NBSIR 76-840

FM-CW ELECTROMAGNETIC TECHNIQUE OF

MEASURING COAL LAYER THICKNESS

Doyle A. Ellerbruch

Donald R. Belsher

Electromagnetics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

May 1976

Prepared for

U.S. Bureau of Mines

United States Department of the Interior

Pittsburgh, Pennsylvania 15222

NBSIR 76-840

FM-CW ELECTROMAGNETIC TECHNIQUE OF

MEASURING COAL LAYER THICKNESS

Doyle A. Ellerbruch

Donald R. Belsher

Electromagnetics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

May 1976

Prepared for

U.S. Bureau of Mines

United States Department of the Interior

Pittsburgh, Pennsylvania 15222

U.S. DEPARTMENT OF COMMERCE, Elliot L. Richardson, Secretary

Edward 0. Vetter, Under Secretary

Dr. Betsy Ancker-Johnson, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

FOREWORD

This report was prepared by the National Bureau of Standards, Boulder,

Colorado, under USBM Contract No. J0155124 . It was administered under

the technical direction of the Pittsburgh Mining and Safety Research Center

with Mr. M. Pazuchanics acting as the technical project officer.
CONTENTS

Page

1 . INTRODUCTION 1

2. MEASURING SYSTEM 1

3. SYSTEM PARAMETERS 4

4 . ANTENNA IMPROVEMENT 5

5. EXPERIMENTAL RESULTS 6

5.1 Laboratory Samples of Shale and Coal 6

5.2 Bruceton Mine 7

5.3 Loveridge Mine 8

5.3.1 Roof Data 8

5.3.2 Floor Data 11

5.4 Hillsboro Mine 11

6 . CONCLUSIONS 12

7. RECOMMENDATIONS 13

8. ACKNOWLEDGMENTS 15

9 . REFERENCES 15

APPENDIX 16

iv
LIST OF ILLUSTRATIONS

Page

Table I. Laboratory Sample Data 17

Table II. Bruceton Mine 18

Table III. Bruceton Mine 18

Table IV. Loveridge Mine 19

Figure 1. Block diagram of the FM-CW electromagnetic system 20

Figure 2. Sample of raw FM-CW system data taken in the Loveridge

Mine 21

Figure 3. H-Plane field strength pattern 22

Figure 4. E-Plane field strength pattern 23

Figure 5 . Antennas in the Bruceton Mine 24

Figure 6 . Test areas in the Bruceton Mine 25

Figure 7. Measurement setup in the Bruceton Mine 2 6 Figure 8. Coal layer depths in the Loveridge Mine 27

Figure 9. Relative amplitude of coal surface response in the

Loveridge Mine 28

Figure 10. Raw FM-CW data from station 10 in the Loveridge mine 29

Figure 11. Raw FM-CW data taken at station 15 in the Loveridge mine-- 3 0 Figure 12. FM-CW floor response data in the Loveridge mine 31

Figure 13. Coal layer depths in the Hillsboro miine 32

V

FM-CW ELECTROMAGNETIC TECHNIQUE OF MEASURING COAL

LAYER THICKNESS

Doyle A. Ellerbruch and Donald R. Belsher

ABSTRACT

An FM-CW microwave system was investigated for measuring coal

layer thickness. Measurements were made in three different mines

near Pittsburgh, Pennsylvania, near Fairview, West Virginia, and near Coffeen, Illinois. Microwave frequencies in the range 1-2 GHz were used to measure samples up to 55 cm thick. All samples were backed with a naturally occurring shale. Measurements were also

made on coal and shale samples compounded in the laboratory at

the Bureau of Mines Pittsburgh Mining and Safety Research Center

near Bruceton, Pennsylvania.

The results indicate that layer thickness can be determined

in most cases, although large anomalies may, in some cases, produce misleading results. Many anomalies that were detected with

the FM-CW system were verified visually by drilling into the coal

layer

.

The dielectric constant of coal apparently varies significantly within a coal seam. The form of the output signals from the FM-CW system seem to

simplify the data interpretation and analysis process as compared

to the manually swept microwave system used previously. It appears

that this technique has the potential of measuring changes in the

dielectric constant of a coal seam and providing an output that

can be used for real-time corrections in layer thickness measurement.

Key words: Automation; coal; coal mine safety; dielectric constant; energy; microwave measurement; nondestructive testing;

thickness of coal layer.

1 . INTRODUCTION

The possibility of using a microwave system to measure coal layer

thickness in a mine was investigated by August 1974, and the results were reported in NBSIR 74-387, Microwave Measurement of Coal Layer Thickness,

dated September 1974. The basic measurement capability was demonstrated in

that microwaves penetrated the coal and were reflected by a shale background.

The primary objectives of this phase of the proposed effort were to

develop improved methods for microwave measurement procedures, to determine

the thickness of undisturbed coal layers in situ, and to demonstrate these

procedures in actual mines.

