NISTIR 3982

QUANTIFYING STANDARD PERFORMANCE

OF ELECTROMAGNETIC-BASED

MINE DETECTORS

William L. Gans

Richard G. Geyer

Wilfred K. Klemperer

1 1

States Department of Commerce

I XI 1 National Institute of Standards and Technology

NISTIR 3982

QUANTIFYING STANDARD PERFORMANCE

OF ELECTROMAGNETIC-BASED

MINE DETECTORS

William L. Gans

Richard G. Geyer

Wilfred K. Klemperer

Electromagnetic Fields Division

Electronics and Electrical Engineering Laboratory

National Institute of Standards and Technology

Boulder, Colorado 80303-3328

October 1991

U.S. DEPARTMENT OF COMMERCE, Robert A. Mosbacher, Secretary

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY, John W. Lyons, Director
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Table of Contents

I Introduction 1-1

A Work Statement Summaries 1-1

B Report Description 1-3

References (I) 1-4

n Fundamental Electromagnetic Field-Matter Relationships 2-1

A Physical Concepts Governing Electromagnetic Behavior 2-1

B Dielectric Polarization Mechanisms 2-2

C Dielectric Relaxation 2-4

C. 1 Debye Relaxation 2-4

C.2 Generalized Relaxation Distributions 2-4

C.3 Non-Debye Behavior of Rocks and Soils 2-5

D Dielectric Properties of Pure Water 2-6

E Dielectric Properties of Saline Water 2-7

F Heterogeneous Soil Mixtures 2-9

G Magnetic Susceptibility 2-10

References (II) 2-11

in Critical Performance Factors 3-

A Introduction 3-1

B Detector-Based Performance Factors 3-1

B. 1 Noise Factor of Mine Detectors 3-

B. 1 . 1 Ideal System Noise Factor 3-2

B. 1 .2 Overall Operating Noise Factor 3-2

B.2 Sensitivity to Atmospheric Noise 3-3

B.3 Detector Dynamic Range 3-5

B.4 Sensitivity to Detector Height above Earth’s Surface and Pattern

Signatures 3-6

B.5 Sensitivity to Detector Tilt Angle 3-7

B.6 Sensitivity to Detector Velocity 3-7

B.7 Sensitivity to Detector Spatial Positioning 3-7

B

8 Sensitivity to Other Detector-Based Parameters 3-7

C Target-Based Performance Factors 3-8

C. 1 Sensitivity to Specified Complex Permittivity Contrasts 3-8

C.2 Sensitivity to Target Size 3-8

C.3 Sensitivity to Burial Depth 3-9

C.4 Sensitivity to Target Shape 3- 10

C.5 Sensitivity to Ground Clutter 3- 1

C.6 Sensitivity to Target Orientation 3- 1

C.7 Sensitivity to Soil Stratification 3- 1

C.8 Sensitivity to Target Resolution 3-13

References (HI) 3-13

m
rV Measurement Methods and Systems for Electromagnetic Properties of

Materials 4-1

A Introduction 4-1

B Laboratory Sample-Holder Measurements of the Electromagnetic

Properties of Materials 4-2

B.l Dielectric Measurements Using Terminated Coaxial Lines 4-2

B.2 Permittivity and Permeability Measurements with a Two-Port

Measurement Technique 4-4

B.3 Procedures for Determining EM Properties of Test Soils 4-5

C In Situ Methods 4-6

D Measuring the Electromagnetic Properties of Magnetic Materials 4-7

E Recommendations and Conclusions 4-9

E. 1 Laboratory Measurement System 4-9

E.2 Field Measurement System 4-10

References (TV) 4-11

V Electromagnetic Properties of the Earth and Options for Standard Media 5-1

A Typical Electromagnetic Properties of Real Soils 5-

B Dielectric Mixing Rules 5-3

B. 1 Function-Theoretic Rules 5-5

B.2 Generalized Heuristic Rules for Soil Mixtures 5-6

B.2.1 Dry Soil Mixture 5-8

B.2.2 Lossy, Fluid-Saturated Soils 5-9

B.2.2. 1 Partially Saturated Soil Mixtures 5-9

B.2.2.2 Saturated Soil Mixtures 5-10

B.3 Application to Granular Soil Mixtures 5-12

B.4 NIST Measurements of Electromagnetic Soil Properties 5-12

B.4.1 Natural Soils 5-12

B.4.2 Synthetic Soils 5-13

C Loose Matrix Synthetic Soils for Narrowband or Fixed Frequency Test

Lanes for Nonmetallic Buried Objects 5-15

D Loose Matrix Natural Soils for Test Lanes for Metallic Buried Objects

and Broadband Systems 5-17

E NIST Recommendations and Conclusions 5- 1

E.l Natural Soil Test Range 5-18

E.2 Loose Granular Material Test Range 5-20

E.3 Liquid Material Test Range 5-21

E.4 Rigid-Matrix Brick Test Range 5-22

References (V) 5-23

VI Target Standards 6-

A Introduction 6-1

B Metallic Standards 6-1

C Nonmetallic Standards 6-3

D Relationship of Standards to Realistic Targets 6-5

D.l Metallic Objects Relationships 6-5

D.2 Nonmetallic Object Relationships 6-6

IV
E NIST Recommendations and Conclusions 6-6

E. 1 Targets for Metal Detector Testing 6-7

E.2 Targets for Nonmetallic Object Detector Testing 6-7

References (VI) 6-8

Vn Standard Measures of Effectiveness 7-1

A Introduction 7-1

B Test Strategy 7-1

C Recommended Core Tests 7-3

C. 1 Sensor Active Envelope Tests 7-4

C.2 Target Burial Depth Tests 7-6

C.3 Target Volume Tests 7-7

C.4 Target-Earth Contrast Tests 7-8

C.5 Target Shape Tests 7-9

C.6 Target Orientation Tests 7-10

C.7 Sensor Height Tests 7-11

D Future Tests 7-12

D. l Sensor Velocity Test 7-12

D.2 Sensor Tilt Angle Tests 7-13

D.3 Target Resolution Test 7-13

D.4 Surface Clutter Tests 7- 1

D.5 Volume Clutter Tests 7- 1

D.6 Earth Stratification Test 7-14

D.7 False Alarm Tests 7-14

E Test Result Analysis 7-14

References (VII) 7-16

Vm Archival Record Requirements 8-1

DC Conclusion 9-1

A Key Conclusions 9-1

B Recommendations for Future Work 9-2

X Glossary of Terms 10-

References (X) 10-2

Appendix IV.A: BASIC Program to Print Out Values of fi and £ 4A-1

Appendix IV.B: Automatic Network Analyzer System for Measuring e and fi 4B-1

Bibliography B-1

V
List of Tables

Table 5. 1 Magnetic susceptibility of rocks and minerals 5-4

Table 5.2 Experimental results of dielectric measurements on sand as a

function of water saturation 5w and frequency / [5.17] 5-14

Table 5.3 Experimental results of dielectric measurements on sandy loam as a function of water saturation Sw and frequency /[5.17] 5-14

Table 5.4 Experimental results of dielectric measurements on silt loam as a

function of water saturation Sw and frequency/ [5.17] 5-15

Table 5.5 Experimental results of dielectric measurements on clay loam as a

function of water saturation Sw and frequency/ [5. 17] 5-15

\ Table 5.6 Experimental results of dielectric measurements on clay as a

function of water saturation Sw and frequency / [5.17] 5-16

Table 5.7 Preliminary set of standard background media for plastic mine

detector validation testing 5-16

Table 5.8 Preliminary set of standard background media for metal detector

validation testing 5- 1

Table 6. 1 Copper test objects 6-3

Table 6.2 Stainless steel test objects 6-4

Table 6.3 Spherical plastic test objects 6-4

Table 6.4 Cylindrical plastic test objects 6-5

VI
List of Figures

Figure 2. 1 Dielectric dispersion for various types of polarization 2-14

Figure 2.2 Frequency variation of and for pure water at 25°C. Debye

equation predicts linear 45° fall-off which appears, above, as the

dashed line 2-15

Figure 2.3 Cole-Cole diagram for the dielectric constant of pure water at 0°C 2-16

Figure 2.4 Measured concentration dependence of dc dielectric constants for

three salts in highly concentrated solution 2-17

Figure 2.5 Measured concentration dependence of relaxation times of sodium

chloride-water solution at 25°C 2-18

Figure 2.6 Depression of dielectric constant and relaxation time of sodium

chloride solutions as a function of ionic concentration. • = 0°C,

O = 20°C, X = 40°C 2-19

Figure 2.7 Typical power-law exponent variation of dielectric constant as a

function of water saturation for a consolidated sandstone 2-20

Figure 2.8 Dielectric constant as a function of water saturation for typical

sandstone plotted parametrically as a function of frequency 2-21

Figure 2.9 Composition diagram of natural magnetic materials 2-22

Figure 2.10 Magnetic susceptibility versus fractional composition of

magnetite-ulvospinel solid solution 2-23

Figiu^ 3. 1 Minimum (B) and maximum (C) atmospheric noise figure vs

frequency 3-15

Figure 3.2 Minimum (B) and maximum (C) atmospheric noise figure vs

frequency 3-16

Figure 3.3 Oppositely directed dipoles at height h above the earth 3-17

Figure 3.4 Normalized electric field at various heights in air for a fixed

observation point in the earth 3-18

Figure 3.5 Normalized electric field at various heights in air for a fixed

observation point in the air 3-19
Figure 3.6 K-plane pattern of oppositely directed dipoles located at air/earth

interface for various earth permittivities 3-20

Figure 3.7 Scattered far-field for a cube (side = 0.1 A), a circular cylinder

(height = radius = 0.1084A), and a sphere (radius = 0.1241A) for

normal incidence, TE polarization 3-21

Figure 3.8 Sources of ground clutter 3-22

Figure 4.1 Cross-sectional drawing of the coaxial transmission line sample

holder for the single-port measurement scheme 4-15

Figure 4.2 Electromagnetic properties (at 600 MHz) of a blend of SiC and

silica sand for various percentages (by mass) of SiC 4-16

Figure 4.3 Schematic of the ANA materials measurement system 4-17

Figure 4.4 Complex relative permittivity and permeability of Teflon as measured with the ANA 4-18

Figure 4.5 Complex relative permittivity and permeability of nylon as measured with the ANA 4-19

Figure 4.6 Complex relative permittivity and permeability of dry sand as measured with the ANA 4-20

