APPLICATION OF GROUND PENETRATING RADAR TO EVALUATE THE 

EXTENT OF POLYURETHANE GROUT INFILTRATION FOR 

MINE ROOF CONTROL - A CASE STUDY

William D. Monaghan, Electrical Engineer 

Michael A. Trevits, Research Physical Scientist 

National Institute for Occupational Safety and Health 

Post Office Box 18070 

Pittsburgh, PA 15236 

ABSTRACT 

Over the period 2000 to 2003, roof falls have accounted for 4 to 

14% of the fatalities in underground mining operations. The 

National Institute for Occupational Safety and Health (NIOSH) is 

conducting research to reduce the frequency, exposure, and risk of

these events through an ongoing program of field and laboratory

studies. One area of research involves the evaluation of

polyurethane grouting technology that is commonly used to 

stabilize fractured mine roof strata. Concurrently, NIOSH is 

conducting work to evaluate the application of ground penetrating

radar for advanced delineation of problematic mining areas. In this 

study, NIOSH partnered with Sub-Technical, Inc. who injected a 

polyurethane grout (also known as glue) into a roof area of

NIOSH’s Safety Research Coal Mine. The mine roof area was 

scanned using ground penetrating radar technology before and after 

grout injection in an attempt to determine the extent of grout 

infiltration. A comparison of the pre-and post-grouting radar

records showed a significant change at a mine roof depth of about 

four to five feet. The interpreted radar records were then compared 

with drill core information, borescope evaluation of roof-bolt holes

in the study area, and underground observations. At this site, the 

interpretations of the radar records correlated with data obtained 

from the core holes, borescope evaluations and underground

observations. This study showed that ground penetrating radar 

technology can be a useful tool for detecting changes in mine roof 

due to the injection of the grout. 

BACKGROUND 

In 2003, the U.S. mining industry achieved its best safety record

since statistics were first compiled in 1910. Moreover, over the 

past three years mining fatalities have declined by 34 percent and 

injury rates by more than 20 percent (1). Despite the remarkable

downturn in the accident statistics, injuries and fatalities continue

to occur. In fact, during the period 2000 to 2003, mine roof falls 

have accounted for 4 to 14% of the fatalities for underground 

mining operations (2). 

The National Institute for Occupational Safety and Health 

(NIOSH) is conducting research to reduce the frequency, exposure, 

and risk of mine roof falls through an ongoing program of field and 

laboratory studies. One area of research involves the evaluation of

polyurethane grouting technology that is commonly used to 

stabilize rock surrounding a mine opening. As a part of this work, 

ground penetrating radar (GPR) is being used to map the 

distribution of injected polyurethane grout in mine roof strata. SubTechnical, Inc., located in Mars, Pennsylvania, agreed to partner 

with NIOSH to evaluate a polyurethane grout (also known as glue), 

that was injected into a mine roof area at the NIOSH Safety

Research Coal Mine (SRCM). NIOSH operates underground test 

facilities for research aimed at improving the health and safety of 

those working in the mine environment. The Safety Research Coal

Mine, located near Pittsburgh, PA, is a specialized full-scale 

underground test facility that provides for realistic testing and 

research on newly developed equipment, procedures, and

technologies (3). The SRCM became functional in the early 

1970’s and was developed as a room-and-pillar mine that is 

approximately the size of a working section in a commercial coal 

mine (4). 

OBJECTIVE OF THE STUDY

The objective of this study was to test the capability of GPR to

determine the extent of grout penetration into the mine roof strata 

in the 13-G room area of the SRCM (figure 1). We approached this

problem by using a variety of antennas whose frequency spectra 

produced pulses centered at near 200-, 400-, 900-MHz. The area of 

mine was selected for study because a number of holes had been

drilled into the roof for the testing of roof-bolts. It was felt that the

roof-bolt holes would permit borescope evaluation of the mine roof 

conditions (fracturing and strata separations) prior to grout 

injection.

Figure 1. Map of NIOSH underground coal mine facility. 
GROUND PENETRATING RADAR 

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

backscattered electromagnetic waves to image, locate and 

quantitatively identify changes in electrical properties in the 

subsurface. Its primary feature is a very short electromagnetic 

pulse, which allows a vertical resolution ranging from centimeters 

to a few meters, depending on bandwidth. In general, GPR surveys 

can generate large quantities of field data and, under the right 

conditions, can provide detailed subsurface information of a scope 

that is superior to that obtained from single-point sources such as 

drill holes. However, when available, confirming data from drill 

holes should always be used as a means of ground truthing the 

conclusions drawn from the radar records. 

