AN ELECTROMAGNETIC SPATIAL/SPECTRAL SENSOR FOR

GEOLOGICAL MEASUREMENTS

Robert L. Chufo

U.S. Department of Energy

Pittsburgh Research Center

P.O. Box 18070

Pittsburgh, PA 15236

bob@silver.usbm.gov

ABSTRACT

The U.S. Department of Energy Pittsburgh Research

Center, has developed and field tested a spatial, spectral 1

sensor for measuring the thickness of mineral deposits.

Knowledge of the dielectric constant of the material is not

necessary because the electrical properties of the media are

determined automatically along with the thickness or

distance measurement. The technology was developed to

measure the thickness of coal remaining in the roof and

floor of a mine or the thickness of a coal pillar remaining

between two mining entries for the purpose of guiding a

mining machine. However, field tests have shown the new

technique is equally valuable for measuring the thickness of

each layer in a multilayer manmade structure such as

highway paving or building foundations and the thickness

of geological deposits other than coal, such as, salt, granite

and sandstone. The noncontacting electromagnetic

technique uses spatial modulation created by moving a

simple dipole 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 and enables frequency domain signal processing

to solve the problem of media, target and antenna

dispersion. The technique overcomes the problem of

multipath reflections from nearby metal structures and

avoids the use of radio frequency absorbing material. In its

present form the system measures material thickness of zero

to over 3 m to within ±2.5 cm accuracy. The system

parameters are variable to suit the application but for most

applications default values of 40 cm antenna motion with

measurements made at 401 frequencies from 0.6 to 1.4 GHz

at each of 32 antenna positions with a power level of 0 dBm

or less are more than adequate to provide a satisfactory

signal-to noise ratio. In the range of 0 to 10 m the system

operates in the continuous wave mode and by simply

turning off the transmitter for ranges beyond 10 m the

problems of time domain processing are entirely avoided.

The technique can be implemented with a vector network

analyzer, a servo controlled L-band antenna positioner, an

IEEE488 bus controller and a process computer. An

electronics package designed and constructed to operate in

the harsh mining environment and providing 1, 2 or 3

dimensional image presentation is being evaluated as a

replacement for the network analyzer.

Key words: Ground penetration radar, Spatial, Spectral

INTRODUCTION

Ground penetration radar technology has to deal with two

fundamental problems: penetration and focus. Lowfrequency electromagnetic waves penetrate the media but

do not clearly image small objects. When the transmitted

energy does penetrate the media, the returning signal

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

caused by dispersion. Media dispersion coupled with

antenna and target dispersion cause problems too complex

to resolve in the time domain. It is much easier to solve

these problems in the frequency domain. The theory

supporting the Radar Coal Thickness Sensor technology is

in the frequency domain but the signal processing

architecture uses both frequency domain and time domain

signal processing. In the future, all the signal processing

will be in the frequency domain. A time domain graphics

plot of the transformed frequency domain data is provided

to visualize the reflected signal.

The thickness measurement technique uses a continuouswave (CW) signal with a single antenna for simultaneously

transmitting and receiving. The antenna is moved

perpendicular to the surface whose layers are being

investigated. The antenna movement is in 32 discrete steps

over a distance of 40 cm. At each step, CW signals are

transmitted in 401 increments from 600 to 1,600 MHz. The

amplitude and phase of the reflected signal are recorded.

When the antenna has completed its motion, all of the

recorded data are analyzed by a computer. The result of the

analysis shows the location and thickness of layers of

material with different dielectric constants. The boundaries

between air and coal, coal and shale, etc., can be measured

with an accuracy of ±2.5 cm to a depth of at least 3 m.

Field experience with the sensor has provided the

confidence to envision that this sensor technology can be a

general solution that will satisfy other underground imaging

and thickness measurement requirements. These

requirements include the detection of tunnels and the

detection of mining hazards such as water-filled voids and

fractures that may be particularly dangerous to the mining

process. The present technology has been implemented as

a one-dimensional (1-D) sensor and 2-D sensor. The theory

This work originated under the U.S. Bureau of Mines prior 1

to transferring to the U.S. Department of Energy on

April 4, 1996.
for developing this same concept into a 3-D imaging sensor clay and metal vein intrusions of iron pyrite could be

has been developed for implementation and field testing. imaged and the distance from the coal surface to the

DESIGN APPROACH

Equipment

Early in this development effort it was decided that the Measurements were made on mine roof coal, mine rib coal,

thickness measurement scheme would be implemented in granite, sandstone, and salt. The data output plot shows the

software that could control off-the-shelf radio frequency amplitude of the reflected signal in decibels vs time in

(RF) test equipment. This approach was taken to insure that nanoseconds and distance in meters. A large peak on the

no resources were devoted to the expense of hardware vertical axis represents the reflection from the first surface.

design and development and the associated development Signals plotted previous to the first surface reflection

risk. Consequently, the program effort went into the represent discontinuities internal to the measurement

development and validation of software to gather and equipment and between the antenna and the first surface.

