AN ELECTROMAGNETIC NONCONTACTING SENSOR FOR

THICKNESS MEASUREMENT IN A DISPERSIVE MEDIA

Robert L. Chufo

Electrical Engineer

U.S. Bureau of Mines

Pittsburgh, Pennsylvania

Abstract

This paper describes a general purpose imaging

technology developed by the U.S. Bureau of Mines

(USBM) that, when fully implemented, will solve the

general problem of "seeing into the earth." A firstgeneration radar coal thickness sensor, the RCTS-1,

has been developed and field-tested in both

underground and highwall mines. The noncontacting

electromagnetic technique uses spatial modulation

created by moving a simple sensor antenna in a

direction along each axes to be measured while the

complex reflection coefficient is measured at multiple

frequencies over a two-to-one bandwidth. The

antenna motion imparts spatial modulation to the data

that enables signal processing to solve the problems

of media, target and antenna dispersion. Knowledge

of the dielectric constant of the media is not necessary

because the electrical properties of the media are

determined automatically along with the distance to

the target and thickness of each layer of the target.

The sensor was developed as a navigation guidance

sensor to accurately detect the coal/noncoal interface

required for the USBM computer-assisted mining

machine program. Other mining applications include

the location of rock fractures, water-filled voids, and

abandoned gas wells. These hazards can be detected

in advance of the mining operation. This initiating

technology is being expanded into a full threedimensional (3-D) imaging system that will have

applications in both the underground and surface

environment.

Introduction

Early research investigated various high-frequency

radar sensor systems using pulse, impulse, FM-CW,

or synthetic pulse. Electromagnetic waves penetrate

coal, rock, and earth, but when the energy penetrates

the media, the returning information content appears

to be scrambled and out-of-focus. The problem is

dispersion: Media dispersion, coupled with antenna

and target dispersion, cause problems too complex to

analyze in the time domain. These problems are

much easier to resolve in the frequency domain. Both

the time domain and frequency domain are transforms

of only one variable, so either approach is legitimate.

However, it is very difficult to work problems in both

domains at the same time. The theory supporting the

present research is in the frequency domain, but the

resulting architecture for signal processing uses both.

The concept being used is the spatial-domain

technique (i.e., moving the antenna to create a

modulation on the radar output). This concept has

been applied to advantage to reject unwanted

reflections and help cancel out media dispersion and

antenna dispersion.

The problems of designing an underground

imaging sensor were solved by utilizing a sensor

model created from one-dimensional, spherical wave,

scattering matrix theory. By separating surface

reflections from single-layered media reflections,

laboratory and field testing confirmed the validity of the

one-dimensional imaging model. The model, based on

fundamentals, allows the use of a wide range of

design architectures.

Rather than devoting project time to hardware

development, emphasis was placed on the data

processing scheme required to derive coal thickness

and dielectric constant from network analyzer

measurements of the reflection coefficient of the target

media. The sensor data were taken at a wide range

of frequencies and antenna positions (e.g.,

401 frequencies between 600 and 1,600 MHz and

32 equally-spaced positions over a distance of 16 in).

This was accomplished with a vector network analyzer

connected to an antenna that was moved in space by

a linear positioner. Data processing provided a direct

measurement of the thickness of underground media

and also the electrical characteristics of the media.

Prior knowledge of the characteristics of the media is

not necessary.

Measurements made of coal, rock, concrete,

granite, and salt have shown that the technique can

measure thickness from 0 to over 5 ft in single and

multilayer media. The accuracy of the technique is not

affected when the material is rough or wet. These

results and parallel applications such as the

measurement of the depth of hidden tunnels, the

thickness of multilayer highway pavement, and the

location of buried nuclear waste, unexploded

ordnance, and cultural artifacts, have provided the

technical incentive to further develop this unique

technology to take advantage of its broad potential,

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including full 3-D imaging of the underground

environment.

Acknowledgment

The author acknowledges the contributions of

Walter J. Johnson, WALTECH Holography for the

development of the algorithms used in the radar coal

thickness sensor and Raymond J. Helinski, USBM, for

constructing the spatial modulator and assisting in

data collection during both surface and underground

testing.

Experimental Measurement Technique

The original development of the radar coal

thickness sensor measurement technique started with

Kerns plane wave theory [1] and solved for each of

the scattering coefficients in the matrix with a standard

antenna calibration technique. However, when the

antenna was within two wavelengths of the material

being measured, diffraction became too strong an

effect for a plane wave model. A solution was sought

in simple plane wave theory but the cost was the

necessity of an explicit solution for the coefficients of

the model. Several approximation techniques for

linear calibration were tried and it was found that a

solution was possible. The coefficients did not vary

when the antenna was closer than two wavelengths to

a metal calibration plate. This was called the linear

reduction method [2] and it worked quite well except

for some second-order problems. It was assumed that

these problems were nonlinear multipath effects that

were not accounted for in the calibration procedure.