 

Although the first GPR survey was performed in 1929 to sound

the depth of a glacier, it was not applied until the 1940’s to earth 

science problems and the 1950’s for ice sounding and planetary

exploration (5-9). Applications of GPR to evaluate rock and soil 

conditions did not occur until the 1970’s when the systems became 

commercially available (9-11). Today, GPR is commonly used in 

archeological, construction, environmental, and geological studies 

to determine the location, orientation, lateral continuity, and depth 

of strata or buried objects. 

 A GPR system generates an electromagnetic pulse that is 

transmitted into the ground with an antenna that is moved along the 

surface, generally at a uniform speed and direction. The

transmitted energy of the pulse is radiated in an elliptical conical 

pattern roughly 90 degrees wide in the antenna plane and 

60 degrees wide in the perpendicular plane. Whenever there is a 

change in the dielectric constant of the subsurface material, a 

portion of the pulse energy is reflected back to the surface and is

detected by the receiving antenna. This reflected pulse provides 

information on the attenuation characteristics (signal strength)

associated with the subsurface material and the two-way travel time

is recorded. 

 The capability of GPR depends on two electrical properties of the 

geological materials under investigation: dielectric constant 

(relative dielectric permittivity) and electrical conductivity. The 

dielectric constant affects the velocity of propagation of the radar

pulse. In rocks and minerals, dielectric properties are primarily a 

function of mineralogy, porosity, water saturation, frequency, and

depending on the lithology, component geometries, and 

electrochemical interactions (12-14). Variations in each of these 

parameters can significantly change bulk dielectric constants (12). 

The greater the difference in dielectric constant between two 

materials, the stronger the reflected pulse energy becomes. The 

dielectric constant ranges from 1 for air (fastest propagation) to 81 

for water (slowest propagation). In addition, there is a notable 

increase in the value of the dielectric constant for a given rock type 

with an increase in the degree of water saturation.

 

 Electrical conductivity controls the depth of pulse penetration. 

The lower the conductivity of the material, the deeper the pulse can 

penetrate. Conductivity is controlled by water, mineral, and clay

content in the subsurface. The depth of penetration of the pulse

also depends on the antenna frequency. Higher frequency antennas 

(e.g., 1200-MHz) provide high resolution, but shallow depths of 

penetration; conversely, lower frequency antennas (e.g., 80-MHz)

have low resolution, but can detect significantly deeper targets.

The equipment used to conduct the ground penetrating radar 

surveys in this study was a GSSI SIR® System 2 (SIR-2) Model 

No. DC-2 control unit built by Geophysical Survey Systems, Inc 

(GSSI).1 The SIR-2 is a lightweight, portable, general-purpose 

radar system and is available as an intrinsically safe unit (figure 2).

The reflected pulse is processed by the control unit and the data are

displayed on the monitor and stored on an internal hard disk. The 

output display can be (1) a single wiggle trace (analogous to an 

oscilloscope trace), (2) a waterfall plot of the wiggle traces, or (3) a 

multicolored line scan in which the reflected signal amplitudes are

represented by various colors according to a user-selected color 

look-up table. The data can also be printed via an external printer.

Figure 2. Photo of Engineer using the SIR-2 unit. 

The two-way signal travel time is determined by measuring the 

time interval between the start of the transmit pulse and start of the

received reflected signal. The amplitude of the reflected signal is 

influenced by the size and geometry of the target, the signal

attenuation characteristics of the geological materials, and the total 

distance that the pulse has to travel. In this study, the resulting 

radar records were processed using GSSI’s Radar Data Analyzer

for Windows NT (RADAN) software1

. This package allows the

user to operate in the Windows environment with applicationspecific modules. 

POLYURETHANE RESIN TECHNOLOGY 

From a historical perspective, Dr. Otto Bayer and his associates 

were awarded a patent in Germany on November 11, 1937, for a 

process to produce polyurethanes and polyureas. This work has 

long been considered to be the foundation patent for the 

polyurethane (polyisocyanate-polyaddition) process (15-16). 

During the 1960’s, the mining engineering community in Germany

began experimenting with the use of a low-density polyurethane

grout (20 lb/ft

3

) to consolidate or secure incompetent strata in coal

mines. It was not until the 1980’s, that high-density polyurethane 

grout (70 lb/ft3

) was introduced (17). This application became

quite popular as a cost-effective alternative to traditional roof 

control maintenance plans. The polyurethane grouting process was 

introduced and pioneered for the U.S. mining industry by the 

former Mobay Corporation (18). Today, several companies offer a 

variety of polyurethane-based mine stabilization services and the

 

1

Mention of specific product or trade names does not imply 

endorsement by NIOSH. 
technology has evolved to the point where special formulations can 

be mixed to meet a variety of strata and in-mine conditions. 