Figtire 4.7 Complex relative permittivity and permeability of a 60% dry sand

and 40% (by weight) SiC mixture as measured with the ANA 4-21

Figure 4.8 Schematic diagram for a typical susceptibility bridge 4-22

Figure 4.9 Conducting permeable sphere of radius a in a uniform alternating

magnetic field. The conductivity, permeability, and permittivity of

the host medium are cTj, and Cj, respectively. The conductivity,

permeability, and permittivity of the sphere are 02, 1^2

^

respectively 4-23

Figure 4.10 In-phase (M) and out-of-phase (N) components of the induced

dipole moment for a sphere in a uniform alternating magnetic field

for the case « 1. See reference [4.43] 4-24

Figure 4. 1 1 Circuit diagram for the susceptibility meter 4-25

Figure 4.12 Half-space multiplicative correction factor for susceptibility

measurements 4-26

viii
Figure 5. 1 Particle properties for different soil types 5-25

Figure 5.2 Particle distribution for different soil types 5-26

Figure 5.3 Measured permittivity and loss tangent vs frequency for tap water

at 20°C 5-27

Figure 5.4 Measured permittivity and loss tangent vs frequency for sand with

4% water 5-28

Figure 5.5 Measured permittivity and loss tangent vs frequency for sand with

8% water 5-29

Figure 5.6 Measured permittivity and loss tangent vs frequency for sand with

14% water 5-30

Figure 5.7 Measured permittivity at 600 MHz vs drying time at 20°C for

various sand and water mixtures 5-31

Figure 5.8 Measured permittivity and loss tangent vs frequency for 100%

silicon carbide 5-32

Figure 5.9 Measured permittivity and loss tangent vs frequency for 90%

silicon carbide 10% sand mixture 5-33

Figure 5. 10 Measured permittivity and loss tangent vs frequency for 80%

silicon carbide 20% sand mixture 5-34

Figure 5.11 Measured permittivity and loss tangent vs frequency for 70%

silicon carbide 30% sand mixture 5-35

Figure 5.12 Measured permittivity and loss tangent vs frequency for 100%

sand 5-36

Figure 5.13 Measured permittivity and loss tangent vs frequency for various

silicon carbide and aluminum powder mixtures 5-37

IX
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QUANTIFYING STANDARD PERFORMANCE OF

ELECTROMAGNETIC-BASED MINE DETECTORS

William L. Cans, Richard G. Geyer, and Wilfred K. Klemperer

National Institute of Standards and Technology

Boulder, Colorado 80303

This is a final report to sponsor on work performed by National

Institute of Standards and Technology (NIST) personnel from

January 1, 1985 to December 31, 1990. An overview of the theory of

the electromagnetic properties of soils is presented along with a brief

review of existing technologies for the detection of buried objects

using electromagnetics. The critical electromagnetic performance

factors for portable EM mine detectors that NIST has identified are presented, along with a discussion of measurement systems for

measuring the constitutive properties of soil and mine-like materials.

Recommendations are then presented for a measurement system

configuration that should meet most of the Army’s requirements. A

recommended mine detector testing strategy is tiien presented along

with a set of instructions for specific tests and an alogrithm for

comparatively scoring the performance of detectors. The tests and the

scoring algorithm are as specific and as detailed as is possible at this

stage of development. Last, a section is included that contains NIST’s

recommendations for the test data that should be archived.

Keywords: buried object; constitutive properties; conductivity;

dielectric constant; dielectric loss; elecromagnetic detection; mine;

mine detector, permeability; permittivity; remote sensing; sensor;

target.

I Introduction

The purpose of this final report is to provide documentation describing the work performed

by the National Institute of Standards and Technology (NIST) under two sequential contracts to

the U.S. Army Belvoir Research, Development and Engineering Center (BRDEC). The first

contract period was from January 1, 1985 to September 30, 1988, and the second period was

from January 1, 1989 to December 31, 1990. This introduction summarizes the original and

modified work statements and contains a brief description of the contents of the remaining

chapters of this report. As closely as possible, the title of this report, the table of contents, and

the chapter titles are those requested by BRDEC personnel.

A Work Statement Summaries

The original work statement defining the tasks to be performed over approximately a four

year period by NIST for BRDEC is outlined below.

1-1
Phase I.

1. Create a joint National Academy of Sciences-NIST-U.S. Army review panel to

review annually the work performed by NIST.

2. Review information provided by the Army pertaining to past and present mine

detection systems, measurement methods, and standards, as necessary.

3. Provide the Army a suggested selection of standard test targets and soil backgrounds.

4. Provide the Army a preliminary set of definitions, parameters to be controlled, and

standard test conditions.

5. Investigate the possibility of developing a performance effectiveness measure for

mine detectors.

Phase II.

1. Recommend a final set of standard test targets.

2. Recommend a final set of standard test soils.

3. Recommend a set of measurement methods to ensure replication of test conditions.

4. Formulate (if possible) a definitive set of effectiveness measures.

Phase III.

1.

Specify a measurement system to ensure standard test conditions.

‘2. Provide an uncertainty estimate for the measurement process.

Phase IV.

1. Provide initial guidance, training, technology transfer.

This original work statement covered the first contract period. The documentation that

reported the results of NIST’s work during this contract period was presented to the Army at Fort

Belvoir in September, 1988, and published as a Restricted, National Institute of Standards and

Technology Inter-Agency Report in March, 1990 [1.1].

A summary of the second work statement, as modified over the course of the second

contract period, is outlined below.

1. Perform engineering studies, with the assistance of an outside contractor, to choose

the optimum material(s) to serve as rigid-matrix soil simulation standards.

2. Produce a small number of rigid-matrix soil standard “bricks” as a “proof of concept,”

and provide the Army with necessary design specifications for these bricks.

3. Provide the Army with a laboratory measurement system for use in measuring the

constitutive EM properties of both standard soil materials and standard target

materials.

1-2
4. Develop a comprehensive set of performance measures (tests) for mine detectors.

This final report contains the documentation for all of the work performed during both

contract periods with one exception. The rigid-matrix brick design and specification work will

be given to the Army in a separate report because this work is not yet completed. Also, NIST

recognizes that more work is required in order to actually translate this research and development

into an operational mine detector test range at Fort Belvoir. Therefore, Chapter DC contains an

outline of NIST’s recommendations for the future work required in order to achieve this goal.

B Report Description

What follows is a brief, chapter-by-chapter description of the contents of this report.

Chapter II contains a review of the existing theoretical relationships between

electromagnetic fields and matter.

Chapter IQ contains a discussion of critical electromagnetic performance factors for

portable mine detectors that NIST personnel have identified.

Chapter IV contains a review of past and present methods developed for the measurement

of the constitutive properties of materials relevant to electromagnetic mine detection, including

the methods used at NIST along with some representative data. Also included in this chapter is a

brief description of the measurement system that we have chosen for delivery to the Army.

Chapter V contains a comprehensive review of the existing theory and practices related to

the electromagnetic properties of soils, the options for realizing soil or soil-like standards (with

advantages and disadvantages discussed for each option), and a description of NIST’s

recommendations for achieving optimal soil standards by use of rigid-matrix brick soil

simulations.

Chapter VI contains a discussion of the desired properties for mine-replica target standards

along with NIST’s recommendations for both plastic and metal standards. Included is a

description of how the standards were fabricated along with possible commercial sources for the

required materials.

Chapter Vn contains a description of the mine detector testing strategy that has been

developed and recommended by NIST, along with our reasons for creating and recommending it. This is followed by a description of seven objective core tests for EM mine detectors that NIST

recommends that the Army implement as soon as is practical. The descriptions for these tests are both as general as is possible, allowing the Army to configure them, as requested by the Army, to

any of the soil and target standards options discussed in chapters V, and VI, and as detailed as

possible considering the fact that the Army has not yet committed to a specific test range

configuration. Also included in this chapter are more general descriptions of a set of advanced

(more difficult to implement) tests that NIST recommends be implemented after the army has

1-3
implemented and gained experience with the first core set. Last, an algorithm for objectively

quantifying (scoring) the comparative performance effectiveness of portable, EM mine detectors

is presented, along with a recommendation for how to include the required subjective evaluation

aspects into the algorithm.

Chapter VIII contains a discussion and a listing of all of the data that NIST has identified

which should be permanently recorded in order to maintain adequate records for both individual

mine detector tests and the maintenance of the test range as a whole.

Chapter IX contains an outline of NIST’s recommendations for future work.

Along with the references cited at the end of each chapter, there is a comprehensive,

alphabetized bibliography included at the end of this report. Also included at the end of this

report is a brief, alphabetized Glossary of Terms.

References (I)

[1.1] Cans, W. L.; Geyer, R. G.; Klemperer, W. K. Suggested Methods and Standards for

Testing and Verification of Electromagnetic Buried Object Detectors. National

Institute of Standards and Technology NISTTR 89-39 15R; 1990.

1-4
II Fundamental Electromagnetic Field-Matter

Relationships

A Physical Concepts Governing Electromagnetic Behavior

Any material is electromagnetically characterized by its permittivity e in F/m, magnetic

permeability fi in H/m, and electrical conductivity crin S/nx Maxwell’s equations, together with

the constitutive equations relating field quantities in terms of material properties, completely

govern electromagnetic wave propagation and behavior in that medium. Therefore, the use of

electromagnetic energy to detect buried objects must involve detecting differences between the

object and the surrounding medium in one or more of the above quantities.

The constitutive equations for a linear, homogeneous, and isotropic medium may be

expressed in the firequency domain as

electric field by the permittivity. Any deviation from linearity is usually included by making e,

H, or G field dependent For anisotropic media, e, fx, and/or cr become 3x3 tensor matrixes as opposed to constants or scalar functions of frequency.

The solution of Maxwell’s equations yields all of the quantities that describe the

propagation of electromagnetic waves in terms of the propagation constant, k, where

In general, the constituent electrical properties may be written as complex quantities; that

J=crE,

D = £E,

(2. 1 )

where 5 is the magnetic flux density (Wb/m^) related to the magnetic field intensity, H (A/m),

by the magnetic permeability; J is the current density (A/m^) related to the electric field

intensity, E (V/m), by the conductivity; and D is the electric flux density, (C/m^) related to the

- (0/l(C0£- jc). (2. 2)

IS,

(2. 3)

2-1
The imaginary part of the propagation constant contains all necessary information about

energy loss in a material medium during wave propagation. If, for the moment, magnetic

properties are ignored, we may consider only the complex forms of e and a in eq (2.2):

co{e' - j{(f + = ^ COE')- y(cr'+ cue"). (2.4)

Here (cue'+ <j") may be considered an effective real permittivity and (ex' + cue") as an

effective conductivity. The (cr' + ycr") physically represents carrier transport due to Ohmic and

Faradaic diffusion mechanisms, whereas the (e' - ye") represents dielectric relaxation

mechanisms. The loss tangent is defined from eq (2.4) as

tan 5 = tan

V

<7^4- cue"

<t" + cue'

’ (2.5)

where iff is the phase angle between E and J. If there are no dielectric losses, e" —> 0.

