). Activity continued at a high 

rate of 52 events/day in conjunction with the roof fall 

extensions that occurred between May 22 and June 6 

(period B-5). 

COMPARISON BETWEEN CUMULATIVE 

CONVERGENCE, MICROSEISMIC ACTIVITY, AND 

ROOF ROCK STABILITY CONDITIONS 

 All of the data presented represents conditions other 

than stable. Stable conditions imply no measurable roof 

deflection that would result in roof sag or roof-to-floor 

convergence. Additionally, the microseismic activity 

would be virtually absent in a stable area with rock 

fracture events occurring only once every few days. 

Stable roof rock rarely fractures and only minor amounts 

of elastic beam deflection could be measured. 

 Comparisons were made between each convergence 

station, the local microseismic event count, and the local 

stability conditions for the semi-stable and unstable 

periods (Figure 5). Clearly, agreement exists between the 

relative trends in convergence and microseismic activity. 

In general, periods of semi-stable roof rock conditions are 

accompanied by lower levels of convergence and 

microseismic activity. Periods of unstable roof rock 

conditions are accompanied by higher levels of 

convergence and microseismic activity. These trends are 

shown in Table 1. Here the nine different roof rock 

stability conditions are listed along with their 

corresponding dates, convergence measurements, and 

microseismic event counts. Microseismic events were 

0

0.5

1

1.5

2

2.5

3

3.5

3/28 4/11 4/25 5/9 5/23

Time

Cumulative Convergence, in

475

525

575

625

675

725

775

825

Cumulative Microseismicity

CS2 Roof Fall A events

0

0.5

1

1.5

2

2.5

3

5/22 5/29 6/5 6/12

Time

Cumulative Convergence, in

800

1000

1200

1400

1600

Cumulative Microseismicity

CS4 Roof Fall B events

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

3/14 3/28 4/11 4/25 5/9

Time

Cumulative Convergence, in

450

500

550

600

650

Cumulative Microseismicity

CS1 Roof Fall A events

Unstable 

period A-3

Unstable 

period A-3

Semi-stable period A-2

Unstable 

period A-4

c)

Semi-stable 

period A-2

Unstable 

period B-5

a)

b)

0

0.5

1

1.5

2

2.5

3

3.5

3/28 4/11 4/25 5/9 5/23

Time

Cumulative Convergence, in

475

525

575

625

675

725

775

825

Cumulative Microseismicity

CS2 Roof Fall A events

0

0.5

1

1.5

2

2.5

3

5/22 5/29 6/5 6/12

Time

Cumulative Convergence, in

800

1000

1200

1400

1600

Cumulative Microseismicity

CS4 Roof Fall B events

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

3/14 3/28 4/11 4/25 5/9

Time

Cumulative Convergence, in

450

500

550

600

650

Cumulative Microseismicity

CS1 Roof Fall A events

Unstable 

period A-3

Unstable 

period A-3

Semi-stable period A-2

Unstable 

period A-4

c)

Semi-stable 

period A-2

Unstable 

period B-5

a)

b)

Figure 5. Comparison between convergence, 

microseismic event frequency, and 

local stability conditions. 
6

Table 1. Convergence and microseismic event data for the nine different roof rock stability conditions observed during the 

study. 

Convergence CS1 Convergence CS2 Convergence CS4 Microseismicity 

Stability State Period Days inches in/day Days inches in/day Days inches in/day Days Events Events/

day 

Unstable A-1 March 4 - March 14 11 473 43 

Semi-stable A-2 March 15 - May 3 51 1.71 0.03 37 0.11 0.00 51 61 1.2 

Unstable A-3 May 4 - May 17 13 2.82 0.22 14 0.24 0.02 14 119 8.5 

Unstable A-4 May 18 - June 3 15 2.87 0.19 17 183 10.8 

Semi-stable B-1 March 12 - April 17 37 71 1.9 

Unstable B-2 April 18 - April 23 6 373 62.2 

Semi-stable B-3 April 24 - May 12 19 74 3.9 

Unstable B-4 May 13 - May 22 10 375 37.5 

Unstable B-5 May 23 - June 6 15 2.20 0.15 12 624 52.0 

measured during all nine periods. This is not the case 

with convergence because these stations were located in 

specific areas and only operated during a portion of the 

periods observed. 