2. MEASURING SYSTEM

The microwave hardware used in these experiments was basically the same

as that used previously [1] ; however, the antenna design was modified to

provide better directivity and to decrease the coupling between transmit and

receive antennas over the frequency bandwidth. Details of these modifications will be presented later in this report.
Appropriate microwave plug-in units were acquired to minimize the

quantity of electronic equipment needed for in-mine experiments, and the

entire system was powered from a 12 Vdc automobile battery. Data recording

was done with a magnetic tape recorder and, in some cases, with an X-Y

plotter

.

The fundamental difference between the system used previously and the one used for this work is that a greater degree of data processing was ac- complished in real time prior to recording. The microwave system was operated

as a frequency modulated-continuous wave (FM-CW) radar.

The microwave signal generator in figure 1 is swept in frequency over

its bandwidth so that a linear frequency vs. time output signal is produced.

The microwave signal travels from the generator to the mixer via more

than one path. If the electrical lengths of the paths are identical, the

reference and test signals going into the mixer would arrive at the same

time and the instantaneous rf frequencies would be identical at all times.

But the test signal will arrive at a later time because that signal

goes through the antennas via the coal sample. A portion of the test signal

is reflected at the air-coal interface and another portion is reflected at

the coal-shale interface. These reflected signals will arrive at the mixer

at times t^ and t^ respectively as shown on figure 1. Because all inputs to the mixer arrive at different times, the instantaneous microwave frequencies differ. The mixer is a product demodulator

and has an output that is a function of the product of the inputs. Only the

lowest frequency components in the output are preserved; the higher frequencies are filtered out.

The lower frequency components are displayed on a spectrum analyzer.

The location of the first peak is a function of the distance from the antennas

to the coal surface. The location of the next peak is a function of that

distance plus the coal layer depth and its dielectric constant.

Assume for the moment that the microwave signal is incident upon an infinitely thick layer of coal. Let the reference signal arriving at the

mixer be presented as

e.^ = cos o)j^^ t, (1)*

and the test signal reflected from the coal surface as,

where 2R/c is the time required for the signal to travel a distance R from

the transmitting antenna to the coal surface and back to the receiving antenna.

It is assumed that both antennas are equidistant from the coal surface and that

the angle of incidence is zero degrees.

*See appendix for definition of terms.
Going through the product demodulation process and neglecting the harmonic microwave frequency component, the mixer output signal is,

Letting,

-1^1^2 2R

^0 " —2 -(t) —

2R

'^(t) = "(t) — '

(3)

(4)

the frequency of the signal out of the mixer is,

d; f = ^ 0 2r dt

ty

(5)

d-r f = 2R (t)

0 c dt (6)

If the frequency of the microwave signal is changed at a constant rate,

the frequency of the signal out of the mixer is proportional to

the distance from the antennas to the coal surface.

Extending the analysis to include a finite layer of coal comprised of

the air-coal surface and the coal-shale surface, the total received test

sicrnal [2] is.

-2vd 2R.

1" 2

-2-. d

e J

(7)

Recognizing that coal is a lossy microwave material and utilizing typical

dielectric constants of coal ( e; = 5 ) and shale {z^ = 36) , the amplitude of

the denominator in eq. (7) is found to be approximately unity. Thus ne- glecting the multiple reflection terms contributed by the higher order

reflection term in the denominator, eq. (7) can be written as [2]

'1^2 ^ , 2R

+ r2E^-{cos

(t) c ^ c

J

-2:xd

(8)

Mixing this received signal with eq . (1) will provide a mixer output

signal

'1^2 ' 2R

•/(t) c J

'2^1^3

r cos to

when the harmonic microwave signal is neglected.

'2R _^ 2d/E

C -C -2ad

(9)

3
The frequency of the first component is given by eq. (6) and the frequency of the second component is

f = f2R. , 2d/e

02 c c

df

dt . (10)

The difference between eqs. (10) and (6) is a frequency that is proportional to the thickness and the electrical properties of the coal layer.

2dvT ^^(t) . "^0 c dt •

The procedure has been to measure the dielectric constant, e, on site

and reduce the working equation to one unknown, d, the coal layer thickness,

AfQ. (12)

2/e df ^^j/dt

It has been found that more than one finite layer of coal may exist at

a given point in a mine. The analysis given here for one layer may be ex- tended for stratified media.

3. SYSTEM PARAMETERS

Equation (12) is the working equation for the FM-CW system. Based

upon the measurements made last year [1] and anticipating that up to 40 cm

of coal will be measured again, the microwave frequency band 1-2 GHz was

used. That bandwidth was swept in 7.48 milliseconds thus eq. (12) becomes

Af

d - 0.1122 —- , (cm) (13)

This equation contains two unknowns (d,e) both of which will be

determined by measurement.

