NBSIR 74-387

MICROWAVE MEASUREMENT OF COAL LAYER THICKNESS

Doyle A. Ellerbruch

John W. Adams

Electromagnetics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

September 1974

Final Report

Prepared for

U. S. Bureau of Mines

United States Department of the Interior

Pittsburgh, Pennsylvania 15222

Working Fund Agreement SO 144086

NBSIR 74-387

MICROWAVE MEASUREMENT OF COAL LAYER THICKNESS

Doyle A. Ellerbruch

John W. Adams

Electromagnetics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

The views and conclusions contained in

this document should not be interpreted

as necessarily representing the official

policies or recommendations of the Interior

Department's Bureau of Mines of the U. S. Government.

September 1974

Final Report

Prepared for

U. S. Bureau of Mines

United States Department of the Interior

Pittsburgh, Pennsylvania 15222

Working Fund Agreement SO 144086

U.S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS Richard W Roberts Director

FOREWORD

This report was prepared by the National Bureau of Standards,

Boulder, Colorado, under USBM Contract No. SO 144086. It was

administered under the technical direction o£ the Pittsburgh

Mining and Safety Research Center with Mr. M. Pazuchanics

acting as the technical project officer.

This report is a summary of the work completed as part of this

contract during the period June 1974 to September 1974. This

report was submitted by the authors in October 19 74.

iii
3.0

4.0

5.0

6.0

7.0

CONTENTS

Page

INTRODUCTION 1

ANALYSIS 3

2.1 Properties of a Medium 3

2.2 The Model 4

2.3 Dielectric Constant Determination 7

EXPERIMENTAL RESULTS 8

3.1 Loveridge Mine 9

3.2 Bruceton Mine 10

CONCLUSIONS 12

RECOMMENDATIONS 13

ACKNOWLEDGMENTS 14

REFERENCE 15

APPENDIX 16

LIST OF ILLUSTRATIONS

Table I.

Table II.

Figure 1.

Figure 2.

Figure 3 . Figure 4.

Figure 5 . Figure 6.

Figure 7.

Figure 8.

Figure 9 . Figure 10.

Figure 11.

Page

17

18

19

20

21

Loveridge mine results

Bruceton mine results

Model used for illustration and calculations

Vector diagram

Block diagram of measurement system

Measurement setup inside Bruceton Mine 22

Measured data, 10 cm-thick layer of coal 23

Calculated data, 10 cm-thick layer of coal 24

Measured data, 20 cm-thick layer of coal 25

Calculated data, 20 cm-thick layer of coal 26

Measured data, 40 cm-thick layer of coal 27

Calculated data, 40 cm-thick layer of coal 28

Photo of 40 -cm thick layer of coal 29

LV
MICROWAVE MEASUREMENT OF

COAL LAYER THICKNESS

The possibility of using a microwave system

to measure coal layer thickness in a mine was investigated. Measurements were made in two different

mines near Pittsburgh, Pa., and near Fairview, West

Virginia. Frequencies in the range 0.5 GHz - 4.0 GHz

were used to measure samples between 10 - 40 cm

thick. All samples were backed with a naturallyoccurring draw slate.

The results indicated a definite possibility

of determining layer thickness in most cases, although anomalies may introduce enough error to give

misleading results in some cases. Anomaly detection may be very useful in some cases. More experimental data and theoretical study are needed for

complete verification.

Data from a model and measured data show considerable agreement. The real value of this model

is to point out causes and effects, and what additional information is needed in order to obtain a measurement system that is optimum under most

conditions

.

The dielectric constant of coal is apparently

a function of moisture content. Draw slate seems to

have a significantly higher dielectric constant than

coal. Also, a layer of coal having different electrical characteristics within the layer was detected

with the microwave system.

This technique has great potential if fully

developed. It would have significant impact in areas

of energy, safety, and productivity.

Key words: Automation; coal; coal mine safety;

dielectric constant; energy; microwave measurement;

nondestructive testing; thickness of coal layer.