The process typically used involves drilling of 1-3/8-in diameter

injection holes to an average depth of the roof-bolts used in the 

area. The injection holes are drilled on a 10-ft spacing along the rib 

areas on both sides of the entry, and in some circumstances a row 

of holes may also be drilled along the centerline of the entry. The

drilling plan calls for the drilling of every other hole followed by

grout injection. A second set of holes are then drilled between the 

first set of holes and injected with grout. The holes are completed 

using a specialized packer and an extension rod. The packer serves

to seal the near end of the hole and also contains a static mixing

unit (figure 3). The setting depth of the packer is usually 

determined by the conditions at the underground site. During grout 

injection, the two components of the grout are pumped under a 

controlled pressure to the extension rod and then to the packer 

where the grout components are thoroughly mixed and injected into 

the rock mass (figure 4). Pumping continues until either a predetermined amount of material is injected or specified pressure 

increase is achieved (17).

Figure 3. Downhole packer and a static mixing unit. Note, static

mixing unit is contained in the packer assembly (mixing unit is 

shown is for illustration purposes only). 

Figure 4. Grout injection process. 

The site selected for study at the SRCM was previously used for 

testing roof-bolts and, as such, numerous holes had been drilled 

into the mine roof and some of the holes were outfitted with roofbolts of various configurations. Some of the holes containing roofbolts were open and others were sealed as part of the roof-bolt 

testing program. These holes were not included as part of this 

study and were also not shown on the location maps of this study. 

In the study area, six, 1-3/8-in diameter, injection holes were drilled 

into the mine roof (figure 5). The holes were drilled to a depth of 

eight feet and the mixer/packer-extension rod unit was placed to a 

depth of three feet in each hole. During injection operations the 

pressure on the system varied from 200-1200 psi with no fixed 

grout injection rate. In all, a total of 100 gallons of 70 lb/ft3 grout 

was injected into the holes. Table 1 shows the volume of grout 

injected into each hole. The order of grout injection began with 

hole No. 2 and concluded with hole No. 7. 

Figure 5. Detailed layout map of SRCM study site. 

Table 1. Volume of grout injected into each hole.

Hole 

No.

Volume of Grout 

Injected, gallons

21 20 

3 3

4 7

5 5

6 30

7 35

Total 100

1

 Hole No. 1 was not used for grout injection. 

Prior to grout injection, each hole was inspected with a borescope

to determine if any void spaces could be detected and none were

found. In addition, a core hole was drilled near the center of the 

intersection to a mine roof depth of 17-ft. Again, no significant

void spaces or fracture systems were observed in the recovered 

core samples (18). During injection operations, grout was observed

to be flowing from nearby open holes and roof-bolts in the area (as

discussed earlier the holes and bolts were from a from previous 

roof-bolt testing program) (figure 6). The actual amount of grout 

material that flowed into the mine opening is unknown. 
Figure 6. Photo of grout flowing from drill holes. 

Upon completion of the grouting operations, seven drill holes 

(drilled to about 8.5 to 9-ft deep) and two core holes (drilled to 

about 12-ft deep) were completed into the mine roof in the study 

area (figure 5). The drill holes were inspected with a borescope to

see if grout material had penetrated to the location of the drill hole.

The samples recovered from the core holes were logged and 

similarly inspected for grout infiltration (figure 7). In all holes, the 

most significant show of grout was 0.5 inch or less in

thickness (19). The data from the drill hole and core holes form the 

basis for ground truthing the post-processed GPR records. 

Figure 7. Photos showing samples recovered from after injection

core holes (19). 

GPR SURVEYS

The SRCM was developed in the Pittsburgh Coalbed and the

mine opening averages about 19-ft wide along the “G-Entry” and

15-ft wide tapering to 12.5-ft wide along the “13-Entry”. The mine 

opening is about six feet in height. In general, the immediate mine 

roof is composed of about 2.5-ft of interbedded coal and 

carbonaceous shale that is overlain by 1.5-ft of shale, 0.5-ft of coal 

and then a shale unit to a height of seven feet (19). 

A GPR survey grid was established in each entry area with

survey lines trending across the room to each face area. In the “GEntry”, three parallel survey lines (labeled A to C), each 

approximately 35-ft long, were located and spaced five feet apart.

