ASSESSMENT OF GROUND CONDITIONS NEAR A MINE PORTAL 

USING GROUND PENETRATING RADAR 

Michael A. Trevits, Research Physical Scientist 

William D. Monaghan, Electrical Engineer 

NIOSH - Pittsburgh Research Laboratory 

Pittsburgh, PA
 

Thomas P. Mucho, President, 

Thomas P. Mucho & Associates, Inc. 

Washington, PA
 

ABSTRACT 

The NIOSH Lake Lynn Laboratory (LLL) is a unique research 

facility, located about 50 miles southeast of Pittsburgh, 

Pennsylvania, that is designed to provide a full-scale mining 

environment for testing and evaluation of mine health and safety 

technologies. The LLL occupies more than 400 acres and is 

composed of surface test and training areas and an underground 

mine. The Lake Lynn Experimental Mine (LLEM) was built at an 

abandoned commercial limestone quarry where surface mining 

ceased in the late 1960's. Later, new underground development 

was constructed to simulate modern-day coal mining scenarios; 

including room-and-pillar and longwall mining layouts. In 

January 1994, a sinkhole opened southeast of the No. 4 Portal of 

the LLEM and since then, the area of the underground failure and 

surface deformation has continued to expand and now includes 

several interconnected sinkholes. A concern developed that the 

overburden instability could expand and affect the structural 

integrity of the nearby highwall and the No. 4 Portal. It was unsafe 

to conduct a detailed underground survey of the mine in this area 

because the mine roof conditions near the No. 4 Portal had 

deteriorated significantly. It was decided to investigate the 

overburden conditions near the No. 4 Portal using ground 

penetrating radar (GPR). A GPR survey grid was located along an 

access road that passes over the top of the portal. To delineate the 

mine conditions near the portal, antennas whose frequency spectra 

produced pulses centered near 100- and 200-MHz were used. The 

results of the GPR survey suggests that the overburden rock units 

appear to be laterally consistent to a depth of about 15-ft. Below 

that point, the overburden appears to be significantly disturbed and 

it was hypothesized that this area contains a large roof fall. A 

follow-up ground truth survey of the mine roof conditions was 

conducted from the mouth of the portal. It is concluded that the 

observations and measurements made from the radar records 

appear to be correct as a large roof fall was observed and measured 

from the mine portal area. 

INTRODUCTION 

The NIOSH Lake Lynn Laboratory (LLL) is a unique research 

facility, located about 50 miles southeast of Pittsburgh, 

Pennsylvania. LLL is designed to provide a full-scale mining 

environment for the testing and evaluation of mine health and 

safety technologies. LLL occupies more than 400 acres and is 

composed of surface test and training areas and an underground 

mine (Lake Lynn Experimental Mine (LLEM)) (figure 1). From a 

geological perspective, the mine has been developed in the 

Mississipian Age Greenbriar Limestone unit of the Mauch Chunk 

Formation and is locally known as the Wymps Gap Limestone 

(formally known as the “Greenbriar of Pennsylvania”). The units 

exposed are typically gray, hard, and massive with a few 

interbedded shale units (1). 

Figure 1. Lake Lynn Laboratory 

The LLEM was built at an abandoned commercial limestone 

quarry (figure 2). The mine workings on the west side of the 

property are known as the “old workings” with entries 49-ft wide 

by 33-ft high. These workings were developed by a commercial 

limestone aggregate operation when surface mining ceased in the 

late 1960's. Later, under the direction of the U.S. Bureau of Mines, 

7,545 ft of new underground development was constructed on the 

east side of the property using 19.7-ft wide by 6.6-ft high entries 

(this area is known as the “new workings”). The new workings 

have been configured to simulate modern-day coal mining 

scenarios; including room-and-pillar and longwall mining layouts 

(2, 3). Roof support in the primary escapeway areas of the mine is 

provided through the use of fully-grouted roof bolts and screens. 

During development of the new workings, fully-grouted roof bolts 

were installed. 
 