 The three periods of semi-stable roof rock conditions 

(A-2, B-1, and B-3) have microseismic event rates 

averaging 1.9 events/day. Convergence was monitored 

during period A-2 by both station CS1 and CS2. The 

mean convergence rate for these two instruments is 0.02 

in/day. Obviously, the rate for station CS1 is much higher 

than CS2 because it was much closer to the roof fall 

during that period. The data suggest that average 

convergence rates on the order of a few hundredths-of-aninch per day and microseismic activity averaging only a 

few events per day are indicative of a semi-stable roof 

rock condition as defined in this study. 

 The six periods of unstable roof rock conditions (A-1, 

A-3, A-4, B-2, B-4 and B-5) have microseismic event 

rates averaging 30.7 events/day. Convergence was 

monitored during periods A-3, A-4, and B-5. The average 

convergence rate for these periods is 0.14 in/day. Again, 

the rate for station CS1 is much higher than CS2 because 

of the closer distance to the existing roof fall. The data 

suggests that average convergence rates of a tenth of an 

inch or more per day and daily microseismic event rates 

in double figures indicate unstable roof rock conditions 

for this study. 

UNDERSTANDING ROOF FALL BEHAVIOR WITH 

COMBINED CONVERGENCE AND 

MICROSEISMIC MONITORING INFORMATION 

 The convergence and microseismic monitoring data 

along with in-mine observations provide a clear picture of 

the behavior of strong roof rock failing from excessive 

levels of horizontal stress (Figure 6). The following 

generalized sequence of events was identified during this 

study: 

• Initial shear failure occurs in individual roof beams 

subjected to excessive levels of horizontal stress 

(Figures 6a and 6c). 

• Adjacent layers begin to fail in shear as stresses are 

added from a) dissipated stresses from prior beam 

failure, and b) layers experiencing volumetric 

increase in mass as a result of failure (Figure 6d). 

• At some point in this process the roof rock can 

begin to deflect enough that its progress can be 

measured over a significant area with convergence 

monitors (Figure 6e). 

• The roof begins to fall into the mine opening once 

enough beam deflection or sag from cantilevering 

occurs, causing tensile failures along the perimeter 

of the beam (Figure 6e). 

• The roof fall can happen as a single, dramatic event 

or over a series of smaller events spanning days or 

weeks. 

• The roof fall cavity eventually assumes a stable arch 

shape (Figure 6b). 

• The axis of the roof fall is defined by the orientation 

Tensile failure occurs along the 

arched roof cavity

Beam begins to shear

Time initial

Tensile

Beam cantilevers

Shear

Tensile

Mine opening

Shear

Time final

Shear propagates and beam dilates 

b)

c)

d)

e)

a)

Tensile failure occurs along the 

arched roof cavity

Beam begins to shear

Time initial

Tensile

Beam cantilevers

Shear

Tensile

Mine opening

Shear

Time final

Shear propagates and beam dilates 

b)

c)

d)

e)

a)

Figure 6. Examples of roof rock failure 

characteristics in high horizontal stress 

conditions.
7

of the horizontal stress field where this axis will be 

perpendicular to the maximum horizontal stress 

direction (Figure 7). 

• Dissipated stresses from the roof failure process are 

concentrated at the axial ends of the roof fall and in 

the remaining competent beams at the crest of the 

roof fall cavity. 

• Higher stresses, rock fractures, and convergence 

occur in areas adjacent to the axial ends of the roof 

fall cavity, causing additional rock falls (Figure 7). 

• Areas adjacent to the long axis of the roof fall and 

away from its axial ends have been destressed and 

are typically stable. 

• Roof falls progress as described above until some 

outside agent causes a reduction in the horizontal 

stress, the roof is strengthened sufficiently by rock 

reinforcement, or the roof encounters a solid rock 

abutment. 