1.0 INTRODUCTION

The present-day needs for determining the thickness of

the coal layer left on mine -haulageway roofs is related

primarily to mine safety. A layer of sufficient thickness

is considered necessary for roof -strength purposes, and in

the case of draw slate (in the Pittsburgh seam) , to prevent
exposure of the slate to air. When exposed to air, draw

slate deteriorates very rapidly and greatly increases the

chance of a roof fall. In addition to the safety aspect,

a measurement system that could non-destructively monitor

layer thickness would aid production and possibly allow

further automation of control of a continuous miner.

Present mining practice is to stop mining every ten feet

to measure coal-layer thickness on the roof by drilling through

the coal to the draw slate. The drill operators determine

when the slate has been reached as verified by a change in

color of the drill chips. The color changes from coal black

to dull gray. Holes are drilled at approximately ten-foot

intervals along one side of the mine haulageway.

The future need for thickness measurements is related

to the need for automated mining machines. A sensor is

required to continuously monitor coal-layer thickness and

provide a signal for vertical control of the automated machine.

It is desirable that the sensor be non -contact ing with the

coal, and it is necessary that the sensor be permissible,

operate remotely, and be able to operate in a very wet and

dusty environment.

The application of microwave technology to the roof coal

thickness measurement problems presents a possibility of

satisfying many present and future needs. Microwave signals

are capable of penetrating and interacting with coal in such

a way that they do not physically affect the coal, and yet the

interaction will provide the necessary information about coal.

A sensor which utilizes microwave signals can be constructed

to be non-contacting, to provide data continuously for monitoring and control purposes, and to be permissible.

There is exciting potential for a fully-developed system

applying this microwave technique through non-destructive,

real time measurements. Safety, energy, self sufficiency,

automation, and increased productivity all would be impacted.

2
This effort consisted of investigating the relationship

between coal electromagnetic penetrability, as a function of

microwave frequency, and of reflections set up by the drawslate -coal interface. Measurements were made in the Loveridge

Mine near Fairview, West Virginia, and in the Bruceton Mine

near Pittsburgh.

In all cases the coal samples tested consisted of an

undisturbed coal layer backed by a layer of draw slate approximately 40 cm thick. The draw slate was backed by a layer of

"wild" coal (definitive description unknown) of unknown

thickness

.

2.0 ANALYSIS

2 . 1 Properties of a Medium

The following equations outline the properties of a complex medium [1]. In general, waves propagating in a medium undergo attenuation and phase shift as they propagate

The complex propagation constant for plane waves is

Y

= ico/ye [1 - i —]^^^

and can be written as

Y

= a (w) + ig (o)) . Solving for a (w) and 3((jo) gives

a (o)) =

3(0)) =

1^

0

2

- 1

+ 1

|l/2

|l/2

The intrinsic impedance of a medium is given by

V e(i-i

a)e

(1)

(2)

(3)

(4)

(5)

3
However, the assumption that — << 1 will be made in the

following model and analysis, except for calculations of

a (oj) .

2.2 The Model

The following discussion outlines the model and the

equations used in these measurements.

The model used consists of two layers of finite thickness

sandwiched between two semi -infinite half -spaces. In the

real world, these layers are air, coal, draw slate, and "wild"

coal (see figure 1) . The media are assumed to be isotropic

and homogeneous with plane boundaries. Electrical properties

of each layer are described by permeability (y) , permittivity

(e) , and conductivity (a). All layers are assumed non-magnetic,

_ 7 i.e., = y^, where y^ = 4Tr x lo hy/m. Additionally, e is

assumed to be independent of frequency. Ray paths for electromagnetic energy are also assumed.

Due to the directivity of the ideal horns assumed in the

model, only reflected waves are received. In practice, considerable design, placement, and polarization strategy are

required to reduce the direct coupling to an acceptable level.