A measured value for the dielectric constant (e) can be obtained for

the coal by any of the three techniques discussed in [1] . The difference

frequency (^fg) out of the mixer is also a measured value; thus, the layer

depth (d) is calculated from eq. (13).

A sample of raw data obtained with the system at one of the test points

is shown in figure 2. The high amplitude response at 8.05 kHz is the reflection from the air-coal interface. The low amplitude ripple preceding that

response was caused by imperfections in the microwave system, such as residual mismatch reflections in the components. The response at 8.4 kHz was caused by reflections from a sulphur ball (pyrite concretion) within the

coal seam at that particular measurement point.

4
The response at 8.9 kHz was caused by reflections from the coal-slate

interface. The measured dielectric constant for the coal at that test point

was 7.6. Using this with the difference frequency from figure 2 in eq.

(13), the depth of the coal seam at that test point is 34.6 cm. The physical

thickness measured at that test point was found to be 36.8 cm. The physical distance between the sulphur ball and the shale at that

test point is 20 cm. In discussions with some of the miners in that particular mine it was learned that a thin layer (up to 0.6 cm thick) of impure

coal exists about 20-25 cm below the coal-shale interface throughout the

Pittsburgh seam. The impure coal layer is referred to as the "top binder,"

or the "soot layer." It was noted that many dielectric discontinuities,

such as sulphur balls and shale lenses, occur at the top binder level.

4. ANTENNA IMPROVEMENT

The theories developed for all microwave measurements in coal mines

assumed ray paths for the electromagnetic energy radiated from the antennas.

It was realized that the antennas used were not directive enough to fully

justify use of a single ray model; however, in the initial measurements last

year the antennas were selectively positioned in the mine to reduce undesired

direct coupling.

A portion of the effort this year was expended in improving the directivity of both antennas and providing better isolation between them. Improvements like this decrease the susceptibility of the system to spurious reflections from surrounding obstacles.

The antennas are broadband rectangular aperture horns which utilize

double ridged waveguide techniques [3]. Initially two sides of the horns

were made from a printed circuit board. These were removed and replaced

with solid aluminum to reduce the H plane beamwidth. All the mechanical

junctions in the horn assembly were taped with conducting tape to provide

continuous current paths inside the horn and to minimize current leakage and

resulting spurious radiation. Both antennas were covered externally with rf

absorbing material to minimize the effect of spurious radiation caused by

current flowing on the outside of the antennas.

Finally, because in an operational mine the horns will be pointed up,

it was realized that some type of dust cover would be necessary to prevent

filling the horns with coal dust. A lucite lens was designed and evaluated

for satisfying two purposes. First, it would cover the entire aperture and

would serve as a dust cover, and second, it would serve to focus the radiated

energy and more closely validate the ray theory assumption. Two different

lens designs were evaluated. One has a radius of 7.5 cm and the other had a radius of 10 cm.

5
The performance characteristics of the antennas were determined by

measuring radiation patterns at 1 and 2 GHz in an anechoic chamber. The

intent was to make comparative field strength measurements. Results of the

antenna measurements are shown in figures 3 and 4 . The narrowest beamv/idth

and the lowest level sidelobes occur when the 10 cm radius lens was flush

mounted on the aperture. Thus a 10 cm radius lens was fabricated for each

antenna.

Figure 5 is a photograph of the antenna assembly positioned under a test area in the Bruceton mine.

5. EXPERIMENTAL RESULTS

5 . 1 Laboratory Samples of Shale and Coal

The Bureau of Mines had prepared samples of a coal layer on a shale

layer. The shale layer was comprised of 92 percent (by volume) shale dust

and 8 percent cement. These were mixed with water and poured into a form.

Similarly a 92 percent coal dust and 8 percent cement mixture was prepared

and poured on top of the shale.

Several different laboratory samples were measured by placing the FM-CW

system antennas above the samples. A summary of sample size and measured

results are given in table I. Sample I was the only one considered to have sufficient surface area

for measurements with the FM-CW system. Nevertheless, all of the measured

results obtained from those samples are given on Table I. With samples 2,

3, and 4, the FM-CW system illuminated not only the desired coal surface but

also the floor and, in some cases, other samples adjacent to the one being

measured. Thus, undesirable responses complicated much of the data collected.

Most of the undesired responses were related to their sources at the time of

the measurement with the use of a small metallic reflector.

The dielectric constants for samples 1, 2 and 4 were measured using

return loss techniques. Those values, along with the FM-CW frequency output, were used in eq. (13) to arrive at the tabulated layer thickness.

No return loss measurement was made on sample 3 . The measured dielectric constant values for sample 3 were determined by using the physical

depths of the coal and shale along with the measured difference frequencies

in eq. (13)

.