 The application of the above knowledge suggests that 

mines with high horizontal stresses will have non-uniform 

convergence profiles associated with roof falls. 

Typically, the areas of highest convergence occur along 

the axial ends of the roof fall where the stresses and the 

concentration of new fracture surfaces are highest. This 

knowledge was used to locate the three roof-to-floor 

convergence stations after the development of a roof fall 

from March 4 to 14 (Figure 7). Two of the convergence 

stations (CS1 and CS2) recorded measurable ground 

movements. As a control measure, station CS3 was 

placed slightly outside the expected trend of the roof fall 

and did not measure roof movement. The roof rock 

conditions immediately adjacent to this station were 

considered to be stable. 

 As has been demonstrated, the zones of highest 

convergence are close to the axial ends of the roof fall and 

dissipate gradually along the axis for distances ranging 

from 50 to 100 ft. Conversely, convergence decreases 

significantly perpendicular to the axial trend of a roof fall. 

This direction is parallel to the direction of maximum 

horizontal stress (Figure 7). The areas adjacent to the 

roof fall cavity, but away from its axial ends, have lower 

stresses and are thought to be within the roof fall’s stress 

shadow. Understanding these trends is important in 

identifying the appropriate ground control response 

(Emery, 1964; Parker, 1966; Mark and Mucho, 1994; 

Iannacchione, et al., 1998). 

 Perhaps the most intriguing finding in this 

investigation is that there seems to be some direct and 

stress-related connection between the two distinct roof 

fall areas. This statement is supported by the fact that 

microseismic activity in Roof Fall B area did not initiate 

until the activity in Roof Fall A area diminshed after the 

March 4 to 14 roof fall (Figure 4). The data suggest that 

roof rock failures, and the corresponding high stress 

conditions that cause these failures, seem to migrate back 

and forth between the two roof fall areas (Figure 4). 

These trends imply that stability conditions are very 

dependent upon what is happening in the adjacent areas of 

the mine. The complexities of geology, mining layout, 

and in-situ stress conditions seem to interact in ways not 

entirely understood to influence the development of 

unstable roof rock zones. 

ANTICIPATING ROOF FALLS WITH COMBINED 

CONVERGENCE AND MICROSEISMIC 

MONITORING INFORMATION 

 Knowledge of roof rock behavior provides a 

framework for constructing strategies to anticipate 

impending roof falls. The use of convergence 

measurements for this purpose has long been studied and 

represents the state-of-the-art in anticipating roof falls. If 

the instruments are placed in ideal locations and at the 

proper time, convergence values can provide an 

invaluable estimation of the roof rock stability condition. 

Unfortunately, these instruments are not always placed in 

the most appropriate locations and often are installed only 

after some rock failure has been identified in the area. 

Microseismic monitoring helps to minimize these 

deficiencies by aiding in determining when and where 

convergence monitors should be deployed. 

Awareness of microseismic activity is critical in 

knowing when to deploy convergence stations

 Perhaps the most important role for microseismic 

monitoring is that it can provide very early indications of 

roof instabilities. Detailed analysis of Figure 4 shows that 

microseismic activity often precedes convergence. Prior 

to the first signs of roof rock failure on March 7, 

Figure 7. Idealized distribution of roof-to-floor 

convergence associated with the March 

4 to 14 roof fall. 
8

microseismic activity in Roof Fall A area was already 

high. From March 4 to 6, more than 200 events emanated 

from this area. A second example occurred on May 4, 

when microseismic activity increased significantly in the 

Roof Fall A area. Approximately one day later, increases 

in convergence from CS1 were observed (Figure 5a). A 

third example occurred on May 14, when microseismic 

activity increased dramatically a full day prior to a 

significant increase in convergence from CS1 (Figure 5a). 

This same trend was observed between microseismic 

activity and convergence from CS2 (Figure 5b). 