Similarly, if there are no Faradaic diffusion losses, a" —> 0. Hence,

tan5 = -^, (2.6) cue

which physically describes the Ohmic losses.

What follows in this chapter is a detailed discussion of the dielectric and magnetic

properties of those materials relevant to this project.

B Dielectric Polarization Mechanisms

We can describe the dielectric properties of a material by the complex dielectric constant,

€ = e'- je'\ where e' contains all the information about energy storage and e" contains all of

the information about energy loss (both Ohmic and dielectric losses) in the material during wave

propagation. The dialectric constant of a soil is the real effective permittivity of the soil

normalized with respect to the permittivity of a vacuum, Eq . The quantity e' is a measure of the amount of polarization in the material. There can be a

number of different polarizing mechanisms present; each has a characteristic relaxation

frequency and an associated dielectric dispersion centered around this relaxation frequency.

Figure 2. 1 illustrates the dispersions of dielectric constant and conductivity that are observed in

materials in the frequency range of 10^ to 10^^ Hz. At the highest frequencies, the polarizing

species in a material are the electrons. Electronic polarization occurs when an applied electric

field causes a net displacement of the electron cloud of an atom with respect to its nucleus. At

frequencies below about 10^^ Hz, there is also a contribution from atomic polarization. Atomic

2-2
polarization occurs in structures (molecules or solutions, for example) in which atoms do not

share electrons equally and electric fields displace the electron clouds preferentially towards the

stronger binding atoms. It also occurs when charged atoms are displaced with respect to each

other. Dipolar polarization, that is, the orientation of polar molecules (molecules with

asymmetric charge distributions), occurs at frequencies below about 10^® Hz [2.1].

At frequencies below about 10^ Hz, various types of charge polarizations occur which may

be collectively referred to as Maxwell-Wagner mechanisms [2.2, 2.3]. One of these, interfacial

(space-charge) polarization, occurs when migrating charge carriers are trapped or impeded in

their motion by local chemical or electric potentials, causing local accumulations of charge and a

macroscopic field distortion. Another low-ffequency mechanism that can occur in soils is due to

mixtures of materials that have differing electrical properties (such as conducting spheres

embedded in a dielectric). Several different equations are available to describe the resultant

properties [2.4, 2.5] for various geometries of the embedded conductor: conducting spheres or

rods in a dielectric, alternating layers of dielectrics and conductors, for example. The common

cause of these effects is the distributions of charge that occur at conductor-dielectric boundaries

and the resultant action under applied electric fields which can yield very large, low-frequency

dielectric constants. This mechanism, when we use synthetic soils whose conductivities are controlled by embedded conductors, is very important to examine and to relate to dispersion

phenomena normally confronted in natural soil environments. This is true particularly when

performance testing of broadband land mine detection systems is done and a relative detector

figure of merit is estimated (broadband in this case means that the bandwidth extends into

frequencies less than 1 MHz).

Still another dispersion mechanism for dielectric behavior at low frequencies, which is

often distinguished from Maxwell-Wagner effects, is that which occurs in colloidal suspensions.

Maxwell-Wagner effects are based on the assumption that the charge around conducting particles

in a dielectric medium is a thin coating which is much smaller than the particle dimensions [2.5]

and that the charge responds to an applied electric field independently of the charge on nearby

particles. In colloidal suspensions, on the other hand, the charge layer is on the same order of

thickness as or larger than the particle dimensions; hence it is affected by the charge distributions

of adjacent particles. The theory of dispersion phenomena in colloidal suspensions is presently a

fertile area of research. Dukhin [2.5] has shown that colloidal responses result in far higher lowfrequency dielectric constants than those resulting from typical Maxwell-Wagner mechanisms,

with dielectric constants on the order of 10^ not uncommon. This could be an important

phenomenon that spectrally increases the visibility of buried plastic land mines when broadband

system performance is considered.

2-3
C Dielectric Relaxation

Polarization processes occurring in material media as a result of electromagnetic wave

propagation are physically damped by relaxation. This relaxation is analogous to a critically

damped or overdamped oscillator. In the relaxation process, maximum energy is dissipated at a

preferred relaxation frequency, or resonance (which is the reciprocal of the relaxation time

constant), with no dissipation at either zero or infinite frequency. Relaxation processes are the

only ones observed in the natural soil environment at microwave frequencies and below. Thus, it would be useful to consider some relaxation models (cited in subsequent comments on soil

specifications for broadband systems). The following relaxation models are based on the general

equation of charge motion

g + (Ha) ^g + (jie) ^2 = 0 , (2.7)

where q is the charge, and • represents differentiation with respect to time. All derivatives are with respect to time. An important current research area is considering diffusion of charged ions

whose concentration is spatially variable. For this case, spatial derivatives must be taken in

determining diffusion relaxation and this leads to generalized distributed impedances and

nonlinear behavior [2.6].

C.1 Debye Relaxation

Debye relaxation occurs in materials that have single relaxation time constants. Relative

complex permittivity in a Debye material is given by [2.7, 2.8]

f -ye =£o

{Es-E„)le,

1 + jctn„ ie,-e^)(or

(2. 8)

where is the measured relative real effective permittivity at dc (e^c ~ is the relative

dielectric permittivity at infinite frequency {SinfinUe - the time constant of relaxation,

—1

and Cq is the free space or vacuum permittivity (8.854 x 10 F/m).

C.2 Generalized Relaxation Distributions

Wyllie [2.9] has given an expression for material media in which multiple relaxations or

distributions of relaxations are found. Such behavior is more typical of soils at frequencies

below 1 MHz than Debye behavior, and the complex dielectric permittivity may be written

2-4
e-je" = e^ + (2.9)

D(tXl-jcor)

l + co^r^

dr,

where D(t) is the time constant distribution function normalized so that

A commonly observed simple relaxation distribution in soils is the Cole-Cole distribution

[2.10]. In the Cole-Cole distribution, eq (2.9) reduces to

e'-je" = e.

^OO

l + (yG>T)

1-m » (2. 10)

where 0 < m < 1. The m = 0 case corresponds to a Debye material that has a single relaxation.

The m = 1 case corresponds to an infinitely broad, continuous distribution (one that has no

relaxation). Usually 1 - m = c < 0.25 for water-saturated soils, and the exact power-law

exponential frequency behavior is critically dependent on water saturation, as discussed later in

this report (c is a constant defined in the next section and related to m as above.) This would

necessitate methods for keeping c constant (as a function of frequency) in natural (or synthetic

water saturated) soils when testing of land mine detectors is performed.

C.3 Non-Debye Behavior of Rocks and Soils

Examples of studies showing the non-Debye behavior of rocks and soils are given by SaintAmant and Strangway [2.11], Alvarez [2.12], Olhoeft et al. [2.13], Pelton et al. [2.14], Knight

[2.15], and Lockner and Byerlee [2.16]. Observed deviations from Debye behavior have led to

modifications of the Debye equation [2.12], including the Cole-Cole expression, eq (2.10) and

the corresponding Cole-Cole equivalent circuit [2.10]. The significant feature of the Cole-Cole

circuit is the inclusion of a constant phase circuit element to model a distribution of relaxation

times. The Cole-Cole expression has been found useful in fitting data from lunar soil samples

[2.13], and an expression for the complex impedance of a material (as measured in the series

mode) that contains a term analogous to the constant phase element in Cole-Cole [2.10] has been

used in modeling the impedance of mineralized rocks [2.14, 2.17].

In addition to the Cole-Cole expression, there are three other empirical relations commonly

used to describe a non-Debye response. These are the Cole-Davidson [2.18], the combined ColeCole, and the Williams-Watts [2.19] expressions. A characteristic feature of all these empirical

relations, besides being based on eq (2.7), is that at frequencies away from the relaxation

frequency they reduce to expressions showing a power-law dependence on frequency for both e'

2-5
and e". This common characteristic led Jonscher [2.20, 2.21] to define this power-law

dependence as the “universal dielectric response,” that is.

e\o)) = Ka) (2. 11 )

where K and c are constants that depend on the sample material and texture.

D Dielectric Properties of Pure Water

Usually, natural soils contain water, so the way in which dielectric constants combine is

important in determining the bulk, or effective, dielectric constant of the soil at hand. The bulk

dielectric constant of any soil (whether natural or synthetic) determines the electromagnetic

visibility of buried land mines and, therefore, intrinsically affects the performance of any

detection system. Before discussion of water saturation of heterogeneous soil mixtures, it is

necessary to review fundamental dielectric behavior of pure and saline water.

The frequency dependence of the complex dielectric constant of pure water, is well

known and is given by the Debye equation [2.1, 2.7],

where = high frequency (or optical) limit of (dimensionless), = dc dielectric constant of (dimensionless), = relaxation time of pure water (seconds), and

G) = frequency (radians per second).

Equation (2.12) may be simply written

(2. 12)

p = e; - = e; ( 1 - y tan )

,

(2.13)

where tan 5^ s is the loss tangent of water.

Rationalization of eq (2.12) yields

(2.14)

and

(2.15)

2-6
The magnitude of the high-frequency dielectric constant has been determined by Lane

and Saxton [2.22] to be

£ =49 (2.16)

and is practically temperature independent. The relaxation frequency of pure water,

occurs in the microwave region and is temperature dependent. At 0°C,/^ » 9 GHz

and at 20°C,/^ » 17 GHz. Equation (2.15) shows that e" has its maximum value at ft) = . A plot of the frequency variation of and for pure water at 25°C is shown in figure 2.2

[2.4]. A Cole-Cole plot of e" versus with /as a variable parameter, is shown in figure 2.3

for pure water at T = 0°C [2.23]. This plot is a semicircle with end points defined by = e^q in

the low-frequency limit and by in the high-frequency limit. The point on the circle at

which e" is maximum occurs at the relaxation frequency, /=/v^, whose coordinates are simply

given by _ (^wO ^woo) ^

_ (^wO ^eo)

2 2

The dc dielectric constant of nonconductive water, is a function of temperature. Klein

and Swift [2.24] have generated a regression fit for e^q(J) from dielectric measurements

conducted between 1 GHz and 3 GHz,

e^(J) = e’^QiT) = 88.045 - 0.4147T + 0.6295 x 10‘^r^, (2.17)

where T is in °C, and the loss term, e"q, is 0.