Moisture contents (by weight) for these samples were measured by the

USBM [4]. Two specimens were chipped from each sample for laboratory moisture

analysis. Average results are shown in parenthesis on table I.

6
5.2 Bruceton Mine

As was done last year [1] , an existing mine face was undercut from the

mine floor up toward the shale to prepare three test areas having different

roof coal depths. The physical arrangement is shown in figure 6. Figure 7 is a photograph of the total area, with the FM-CW system in the foreground

between the miners. The undercut extended approximately 1.2 5 meters into

the coal face so that the antennas could easily be placed under each test

area. Each test area was at least 1.25 meters wide. The coal surfaces

were dry.

The dielectric constant was measured in the three test areas by using

the measured difference frequency and the physical depth in eq . (13). A

summary of that dielectric constant data is given in table II.

As was done last year, the dielectric constant was also measured by

utilizing known step changes in coal depth, when the absolute coal depths

are not known. Let a difference frequency measurement be made in one of

the test areas of figure 5. Equation (13) can be rearranged into the form,

If^ = 0.1122 d^.^. (14)

Now let a difference frequency measurement be made in another of the

test areas of figure 6. The equation for that measurement becomes,

L±^ = 0.1122 d^/e. (15)

Taking the difference between eqs . (14) and (15) and solving for the

dielectric constant results in

79.4 (Af--Af )

2

£ = (16)

(d2-d^)2

After the dielectric constant has been determined with eq . (16), it can

be used in eqs. (14) and (15) to calculate the depths of the head coal at

those test points.

This measurement and data processing procedure was utilized in the

Bruceton mine. A summary of the results are given on table III.

The results given on tables II and III are calculated from data collected at various points along approximately 3 meters of the face. This

year, as was observed last year, there was some variation in the dielectric constant data. It is believed that the variation is real and is caused

by variations in the constituency and homogeneity of the coal along the seam For example, the coal in test area 3 appeared to have a 17.7 cm lower layer

of friable coal, as observed on site. The upper 22.9 cm of coal was harder.

Such stratification was noted last year also in the Bruceton mine [1] , and i

consistent with the location of the "top binder" in the Pittsburgh seam.
It was also noted that although the coal surfaces and the floor under the

undercut were dry, some water was dripping from the roof 1.25-1.5 meters

back from the coal face. Variable amounts of moisture within the coal at

the various test points can result in different dielectric constants.

The head coal depth is related to the dielectric constant as given in

eq. (13). Variations in the dielectric constant will produce errors in the

measured depth unless those variations are taken into account. The relative

magnitude of the error can be determined by differentiating eq. (13) with

respect to /e, and utilizing the change in dielectric constant.

lAdL= 1 M_ |A/I| . (17)

d d 9/e

After going through the differentiation, eq. (17) becomes

iMi = lA/Ii. (18)

d /e

Using an average value for /e and the variation from tables II and III

in eq. (18), the error in the depth calculation can be as high as approximately

4 percent.

Much larger variations were noted in subsequent measurements in another

mine; thus, a method has been devised for the FM-CW system to continuously

monitor the dielectric constant. That method will be discussed in the next

section

.

5 . 3 Loveridge Mine

5.3.1 Roof Data

Measurements were made at a site where the coal had been mined at least

six years ago. The actual site was also used by Foster-Miller Associates,

Inc., for roof coal measurements with a pulse radar (USBM Contract No.

H0357000). That site was approximately 17 meters (55 feet) long. Measurements of the roof coal depth were made with the FM-CW system every 1.5

meters (5 feet) along the test site.

A physical analysis of the test site was conducted jointly with

Mr. Gregg Riley of Foster-Miller Associates, Inc., immediately after the

electromagnetic measurements were done. This analysis consisted of drilling

4 cm (1 5/8 inch) diameter holes into the coal layer at approximately

1.5 meter intervals along the test site to visually and physically note the

constituency, stratification, and thickness of the coal layer. Core samples

of 10 cm diameter were taken at stations 20 and 30 for moisture analysis.
Mr. Riley has recently completed a survey of the test site by using a

4 cm diameter probe drill at approximately 3 0 cm intervals to determine

stratification and thickness of the coal layer. He has provided us with a copy of those results and it has been decided that those should be used as the reference data for the FM-CW measurement.

The results obtained in the Loveridge experiments are shown in figure

8. The radar data have been corrected for apparent changes in the dielectric

constant of the coal along the test site. Changes in dielectric constant

were measured as a function of changes in the amplitude of the signal re- flected at the air-coal interface. Because no automatic gain control circuits were used in the system, a condition external to the measurement

system must cause the amplitude of the coal surface response to vary. As

shown previously [1] , the amplitude of that signal is a function of the

dielectric constant of the coal. Thus the changes in the amplitude of the

coal surface reflected signal correspond to changes in the coal dielectric

constant used in (13). A tabulation of delectric constants measured over

the test sight is given in table IV.