 Trends of microseismic activity proceeding 

convergence indicate that microseismic monitoring 

provides an earlier detection capability than convergence 

monitoring. As would be expected, rock fracturing 

precedes significant non-elastic roof deflection. This type 

of information can prove crucial in helping mine 

personnel to know when to install convergence stations. 

It is important to place the convergence stations in the 

earliest stage of semi- or unstable roof rock conditions. 

Only in this way can a complete cumulative convergence 

profile be obtained. The greatest value to miner safety is 

achieved when monitoring data becomes available early 

in the roof failure process. 

Knowledge of local roof rock behavior is critical in 

knowing where to deploy convergence stations

 In order to have complete confidence that no 

convergence of unstable ground goes undetected, an 

unrealistic number of stations may need to be deployed. 

This has been one of the major hindrances to the full 

utilization of this technology. It therefore becomes 

essential to place the fewest number of convergence 

stations in the most optimal locations. In this case, 

microseismic activity may supply some direct input. If 

the events can be accurately located, then the source of 

the instabilities can be identified. Unfortunately, the 

location of individual events is highly dependent upon the 

equipment characteristics and the location and number of 

geophones used. For this study, the locations were 

accurate within a range of 60 to 100 ft. If enough events 

are located then the center of the activity will cluster in a 

location representative of the unstable ground. This is an 

example of the direct application of the microseismic 

information to locate convergence stations. 

 An indirect application comes from the enhanced 

understanding of roof fall behavior discussed in previous 

sections. Areas of critical convergence can be predicted 

with sufficient understanding of the mechanism 

controlling roof failure. The value of this knowledge 

should not be understated. Knowing how local roof rock 

fails is key in knowing where to place convergence 

stations. As demonstrated in this study, the understanding 

of roof rock failure processes allowed for the optimal 

placement of stations. 

SUMMARY AND CONCLUSIONS 

 This paper examines the relationship between the 

fracturing of roof rock and the corresponding roof beam 

deflection into the mine opening in an underground stone 

mine. Both of these pre-roof fall behaviors are compared 

to local roof rock stability conditions so that more can be 

learned about using these technologies to better 

understand and anticipate roof falls in underground mines. 

A connection between roof rock instabilities and trends in 

convergence and microseismic activity was demonstrated. 

Additionally, microseismic emissions were observed 

sometimes to occur prior to convergence, demonstrating 

that rock fracturing precedes roof deflection. The results 

of this analysis suggest that these measurement 

technologies could be used to better determine the relative 

stability of local roof rock conditions. 

 This paper outlined three categories of stability and 

defines them as follows: 

• Stable - No measurable roof deflection that would 

result in roof sag or roof-to-floor convergence. The 

microseismic activity would be virtually absent in a 

stable area with rock fracture events occurring only 

once every few days. Stable roof rock rarely 

fractures and only minor amounts of elastic beam 

deflection could be measured. 

• Semi-stable - Relatively low convergence rates in 

the range of several hundredths-of-an-inch per day 

and few microseismic events from a specific area 

per day. During these periods, roof rock failures are 

typically not observed, but there is a sense from the 

monitoring data that the roof is only temporarily 

stable and can move to a state of instability at any 

time. 

• Unstable - Relatively high convergence rates in the 

range of a tenth of an inch per day or more and 

many microseismic events per day in the range of at 

least 10 to 20 events per day. During these periods, 

a combination of observational and measurement 

information provides a sense that roof failure is 

imminent. Roof failures were often observed or 

noted by increases in the rock piles beneath roof fall 

cavities. 

 This study suggests that current state-of-the-art 

technology in roof fall anticipation could be improved 

upon. These techniques rely only on convergence 

monitoring and observational data to develop local 
9

ground control strategies. Actual experience has shown 

that convergence information provides a non-ambiguous 

picture of local roof rock conditions. However, 

convergence information suffers from difficulties in 

knowing the right time and location for placement of 

instruments. Data presented support the use of 

microseismic monitoring to improve convergence station 

placement and to aid in collaborating trends developed 

from the convergence monitors. 

 This study also demonstrates the use of monitoring 

information to better understand roof failure processes. 