E Dielectric Properties of Saline Water

Although the dielectric properties of pure water and ice obey the Debye relaxation

equations and are fairly well understood, ionic salts dissolved in water produce an electrolytic

solution whose microwave dielectric properties may differ greatly from those of pure water. The

salinity 5 of a solution is defined as the total mass of solid salt in grams dissolved in 1 kg

solution. Thus, 5 is normally expressed in parts per thousand by mass. Little will be said about

broadband dielectric characterization of electrolytic solutions, except that the real and imaginary

parts of the dielectric constant of a saline water solution are given by [2.24, 2.25]

'SW = £

5Vtx>o JWoo

l + (©Tw)

(2.18)

and

2-7
(2.19) p» ^'^SW ( ^SW0 ^SWOO ) , 1 . / n2 ’ i+(o>T^r

where the subscript sw refers to saline water and Gi is the ionic conductivity of the aqueous

solution in S/m. The form of the complex permittivity for saline water differs from that of pure

water only in the loss term, where the added term, Gile^co , resulting from the ionic

conductivity of the aqueous solution is present The ionic conductivity of saline water has a

marked effect on the loss factor below lO GHz. Therefore, high soil salinity will probably

significantly affect the dielectric properties of wet soil. As Jedlicka [2.26] notes, few

measurements and analyses have been reported relating soil salinity to effective soil dielectric

constant. Consequently, the dependence of permittivity on soil salinity is not well understood.

We do know, however, that the salinity of (free) pore water within a soil matrix depends directly

on the cation exchange capacity of the matrix material. This fact should provide a direct

relationship between soil type, amount of volumetric moisture present, and effective soil

complex permittivity. Further work will enhance understanding between these physical soil

parameters and measured electromagnetic properties.

Stogryn [2.27] points out that there is no evidence to indicate that depends on salinity;

hence, we are safe to assume that = 4.9. He also empirically determines the

dependence of permittivity on both the temperature and salinity of saline water by writing

as a factorable product, £s^q{T, S) = e^Q{T)F{S). Polynomial fits to this relation have been

obtained [2.24, 2.28] on measurements performed by Ho and Hall [2.29] and Ho, et al. [2.30] for

4 < S < 35 to obtain the following functional (nonmultiplicative) dependence of the dielectric

constant of water with respect to salinity,

(T". •S) = £w0 (T") - 0. 1 556 - 4. 1 3 X 1 0^5 + 1. 5 8 X 10"^ (2.20)

and

S) = e;„o = 5.66 + 2.65 x 10"^S-4.5 x 10"^ 5^. (2.21)

Equations (2.17), (2.20) and (2.21) will be used in subsequent development relating

effective dielectric constant measurements (real part, £' , and loss tangent, tan5) of a lossy fluidsaturated soil mixture to the in situ fluid dielectric properties, as well as to the water-filled

porosity of the soil mixture.

Several figures are included here to illustrate the relationship between the addition of salts

to pure water and the dielectric properties of the resulting solution. Figure 2.4 [2.31] is a plot of

dc dielectric constant, versus solution concentration for three different salts. Figure 2.5

[2.31] is a plot of normalized relaxation time for a NaCl solution versus salt concentration at

2-8
25°C. Figure 2.6 [2.31] is a plot of the depression (change) of dielectric constant and relaxation

time for a NaCl solution versus salt concentration at 0°C, 20°C, and 40°C. These figures show

that the presence of salt(s) in water (and thus in damp or wet soils) can drastically change the

medium’s dielectric constant.

F Heterogeneous Soil Mixtures

By definition, natural soils are mixtures of host matrix mineral(s), air, and water.

Generally, the real part of the effective dielectric constant, of the mixture rarely exceeds 8 in

the microwave region—as long as there is no liquid water in the mixture. Similarly, the

imaginary part usually does not exceed 1 in the absence of liquid water. By way of

comparison, we see from figure 2.2 that of water is one order of magnitude larger than the

relative permittivity of dry materials, whereas is two orders of magnitude larger than that of

dry materials, particularly at frequencies less than 2 GHz. Because of the large contrast between

(complex) e of the pore water and that of the host matrix in soils, the dielectric constant of the

mixture is generally dominated by the dielectric behavior of water. For this reason, many

investigators [2.22, 2.27, 2.32, 2.33] have generalized the Debye formulas given by eqs (2.14)

and (2.15) to mixtures in the following way.

(2.22)

1 + (<BT^)^

(2.23)

where ^re functions of the dielectric constant of the matrix mineral(s), the

water saturation, 5w, fraction in the pore space of the matrix, and the shape and orientation of the

water inclusions. The dielectric constants of the matrix minerals are presumed to be

nondispersive (frequency independent or lossless) as well as independent of the applied direction

(polarization) of the incident electric field. However, caution must be exercised in the use of eqs

(2.22) and (2.23) for soil mixtures over a broad range in frequency, since multiple relaxation

phenomena not taken into account by the Debye rules can occur [2.1].

An example of a typical power-law variation of the dielectric constant as a function of

water saturation for a consolidated sandstone is shown in figure 2.7 (recalling eq 2.1 1). As the

saturation decreases from full saturation, the power-law exponent, c, increases until at some

critical saturation point (usually 5w = 0.2 for critical saturation) it drops rapidly to near 0. Most

2-9
surface sands would probably fall below critical saturation, whereas most clay soils might fall in

the region above critical saturation.

Typical behavior of dielectric constant as a function of both water saturation and frequency

is shown in figure 2.8. The observed dependence of f// on Sw shows that it increases rapidly up

to a critical water saturation, whereupon it increases more gradually and linearly with increasing

saturation. For a completely dry soil and one which is only a single component system, e' goes

to some threshold value independent of frequency. For a water saturation above critical

saturation, e' not only changes less with increasing frequency, but also has a lower value, almost

by a factor of 4, from 50 kHz to 1 MHz. The significance of this observation is that mine

detectors operating at differing frequencies are likely to be observing different contrasts in

complex permittivity as long as water saturation (or equivalent metallic concentration in a

synthetic soil) stays above critical with a greater dielectric contrast skewed toward lower

detection frequencies.

Thus, there may be valid reasons to operate in this low frequency range for enhancing

dielectric visibility contrast, even at the expense of spatial target resolution. The purpose of

drawing attention to these phenomena is (1) to better compare detection systems that have

different operating frequencies (whether fixed or broadband) in terms of the actual electrical

contrast, f// - ff , seen between the host soil environment and buried land mine; and (2) to allow

the removal of a variable, nonlinear frequency dependence of the soil’s electrical properties in

the case of water saturated soils by keeping water saturation above the critical point for differing

target size and burial depth parameters.

In summary, the functional dependence of c and on water saturation in a porous

medium is typified by figures 2.7 and 2.8. The actual power-law exponent observed depends on

the texture (microgeometry) of the soil and resultant pore surface water or pore surface area-tovolume ratio.

G Magnetic Susceptibility

Magnetic susceptibility is a fundamental physical property of a material medium. The

degree to which a body is magnetized when placed in an external magnetic field is given by

^ = 2.24

where M is the magnetization in A/m, H is the applied external magnetic field intensity in A/m,

and Xm is magnetic susceptibility (dimensionless) relating the applied field to the intensity of

magnetization. We will use the International System of Units (SI) here.

2-10
For a magnetically linear, isotropic substance, the constitutive equation relating the

magnetic flux density in Wb/m^ within a substance to the external magnetic field H due to

magnetization is simply expressed by

B = + = (2.25)

where fJ^ is the permeability in vacuum (4n x 10"^ H/m) and Xm is the dimensionless magnetic

susceptibility defined in eq (2.24).

The magnetic susceptibility of soils depends on the component magnetic minerals derived

from chemical and mechanical breakdown of bedrock. Magnetic minerals of importance are few, and those most commonly encountered are the iron and titanium oxides which form several

solid solution series in rocks (figure 2.9). Depending on the fractional composition of any solid

solution series, the susceptibility may vary widely (figure 2.10). In addition to specific chemical

composition, the susceptibility depends on grain size and the intensity of the magnetizing field.

Thus, considerable variation in the susceptibility of rocks (and soils) can occur.

References (II)

[2.1] Geyer, R.G. Experimental parameters for land mine detection performance

evaluations. NIST Report, Project 7232457, 1(X) p.; November 1987.

[2.2] Maxwell, J.C. A treatise on electricity and magnetism. Dover Pub.; 1891.

[2.3] Wagner, K.W. Erklarung der dielektrischen Nachwirkungsworgange auf Grund

Maxwellscher vorstellungen. Archiv. Electrotechnik. 2: 371; 1914.

[2.4] Hasted, J.B. Aqueous dielectrics. London: Chapman and Hall, 302 p.; 1973.

[2.5] Dukhin, S.S. Dielectric properties of disperse systems, in Surface and colloid

science, Vol. 3, Matijevic, E., ed. New York, NY: Wiley Interscience, 83-166; 1969.

[2.6] Sluyters-Rehbach, M.; Sluyters, J.H. Sine wave methods in the study of electrode

processes, in Electroanalytical Chemistry, Vol. 4, A.J. Bard, ed. New York, NY:

Marcel Dekker, 1-128; 1970.

[2.7] Debye, P. Polar molecules. New York, NY: Chemical Catalog Co.; 1899.

[2.8] Smyth, CP. Dielectric relaxation times in molecular relaxation processes. London:

The Chemical Society, 1-14; 1966.

[2.9] Wyllie, G. Dielectric relaxation and molecular correlation in dielectric and related

molecular processes. London; The Chemical Society; 1972.

[2.10] Cole, K.S.; Cole, R.H. Dispersion and absorption in dielectrics. J. Chem. Physics. 9:

341-351; 1941.

2-11
[2.1 1] Saint-Amant, M.; Strangway, D.W. Dielectric properties of dry geologic materials.

Geophysics. 35: 624-645; 1970.

[2.12] Alvarez, R. Complex dielectric permittivity in rocks: A method for its measurement

and analysis. Geophysics. 38: 920-940; 1973.

[2.13] Olhoeft, G.R.; Frisillo, A.L.; Strangway, D.W. Electrical properties of lunar soil

sample 15301, 38. J. Geophys. Res. 79: 1599-1604; 1974.

[2.14] Pelton, W.H.; Ward, S.H.; Hallof, P.G.; Sill, W.R.; Nelson, P.H. Mineral

discrimination and removal of inductive coupling with multifrequency IP.