 

 

 

In late January 1994, a large roof fall occurred in the old workings 

near the Nos. 3 and 4 Portals and in close proximity to the highwall 

(figure 3). Since the area of the fall occurred near the outcrop, the 

overburden was relatively shallow (about 100 ft) and the upward 

growth of the roof fall broke through to the surface forming a 

sinkhole (4). With time, the area of the underground failure and 

surface deformation has continued to expand and now includes an 

area of several interconnected sinkholes (figure 4). 

Figure 2. Lake Lynn Experimental Mine 

Figure 3. Map of roof fall area in the old workings. 

Figure 4. View of mine portal Nos. 3 and 4 and sinkhole area. 

The conditions leading up to the roof collapse have been studied 

in detail from a geologic and engineering perspective and several 

factors have been identified that most likely contributed to the roof 

collapse. These factors are summarized as follows from the 

literature (4-6). As mentioned earlier, the Greenbriar Limestone at 

this site contains several interbedded limestone and shale units. 

Rock mass ratings of the roof material using two different systems 

(Norwegian Geotechnical Institute and Bieniawski’s Rock Mass 

Rating systems) showed the mine roof at the LLEM to be in the 

“good” range with recommended permanent width openings no 

greater than 50 ft and between 90 and 150 ft for temporary 

openings (4). Although the mine entry spans in the old workings 

are about 50 ft, intersection spans approach 75 ft. In addition, the 

staggered pillar design employed in the old workings also resulted 

in significant roof spans (5). It is interesting to note that the mine 

roof failure in January 1994 did indeed occur in an intersection. 

 At the mine site, there are three nearly vertical sets of joints sets. 

A prominent joint set trends nearly parallel to the highwall at 

N70o

E and two less significant joint sets are oriented at N20oE and 

N45o

W (figures 3 and 5). In some areas of the old workings, water 

can be observed dripping intermittently from the exposed joints 

especially during periods of precipitation. In the area of the 

collapse, the joints outlined large blocks of limestone in the mine 

roof. It is believed that over time, water percolated through and 

eroded the joint sets, followed by an infilling of sandy clay material 

(thereby reducing the overall rock mass strength) (4, 5). 

Figure 5. View of mine roof near sinkhole. 

 Finally, weather conditions may have been the triggering 

mechanism leading to the mine roof collapse. The cold winter 

weather at LLL typically causes significant ice build-up along the 

highwall and in the portal areas. During the month of January 

1994, temperatures were extremely cold followed by a sudden 

warming trend that allowed snow to melt on the surface above the 

mine thus increasing the amount of water in the overburden. The 

highwall ice barriers could have restricted water flow from the 

overburden thus causing a build-up of water pressure and thereby 

reducing the stress applied normal to the joints. The reduction in 

normal stress reduced the frictional resistance along joint surfaces. 

This process could have also caused the joint-outlined roof rock to 

slide along the joint planes to a level that allowed the roof to 

collapse (4). 

There was a concern that the roof failure could expand in two 

directions to include the secondary escapeway of the LLEM. It 

was decided to reinforce and protect the secondary escapeway and 

to install instrumentation to monitor the changing conditions in the 
 

 

 

 

 

 

mine roof (figure 6). This work is described in detail by Dolinar 

(4). There was additional concern that the overburden instability 

could also expand and affect the structural integrity of the highwall 

and the No. 4 Portal. To monitor conditions along the highwall 

areas, several survey monuments were installed along the edge of 

the highwall and surveys of the position of the monuments are 

collected monthly. In addition, four biaxial tiltmeters were also 

installed along select areas of the surface deformation (figure 7). 

These instruments are connected to a datalogger that collects 

ground movement on an hourly basis. Over time, the mine roof 

conditions near the No. 4 Portal had deteriorated significantly and it 

was too dangerous to conduct a detailed underground survey of the 

mine in this area. It was therefore decided to investigate the 

overburden conditions near the No. 4 Portal remotely using GPR. 

Figure 6. Crib wall to protect the secondary escapeway. 

Figure 7. Biaxial tiltmeter installed at edge of sinkhole. 