This knowledge is critical in developing the most 

effective response to combat local unstable roof rock 

conditions. Better understanding of roof fall behavior and 

improved techniques to anticipate unstable roof rock 

conditions are expected to lead to safer and less hazardous 

working environments for our nation’s miners. 

REFERENCES 

 Altounyan, P.F.R., D.N. Bigby, K.G. Hurt, and H.V. 

Peake, 1997, "Instrumentation and Procedures for Routine 

Monitoring of Reinforced Mine Roadways to Prevent 

Falls of Ground," 27th Int. Conf. of Safety in Mines 

Research Inst., New Delhi, India, pp. 759-766. 

 Brady, B.T. and K.Y Haramy, 1994, "High Amplitude 

Stress Wave Generation and Damage Induced by Roof 

Caving in Mines," Proceedings of the 1st North American 

Rock Mechanics Symposium, Univ. of Texas, Austin, 

TX, June 1-3, pp. 1033-1040. 

 Emery, C.L., 1964, "In Situ Measurements Applied to 

Mine Design," Proceedings, 6th Symposium on Rock 

Mechanics, Rolla, MO, pp. 218-230. 

 Gale, W.J, K.A. Heasley, A.T. Iannacchione, P.L. 

Swanson, P. Hatherly, and A. King, 2001, "Rock Damage 

Characterization from Microseismic Monitoring, Rock 

Mechanics in the National Interest," ed. Elsworth, Tinucci 

and Heasley, Washington, DC, pp. 1313-1320. 

 Hardy, R., 1975, "Emergence of AE/MA as a tool in 

Geomechanics," First Conference on Acoustic EmissionMicroseismic Activity in Geologic Structures and 

Materials, University Park, PA, June 9-11, Series on Rock 

and Soil Mechanics, Vol. 2, No. 3, By Trans Tech 

Publications, Rockport, MA, pp. 13-31. 

 Maleki, H.N. and J.R. McVey, 1988, "Detection of 

Roof Instability by Monitoring the Rate of Movement," 

U.S. Bureau of Mines RI 9170, 12 p. 

 Iannacchione, A.T., D.R. Dolinar, L.J. Prosser, T.E. 

Marshall, D.C. Oyler, and C.S. Compton, 1998, 

"Controlling Roof Beam Failures from High Horizontal 

Stresses in Underground Stone Mines," Proceeding, 17th

International Conference on Ground Control in Mining, 

Morgantown, WV, Aug. 4-6, pp. 102-112. 

 Iannacchione, A.T., L.J. Prosser, R. Grau, D.C. Oyler, 

D.R. Dolinar, T.E. Marshall, and C.S. Compton, 2000, 

"Roof Monitoring Helps Prevent Injuries in Stone Mines, 

Mining Engineering," Nov., pp. 32-37. 

 Iannacchione, A.T., T.E. Marshall, and L.J. Prosser, 

2001, "Failure Characteristics of Roof Falls at an 

Underground Stone Mine in Southwestern Pennsylvania," 

Proceedings, 20th International Conference on Ground 

Control in Mining, Morgantown, WV, Aug. 7-9, pp. 119-

125. 

 Iannacchione, A.T. and P.R. Coyle, 2002, 

"Examination of the Loyalhanna Limestone’s 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. 

 Mark, C. and T.P. Mucho, 1994, "Longwall Mine 

Design for Control of Horizontal Stress," U.S. Bureau of 

Mines Special Publication 01-94, New Technology for 

Longwall Ground Control, pp. 53-76. 

 Obert, L. and W.I. Duvall, 1967, "Rock Mechanics 

and the Design of Structures in Rock," John Wiley and 

Sons, Inc., New York, pp. 113-188. 

 Parker, J., 1966, "Mining in a Lateral Stress Field at 

White Pine," Canadian Institute of Mining and Metallurgy 

Transactions, Vol. LXIX, pp. 375-383. 

 Parker, J., 1973, "How Convergence Measurements 

Can Save Money," Engineering and Mining Journal, 

August, pp. 92-97.