The received signal amplitude for the coal surface response is shown on figure 9. The plots for the two experimental days run essentially parallel

at all except station 40. Parallel responses indicate that the receiver

responded to the same change in coal dielectric constant both days but the

receiver gain was set at different, but constant, levels.

Moisture analysis of samples taken from stations 20 and 30 show the

coal to be relatively dry [5], which is consistent with the lower dielectric

constants measured at those stations.

Near the end of the first experimental day the roof area of station 40 was soaked with water in conjunction with an experiment being conducted by

Foster-Miller Associates, Inc. The water was sprayed on the roof after the

FM-CW amplitude responses on figure 9 were obtained.

It was noted the following morning that the roof area was still wet.

A higher received signal amplitude on figure 9 corresponds to a higher

dielectric constant. Addition of water to a coal sample would increase the

dielectric constant; thus, the received signal amplitude data seems to

correlate well with the sequence of experimental events.

Not shown on figure 8 are other microwave responses, some of which

correlate with the "top binder" and other discontinuity locations. Other of

the microwave system responses probably were caused by discontinuities that were undetected during the physical analysis.

9
The sample of raw data shown in figure 2 was one of the most straightforward to analyze. Many of the data were that straightforward; however,

some were more complicated. Consider the data shown in figures 10 and 11

for example. For figure 10, the closest physical analysis drill hole for

station 10 was at station 11. That analysis showed that a thick sulphur

ball was located 17.1 cm up from the coal surface and was 13.4 cm thick. An

additional 12.7 cm layer of coal existed above the sulphur ball.

In figure 10, the 8.15 kHz response corresponds to the reflection at

the coal surface. The response at 8.5 kHz, along with the measured dielectric constant at station 10 (e = 5.2) indicated that a reflection occurred

17.2 cm in from the coal surface. That distance correlates very well with

the noted location of the lower edge of the sulphur ball; thus, it is con- cluded that 8.5 kHz response was indeed caused by the sulphur ball. It

probably lies on the plane of the "top binder."

Because the sulphur ball was noted to be 13.4 cm thick at the drill

hole point, it is quite likely that the response at 8.9 kHz was caused by a reflection at the top of the sulphur ball. Using a dielectric constant

measured for shale in the mine (e - 24.2) along with the difference frequency of 0.4 kHz (8.9 8.5 kHz) results in a FM-CW measured thickness of

9.3 cm.Finally, the frequency response at 9.2 kHz was probably caused by

reflection at the coal-shale interface. The thickness of the coal above the

sulphur ball was computed to be 13.4 cm, giving a total depth from the coal

surface to the shale at 39.9 cm at station 10.

For figure 11, the closest physical analysis for station 15 was done at

station 14, where the total physical thickness of the coal was 52.1 cm. The

response at 8.45 kHz corresponds to a reflection at a depth of 17.2 cm. Based upon all the data analyzed and considering the relatively large amplitude of that response, it is assumed that reflection was caused by the top

binder

.

The weak response at 8.7 kHz corresponds to a reflection at 30.4 cm within the coal layer. No discontinuity was noted at that depth in the

physical analysis at station 14. . The large amplitude response at 9.1 kHz is interpreted as the reflection

at the coal-shale interface, thus the total coal thickness is found to be

52.6 cm at station 15.

In both of these examples the response from the coal-shale interface is

quite definite; however, there can be some problems in interpreting all of

the responses without a complete physical analysis. This measuring system

10
can be readily adapted to continuous recording and then some of the anomalies

such as sulphur balls become obvious because they are small in extent.

5.3.2 Floor Data

Figure 12 is data taken with the FM-CW looking into the floor. There

was no coal on the floor at any point, except for some dust that was mixed

with rock and shale dust at station 40.

The data indicates that some floor layering is present. A very limited

physical analysis revealed the following.

No coal was on the floor at station 50. The first 16 cm of slate was easily fractured and very easy to remove. At a depth of approximately 16 cm,

the slate become very hard and was essentially impenetrable with a hand

pickax. Most of the excess water that was sprayed on the roof the previous

day collected on the floor at station 50. Water penetrated the shale approxi

mately 2.5 cm. Approximately 1 cm of coal, shale, and rock dust was on the floor at

station 40. This was the only point where floor coal in any form existed.

The first 2.5 cm of slate fractured very easily and was removed. The

hard slate layer came next. No water collected in this test area. At station 30, 6.5 cm of shale and rock dust was on the surface.

A 5.0 cm layer of easily fractured shale came next followed by the hard

shale. The entire area was dry.

At station 10, the upper 7 cm of slate was easily fractured. This was followed by the hard slate. The entire area was dry.

5 . 4 Hillsboro Mine

Measurements were made at a site where the coal had been mined 4-6

week previously. This site was also used by Foster-Miller Associates, Inc.,

for measurements with their pulse radar. This site was approximately

31 meters (100 feet) long. Measurements were made every 1.5 meters

(5 feet) along the test site.