process the measurement data. The antenna is a simple These reflections are reduced to at least 30 dB below the

dipole with a polarized screen reflector and matching balun. first interface reflection by the calibration and spatial

The antenna positioner that moves the antenna to produce integration scheme. Peaks following the first reflection are

the spatial modulation is constructed entirely of common reflections from discontinuities internal to the media being

hardware store PVC tubing. A stepper motor is enclosed in measured. A table lists the calculated real part of the

the base of the positioner and drives a nonmetallic screw to dielectric constant, the loss in decibels-per-meter, and the

position the antenna with sub-millimeter accuracy. The thickness of the target material. Measurements have

positioner assembly includes limit switches that interrupt identified both the thickness of the coal and the thickness of

the data collection software if antenna positions beyond the the next layer, usually shale, above the coal roof. At the Lrange of travel are entered into the computer data band frequencies presently used the depth of penetration is

statements. Originally it was thought that the antenna usually about 3 m. The present roof thickness measurement

positioner should be constructed of nonmetallic research provides a direct readout of the thickness of each

components, and with the exception of the stepper motor at layer of geological material within the penetration range of

the far end of the assembly and the small limit switches, the the signal along with the permeability and permittivity. A

positioner is nonmetallic. transmitter power level of 0dBm (1mW) is adequate to

The instrumentation used is a HP8753 network analyzer a increased power levels are not necessary. Both the 2

HP R/382 68040-based computer controller, and a general hardware and software will operate from 300 kHz to

purpose IEEE488 bus controller for driving the stepper 3,000 MHz.

motor. A fiber optic IEEE488 data link is used when it is

desired to locate the control computer in a sheltered location Similar results are obtained when the sensor is used to

away from rock falls and the mining machinery. The measure the thickness of concrete or asphalt paving.

computer code is written in Rocky Mountain Basic (RMB). Multiple layers of paving can be detected and the thickness

FIELD TESTS

The initial purpose of this research was to develop a coal

and rock thickness sensor of sufficient accuracy to provide An electronics package has been constructed that will

vertical and horizontal guidance of both room-and-pillar replace the off-the-shelf test equipment that has been used

and highwall mining machines. In order to validate the on the tests described in this paper. This second-generation

theory developed for thickness measurement, extensive effort has resulted in a small portable instrument that will

underground and surface mine testing was conducted over greatly ease the problems associated with transporting the

a period of 2 years in mines with a variety of geological and laboratory test equipment to the various test sites and into

environmental conditions. Test areas of both freshly mined and out of operating underground and surface coal mines.

and aged coal from 7 cm to 150 cm thick were measured. The sensor package, including the antenna, are packaged so

The areas measured ranged from very dry to extremely wet that it can be installed on an operating highwall mining

with water dripping from the roof test area. The wet coal machine to measure the thickness of the coal rib separating

did not affect the thickness measurement. Coal seams with the active cut from the previous cut. This information will

intrusion could be 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.

produce a good SNR for the return signal measurement and

and electrical characteristics of each layer measured. 

FUTURE PLANS

be used for automated machine guidance so that a rib

thickness of 1 m to 1.5 m can be maintained for roof

support. The actual thickness maintained will be

determined by the conditions at each mine site.

Reference to specific products does not imply endorsement 2

by the U.S. Department of Energy Pittsburgh Presearch

Center.
This sensor will measure multilayer targets in 1-D with a equipment operates in the presence of nearby and adjacent

spatial modulator that moves the antenna on an axis metal structures and will be packaged as a portable

perpendicular to the target. A new antenna positioner instrument that can be used for other applications for

moves the antenna in the necessary two or three dimensions measuring the thickness of rock and concrete and measuring

as required to scan the target to produce a 2-D or 3-D the size and depth of tunnels. The technology when fully

image. A low-cost electronics package was designed and developed appears to be usable as a general solution to the

constructed to replace the function of the laboratory network problem of locating and imaging underground artifacts.

analyzer. The frequency synthesizer included in the new

analyzer package covers a three-to-one bandwidth with submicrosecond coherent switching between programmed

frequency steps. The success of the system depends on the

accuracy and speed of the frequency synthesizer, the data Kerns, D. M., "Plane-Wave Scattering-Matrix Theory of

processing speed and data acquisition, and transmission Antennas and Antenna-Antenna Interactions",

rate. For low-frequency ground probing applications, it is June 1981, Nat. Bur. Stand., NBS Monogram

now possible to construct an all-digital programmable 162.

receiver and transmitter at low cost.

CONCLUSIONS

An electromagnetic, noncontacting thickness sensor has 4, 1993.

been designed, constructed, and field tested. The results

were of sufficient accuracy to justify continuing with the Chufo, R. L., "Noncontacting Coal and Rock Thickness

engineering work necessary to develop a practical sensor Measurement with a Vector Network Analyzer",

that can be mounted on mining machines for the RF Design Expo East, Tampa, FL., September

determination of roof, floor, and rib thickness. The 22-24, 1992.

BIBLIOGRAPHY

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

Thickness Sensor", Conf. Proceedings of the

1991 IEEE Industry Applications Society Annual

Meeting, Dearborn, MI., September 29-October