Geophysics. 43: 588-609; 1978.

[2.15] Knight, R.J. The use of complex plane plots in studying the electrical response of

rocks. J. Geomag. Geoelectr. 35: 767-776; 1983.

[2.16] Lockner, D.A.; Byerlee, J.D. Complex resistivity measurements of confined rock. J. Geophys. Res. 90: 7837-7847; 1985.

[2.17] Madden, T.R.; Cantwell, T. Induced polarization, A review, in Mining Geophysics,

Vol. 2, A.W. Musgrove, ed., Soc. Explor. Geophys., 373-400; 1967.

[2.18] Davidson, D.W.; Cole, R.H. Dielectric relaxation in glycerol, propylene glycol, and

n-propanol. J. Chem. Phys. 29: 1484-1490; 1951.

[2.19] Williams, G.; Watts, D.C. Non-symmetrical dielectric relaxation behavior arising

from a simple empirical decay function. Trans. Faraday. Soc. 66: 80-85; 1970.

[2.20] Jonscher, A.K. The interpretation of non-ideal dielectric admittance and impedance

diagrams. Phys. Stat. Solutions (a), 32: 665-676; 1975.

[2.21] Jonscher, A.K. The universal dielectric response, a review of data and their new

interpretation. Chelsea Dielectrics Group, University of London; 1979.

[2.22] Lane, J.; Saxton, J. Dielectric dispersion in pure polar liquids at very high radio

frequencies, HI. The effect of electrolytes in solution. Proc., Roy. Soc. 214(A): 531-

545; 1952.

[2.23] Hasted, J.B. Liquid water: dielectric properties - Chapter 7 in Water—

comprehensive treatise, I, the physics and chemistry of water. New York: Plenum

Press; 1972.

[2.24] Klein, L.A.; Swift, C.T. An improved model for the dielectric constant of sea water

at microwave frequencies. IEEE Trans. Antennas Propag. AP-25: 104-111; 1977.

[2.25] Dobson, M.C.; Ulaby, F.T.; Hallikainen, M.T.; El-Rayes, M.A. Microwave dielectric

behavior of wet soil. Part II: dielectric mixing models. IEEE Trans. Geosci. Remote

Sensing. GE-23(1): 35-46; 1985.

[2.26] Jedlicka, R.P. Saline soil dielectric measurements. M.S. Thesis, New Mexico State

University, NM; 1978.

2-12
[2.27] Stogryn, A. Equations for calculating the dielectric constant of saline water. IEEE

Trans. Microwave Theory Tech. MTT-19: 733-736; 1971.

[2.28] Wharton, R.P.; Hazen, G.A.; Rau, R.N.; Best, D.L. Electromagnetic propagation

logging: advances in technique and interpretation. SPE of AIME Annual Technical

Conference and Exhibition (Paper SPE 9267), 21-24; September 1980.

[2.29] Ho, W.; Hall, W.F. Measurements of the dielectric properties of sea water and NaCl

solutions at 2.65 GHz. J. Geophys. Res. 78: 6301-6315; 1973.

[2.30] Ho, W.W.; Love, A.W.; Valle, J.J. Measurements of the dielectric properties of sea water at 1.43 GHz. NASA Contractor Report CR-2458. NASA Langley Research

Center, Langley, VA; 1974.

[2.31] Debye, P. and Huckel, E.Z., Z. Phys. 24: 133, 305; 1923.

[2.32] Pottel, R. Water: A comprehensive treatise, Vol. II, Ed. F. Franks, Plenum Press,

New York-London, ch. 17; 1973.

[2.33] Collie, C.H., Ritson, D.M. and Hasted, J.B. J. Chem. Phys., 16(1); 1948.

2-13
DIELECTRIC

CONSTANT

OR

CONDUCTIVITY

Figure 2 .

Dielectric dispersion for various types of polarization.

2-14
f (GHz)

Figure 2.2 Frequency variation of and c” for pure water at 25 ®C.

Debye equation predicts linear 45" fall-off which appears,

above, as the dashed line.

2-15
Figure 2.3 Cole-Cole diagram for the dielectric constant of

pure water at O^C.

2-16
Figure 2.4 Measured concentration dependence of dc dielectric

constants for three salts in highly concentrated

water solution.

2-17
Q.J < [ I

1 i

I L

0 1 2 3 4 5 6 7

c (mol/kgH20 )

Figure 2 . 5 Measured concentration dependence of relaxation

times of sodium chloride-water solution at 25 °C.

2-18
ivpue

(

)

Figure 2.6 Depression of dielectric constant and relaxation

time of sodium chloride solutions as a function of

ionic concentration. • = 0“C; o = 20°C; x = 40'’C.

2-19
0.4

Figure 2.7

WATER SATURATION,

Typical power- law exponent variation of dielectric constant as

a function of water saturation for a consolidated sandstone.

2-20
LINEAR, NEARLY CONSTANT BEHAVIOR

Figure 2.8 Dielectric constant as a function of water saturation for

typical sandstone plotted parametrically as a function of

frequency.

2-21
Ti02

3 (HEMATITE)

Figure 2.9 Composition diagram of natural magnetic minerals.
SUSCEPTIBILITY/4

0.12

0.08

0.04

0.00 I 0.0

(MAGNETITE)

0.2 0.4 0.6

FRACTION Fe2Ti04

0.8 1.0

(ULVOSPINEL

Figure 2.10 Magnetic susceptibility versus fractional composition of

magnetite -ulvospinel solid solution.

2-23
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’'JA-j

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iVrN } T . f \ D,

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ti?

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'r.':i:
ill Critical Performance Factors

A Introduction

Parameters affecting performance of portable electromagnetic mine detection systems may

be divided into three main categories. In the first category are those factors that are not directly

related to electromagnetic waves. Some of these parameters are cost, weight, battery life,

reliability, ease of operation, and ease of repair. Any attempt to assign an overall figure of merit

to a detector system should include weighting factors for this category of parameters, but this

task is not within the expertise of NIST personnel and is best left to the Army.

Of the parameters which are directly related to electromagnetic waves, we have chosen to

place all of them into two additional categories. The first category discussed in this chapter

contains those parameters which we have chosen to call “detector-based.” The second category

contains those parameters which we call “target-based.” The category of detector-based

parameters contains all parametersfound above the air!earth interface such as atmospheric

noise; detector noise levels and dynamic range; and sensor height, velocity, and tilt angle. The

category of target-based parameters contains all parametersfound at or below the air-earth

interface such as target-soil permittivity, conductivity, and permeability; target-soil contrast

ratio; target depth, size, and shape; and ground clutter. What follows in this chapter is a listing

and some discussion of all of the detector-based and target-based parameters that NIST personnel

have been able to identify. Chapter Vn contains detailed descriptions of NIST-suggested

performance tests that address a selected set of these parameters, along with a suggested

methodology for analysis of the test results.

B Detector-Based Performance Factors

B.1 Noise Factor of Mine Detectors

If there were no noise at the receiver input and none were generated by the receiver,

sufficient amplification would result in the detection of any signal, even for a small contrast

between the electromagnetic properties of the land mine and host soil. In reality, noise is always

present; consequently, amplification of a signal results in amplification of noise as well.

Furthermore, the minimum detectable signal power in the receiver is limited by noise which can

be attributed to the following internal and external sources: (a) equipment noise (amplifier or

sensor noise); (b) cosmic and atmospheric noise picked up by antenna; (c) man-made

interference (machinery, other radio transmitters); and (d) ground clutter. If these noise sources

3-1
were the only limiting factors affecting mine detection, then a suitable requirement would be that

the secondary electromagnetic field due to the presence of a land mine would not be exceeded by

these noise sources.

B. 1. 1 Ideal System Noise Factor

The system noise factor of an ideal receiver may be simply defined as a measure of the

degradation of the signal-to-noise ratio as the signal passes through the receiver (which depends

on the design of the input stages and frequency of operation of the receiver). The system noise

factor, SNFy of a mine detector is defined, therefore, as the power ratio of (5/A0/„ to

5NF = 10 logic

iS/N),^ • (3.1)

For an ideal receiver (S/N),„ = so SNF = 0 dB. Equation (3.1) is a measurable

quantity for evaluating mine detector performance and, in general, depends on both operating

frequency and another measurable quantity of the system, its dynamic range.

B.1.2 Overall Operating Noise Factor

For a general receiving system free from the spurious effects of ground clutter, the overall

operating system noise factor [3. 1-3.3] is given by f = fa

or

/ = /. + (^c-l)

(Tr] (tA

+ 4(/,-l)

where/a = the external noise factor defined as L=Pj^ob = TJTc,

(3.2)

(3.3)

and= noise power available from an equivalent loss-free antenna, in W,

K = Boltzmann’s constant = 1.38 x 10“^ J/K,

Tq = reference temperature, taken as 288 K,

b = effective noise power bandwidth of the receiving system, Hz, = effective antenna temperature in presence of external noise, K, = antenna circuit loss (available input power/available output power),

Tc = actual temperature, K, of the antenna system and nearby ground, = transmission line loss (available input power/available output power).

3-2
Tf = actual temperature, K, of the transmission line, = noise factor of the receiver.

fi = internal system noise factor

Fa = the external noise figure defined as

Fa=mOgfa{dBW),

and F^ = the noise figure of receiver defined as

F^ = mogf,{dBW).

If 7c = = 7^, eq (3.2) becomes

/ = /a-l + /c/,/<i.

where

^ 'T \

_£

7

is the noise factor associated with the antenna circuit losses and

(3.4)

(3.5)

/, = l + (f,-l)

^ 'r \

j_C

\To j

is the noise factor associated with transmission line losses. For no antenna circuit or

transmission line losses, /is given by

(3.6)

(3.7)

Internal system noise can be closely controlled by the manufacturer for most mine detection

systems. External noise sources are less controllable; of these, atmospheric and ground clutter

are the least controllable.

B.2 Sensitivity to Atmospheric Noise

Spaulding [3.3] has given the minimum and maximum external noise figure due to

expected atmospheric noise as a function of frequency (from 0.1 Hz to 10® Hz). These are shown in figs. 3.1 and 3.2, and take into account the entire earth’s surface, all seasons, and times

of day. In the frequency range 0.1 Hz to 10 kHz (that of most operating metal mine detection

3-3
units), there is very little seasonal, diurnal, or geographic variation. The variability in the 100 to

120 kHz range is principally due to the variability of the earth-ionosphere waveguide cutoff.