Each line contained reference stations located five feet apart. In the 

“13-Entry”, two sub-parallel survey lines (labeled D and E) were

located and spaced eight feet apart at one end and then converged

to five feet apart (due to the reduced width at the “13-Face” area).

These lines also contained reference stations located five feet apart. 

In each entry, survey lines were positioned parallel to the trend of 

the rib with the exception of the D line (figure 5). 

During calibration of the radar equipment, the dielectric constant 

for the mine roof material was adjusted until the arrival time of the 

reflected signals (expressed in terms of depth) matched the 

stratigraphic information obtained from the pre-grouting core

samples and drill holes (the dielectric constant for the mine roof 

was determined to be 7.5). 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 high-and low-pass filter settings, 

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 (20). 

Table 2. SIR-2 System set-up used for GPR scans at the SRCM. 

Parameter 

Setting for 

200-MHz

Antenna 

Setting for 

400-MHz

Antenna 

Setting for 

900-MHz 

Antenna 

Data collection mode Continuous Continuous Continuous

Range, ns Varied1 Varied1 Varied2

Samples per scan 512 512 512 

Resolution, bits 16 8 8 

Number of gain points 

5 5 4 

Vertical high pass 

filter, MHz 30 30 100 

Vertical low pass filter,

MHz 400 800 1800 

Scans per second 32 32 32 

Horizontal smoothing, 

scans 5 4 4 

Transmit rate, KHz 64 64 64 

Dielectric constant 7.5 7.5 7.5 

1. Varied from 300 to 1000 ns. 

2. Varied from 300 to 1200 ns. 

In order to determine the extent of grout penetration into the mine

roof, dynamic GPR surveys were performed before and after the 

grouting process at the “13-G Room” study area. Dynamic GPR

surveys of the mine roof were conducted by placing the antenna in 

the immediate vicinity of the mine roof and then the antenna was

moved along the survey line at a constant speed (figure 8). In each 

case, the surveys were conducted in both directions and the 

resultant data were compared to ensure repeatability. If a problem

was observed in the record, then the survey was repeated. 

 

DISCUSSION 

It was decided to only use the radar data from the 900-MHz 

antenna because this antenna provided the highest resolution and 

the required depth of penetration. Here, we were trying to discern 

the location where grout had been injected into the mine roof (at a 

depth of four to five feet) and we were aware that the zones where 

the grout was observed were on the order of 0.5-in or less. Figure 

Nos. 9-11 show selected post-processed results of the GPR surveys
before and after the grout was injected in the “13-G-Room” area 

(refer to figure 5 for the layout map of the GPR survey lines). In

the figure, the survey begins at the left margin of the figure. The 

depth into the mine roof is shown in the left hand column and the 

boxes with numbers and short line segments at the top of the figure 

show the location of the reference stations along the survey line. 

The designation “before” represents the scan information before the 

grout was injected (pre-grouting) and the designation “after” 

represents the scan that was conducted after grout injection (postgrouting). A generalized stratigraphic column has been added to 

aid in the interpretation and is shown in the right side of both the 

pre-and post-grouting radar records. The vertical white dotted lines

that transect the post-grouting scan represent the location of the

post-grouting drill and core holes projected onto the GPR survey

line. In general, data from an observation hole was used only if the 

hole was in the immediate vicinity of the subject survey line. The

symbol used to identify the type of hole is the same as used in 

figure 5. The white stars are used to show the observed location of

grout (19). 

Figure 8. Researcher conducting GPR study in SRCM. 

Figure 9 shows the post-processed results of the dynamic GPR

survey for the B-Line. The radar records show a good correlation 

with the generalized stratigraphic section shown on the right side of

each record. In 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. As discussed earlier, the

greater the difference in dielectric constant between two materials,

the stronger the reflected pulse energy. In the pre-grouting radar 

record, there is a band of reflected pulse energy at a mine roof 

height of four to five feet. At this depth, there is a coal unit that

most likely contains some water which could account for the high

level of reflected pulse energy at this location. Recall that water

has a dielectric constant of 81 as compared to the value of 7.5 used 

for the mine roof strata in this study and that the level of water 

saturation can cause a notable increase in the value of the dielectric

constant for a given rock type. In the post radar record, the higher 

level of reflected pulse energy does not appear, and this is also the 

depth where grout was noted in the post-grouting core and survey

holes. This could have occurred because grout may have infiltrated

and displaced water that was contained in the coal unit and would 

then explain the absence of the high-level pulse energy reflections

in the post-grouting radar record. 