The math model was then expanded to include the

higher order effects and named the "quick reduction

method." Many of the higher order coefficients were

lumped together to accommodate a metal plate

calibration technique preformed with a 4-ft-square

metal plate placed between the sample and the

antenna. This in-situ calibration technique corrected

for signals returned from reflectors beyond the edges

of the calibration plate. In an underground mine these

reflections would be from material similar to the target

coal but outside the measurement area. Presently a

self-calibrating technique is being evaluated that will

improve upon the metal plate calibration technique.

The thickness measurement process begins with

a measurement of the input reflection coefficient of an

antenna in close proximity to the coal surface. This

measurement is taken at a wide range of frequencies

and positions (e.g., 401 frequencies between 600 and

1,400 MHz, and 32 positions between 4 and 20 in

from the coal. This is accomplished using a vector

network analyzer connected to an antenna moved in

space by a linear positioner. The measurement plane

is then electrically moved from the instrument

measurement plane to the plane of the antenna.

Figure 1 shows the instrumentation setup.

Fig. 1. Instrumentation

Data

The data from this measurement are a function of

both frequency and position. The data contain both

amplitude and phase information. Transforming the

data to the time domain at this point in the process

and inspecting the time domain history for this one

antenna position shows the absence of any sharp

peaks around the antenna, indicating that the

information for the coal surface is corrupted by other

effects such as the antenna dispersion, diffraction, and

multipath. These effects must be characterized and

accounted for by considering the frequency domain

history at each antenna position.

Antenna Transfer Functions

To characterize the antenna, a separate test is run

with a metal surface substituted for the coal surface;

the same frequencies and positions are used. This

provides data from a known reflection surface to

obtain the antenna transfer functions. These functions

are used in removing antenna dispersion from the data

taken at the corresponding antenna position.

Removing Antenna Dispersion

When the antenna transfer functions are

accounted for in the data, the result is the product of

the antenna-to-surface-to-antenna distance,

represented by the spatial delay and the coal surface

reflection coefficient. It is the reflection coefficient that

contains the information for the coal thickness. Other

reflections (i.e., multipath) are also present in the

resulting transfer function.

Shifting Image Plane to Coal and Removing Diffraction

Dividing the reflection coefficient expression by the

spatial delay shifts the image plane from the antenna

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to the surface of the coal and removes diffraction.

With the antenna-to-surface-to-antenna distance a

known quantity, the true reflection coefficient in the

frequency domain can be determined.

Integrating Space

By performing a spatial integration, the multipath

can now be decorrelated. Since spatial integration is

coherent with the coal but not coherent with any other

spatial distances, the multipath will become zero sum

or at least small compared with the surface reflection.

Transforming to the Time Domain and Range Gating

Transforming the data to the time domain and

range gating, removes unwanted reflections from the

data by gating out all the information on either side of

the desired peak. However, the data as presented in

the results section of this paper have shown that

range gating is unnecessary as the signal return from

the dielectric interfaces between the coal and rock are

sufficiently distinct to make the interface easily

discernible. If range gating were used, transforming

the reflection coefficient back to the frequency domain

would yield the composite reflection coefficient for just

the reflections within the range gate.

Validation of the Model

An earlier method [2] for determining the thickness

transformed the time domain reflection coefficient back

to the frequency domain so that the measured

reflection coefficient could be correlated with

theoretical reflection coefficients for various

thicknesses and dielectric constants. The theoretical

reflection coefficient that correlated best, provided a

statistical determination of the thickness and dielectric

constant of the coal being measured. For example,

the theoretical reflection coefficient that correlated best

with the measured reflection coefficient was for a coal

thickness of 5.3 in (for a relative dielectric 4 and loss

tan of .03). The actual thickness of the rough wet coal

measured in this underground test was a nominal 6 in.

This result provided the encouragement to refine the

model and proceed with the development of a method

to directly measure the coal thickness from data

acquired by the network analyzer frequency domain

measurements.

Field Test Results

The purpose of this research was to develop a

coal and rock thickness sensor of sufficient accuracy

to provide vertical and horizontal guidance of both

room-and-pillar and highwall mining machines. In

order to validate the theory developed for thickness

measurement, extensive underground and surface

mine testing was performed. Over a period of 2 years,

tests were conducted in mines with a variety of

geological and environmental conditions. Test areas

of both freshly mined and aged coal from 3 to 60 in

thick were measured. The areas measured ranged

from very dry to extremely wet with water dripping

from the roof test area. The wet coal did not affect the

thickness measurement. Coal seams with clay and

metal vein intrusions of iron pyrite could be imaged

and the distance from the coal surface to the intrusion

was accurately measured. Surface roughness and

cleating was not a problem. The average thickness of

rough cleated surfaces was measured accurately.

Accurate measurements were obtained even when

water filled the cracks between the cleats.