GROUND PENETRATING RADAR TECHNOLOGY 

GPR has been used in agricultural, archeological, construction, 

environmental, forensic, geological, groundwater, military, and 

mining applications (7)1

. Under the proper conditions, GPR can 

provide valuable information that can be integrated into the mining 

plans to help define problematic areas or identify the problem 

Additional information on GPR can be found on the Internet 

using search terms such as “ground penetrating radar”, “GPR”, 

“georadar”, “ground probing radar”, “subsurface radar”, “borehole 

radar”, borehole GPR”, etc (www.g-p-r.com is a good starting 

point) . 

sources. Advance identification of the areas can allow mine 

operators to plan for changing ground conditions or avoid predicted 

problem areas (8). Use of dynamic GPR surveys (surveys 

performed while moving the antenna at a constant pace) can 

generate large quantities of field data and, under certain 

circumstances, can provide detailed subsurface information of a 

scope that is superior to that obtained from single-point sources 

such as drill holes (note, the collection of ground truth information 

to verify the radar interpretations is always recommended). 

Because of greater sample density, anomalous zones are more 

likely to be detected by GPR as compared to drilling, resulting in a 

more accurate characterization of subsurface conditions (9). 

GPR is a non-invasive geophysical method that uses reflected and 

backscattered electromagnetic waves to locate and identify 

variations in the electrical properties of subsurface materials (7). 

The basic principles of reflective ground penetrating radar are 

summarized as follows (10). An electromagnetic pulse wave is 

radiated into the ground from a transmitting antenna and travels at a 

velocity that is primarily determined by the relative permittivity of 

the material. The wave radiates outward until it encounters 

material with different electrical properties. At that point, a portion 

of the energy passes through the material, some of the energy is 

refracted away and part of the energy is reflected back to the 

source. The reflected portion of the wave is captured by a 

receiving antenna and is recorded for processing. The round trip 

time (or two-way travel time) of the pulse is greater for deeply 

buried material or objects than for shallow material or objects. 

Therefore, the time of arrival for the reflected wave can be used to 

determine the approximate depth to a feature (fracture, void, 

geological structure, etc), if the velocity of the wave in the 

subsurface is known. 

There are two important electrical properties of geological 

materials that affect the pulse wave – dielectric constant (relative 

permittivity) and conductivity. Dielectric constant is a critical 

parameter because it controls the propagation velocity of 

electromagnetic waves through a material and the reflection 

coefficients at interfaces, as well as affecting the vertical and 

horizontal imaging resolution (11). In rocks and minerals, 

dielectric properties are primarily a function of mineralogy, 

porosity, water saturation, frequency, lithology, component 

geometries, and electrochemical interactions (11-13). Variations in 

each of these parameters can significantly change bulk dielectric 

constants (11). The value of the dielectric constant ranges from 

1 for air (fastest propagation) to 81 for water (slowest propagation) 

and the greater the difference between the two materials, the 

stronger the reflected pulse wave (14). Table 1 shows select 

reported values for dielectric constants. 

Table 1. Reported values of select bulk dielectric constants. 

Rock Type, Condition Bulk Dielectric Constant 

Coal, dry 3.5 (15) 

Coal, wet 8 (15) 

Limestone, dry 5.5 (14) 

Limestone, wet 8 (15) 

Sandstone, dry 2-3 (15) 

Sandstone, wet 5-10 (15) 

Shale (Condition Not Reported) 5-15 (15) 

Shale, wet 6-9 (15) 

1 
 

 

 

 

 

 

 

 

 

 

 Electrical conductivity is the ability of a material to conduct 

electrical current, and it determines the depth of penetration of the 

pulse wave, i.e., greater conductivity results in lesser depth of 

penetration. Conductivity can vary greatly and is primary governed 

by water content and dissolved salts, as well as the density, 

permeability, porosity, and the temperature of the material (16). 