A physical analysis of the test site was conducted jointly with

Mr. Riley of Foster-Miller Associates Inc. This analysis consisted of

drilling 4 cm diameter holes into the coal layer at approximately 30 cm intervals along the test site to visually and physically note the con- sistency, stratification, and physical thickness of the coal layer. Samples were taken for moisture content analysis in the laboratory [5].

The results obtained from these experiments are shown in figure 13.

In these experiments also the amplitude of the signal reflected at the

air-coal interface was noticed to vary from test point to test point; however, the range of variation at Hillsboro was much greater than that at
Loveridge. Perhaps the wider range of variation at Hillsboro is attributed

to the fact that the coal layer thickness ranged from essentially zero to

approximately 20 cm, thus the dielectric constant at the surface ranged from

values of slate to values of coal. In any event, the wide range of amplitude

variation drove the spectrum analyzer response off scale at times. This

large change in received signal strength was counteracted on site by adjusting the system gain. Changes in system gain were not recorded, thus the

measured results shown on figure 13 are not corrected for changes in dielectric constant. A dielectric constant of 4.6 was used for all the results

given in figure 13. , Another problem that was present with these experiments (not realized

until the data was being processed) was that the system had not been set up

for very thin coal layers. The rf sweeper used had the capability to be

swept over the bandwidth 2-4 GHz. For coal depths less than 10 cm the rf

bandwidth should have been increased to increase the system resolution.

Thus, measured data was not obtained at all of the thinner test points.

6. CONCLUSIONS

6.1 The FM-CW system can be used to measure the thickness of a layer of coal

under most conditions, as well as detect other anomalies within that

layer. The thickest layer measured with this system was 55 cm. However, that does not appear to be the upper limit.

6.2 The FM-CW system provides an output signal in a format that is amenable

to data interpretation and analysis processes. Information other than

coal thickness is inherently present in the data output. The distance

from the antennas to the coal surface, for example, can be extracted.

Anomaly presence is also indicated.

6.3 The dielectric constant of coal apparently varies significantly within

a coal seam. The dielectric constant in the Bruceton mine was lower

this year than last year. Last year the coal surfaces were wet be- cause of the high humidity within the mine. This year the coal surfaces were dry; thus moisture was evidentally a significant factor in those

measurements

.

6.4 This FM-CW measurement technique has the potential of continuously measuring changes in the dielectric constant of a coal seam and providing

an output signal that can be used for real-time measurement of layer

thickness. It should be pointed out, however, that this measurement

is related to the dielectric constant of the coal at the surface. It

is believed that for measurements near the face in operational mines a measure of the dielectric constant near the surface of a layer of coal

should be representative of that for the entire layer at that point.
6.5 The realization of the existence of the "top binder" in the Pittsburgh

seam immediately explained some of the "spurious" responses last year

in the initial experiments as well as this year. The fact that the top

binder varies in thickness from essentially zero up to 0.6 cm indicates

the ability of the FM-CW system to respond to thin layers whose physical

and electrical differences from the surrounding coal are very slight.

6.6 The dielectric constant and thickness of several laboratory prepared

samples of coal were measured; however, only one sample was considered

to have sufficient surface area for measurements with the radar system.

6.7 The directivity of both antennas and the isolation between them was improved by the use of lenses and by taping the mechanical junctions

to provide continuous current paths

.

6.8 The presence of sulphur balls, shale lenses, and other dielectric discontinuities within a coal layer can complicate the data output from

the system and consequently its analysis. Most of those discontinuities are small in extent and will only temporarily appear in the

data output. In an operational system the coal removed may extend

above the top binder; thus many of the discontinuities will be removed

prior to measurement.

6.9 Floor data were collected at several points in the Loveridge mine. The

data indicates floor layering; however, a detailed physical analysis

of the floor was not done. No coal was on the floor where these mea- surements were made

.

6.10 Penetration of the shale layer with the microwave signal was accomplished

last year. The results obtained this year also indicate penetration

of the shale layer; however, a physical analysis of that layer was not done. Shale layer data may be useful to delineate the layering

within the shale and for guidance during roof bolting operations.

6.11 The FM-CW technique is much more amenable to automation than the technique used last year.

7 . RECOMMENDATIONS

A program could profitably be undertaken to develop and apply this

microwave measurement technique. The following specific efforts are recom- mended for advancing the technology toward developing a practical, reliable

coal interface detector (CID) to determine coal-shale interface distance into

the roof and the floor for anomaly detection and for distance measurements.