Relation (3.3) can be written

P„ = F^ + 5-204dfiiy, (3.8)

where = 10 log /?„, 5 = 10 logZ?, and 10 log KT^ = -204. Lauber [3.4] has given the expression

for the vertical component of the rms* electric field strength for a half-wave dipole in free space:

En=Fa + 20 log +5-99.0 dB{^VIm). (3.9)

If we use a conversion factor of 1 jiV/m = 33 pT for relating electric field strengths to

orthogonal magnetic field strengths and note that figs. 3.1 and 3.2 may be used for vertical as well as horizontal magnetic field strength noise levels at any frequency and any effective antenna

temperature, an approximate value for the magnetic field strength atmospheric noise is given by

H^=Q.\nT-Jb.

(310)

Cohn [3.5] has used this nominal value for //„ to plot primary coil ampere-tums-per-coil

diameter required to achieve a specified detection range for a normalized target size of 1 when

external atmospheric noise is the only limiting factor. An operating metal detector frequency of

2.5 kHz is close to a local minimum of the external noise figure shown in figure 3.1 (127 dB),

which takes into account the entire surface of the earth, all seasons, and all times of day.

Therefore, it is an optimal operating firequency with respect to noise figiu^s based on

atmospheric sources. Figures 3.1 and 3.2 may be used in specifying the external atmospheric

noise figure for any operating frequency and effective antenna temperature.

A report [3.6] on the noise factor for mine detection systems where signals due to ground

clutter are specifically excluded has been written. In this paper, an example is taken to show how

analytic work may be used with external atmospheric noise measurements to predict whether a

given system operating at a specified frequency and effective temperature would allow detection

of metal mine targets of specified sizes. In another report [3.7], Geyer has pointed out that there

are optimal operating frequencies that maximize the quadrature-induced multipole moments of

buried metallic spheres for any permeability and conductivity contrast between the sphere and

host soil and for a buried sphere with any diameter. Thus, there is a means in metal-mine

detection to choose an operating frequency that maximizes the quadrature response for a given

size mine target. Together with the results described here, signal-to-noise ratios could then be

3-4
determined to predict whether that size target, at a given depth of burial, would yield a threshold

detection signal (signal exceeding overall operating noise factor by at least 2 or 3 dB).

B.3 Detector Dynamic Range

One of the most important measures to be considered in the evaluation of mine detection

systems is the mine detection probability that can be ascribed to a given system when the

electromagnetic characteristics of the test target and background soil are specified. The detection

probability is defined as the probability that the (target) signal, if present, will be detected. Given

the electromagnetic characteristics of metallic (or plastic) test standards and background soil(s),

as well as the overall operating noise factor of the detection system, a threshold detectability

level may be ascertained in principle for any burial depth and size of the test standard.

The function of a mine detection system is to determine the presence or absence of a buried

mine. The maximum sensitivity of any system is governed by the threshold of detection. The

threshold in turn is set at a level based on a tolerable false alarm rate. A false alarm results from

an occasional level of noise exceeding the system threshold. Since system sensitivity is of prime

importance, it is necessary to examine the relationship among noise (for the types of noise

previously categorized), bandwidth, threshold, and false alarm rate.

Threshold and signal-to-noise requirements for mine detection are reviewed by Geyer [3.6].

The probability of a false alarm can be ascertained once a tolerable false alarm rate and the

bandwidth of the detection system are determined. From the estimated false alarm probability

and noise variance, a detection threshold may be simply calculated. Finally, minimum

acceptable signal-to-noise ratios for any probability of successful detection and any false alarm

probability can then be evaluated. Any mine detection system having a high threshold of

detection (or low sensitivity) would be given a low figure of merit. Conversely, any detection

system that has high sensitivity, or at least variable threshold levels of detection, would be given

a higher performance rating. High sensitivity for variable detection thresholds usually means

that the system intrinsically has greater dynamic range.

A minimal detection requirement is that the signal output from measured electromagnetic

fields perturbed by a land mine (for systems with earphones) be at least 2 or 3 dB above the

overall operating noise factor of the system in order to be resolvable. Systems that depend on the

normal hearing threshold (approximately a rms sound pressure of 2 x 10 N/m at 1 kHz), as well as the smallest discemable change in audible level (2 or 3 dB), are therefore inherently

limited in sensitivity. This is a critical systems engineering requirement that should be

addressed.

Ensuring signal levels that are at least 2 or 3 dB over the overall operating noise factor of

the system (defining signal as that due to the presence of a mine) may or may not always be

3-5
possible, even when the mine is electromagnetically visible. Such assurance will demand a thorough broadband analysis of the ground clutter problem, since system internal noise can be

controlled and since system engineering considerations can be used to overcome external

atmospheric noise sources.

B.4 Sensitivity to Detector Height above Earth’s Surface and Pattern Signatures

Smith [3.8] has investigated the directive transmission characteristics of horizontal electric

and magnetic dipoles over a dielectric half-space by expressing the field of a general antenna

over that half-space as a spectrum of plane waves [3.9]. He then asymptotically evaluates the

integrals representing the far field so as to define pattern function, gain, and directivity of the

horizontal electric or magnetic dipole antenna as a function of height above the earth.

More recently. Hill [3.10] has considered the specific case of oppositely directed horizontal

electric dipole antennas, such as are found in the AN/PRS-7 plastic mine detector (see figure

3.3). A null always exists for the AN/PRS-7 in the directions normal to the air-earth interface

from the detector head (midway between the oppositely directed dipole antennas)—in the

absence of a lateral change in complex permittivity.

We can, however, examine the effect of antenna height on the field for fixed observation

points either in the earth or in the air. This is illustrated in figs. 3.4 and 3.5 which clearly show

the desirable and expected increase in subsurface fields (and decrease in the fields in the air), as

the antenna is brought closer to the earth’s surface. Generally, electromagnetic field

enhancement in the earth will always occur as the source antenna is brought closer to the airearth interface, regardless of the type of source excitation or antenna configuration. The actual

enhancement will depend on the specific antenna.

Hill [3.1 1] has also shown the far-field H-plane multiple-lobe pattern that can occur over a

uniform half-space having high dielectric contrast, e'/So = 80 (see figure 3.6). For 1 < ^7^o ^ 4,

this complex pattern does not occur. This H-plane multiple-lobe pattern (more nulls than at

0 = 0°, 90°, and 180°) is undesirable in any plastic mine detection system and should be avoided,

since it would be difficult to distinguish several closely spaced targets fi'om single buried ones. Any mine detector that has a multiple-lobe signature pattern (more than two side lobes) for the

maximum expected ratio of e'jeQ should be ascribed a lower figure of merit.

The variations in sensor head height over a given sweep time (which are likely to occur in

field operations) could lead to signals greater than what might be expected from a buried weak

scattering target (such as a plastic mine in dry sand). One seemingly simple systems approach

for handling this problem would be to incorporate within the detection head an ultrasonic or

infrared range finder that would allow feedback system signal corrections to be made in real time

in accordance with theoretical near-field analysis [3.1 1].

3-6
B.5 Sensitivity to Detector Tilt Angle

Depending on a particular detector’s antenna configuration and electromagnetic field

radiation pattern, the angle between the detector and the earth may be an important parameter.

Some configurations might be very sensitive to tilt angle, so that even small deviations from

horizontal would seriously degrade the detector’s ability to detect targets.

B.6 Sensitivity to Detector Velocity

Depending on a particular detector’s antenna configuration, electromagnetic field radiation

pattern, and detection electronics circuitry design, the velocity of the detector with respect to the

earth may be an important parameter. Again, some configurations might be very sensitive to

small deviations from optimal velocity (as specified by the manufacturer) such that the detector’s

ability to detect targets would be seriously degraded. Inability to respond to rapidly changing

signals would severely limit search speed and system utility.

B.7 Sensitivity to Detector Spatial Positioning

Every detector will possess some spatial positioning “sweet spot” through which the

detector must pass in order to successfully detect targets. One way to define this spatial

positioning problem is to move the detector along lines which have increasing perpendicular

distances from the mass center point of the target. For all detectors, there is some distance so

defined beyond which the target will not be detected. Also, depending on the rotational

orientation of the detector with respect to the axis perpendicular to the earth, this maximum

distance from the target may not be constant. As with the detector velocity case mentioned

• above, the dimensions of this sweet spot will be a function primarily of the detector’s antenna

configuration, radiated electromagnetic field pattern, and detector electronics circuit design. The

effects of variations in this parameter are that some detectors would require very precise

positioning of the sensor in order to detect targets while others would be more forgiving.

B.8 Sensitivity to Other Detector- Based Parameters

A large number of other detector-based parameters might play an important role in

determining the detector’s overall figure of merit. For example, a bistatic detector system, with

separate transmit and receive antennas, would require the consideration of antenna spacing.

Another example would involve the “intelligence” of the detector system. “Smart” detectors,

with otherwise similar designs to “dumb” detectors, could far outperform the latter due to their

signal processing, image processing or false-target-rejection capabilities. As will be discussed in

chapter VII, NIST recommends that the Army avoid attempting to perform detector-based tests

3-7
whenever possible, and instead, require the detector manufacturer to supply this information if so

desired.

C Target-Based Performance Factors

C.1 Sensitivity to Specified Complex Permittivity Contrasts

One of the important parameters needed in evaluating the performance of any land mine

detection system is that system’s sensitivity to specified complex permittivity contrasts. Because

mine detectors may have differing operating frequencies or be broadband operationally, it is

important to understand dielectric dispersion in test soils so as to be cognizant of various

dielectric relaxation processes affecting the validation testing. It is also important to recognize

these processes so that standardized test conditions may be properly controlled and so that mine

detector performance evaluations are properly made (that is, mine detectors correctly compared)

in the light of actual measured electrical contrasts.

C.2 Sensitivity to Target Size

Previous analytic studies performed for BRDEC [3.12, 3.13] have demonstrated that

anomalous signals generated by a buried land mine, whether metal or plastic, are proportional to

target volume in the frequency range used by most detection systems. Most continuous wave

(cw) mine detection systems currently available operate in the Rayleigh scattering region insofar

as mine detection is concerned; that is, the electrical size of the target mine is small compared to

the wavelength in the surrounding medium. Hence, shape information, which could lead to a

reduction in the false alarm rate, cannot be directly ascertained from signals measured by most

mine detectors.