If horns were close enough to ideal to prevent a direct coupling

of energy, lower minima could be detected. Also, the horns

are not directive enough to fully justify use of a singleray model. Nevertheless, this model is instructive in

determining what is important.

For E normal to plane of incidence, impedance, Z, from

medium o to medium 1 is

Z = n sec 9 o o o

n

1 sec G 1'

and

4
ol

'sec ( 1

sec 9

0

Z -Z /FT

z,+z

1 0

sec i sec 6

0

L 0

cos 9 /FT - cos 9T/Fr o o 11

cos e /e' + cos 9i/£' 0 0 11

is calculated according to Snell's law as

(6)

(7)

The resultant field from single reflected waves from each of

the three boundaries are

E = E e y o

-123 s o

r + (1- I r I 2)e

+ r2(i-|r^|2)(i-|rJ2)e

2 (a^£+a2d)

(8)

where E^ is the component of E normal to the page in figure 1.

Multiple reflections in layer one can be described by an

infinite series of terms

-2(n + l)aT£ -i239i^(n+l)

I r^(r^r^)^(i-|rj^)e ^ e

^ n=l

(9)

^

-2(n+l)a^Jl

However, due to the term (T^r^) e , only the first and

second terms of the series will be necessary in typical coal

layer cases. Multiple reflections in layer 2 are neglected.

When parameters that determine values of 3 are inserted,

along with two reflections in layer one, then E^ is given by

5
E e o

-i23 sf -2a, £ -i^^ /e^ cos0,

° r +r, (1-1 r r)e ^ e ^ ^

^ o 1 ^ ' o

-1 /e' COS0-

c 2 2

2aTil -2aTd

2^„ 1_ 2_ c 1

2a)£ /—

r

/e' CO SI

-4a-,il

r 1 2) (r rje e o ' ^ o 1^

4ooil ej^ cos0-j^

-6a-, £ -1 /£

r |2)(r rj^e J- ^

c cos

(10)

The various e' denote relative dielectric constants,

where e = £ ' , and is permittivity o£ free space,

12

8 . 854 X 10 "^^ £d/m. At least to first order, the spacing

between the antennas and the air-coal interface makes no difference in this technique, as its effect is factored to the

outside of the bracket (in eq. (10)). Examination of the

first and second terms inside the bracket shows how the thickness, £, of layer one is determined. The reflection coeffi-

-2a^£

cient, r^, is a stationary vector, while r^(l-|r^|^)e is

a rotating vector as shown in figure 2. The magnitude of the

resultant vector will go through successive minima (or maxima)

every 2Tr radian rotation of the second vector. In this case,

0) is varied. Thus, solving for I in terms of frequencies of

two successive minima in the argument,

/iy cos Q-^,

gives

2(f2-fi)/eJ cos

(11)

6
where successive minima of the resultant vector occur at f^^ and f.2' Argument is very close to zero for all reasonable

angles of incidence, and even for grazing incidence, would be

only about 20°. Therefore the most important parameter is e-j^.

Unfortunately, this is not the only parameter needed, as there

are other rotating vectors present also. Their magnitudes

may obscure the desired layer thickness if attenuations and

reflection coefficients do not reduce their amplitudes

sufficiently

.

The simplifying assumptions made throughout this analysis

are subject to question; no judgment can be made as to the

validity of these assumptions without adequate information on

properties of materials, antennas, etc. This information can

be obtained from extensive measurements made in situ.

Obviously, e' of coal, as well as slate and other roof

materials, must be known with fairly good accuracy. Unfortunately, coal is not isotropic, and its dielectric constant

is strongly influenced by the amount of moisture present.

Results in the next section show this to be true for the two

mines visited, where e' varied from less than 5 in a relatively

dry mine to over 7 in a relatively wet mine. The exact moisture content is not known. The fact that moisture influences

the properties of materials has been documented in other cases

[2]. Thus, values taken from laboratory measurements are not

likely to be suitable in this situation [3]

.

There are at least three ways of measuring e' in situ.