13
7.1 Instrumentation effort. This effort would be aimed at obtaining a fully automated, permissible system. The starting point would be the

FM-CW system as it now exists; however, some modifications should be

done in attempts to simplify this existing design. For example, the

system should be modified for one antenna experiments that would, if

successful, simplify the amount of hardware required for an operational system. This will involve use of state-of-the-art directional

couplers. Solid state microwave sources in the frequency range 1-4 GHz

should be selected for incorporation into a permissible system.

Appropriate solid state circuitry ' must be developed to process the

microwave system output and to display the coal layer thickness on

a continuous basis.

7.2 Material properties measurement effort. The objective is to measure

the permittivity of coal, shale, sulphur balls, etc., in mine environments and with laboratory samples from operational mines to determine

the ranges of dielectric constant values encountered in situ. Included here is a determination of the magnitude of changes of dielectric constant caused by localized conditions in a mine particularly

near the front face where actual conditions occur in an operational

mine .

7.3 The penetrability of electromagnetic signals into the front face of

a stratified coal seam should be studied to determine the feasibility

of detecting sulphur balls, shale lenses, and other anomalies up to

1 meter ahead of the front face. Knowledge of the electrical properties

of sulphur balls and shale lenses with respect to the surrounding coal

is of importance in this effort. Those materials must continue to be

studied in situ; however, it may be well worthwhile to take samples of

them into a laboratory for sustained evaluation in less hostile environments. Some possible findings here may include the change of electrical properties of shale for example, when exposed to the atmosphere.

It may be possible to relate microwave response to strength of the

material. The magnitude of changes in the dielectric constant as a function of moisture content, and atmospheric exposure is of importance.

7.4 In situ mine measurements effort. The study of the electrical characteristics of materials in a mine must continue. Additional measurements need to be made in operational mines to acquire data to establish

feasibility of measuring floor material thicknesses when the floor is

either wet or dry, for probing the front face to locate discontinuities

in the undisturbed coal seam, and to measure distances from a reference

point to the roof, to the floor, and to the front face.

14
7.5 Modeling effort. Some mathematical modeling was done the last two years

to predict responses and in some cases, to verify some of the experimental data. Mathematical modeling efforts should be continued. In

addition, some experimental modeling should be done to determine what

effects the presence of a metallic miner might have on the performance

of the radar system.

8 . ACKNOWLEDGMENTS

None of this investigation would have been possible without the complete

cooperation and excellent assistance of the following: John Burr of Lee

Engineering, A Division of Consolidation Coal Company; Paul Carter, Darrel

Auch, Hershel Moats and Walter Gull at Loveridge mine; Emil Teisa and

Mike Caldwell at Hillsboro mine; Mike Pazuchanics, Bert Nagy and others at

the Bruceton Mine of the USBM Mining and Safety Research Center; Gregg Riley

of Foster-Miller Associates, Inc.

Jocelyn Spencer provided the drafting service, and Sharon Foote provided

the typing service

.

9 . REFERENCES

[1] Ellerbruch, Doyle A. and Adams, John W. , "Microwave Measurement of Coal

Layer Thickness," NBSIR 74-387 (Sept. 1974).

[2] Javid, Mansour and Brown, Philip Marshal, Field Analysis and Electro- magnetics (McGraw-Hill Book Company, Inc., New York, N.Y,, 1963).

[3] Kerr, John L. , "Short Axial Length Broadband Horns," Proceedings of

the 22nd Annual Symposium on USAF Antenna Research and Development,

University of Illinois (Oct. 11-13, 1972).

[4] United States Department of the Interior, Bureau of Mines Analytical

Report on Coal and Shale Samples, Laboratory No. K57384-K57399

(Oct. 28, 1975)

.

[5] United States Department of the Interior, Bureau of Mines Analytical

Report on Coal, Laboratory No. K60552-K60568 (Jan. 30, 1976).

15
APPENDIX

ABBREVIATIONS AND SYMBOLS

E^fE^fE^ Electric Intensities in paths 1, 2, and 3, V/m.

R Distance from the antennas to the coal surface, m.

c Velocity of propagation in free space, m/sec

.

d ' Coal layer thickness, measured with the FM-CW system, m.

^l'^2 Instantaneous electric intensity in paths 1 and 2, V/m.

Instantaneous electric intensity out of the receiver antenna, V/m.

fg- Frequency of the signal out of the mixer, Hz.

Af^ Difference between two signal frequencies out of the mixer, Hz.

t Time, sec.

a Attenuation constant, neper/m.

^l'^2 Reflection coefficients.

Y Complex propagation constant,

e Relative dielectric constant,

w Angular velocity, radian/sec.

w^^j Instantaneous angular velocity of the microwave signal out of the

generator, radian/sec.