Hill [3.13] has used the Bom approximation to derive the plane wave scattering matrix

[3.9] for objects of low dielectric contrast. He has also considered the shape dependence of

scattered (plane wave) electromagnetic fields for the cases of a sphere, a cube, and a cylinder

whose electrical properties simulated a plastic nylon land mine. These shapes bracket all those

likely to be encountered in land mines. Although some shape dependence in the scattered field

was determined (see figure 3.7), it would be difficult to determine and use in the field as a basis

for discriminating against unwanted ground clutter signals without a prior knowledge of the

target volume itself—even with multiple look-angles of the target with spatially variable

transmitter-receiver configurations.

For this reason, we have submitted copper right-circular cylinders that might be used as

standards for metallic test objects and both nylon and Teflon spheres that might be used as

plastic test objects. These standards can be used in a discriminatory fashion in mine detector

3-8
validation tests by parametrically changing the target volume for a given burial depth and hosttarget electrical property contrast. If a mine detection system is unable to detect the largest

standard, it will be unable to detect smaller standards (all other factors being equal). Similarly, if

a mine detector cannot sense the presence of a buried land mine where the electrical property

contrast is high, it surely will not sense a mine when the contrast is low (all other factors being

equal).

C.3 Sensitivity to Burial Depth

One of the more important parameters to be considered in assigning a figure of merit to any

mine detection system is its detection sensitivity to a prescribed burial depth of a given land

mine. The functional dependence with depth of plane electromagnetic waves traveling in a

uniform, isotropic earth is exponential, that is, proportional to (+z is positive downward), or

=

^-{y2){nfnG)z^-j{y2){nfnG)z

( 3. 1)

where /x is the (real) magnetic permeability, a is the (real) conductivity, and / is the operating

frequency. The well-known skin depth, that is, the distance within the earth at which the

amplitudes of the electric and magnetic field vectors are attenuated to 1/e = 0.3679 of their

respective values at the surface, is

/

"skin depth

\V2

Ttfua) Re(r)

, <T» (oe. (3.12)

The plane-wave skin depth gives an effective depth of penetration and can be used as a rule

of thumb for effective search depths of a mine detector in conductive soils, even though field

excitation is not plane wave.

For a nominal ground conductivity of 0.1 S/m and magnetic permeability approximately

equal to that of fi*ee space (4;r x 10“^ H/m), we see that the skin depth is roughly 30 m at 2500

Hz. Hence, highly conducting soils would not be expected to inhibit search depths of most metal

mine detectors. However, the effective search depth of a plastic land mine detection unit

operating at 250 MHz in the same conductive soil environment would only be about 0.1 m, a

depth which is less than the maximum probable depth of mine burial (0.3 m). Effective search

depths should at least be 0.3 m in the worst case likely to be encountered—that of an electrically

lossy environment—for all cw mine detection systems. The easiest way to control this is to

lower the operating frequency.

Dispersion is evident whenever the host medium is lossy. Therefore, two mine detection

units operating at widely different frequencies would sense differing electrical property contrasts.

3-9
all other factors being equal. It is important to be able to predict when favorable electrical

contrast between the mine and the background medium can be expected. Such information

allows proper comparisons among various mine detection systems to be made.

Although the expected dielectric contrast and hence visibility of a (plastic or metallic) land

mine increases with decreasing frequency (which would increase the detection range

performance), spatial resolution and shape definition do not. In this regard, Cohn [3.6] has

considered the quasistatic effects of variation in height of the search head on measured signals

and detection range, assuming that the attendant signal variations (due to conductive or

magnetically permeable ground) remain uncancelled. It would be desirable, if practical, to have

as broadband a detection response as possible under the attenuation constraints imposed by a lossy background test medium.

It is advisable to make sensitivity tests of land mine burial depth in a conductive host soil at

the maximum probable depth of mine burial. If any mine detection system is unable to sense a

given mine at the surface (or in free space), it may be unable to do so when buried in a lossy

medium.

C.4 Sensitivity to Target Shape

Anomalous signals generated by a buried land mine, whether metal or plastic, are proportional to target volume in the frequency range used by most detection systems. A means

for discriminating among different target shapes (target classification) is vital to reducing the

false alarm rate at any given detection threshold and for increasing the detection probability.

Such a discrimination capability is closely akin to the susceptibility of that system to unwanted

ground clutter in the external noise factor. Any mine detection system having shape

discrimination capability would, all other factors being equal, command a higher figure of merit.

Target classification may be viewed as a crude form of inverse scattering that determines

which of many body shapes is most consistent with the measured scattered field data. The

scattered electromagnetic field produced by either a conducting or dielectric body in the presence

of an incident wave is described by Maxwell’s equations. Since these fields are determined by

the relative positions of the constituent parts of the scattering body surface (the boundary

condition), it might be expected that, in general, the characteristics of the scattered field would be

different for different object geometries. The most desirable mine classification scheme would

produce an image displaying the shape and size of the object being illuminated. Such a detection

system would effectively solve the inverse scattering problem, which is determining the

boundary conditions from the received waveform.

As expected, there are preferred frequencies for indicating the shape and size of a target. In

the theory of scattering, three roughly defined frequency regions are of interest: the Rayleigh,

3-10
the resonance, and the optical regions, where the wavelength of the incident field is much larger,

of the same order, or much smaller than the size of the target, respectively. With illumination in

the Rayleigh region, the scattered field describes the volume of the target [3.14]. The character

of the backscattered field in the optical region, on the other hand, is indicative of the principal

curvatures of the target’s specular points. Dlumination in the resonance region gives rise to

surface currents that circulate on the body surface of the target (into what would be the optical

shadow areas) and re-radiate. Resonance region illumination is generally considered most

efficacious for target shape and size determination using multiple frequency measurements.

Chan and Peters [3.15], in an elegant attempt to characterize the effects of buried cultural

debris and to distinguish such effects from the expected signals of buried land mines, have

examined some resonant phenomena of targets using broadband (short-pulse ground radar)

detection systems. Preliminary results suggest that schemes either to eliminate or to process

ground clutter will rely on broadband detection systems. The principal limitations in deducing

shape information from resonance phenomena are the attenuation constraints imposed by the

lossy background test medium when a validation test is performed at the maximum expected

depth of burial (0.3 m).

In conclusion, shape discrimination is one of the more important attributes that could give

future (broadband) mine detection systems a high figure of merit in validation tests. Because few

of the land mine detection systems presently available have demonstrated capability for shape

discrimination, however, it will probably have a low weight in any performance effectiveness

testing scheme that BRDEC decides upon.

C.5 Sensitivity to Ground Clutter

While it is possible to evaluate the performance of any mine detection system under

controlled circumstances having minimal or no ground clutter, it is more difficult to predict the

operation of that system on the battlefield. A mine detection system can be tested and shown to

have the highest overall figure of merit based on weighted experimental parameters discussed to

this point—but could fail to detect a mine in the field where the spurious signals due to ground

clutter are high.

Ground clutter includes the natural effects of an inhomogeneous and/or dispersive host

medium in which the mine target resides, as well as effects of surface roughness. Other types of

ground clutter could stem from surface or buried cultural debris such as spent shells or artillery

(see figure 3.8).

Unwanted signals due to ground clutter can be the most important contributor to the overall

operating noise factor. This source of noise, which can give large numbers of nuisance or false

alarms, is currently difficult to quantify in a meaningful way. Only if there is spatial or temporal

3-11
resolvability of the signal in the presence of clutter can there be hope of taking fruitful measures

to minimize the effects of interfering signals on target signals. Furthermore, it would be useful to

know whether, if any mine detection system incorporated an optimally matched filter as part of

its system design, spectral resolvability exists between the mine target signal S(co) and unwanted

noise signals N((o)—and, if so, under what conditions. Thus it would be desirable to have a

generalized solution to the model illustrated in figure 3.8 which (1) is broadband (not frequency

limited); (2) incorporates ground clutter; (3) allows spatially variant and dispersive /i//(r;cu),

£//(r;G)); and (4) can be applied to weak or strong scattering targets.

Such an analysis would provide insight into system design specifications and lead, in

general, to the ultimate goal—better detection capability. It is difficult to conceive how present

mine detection systems undergoing test validation would perform better in a cluttered

environment than in highly controlled, standardized test environments. Consequently, the

BRDEC approach of highly controlled validation testing is probably sufficient for comparative

performance evaluations of existing detection systems. Geyer [3.7] has outlined an exact

deterministic approach to the analysis of ground clutter for an arbitrary dispersive and spatially

variant background medium and for an arbitrary incident source field.

One approach to attacking the ground clutter problem and to increasing the detection

probability is to know the (near-field) frequency spectrum S(co) (or averaged frequency spectrum

<S(G))>) of the input target (land mine) signal. Once 5(g)) is given for an ensemble of specified

targets, it is not only possible to incorporate adaptive thresholding, but also to design an

optimally matched filter transfer function //(g)) for a mine detection system operating in the

presence of white Gaussian noise; that is,

//(G)) = 5* (G)), (3.13)

where 5(G)) is the frequency spectrum of the input (target) signal s(t) and 5 (g)) is the complex

conjugate of 5(g)). A mine detection device that has a system response given by eq (3.13) is the

optimal detection system in the sense of maximizing signals relative to average noise power. We

could therefore place a higher figure of merit on a mine detector configured such that the output

response 0(g)) for a given test standard 5(g)) is given by

0(g)) = 5(g))5*(g)). (3.14)

Ground clutter may or may not be white Gaussian in character. If not, it would be possible

to design a Wiener-Hopf filter, optimal in the least squares sense; that is.

3-12
» (3.15)

when noise is uncorrelated with the desired signal output and where and 0;y(ct)) are the

signal and noise power spectra, respectively. Future detection systems may well incorporate a

Wiener-Hopf filter. Such incorporation will await more definitive work on characterizing

C.6 Sensitivity to Target Orientation

Detectors may have varying sensitivities to the orientation in the earth of otherwise

identical targets. In one orientation the detector may sense the target easily, while in another

orientation the detector may fail to detect it. Probably the most important characteristic of a

detector that would affect this sensitivity is the polarization of the sensor’s electromagnetic field.

For example, the magnitude of eddy cuirents produced by the sensor’s exciting field in a target’s

metal firing pin will depend strongly on the orientation of the pin with respect to the polarization

of the field.

C.7 Sensitivity to Soil Stratification

This parameter sensitivity may be considered a special case of ground clutter. If the

electromagnetic properties of the soil vary as a function of depth, then it is very possible that the

performance of a given detector could be seriously degraded when compared to its performance

under uniform soil conditions. Under extreme stratification conditions, a detector could

effectively be blinded to targets buried below a particular layer of soil, similar to the blinding of

shipboard sonar systems due to different thermal layers in the sea.