The first method involves measuring r at an air-coal

interface. From

2.3 Dielectric Constant Determination

r (12)

1 o 1

7
' = 1, then

o

'

r il- r (13)

This method assumes the medium ej^ is sufficiently lossy to

appear as an infinite half space

.

A second method is to use eq. (8), and physically measure

the coal layer thickness. Then e' can be determined.

A third method for determining the dielectric constant

is to utilize a change in depth of the coal layer where the

absolute depths are not known, but the change of depth is

known. Assuming a step change in depth, (8) is rewritten as

^4 = — , (14)

^ 2/e'Af

where Af = ^2'"^! primed values correspond to another

depth. Taking the difference between (11) and (14) and solving for the dielectric constant results in

2 e' = 2A£j [M AF (15)

All three methods for determining the dielectric constant

of coal and the draw slate were utilized during these measurements. The results will be given in the next section.

3.0 EXPERIMENTAL RESULTS

This measurement technique has been used previously to

measure layer thickness of slush hydrogen. This instrumentation is not practical for coal-mine measurements, but was used

only to determine feasibility.

Approximately 225 kg of off-the-shelf, non-permissible,

microwave, laboratory, general test equipment were transported

to each test location. Two lightweight wideband antennas were

mounted on a tripod immediately under a selected test site.

Measurements could be made at other test sites in a given

location by moving the tripods to those sites.

8
Because the equipment required 115 volts ac, and was

non-permissible, the test locations available for these

measurements were limited.

The equipment used is shown in a block diagram in figure

3. As mentioned earlier, it is not in a form that is suitable

for mine measurements, but was adapted for this application

for this feasibility study.

The microwave frequency is varied by the sweep oscillator,

and the relative, received (reflected) signal amplitude is

then shifted down in frequency, detected, and used for the

vertical input to an x-y plotter and an oscilloscope. The

sweep voltage from the oscillator is used to drive the horizontal input to the plotter and oscilloscope.

The system as shown will not be able to cover the wide

range of conditions that occur in normal mining operations.

3 . 1 Loveridge Mine

Measurements were made at two different locations within

Loveridge. Initial measurements were made at a location where

the coal had been mined approximately ten years ago. Final

measurements were made at a location where the coal had been

mined approximately two weeks before the measurements were

made. It is felt that the results obtained at the latter

location should be representative of those obtained at the

face during mining operations.

Two points were located in the two-week-old test location

where the coal depth changed in a step function. One step was

10.2 cm high and appeared to be homogeneous coal. The dielectric constant of the coal at that step was measured to be 5.2.

The second step was 15.3 cm high, and its measured dielectric

constant was 4.5. It should be noted, however, that petrified

wood of unknown depth was visible immediately above the microwave antennas at this latter step. The geometry at the second

location was such that it was considered impractical to move

9
the antennas to avoid the petrified wood. Thus the dielectric

constant calculation at the 15.3 cm step includes the effect

of the petrified wood, whose electrical properties are, at

this time, unknown. The result in table 1 is an average

dielectric constant for the petrified wood-coal mixture at

that measurement point. It should also be noted that a tolerance of ± 0.6 cm in the step measurements would cause

the dielectric constant bands generated by those tolerances

to overlap.

Measurements were made in an area where the estimated

thickness of the coal layer was 20-25 cm. This thickness

estimate was arrived at by measuring the depth of the nearest

drill holes along one side of the tunnel. Due to the curvature

of the roof, the depth at the drill hole was greater at the

side than that at the center of the haulageway; visual

sighting clearly indicated this. Further, actual penetration

of the drill bit into the slate was unknown, but was generally

thought to be 2.5-5.0 cm. Using an average of the dielectric constants obtained

from the step data, the results given in table I correspond

to test points within the general test location.

Antenna rotation experiments were performed to determine

if coal exhibited anisotropic polarization effects. The data

indicate this effect to be negligible for this particular

case.