Antenna selection for mining applications will be dictated by the 

size, orientation and depth of the feature under investigation and 

the ability to securely hold or move the antenna along the mine 

roof, rib areas or along the ground surface. If sufficiently sized 

samples of the host rock can be obtained or if in-mine calibration 

tests can be performed, then it is possible to accurately estimate the 

dielectric constant of the material and thus make reliable estimates 

of the depth and location of the in-situ material, object or feature 

under study. In general, a contrast in properties has to occur over a 

size larger than 1/3 of a wavelength to be observable (though 1/10 

wavelength is possible under exceptional circumstances) (7). 

Radar survey resolution increases with increasing antenna 

frequency (decreasing wavelength), but at the expense of depth of 

investigation (which generally improves with decreasing antenna 

frequency) (7). In other words, high-frequency antennas can 

provide high resolution, but have shallow depths of penetration and 

lower-frequency antennas have lower resolution and deeper depths 

of penetration (17). For further information about antenna 

wavelength and resolution, an in-depth discussion is provided in 

reference 7. 

OBJECTIVE and APPROACH 

The objective of this work was to determine if GPR could be used 

to help assess the mine roof conditions near the No. 4 Portal area of 

the Lake Lynn Experimental Mine (LLEM). The GPR surveys 

were conducted using shielded antennas whose frequency spectra 

produced pulses centered near 100-MHz (set in the bistatic 

position) and 200-MHz (figure 8). These antennas were selected 

because they were expected to provide the required depth of 

penetration (the 30 ft thickness of the overburden) with the 

necessary resolution (ability to resolve the mine opening or roof 

fall area) and could be easily moved along an old access road that 

served as the survey site over the No. 4 Portal. 

Figure 8. SIR 2 System and antennas used in this study. 

The equipment used to conduct the ground penetrating radar 

survey in this study was a GSSI SIR® System 2 (SIR-2) Model No. 

DC-2 control unit (figure 8) built by Geophysical Survey Systems, 

Inc.2

2

 Mention of a specific product or trade name does not imply 

endorsement by NIOSH. 

 The SIR-2 is a lightweight, portable, general-purpose radar 

system that is available as an intrinsically safe unit. The output 

display can be shown as a single wiggle trace (a waveform display 

plot that is analogous to an oscilloscope trace), a waterfall plot of 

the wiggle traces (shows multiple stacked radar waveforms), or a 

multicolored line scan (reflected signal amplitudes represented by 

various colors according to a user-selected color look-up table). 

The data can also be printed via an external printer. 

RADAR SURVEY 

A GPR survey grid was located along an access road that passed 

over the top of portal No. 4 (figure 9). In the area of the access 

road, the soil and unconsolidated weathered rock units had been 

removed exposing the limestone rock mass, thus allowing for good 

coupling of the transmitted radar wave with the rock units. Three 

survey lines were positioned on the access road and reference 

stations were placed on each line about 10 ft apart (labeled A-I). 

Survey line L1 (survey stations 1A-1I) and L2 (survey stations 2A2I) were each 80 ft long and trended approximately perpendicular 

to the portal. Survey line L3 (survey stations 3B-3G) was 

positioned parallel to L2 and was only 60 ft in length due to a 

nearby hillside that limited surface access (figure 10). A tape 

measurement showed that the overburden at the No. 4 Portal was 

30 ft thick. A layout map of the survey grid is shown in figure 11. 

Figure 9. LLEM Portal No. 4. Note relative position of GPR 

survey line is shown as the black dotted line. 

Dynamic scans of the area were made over the grid following the 

trend of each survey line. Reference marks were placed at 10 ft 

intervals along each line in the radar record to mark the start and 

end of the line, and at the edge of the No. 4 Portal. Table 2 shows 

the SIR-2 system set-up for each antenna used in this study. The 

values shown in the table, with the exception of the vertical highand low-pass filter settings, scans per second and the dielectric 

constant, are standard default values. Adjustments to the vertical 

low-pass filter settings were made to eliminate high-frequency 

noise (snow) from the data. Adjustments to vertical high-pass filter 

settings were made to eliminate low-frequency noise (tilt) from the 

data (14). A dielectric constant of 6.0 was used in this study based 

on results from previous radar work at the LLEM; these studies 

included calibration experiments (18). Although several 

interbedded zones of shale were observed in the outcrop, it was 

believed that the value of the dielectric constant used was 
 

reasonable for this study site since the most prominent rock units 

appeared in the correct location in the processed radar records 

when compared to highwall measurements. 