Note, the vertical anomalies in the pre-grouting record most 

likely represent nearby roof-bolt holes (refer to previous discussion 

about the roof-bolt testing program). As mentioned earlier, most of 

these holes were filled with grout and do not show in the postgrouting record. Two anomalies are also observed in the postgrouting record (as highlighted by the ellipses). The first anomaly 

projects from the lower right to the upper left of the record, 

between stations 5 and 10. The second anomaly projects from the 

upper left to the lower right of the record, between stations 20 and 

30. It is unknown if these anomalies are related to the mine 

opening or geological in nature. 

Figure 10 shows the post-processed results of the dynamic GPR

survey for the C-Line. Again, there are vertical anomalies in the

pre-grouting record that most likely represent nearby holes from the

roof-bolting program. These structures are not shown in the postgrouting record most likely because they were filled with grout. 

Additionally, in the post-grouting record, there is a vertical 

anomaly projecting downward between stations 20 and 25. This is 

most likely a nearby hole from the roof bolting program. Also, an

anomaly is observed projecting downward from the upper left to 

the lower right of the record, between stations 20 and 35. It is 

unknown if this anomaly is related to the mine opening or

geological in nature. 

As shown in Figure 9, the radar records show a good correlation 

with the generalized stratigraphic section shown on the right side of

each record. As in figure 9, there is a notable difference in the 

post-grouting radar record as compared to the pre-grouting radar

record at a mine roof depth of four to five feet. The reason for the 

difference between the pre-and post-grouting radar records is the 

same as offered in the discussion of figure 9. 

Figure 11 shows the post-processed results of the dynamic GPR

survey for the D-Line. There are several vertical structures in the 

pre- and post-grouting radar records. These features are believed to

be holes from the roof-bolting program. The radar records show a 

good correlation with the generalized stratigraphic section shown

on the right side of each record. As seen in figures 9 and 10, there

is a difference in the post-grouting radar record as compared to the 

pre-grouting radar record at a mine roof depth of four to five feet. 

The reason for the difference between the pre-and post-grouting 

radar records is the same as offered in the discussion of figure 9. 

Two anomalies are also observed in the post-grouting record (as 

highlighted by the ellipses). The first anomaly projects from the 

upper left to the lower right of the record between stations 15 and 

25. The second anomaly projects from the upper left to the lower

right of the record, between stations 30 and 45. It is unknown if

these anomalies are related to the mine opening or geological in

nature. 

SUMMARY AND CONCLUSIONS 

In this study, NIOSH conducted research to evaluate the 

application of GPR to determine the extent of injected polyurethane

grout infiltration into a roof area of the NIOSH Safety Research
Figure 9. Post-Processed Radar Records with Ground Truthing – B-Line. 

Figure 10. Post-Processed Radar Records with Ground Truthing – C-Line. 
Figure 11. Post-Processed Radar Records with Ground Truthing – D-Line.

Coal Mine. Sub-Technical, Inc., partnered with NIOSH in this 

effort and provided the materials and grouting services. 

GPR was selected for use in this study because it is a noninvasive technology and would not degrade the mine roof. The 

technology could also provide the level of resolution and depth of 

penetration needed to detect zones of grout infiltration. A grid 

was established in the study area and the mine roof was scanned 

using GPR technology before and after grout injection. A 

comparison of the pre-and post-grouting radar records showed a

significant change (absence of high-level pulse energy

reflections) at a mine roof depth of about four to five feet. The 

changes were correlated to the location where polyurethane grout

had infiltrated the mine roof strata as observed in the postgrouting core and survey holes. The difference between the preand post-grouting radar records could have occurred because 

grout may have infiltrated and displaced water that was contained 

in a coal unit (thus affecting the pulse energy reflections in the

post-grouting radar record). 

Although the results of this study offers promising insight in to 

the application of GPR to determine the extent and effect of

polyurethane grout infiltration, it should be kept in mind that this

technology should be tested at other mine sites under a variety of

mining conditions (varying geology, fractured roof strata and

zones with and without water infiltration, etc). Future GPR

research will focus on laboratory and possibly field studies to test 

the use of tagging material (e.g., metal fines) in the grout 

mixture. These materials should serve as a target and could

possibly enhance the radar signal reflections. 

ACKNOWLEDGMENTS 

The authors would like to recognize Paul Stefko and the 

NIOSH staff at the SRCM facility for their dedication and 

assistance during the conduct of this research effort. 

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