The dielectric constant of the draw slate was determined

by measuring its return loss. This measurement was made at

the ten-year-old location by removing the coal under the slate,

and measuring the return loss of the slate. The relative

dielectric constant for the slate was 63.

3.2 Bruce ton Mine

The objective in the Bruceton Mine was to prepare coal

layers of known thickness and to determine the dielectric

constant through measurement.

10
Measurements were made at one specially prepared location

within the Bruceton Mine. An existing mine face was undercut

from the mine floor up toward the draw slate. The undercut

extended into the face approximately 1.25 meters so that the

antenna- tripod structure could be placed under the test site.

The width of the test location was approximately 3.75 meters.

The test location was divided into three test areas and the

coal layer beneath the draw slate was undercut to depths of

10-13 cm, 20-26 cm, and 39.4-40.6 cm at these three areas.

Roof jacks were used to prevent roof fall at these undercut

test sites. A picture of the test area is shown in figure 4.

Roof jacks, the test areas, antenna- tripod structures and

microwave equipment are clearly visible.

The measured dielectric constant obtained in the three

prepared coal layers are given in table II.

The measured dielectric constant of the draw slate in

the Bruceton mine was 50.3.

Measured data for these three areas are shown in figures

5, 7, and 9. Calculated data are shown in figures 6, 8, and

10. Although the curves from the measured data are not as

smooth as the curves from the calculated data, the basic

minima occur at about the same spacing. The calculated

curves could be made to agree very closely with the measured

curves if enough variation in parameters were used by trial

and error; however the significant factor is the spacing

between minima. The 40 cm cases in figures 9 and 10 show

the same pattern, except a thinner layer, with deeper nulls

(due to less attenuation) spaced further apart, is superimposed on figure 9. Two periodic waves can be extracted

from that data. One has a frequency bandwidth of 140 MHz

and the other 530 MHz. The narrow bandwidth component

corresponds to the dielectric constant given in table II

for that sample. Substituting the \\;ide bandwidth and the

dielectric constant of the sample into (13) results in an

11
interface located 10.6 cm into the coal layer. There are

two possible causes for such an interface. First, there

could be a dielectric discontinuity at a plane 10.6 cm

above the air-coal interface. Such a discontinuity could

be produced by a fracture plane, for example, or a thin

layer of petrified wood. Another possibility is that the

lower 10.6 cm of coal has electrical characteristics that

are different from those for the upper 29.4 cm of coal.

An on-site visual inspection of this sample was made, and no

change in coal texture or color was detected. However, a photographic slide made of that sample shows a change in color

of the lower portion of the sample. The lighting for the

instant of the photograph was much better than was available

from cap lamps. A print of that slide is given in figure 11.

The color change for the lower level suggests that the physical characteristics of that test are different, which may

also produce a slight change in electrical characteristics

and account for the dielectric discontinuity.

4.0 CONCLUSIONS

4.1 Microwave techniques can be used to measure the

thickness of a layer of coal under most conditions; in case

of severe anomalies, the detection of the anomaly may be

more important than measuring the layer thickness. The thickness of a coal layer which can be measured this way is at

least 0.5 meters

4.2 The key microwave parameters of all relevant

material occurring in the layers must be known under the

various conditions that exist in actual mines. This basic

information cannot be determined from laboratory data, and

must be obtained from in situ measurements, as these parameters are strongly dependent on moisture content, and to some

extent, time elapsed since mining. There may be other dependences besides these. The few values measured in this investigation indicate the need for better data.

12
4.3 A practical operational system will have to be able

to adjust to a rather wide range of electrical properties of

materials in order to work under most conditions.

4.4 This limited effort indicates that the frequency

range from 1 to 2 GHz may be the best for this application,

but until better basic data is available, this is a tentative conclusion.

4.5 Data from the model and analysis developed show

general agreement with measured data.

4.6 All of the recommendations in the next section are

well within the present state-of-the-art.