Roof Tests

Roof thickness tests were made in production

mines and in the Safety Research Coal Mine at the

USBM Pittsburgh Research Center. Figure 2 is a

representative measurement of roof coal thickness.

On the vertical axis, the plot shows the amplitude of

the reflected signal in decibels; time in nanoseconds

is shown on the horizontal axis. The large peak on

the vertical axes represents the reflection from the first

interface, the air/coal interface. Signals plotted to the

left of the large peak represent discontinuities internal

to the measurement equipment and between the

antenna and the coal surface. These reflections are

reduced to at least 30 dB below the air/coal reflection

by the calibration and spatial integration scheme. To

the right of the air/coal reflection are reflections from

discontinuities internal to the coal and shale being

measured. The printout on the left is the thickness of

coal between the air/coal interface and the coal/shale

interface. Measurements have identified both the

thickness of the coal and the thickness of the next

layer, usually shale, above the coal roof. At the Lband frequencies presently used, the depth of

penetration is usually about 10 ft. Future roof

thickness measurement research will attempt to

provide a direct readout of the thickness of each layer

of geological material within the penetration range of

the signal. At the present time, the power level of the

transmitted signal is 0 dBm (1 mW). This signal level,

or less, is adequate to produce a good signal-to-noise

ratio for the return signal measurement. The hardware

will permit an increase in transmitted power of 20 dB

to determine if greater penetration is best achieved

through increased signal power or through the use of

a lower transmitter frequency. Both the hardware and

software will operate from 300 kHz to 3,000 MHz.

Rib Tests

Figure 3 is a plot of actual data taken at an

operating highwall mine in West Virginia. 

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Fig. 2. Roof Coal Thickness

Measurements were made in freshly mined entries

immediately following the mining machine. The

determination of rib thickness can usually be

interpreted both visually from the FFT data in the

figure and from the numerical readout from the

automatic thickness measurement software. A large

dielectric contrast is seen at the first air/coal interface

and a somewhat smaller but still pronounced reflection

can be seen as the signal exits from the coal rib at the

coal/air interface in the adjacent drift. The vertical bar

to the right of the main peak at the first interface as

the signal enters the rib indicates that the rib thickness

is 35 in. The dielectric constant and loss in decibels

per meter is also indicated above the rib thickness

measurement printout at the left of the plot. Rib

measurements were also made in underground mines

over the range of 18 to 50 in. The thickness in these

test ribs could be determined to within 1 or 2 in.

Fig. 3. Rib Thickness

Figure 4 is a plot of actual data taken at an

operating highwall mine in Kentucky. In this case the

software presented an amplitude vs range plot, an

improvement over the amplitude vs time plot of

figure 3. The data shown are for a rib thickness of

57.2 in with a dielectric constant of 3.97. The

measured loss was 2.92 dB per wavelength. Also

measured but not shown was the distance from the

antenna measurement plane to the coal surface. This

distance data could be used for control of the position

of the mining machine.

Fig. 4. Rib Thickness, Amplitude vs. Range

Thickness Measurement of Other Materials

The thickness of other materials has been

measured with equal success. Granite, sandstone,

and concrete ranging in thickness from 2 ft to over 4

ft have been measured to within 2% of their actual

thickness. The thickness of each layer of multilayer

pavement can be determined as can the location and

orientation of steel reinforcing bars.

Conclusions and Recommendations

Field testing of the electromagnetic coal thickness

sensor has produced results of sufficient accuracy

(1 in for coal from 3 to 60 in thick) to justify continuing

with the engineering work necessary to develop a

practical sensor that can be mounted on a mining

machine for the determination of roof, floor, and rib

thickness. In addition, this research will be extended

to the development of a full 3-D imaging system

capable of "seeing into" the earth. Algorithms are

presently being evaluated to simultaneously measure

the azimuth, elevation, and range of targets in

multilayered media such as coal and rock as well as

for the location of buried ordnance and nuclear waste.

Future plans are to minimize the size of the data set

to reduce the software processing time now about 1

sec, and facilitate the construction of a compact

sensor suitable for machine mounting or use as a

general geological survey tool.

It was found that vertical E-field polarization

penetrates thicker coal ribs than horizontal E-fields. 

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This is thought to be due to the thin horizontal ash

layers having a higher loss than the coal.

The real part of the dielectric in coal varies very

little from a value of 4 but the loss tangent varies a

great deal. Wet, rough, or heavily cleated coal had

little effect on the dielectric measurement.

The ash content may be related to the loss

tangent of the dielectric measurement. This would be

a helpful means to identify higher quality coal.

References

1. Kerns, D. M., Plane-Wave Scattering-Matrix Theory

of Antennas and Antenna-Antenna Interactions, Nat.

Bur. Stand., NBS Monogram 162, June 1981.

2. Chufo, R. L. and Walter J. Johnson, A Radar Coal

Thickness Sensor, The 1991 IEEE Industry

Applications Society Annual Meeting, September 28-

October 4, 1991, Dearborn, MI.

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