Figure 10. Layout of the GPR survey area near Portal No. 4. 

 

Figure 11. Map of the GPR survey area near Portal No. 4. 

Table 2. SIR 2 set-up used for GPR survey. 

Parameter 100-MHz 

Antenna 

200-MHz 

Antenna 

Data collection mode Continuous Continuous 

Range, ns 300-600 200 

Samples per scan 2048 1024 

Resolution, bits 16 16 

Number of gain points 4 5 

Vertical high pass filter, MHz 15 30 

Vertical low pass filter, MHz 200 400 

Scans per second 16 32 

Horizontal Smoothing, scans 5 5 

Transmit Rate, KHz 32 64 

Dielectric Constant 6 (18) 6 (18) 

RADAR RECORD INTERPRETATION 

The radar records generated during this study were analyzed using 

GSSI’s Radar Data Analyzer for Windows (RADAN)3

 version 6.0. 

This package allows the user to operate in the Windows 

environment with application-specific modules (19). Figures 12-14 

show examples of interpreted radar records from surveys of each 

line using the 200-MHz antenna to a depth of about 20 ft. The 

vertical scale in all figures represents depth into the limestone and 

the horizontal scale represents distance along the survey line. Short 

vertical line segments were added to the horizontal scale showing 

the beginning and end of the survey, reference survey points, and 

the edges of the portal. In all of the radar records, the reflected 

pulse energy is shown in terms of a grayscale, with near-white 

being the highest level of reflected pulse energy and near-black the 

lowest. Areas in the record shown with similar gray tones should 

be interpreted as similar levels of reflected pulse energy. In figure 

12, a disturbed or failed zone is believed to extend across the No. 4 

Portal opening to a depth of about 10-ft below the ground surface 

(as outlined by the dotted ellipse). In figure 13, a fractured or 

disturbed zone is believed to extend near the middle to the left edge 

of the No. 4 portal opening at a depth of about 10-ft below the 

ground surface. It is possible that the high energy reflector (shown 

as near-white in the figure) observed at about 9 ft deep is a zone 

that may contain water (perhaps a zone containing shale). The loss 

of this signal return in the middle area of the image may be due to 

loss of water through the joints or fractures in the disturbed zone. 

These same reflections are seen in figure 12 with a similar lateral 

disruption of the reflected energy wave over the disturbed roof 

zone. In figure 14, it appears that there are two shallow zones, 

extending to a depth of 5- to 10-ft below the ground surface 

(outlined by dotted ellipses), that show a disruption in the lateral 

continuity of the reflected energy wave. These zones may 

represent fractured or jointed areas. Below that, extending from the 

middle to the right edge of the No. 4 Portal (outlined by the dotted 

ellipse), is another zone of disrupted energy wave reflections. 

Again, it is thought that this area may represent a fractured or 

disturbed zone. 

Figure 12. Interpreted radar record for Line 1 (200-MHz antenna). 
 

 

Figure 15. Interpreted radar record for Line 1 (100-MHz antenna). 

 


 

Figure 13. Interpreted radar record for Line 2 (200-MHz antenna). 

Figure 14. Interpreted radar record for Line 3 (200-MHz antenna). 