5.0 RECOMMENDATIONS

A substantial program should be undertaken to fully

develop and apply this microwave measurement technique. The

following factors should be included in such a program in

order to achieve a practical, reliable system:

5.1 The electrical characteristics of the various constituents of the layered media must be determined to much

better accuracy than they are presently known. This effort

should rely mainly on in situ measurements rather than labora

tory measurements, and should cover the widely varying conditions that occur normally during mining operations (e.g.,

varying moisture content). The variations of constituents

from one location to another should also be covered (e.g.,

slate, limestone, etc). Effects of frequency dependence must

be considered, probably between 500 f-IHz and 4 GHz.

5.2 The model and analysis given in this report should

be used extensively to obtain insights, limits, and allowable

uncertainties. It will provide a powerful design tool. The

results will be much improved when better data from 5.1 is

available. The analysis can also be improved by eliminating

assumptions whenever the better data indicates this would be

appropriate

13
5.3 Better instrumentation than was used in this

feasibility investigation can be developed. Areas for

consideration are:

5.3.1 Better horn antennas will give better

directivity, sensitivity, etc.

5.3.2 A coaxial directional coupler (better than

is presently commercially available) might allow use of

a single horn antenna rather than separate transmit and

receive antennas.

5.3.3 Data processing and presentation may be

improved and/or automated, and a feedback control system

added, especially if development of a fully-automated

continuous miner is desired.

5.3.4 The system must be permissible, rugged,

compact, etc. These considerations will affect the

final design.

5.3.5 The system must have enough flexibility

and dynamic range to adjust to the rather wide ranges

of conditions normally encountered in mining.

5.3.6 Time domain techniques should also be considered. However, instrumentation complexity, cost,

and accuracy are the important factors

.

6.0 ACKNOWLEDGMENTS

Neil Larsen assisted in implementing the analysis;

Jocelyn Spencer provided drafting service; Sharon Foote and

Janet Becker provided typing service.

None of this investigation would have been possible

without the complete cooperation and excellent assistance of:

John Burr of Lee Engineering, a Division of Consolidation

Coal Company; Paul Carter, Carl Immel, and Darrel Auch of

Loveridge Mine, of the Mountaineer Division of Consolidation

Coal Company; and Mike Pazuchanics and others at the Bruce ton

Mine of the U.S. Bureau of' Mines Mining and Safety Research

Center, at Bruceton, Pennsylvania.

14
7.0 REFERENCES

Ramo , Simon and John R. Whinnery, Fields and Waves in

Modern Radio , Second Edition, Chapter 7 (John Wiley §

Sons, Inc., New York and Chapman ^ Hall, Ltd., London,

1953) . Hipp, Jackie E., Soil Electromagnetic Parameters as Functions of Frequency, Soil Density, and Soil Moisture,

Proc. of IEEE, Vol. 62, No. 1, January 1974.

Cook, J.C., Electrical Properties of Bituminous Coal

Samples, Geophysics, Vol. 35, No. 6, pp. 1079-1085,

December 19 70.