16
Table I. Laboratory Sample Data.

triiy s xodx Dielectric

Layer Thickness

From FM-CW

1*16 a. o UJ. 61116 nX. o Surface

(cm)

Thickness Constant (cm)

Sample

iNumDer

(cm)

v^oax ofiaxe Coal Shale uoai onaie

1 63.5 X 137 12.7 12.7 4.8

(6.3%)

51.8

** (16.3%)

12.6

2 33 X 107 30.5 10.2 4.8* 24.7* 30.7

3 33 X 107 19.1 11.4 9.0

(2.8%)

36.4

(1.8%)

4 33 X 107 20.3 12.7 16.6

(3.9%)

*No moisture analysis done.

**Values in parentheses are moisture content.

17
Table II. Bruce ton Mine.

Test Area

Physical

Thickness of Layer

Measured

Dielectric Constant

1 6.4

2 * 27.9 cm 5.6

2* 27.9 cm 6.3

3 40.6 cm 5.6

*Dif f erent points in that test area.

Table III. Bruceton Mine.

Test Area

Measured

Dielectric Constant

Coal Layer Thickness From

FM-CW Measurement

(cm)

1 and 2 5.5 19 .2

28.1

2 and 3 5.8 27.4

40.1

18
Table IV. Loveridge Mine.

Station

Measured

Dielectric Constant

-5 7.6

0 5.3

5 4.9

10 5.2

15 4.6

20 4.2

25 3.2

30 2.5

35 3.7

40 *

45 4.5

50 6.8

*4.9 on the first test day (before water spray)

6.8 the second test day (after water spray)

19
(draw slate)

/ (coal) .

(air)

MICROWAVE

SrGNAL

GENERATOR Transmit

Horn

FREQUENCY

Reference

Signal

Input

Ml

SPECTRUM 1

ANALYZER 1

Test Signal Input and

Receive

Horn

Mixer Output

AMPLITUDE

Figure 1. Block diagram of the FM-CW

electromagnetic system

20
3SN0dS3d dO 3anindi/^v

21
o

O O IT' O in O ITTT

I —''I'll — cvj c\J to ro

22
9>

3

<

CIu

d) 4J

4-) COa.

0)

m

a;

CO iH

I W

(U

!-i 3

00

•H

gp * dpni!|dujv aAj|D|9bl

23
24

26
O O O O O O-r

a> lO ^ ro CVJ — '

(UUO) 4id9a J9Ad-| |D03

27
1 1 1

1 o

• IT) to

1 o

1 ro ro

1 X • O O _ o

• X

•

x» — o

ro

•

— O

X • cvJ

•

X • O

•

O

1 1 1 o

0>E

z

c

o

CM O 00 CC ^ CM T

(4U9UJd3D|ds!p mo) jez^iDuy oinjpeds uo dpn^iidaiv |Du6!S

28
6.5 7.0 7.5 8.0 8.5 9.0 9.5

FREQUENCY, kHz

Figure 10. Raw FM-CW data from station 10 in the Loveridge Mine

29
30
ZH>1 ' Aou9nb9Jj indino J9><!IN

31

NBS-lUA (REV. 7-73)

U.S. DEPT. OF COMM.

RIRLIOCRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBSIR 76-840

2, Gov't Accession

INO.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

FM-CW Electromagnetic Technique of Measuring Coal

Layer Thickness

5. Publication Date May 1976

6. Performing Organization Code

276.07

7 AUTHOR(S)

Doyle A. Ellerbruch and Donald R. Belsher 8. Performing Organ. Report No,

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 2767389

11. Contract/Grant No. J0155124

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

U. S. Bureau of Mines

Pittsburgh Mining and Safety Research Center

4800 Forbes Avenue

Pittsburgh, Pennsylvania

13. Type of Report & Period Covered

8/75-2/76

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less (actual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

An FM-CW microwave system was investigated for measuring ooal layer thickness.

Measurements were made in three different mines near Pittsburgh, Pa. , near Fairview,

W. Va. , and near Cof feen. 111. Microwave frequencies in the range 1-2 GHz were used

to measure samples up to 55 cm thick. All samples were backed with a naturally

occurring shale. Measurements were also made on coal and shale samples compounded in

the laboratory at the Bureau of Mines Pittsburgh Mining and Safety Research Center

near Bruceton, Pa.

The results indicate that layer thickness can be determined in most cases,

although large anomalies may, in some cases, produce misleading results. Many

anomalies that were detected with the FM-CW system were verified visually by

drilling into the coal layer.

The dielectric constant of coal apparently varies significantly within a coal

seam. The form of the output signals from the FM-CW system seem to simplify the data

interpretation and analysis process as compared to the manually swept microwave

system used previously. It appears that this technique has the potential of measuring

changes in the dielectric constant of a coal seam and providing an output that can be used for real-time corrections in layer thickness measurement.

i. 17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper

j name; separated by semicolons

)

Automation; coal; coal mine safety; dielectric constant; energy; microwave measureI ment; nondestructive testing; thickness of coal layer.

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1 For Official Distribution. Do Not Release to NTIS

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