Figures 15 and 16 show examples of interpreted radar records 

from surveys of L1 and L2 respectively using the 100-MHz 

antenna. Note, a lower depth of penetration is shown in the record 

than those presented for the 200-MHz antenna (refer to the 

previous discussion on antenna resolution and depth of 

penetration). In figure 15, a downward trending disturbed zone 

extends from survey reference point 1A through to survey reference 

point 1H. The beginning of the disturbed zone appears to start at a 

depth of 15 ft near survey reference point 1A and then gradually 

progresses downward to a depth of about 20 ft near survey 

reference point 1H. In the record, two high-energy reflections 

(areas shown as near-white in the figure) are also seen, one leading 

upward from the bottom of the figure towards reference point 1C 

and the other, though less prominent, leading upwards towards 

reference point 1F. These reflections are most likely the supports 

for the metal structure positioned at the opening of the portal (refer 

to figure 9). In figure 16, the overburden rock units appear to be 

laterally consistent to a depth of about 12 to 15 ft. Below that 

point, from survey reference point 2B extending to the right edge of 

Portal No. 4, the overburden appears to be significantly disturbed. 

In the record, high-energy reflections (again areas shown as nearwhite in the figure) are also seen at the bottom of the figure below 

survey reference points 2D, 2E and 2F. These reflections are again 

most likely the supports for the metal structure positioned at the 

opening of the portal (refer to figure 9). The fact that the energy 

reflections shown in figure 16 are not as prominent as those shown 

in figure 15 is most likely because L2 is positioned farther away 

from the metal structure than L1. It is believed that the disturbed 

area of the mine roof shown in figures 15 and 16 extends one-half 

the thickness of the overburden above the portal, about 15-ft, or 

downward to the first instance of disruption at a depth of 15-ft. 

Figure 16. Interpreted radar record for Line 2 (100-MHz antenna). 

GROUND TRUTH SURVEY 

 A follow-up ground truth survey of the mine roof conditions was 

conducted remotely from a safe area at the mouth of the portal. 

The observations made from the radar records appear to be correct 

as a large roof fall was observed near the mouth of the portal 

(figure 17). A close-up view of the roof conditions near the mouth 

of Portal No. 4 is shown in figure 18. In this figure, the roof fall 

appears to extend to right and towards the A to C survey reference 

points (note in the radar records the disturbed zones also generally 

extended through these survey points). Laser distance 

measurements of the height of the mine roof, made from the mouth 

of the portal show the collapsed area was within about 15 ft of the 

ground surface. These measurements are also in general agreement 

with the observations made from the 100- and 200-MHz radar 

records. 
 

 

 


 

Figure 17. View of conditions near the opening of Portal No. 4. 

Figure 18. Close-up view of roof conditions near the opening of
 

Portal No. 4. 

SUMMARY and CONCLUSIONS 

In late January 1994, a large roof fall occurred in the old workings 

of the LLEM and extended upward to the ground surface forming a 

large sinkhole. Over time, the roof fall area and the surface 

disturbance continued to grow. There was concern that the roof fall 

area could extend and negatively affect the secondary escapeway of 

the LLEM and highwall area. Supplemental mine roof support was 

installed underground along with monitoring stations to chart the 

movement of the mine roof, highwall and ground surface near the 

sinkhole areas. Because of limited underground access due to 

extensive roof falls, it was decided to survey the conditions of the 

mine roof near the No. 4 Portal using GPR. Three survey lines 

were located on an access road that extended over the No. 4 Portal. 

GPR surveys were performed using 100- and 200-MHz antennas. 

The results of the surveys suggest that the effects of the mine roof 

disturbance extend to within 10- to 15-ft of the ground surface. A 

follow-up ground truth survey of the No. 4 Portal area, though 

limited in lateral extent, seems to confirm the observations made 

from the radar records. It is concluded from this study that GPR 

surveys using 100- and 200-MHz antennas can provide detailed 

information about the upward extent of mine roof falls in areas of 

shallow overburden thickness and locations where surface soils and 

unconsolidated weathered rock units are not a factor. 

ACKNOWLEDGMENTS 

The authors would like to recognize the valuable help and 

knowledgeable assistance of the following individuals during the 

review of this paper: Dr. Syd S. Peng, Chairman and C.T. Holland 

Professor, West Virginia University; Dr. Gary L. Mowery, Mining 

Engineer, NIOSH, George Gardner, Mining Engineer, MSHA; and 

Thomas A. Gray, Engineering Manager II, GAI Consultants, Inc. 

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