15
APPENDIX

Abbreviations and Symbols

dBm Signal level with respect to 1 mW, decibels

E Electric intensity, v/m

E Maximum electric intensity, v/m

c Velocity of light in free space, m/sec

e Natural logarithm base

f Frequency,

s,il,d Thickness of various layers, m

t Time , sec . r Reflection coefficient

a Attenuation constant, nepers/m

3 Phase constant, radians/m

Y Propagation constant

e Permittivity, farad/m

y Permeability, henry/m

a Conductivity, mho/m

0) Angular velocity, radians/sec

x,y,z Axes of a rectangular coordinate system

9^ Argument of incident, reflected or refracted waves

in various layers of a medium, radians

Tij^ Intrinsic impedance of a medium, ohms

1^ Intrinsic impedance of a medium for a wave at

oblique incidence, ohms

16
Table I. Loveridge mine results,

e - 4.84

Thickness Measured With

Site Microwave System

1 22.7±0.6cm

2

3 17 . 0 ± 0 .6 cm

3A 18.1 ± 0.6 cm

19.1 ± 0.6 cm

4 17.3±0.6cm

19 .7 ± 0.6 cm

10 .2 cm 32 . 5 ± 0 .6 cm

step 22.7 ± 0.6 cm

15 .3 cm 42 .6 ± 0 .6 cm

step 23.1 ± 0.6 cm

Comments

Estimated 20-25 cm thick

No reliable data obtained

Thinnest layer verified by

visual sightings

Antenna rotated 90° at

approximately same site

Antenna rotated 90° at

approximately same site

Thick side of step

Thin side of step

Thick side of step

Thin side of step

17
Table II. Bruceton mine results.

Physical Measured Dielectric

Site Thickness Constant

1 11 .4 cm 6.4

2 21 .6 cm 7.4

3 40 .0 cm 7.2

18
(wild coal)

1

ci (draw slate)

; (coal)

i LL € CT ) /// ^r^o» o» o' //

/

Transmit

Horn ,

\\\ c

Receive

J Horn

! ^ ^

) (air)

Figure 1. Model used for illustrations and calculations.

19
Resultant

vector

Rotating vector,

magnitude of T, (I- jrj^je-^^i^

argument of 2ujJ? /rj~cos 9^

Stationary

vector, Fq

Figure 2. Vector diagram to show derivation of equation

f

20
>> <

UJ z > z:

UJ

O Z

cr

X < I o

IXER OCAL II

AT

LLA I

ir

LLi X

-Jo

COo

tr o z H X <

0)

J21
22

24

26


Figure 11. Photo of 40 -cm thick layer o£ coal.

29

NBS-114A (REV. 7-731 r U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBSIR 74-387

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

MICROWAVE MEASUREMENT OF COAL LAYER THICKNESS

5. Publication Date

^ YY^ W T* 1 Q v 4.

6. Performing Organization Code

7. AUTHOR(S)

Doyle A. Ellerbruch and John W. Adams

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS, Boulder Labs

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No.

11. Contract /Grant No.

SO 144086

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

6/74-8/74

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most si^ificant information. If document includes a significant

bibliography or literature survey, mention it here.) The possibility of using a microwave system to measure coal layer thickness in a mine was invefetigated. Measurements were made in two different mines near Pittsburgh,

Pa., and near Fairview, West Virginia. Frequencies in the range 0.5 GHz - 4.0 GHz were used to measure sarrples between 10 - 40 cm thick. All samples were backed with a naturally-occurring draw slate.

The results indicated a definite possibility of determining layer thickness in most

cases, although anomalies may introduce enough error to give misleading results in some

cases. Anomaly detection may be very useful in some cases. More experimental data and

theoretical study are needed for complete verification.

Data from a model and measured data show considerable agreement. The real value of

this model is to point out causes and effects, and what additional information is needed

in order to obtain a measurement system that is optimum under most conditions.

The dielectric constant of coal is apparently a function of moisture content. Draw

slate seems to have a significantly higher dielectric constant than coal. Also, a layer

of coal having different electrical characteristics within the layer was detected with

the microwave system.

This technique has great potential i£ fully developed. It would

have significant impact in areas of energy, safety, and productivity.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons) Automation; coal; coal mine safety; dielectric constant;

energy; microwave measurement; nondestructive testing; thickness of coal layer.

18. AVAILABILITY Unlimited 19. SECURITY CLASS

(THIS REPORT)

21. NO. OF PAGES

1 For Official Distribution. Do Not Release to NTIS UNCLASSIFIED

27

1 Order From Sup. of Doc, U.S. Government Printing

Wasfiington. D.C. 20402, SD Cat. No. C13

Office 20. SECURITY CLASS

(THIS PAGE)

22. Price

1

^ Order From National Technical Information Service

Springfield, Virginia 22151

(NTIS)

UNCLASSIFIED

USCOMM-tRl, USCOMM-DC 29042-P74