C.8 Sensitivity to Target Resolution

The ability of a detector to resolve closely spaced targets could, under certain conditions,

become very important. For example, a large vehicle mine could be buried close to a small

antipersonnel mine. If only the large mine were to be detected, personnel could be fooled into

attempting to disable it with disastrous results to those personnel from the accidental detonation

of the small mine.

References (III)

[3.1] Barsis, A. P.; Norton, K. A.; Rice, P. L.; Elder, P. H. Performance predictions for

single tropospheric communication links and for several links in tandem. Nat. Bur.

Stand. (U.S.) Tech. Note 102; 1961.

3-13
[3.2] International Radio Consultative Committee. Characteristics and applications of

atmospheric radio noise data; Report No. 3220-2; 1983.

[3.3] Spaulding, D. A. International Radio Consultative Committee. Worldwide minimum

external noise levels, 0.1 Hz to 100 GHz; Report No. 670; 1982.

[3.4] Lauber, N. R. Preliminary urban UHFA^HF radio noise intensity measurements in

Ottawa, Canada. Proc., 2nd Symp. on EMC; Montreux, Switzerland; 28-30 June,

1977.

[3.5] Cohn, G. I. Low frequency electromagnetic induction techniques; Tech. Essays,

Army Countermine Study Report; 1973.

[3.6] Geyer, R. G. Noise factor for mine detection systems: A performance measure;

Report to U.S. Army Belvoir Research and Development Center, Fort, Belvoir; 1987.

[3.7] Geyer, R. G. Suggested experimental parameters for land-mine detection

performance evaluations; Report to U.S. Army Belvoir Research and Development

Center, Fort Belvoir; 1987.

[3.8] Smith, G. S. Directive properties of antennas for transmission into a material halfspace. IEEE Trans. Ant. Prop. AP-32(3): 232-246; 1984.

[3.9] Kerns, D. M. Plane-wave scattering-matrix theory of antennas and antenna-antenna

interactions; Nat. Bur. Stand. (U.S.) Monogr. 162; 1981.

[3.10] Hill, D. A. Near-field detection of buried dielectric objects, IEEE Trans. Geosci.

Remote Sensing; GE-27(4): 364-368; July 1989.

[3.1 1] Hill, D. A. Fields of horizontal currents located above the Earth. IEEE Trans.

Geosci. Remote Sensing; GE-276(6): 726-732; November 1988.

[3.12] Hill, D. A.; Cavcey, K. H. Coupling between two antennas separated by a planar

interface. IEEE Trans. Geosci. Remote Sensing; GE-25(4): 422-431; July 1987.

[3.13] Hill, D. A. Electromagnetic scattering by buried objects of low contrast. IEEE Trans.

Geosci. Remote Sensing; GE-26(2): 195-203; March 1988.

[3.14] Crispin, J. W.; Siegel, K. M. Methods of radar cross-section analysis. New York,

NY: Academic Press; 1968.

[3.15] Chan, L. C.; Peters, L., Jr. Subsurface electromagnetic mine detection and

identification; Technical Report, Department of the Army, U.S. Army Mobility

Equipment Research and Development Command, Fort Belvoir, VA; 76. p.; 1978.

3-14
•MIcroputsatfons

300

QQ

•D

<0

Frequency (Hz)

Figure 3.1 Minimum (B) and maximum (C) atmospheric noise

figure vs frequency.

3-15
Figure 3.2 Minimum (B) and maximum (C) atmospheric noise

figure vs frequency.

3-16
y

Figure 3.3 Oppositely directed dipoles at height h above

the earth.

3-17
'Xn

Figure 3.4 Normalized electric field at various heights in air

for a fixed observation point in the earth.

3-18
Figure 3.5 Normalized electric field at various heights in air

for a fixed observation point in the air.

3-19
Figure 3.6 H-plane pattern of oppositely directed dipoles

located at air/earth interface for various earth

permittivities

3-20
(r/X)

|e//EoI

6

Figure 3.7 Scattered far-field for a cube (side = O.lX)/ a

circular cylinder (height = radius = 0.1084 X), and

a sphere (radius = 0.1241 X) for normal incidence,

TE polarization.

3-21
Figure 3 .

Sources of ground clutter.

3-22
IV Measurement Methods and Systems for

Electromagnetic Properties of Materials

A Introduction

The measurement of mine detector performance generally requires that certain specified

tests be carried out with test targets buried in a carefully controlled environment or test lane. For

tests of plastic-object detectors that use UHF or microwave frequencies, the returned signals are very much weaker than the transmitted signals. This makes it imperative that the test procedures

be designed so that we can obtain consistently similar results for the returns from a given

combination of test target and background medium. Unfortunately, test conditions vary over

time—especially if the test lane contains damp soil. In this section of the report, we briefly

review various laboratory methods for determining the electromagnetic properties of target and

background materials. We then describe methods for simulating the electromagnetic properties

of damp, sandy soils with dry materials. We also describe methods for accurately determining

the electromagnetic properties of such dry materials and for the long-term monitoring of the

properties of test lanes composed of such materials.

Dielectric measurement techniques have a long history [4. 1-4.3] and will be briefly

mentioned here. Afsar, Birch, and Clarke [4.4] give a comprehensive review of techniques

developed over the past two decades to measure dielectric properties of materials in the

frequency range 1 MHz to 1500 GHz.

Frequency-domain laboratory techniques often employ propagation measurements in a

waveguide transmission line containing the dielectric. Measurements of the input admittance of

a shielded open-circuited coaxial line filled with the material are common [4.5]. Depending on

the frequency range of interest, the admittance at the plane of the sample holder connector is

measured by a capacitance bridge (kilohertz range), Schering or twin-T bridges [4.6] (up to 200

MHz), or an Automatic Network Analyzer (ANA) (0.5 MHz to 18 GHz). These admittances are then converted by suitable formulas into complex permittivity, loss tangent, and so on. Closed

cavity resonator techniques are also used for permittivity measurements in the microwave region

[4.7-4. 1 1]. As the frequency range of interest becomes still higher (greater than 20 GHz) open

cavity resonator techniques are used [4.12-4.21], since at the higher frequencies the open

resonator can be made fairly small and the high-Q values required for the measurement of low

dielectric loss are easily attained.

Time-domain laboratory methods for dielectric spectroscopy are also undergoing rapid

development [4.22^.28]. These methods typically make use of a reflected wave off a length of

4-1
air line, part of which is filled with the material to be tested. The complex permittivity is

determined from the measured reflection coefficients.

B Laboratory Sample-Holder Measurements of the Electromagnetic Properties

of Materiais

B.1 Dielectric Measurements Using Terminated Coaxial Lines

Bussey [4.5] and, more recently, Jesch [4.29] have made measurements of the dielectric

properties of materials using open, coaxial sample holders. The design of the sample holders,

fabricated from precise, 14 mm, 50 Q coaxial air line and equipped with a 900-BT precision

coaxial connector, is shown in figure 4.1. Essentially, soil conductivity and permittivity are determined by the construction of a section of coaxial transmission line, as illustrated in

figure 4.1, with the soil serving as the lossy dielectric environment for the electromagnetic fields

produced within the transmission line.

The impedance at the input end of a transmission line, having a characteristic impedance Zq

and termination impedance z^, is given by

z^ + zptanhy^

^ ZQ + ZgXanhyt

(4.1)

where y= a +jpis the complex propagation constant of the transmission line, a and j3 are the

respective attenuation and phase constants of the transmission line, and i is the length of the

transmission line. When the termination impedance is 0 (short circuit at the termination), eq

(4.1) becomes

^in, short circuit ~ ^0 ^anh (4.2)

whereas, when the termination is an open circuit we have

Zin,^«nc.>cuir=Zo(tanhy«r‘. (4.3)

Dividing the short circuit input impedance by the open circuit input impedance yields

^‘-skor'circui,

^in, open circuit

For simplicity of notation in the following four equations, let

4-2
A =

^in, short circuit

in, open circuit

Then,

y= — tanh

i

or

1 ,

l + ^^A

y - — r= ^ It 1-Va

It

1 + Va 1 (

4_ \-4a 2^

. 1 + Va * ^ phase 7= + 2rm

1-Va

n = 0, ±1, ±2, Hence the attenuation constant a of the transmission line is

1 , a = —In

It

1 + Va

\-4a

(4.5)

(4.6)

and the phase constant ^ is —phase

2t

1 + Va nj:

l-yfA I

The characteristic impedance zq of the coaxial transmission line is

(4.7)

2o

jcop tn{b/a)

(j + jcoe 2 k

where b and a are the outer and inner diameters of the coaxial line, respectively.

From Ampere’s law,

VxH = (<7 + 7cy£)E,

and Faraday’s law.

V xE = -jcofM,

(4.8)

(4.9)

(4.10)

we see that

4-3
= jco^i(a+Jcoe), (4.11)

SO that from measurements of a and p conductivity and permittivity are easily obtained,

a{a) = 2aPI{(on), (4.12)

e(m) = {fi^ -a^ )/toV• (4.13)

The results of a series of measurements using this open-ended coaxial holder technique are shown in figure 4.2. The technique is well-suited to making measurements of the dielectric

properties of granular mixtures. Figure 4.2 shows data obtained at 600 MHz for various mixtures

of sand and silicon carbide as the percentage of sand is varied.

B.2 Permittivity and Permeability Measurements with a Two-Port Measurement

Technique

As described more fully in Appendix IV.B, the complete electromagnetic properties of

materials can be obtained over a wide range of frequencies with a two-port measurement set-up.

We made a large number of measurements using an Automatic Network Analyzer (ANA). This

system [4.30] is composed of an S-parameter test set, sweep frequency generator, and a control

computer. The system is capable of determining S-parameters over the frequency range of 45

MHz to 26.5 GHz. We made all of our measurements between 50 MHz and 1.05 GHz. Before

taking any data, calibrations were carried out with three standards: a precision short, an open

circuit, and a 50 Q matched load. These standards were all terminated in 3.5 mm connectors on

the S-parameter test set. We used a sample holder made from a 10 cm length of precision

beadless 14 mm air line. The holder was terminated with GR-900 connectors. Samples of

plastic material were machined on a lathe so that they fitted very precisely between the inner and

outer conductors of the holder. Samples of the Teflon and nylon used to make test targets were

prepared in this way. To measure the properties of a given sample, one of the GR-900

connectors was removed from the line and the machined sample was then inserted. The distance

from the end of the holder to the sample was measured with a depth gauge and recorded before

reattaching the GR-900 connector. The holder was then carefully connected to the ANA. See

figure 4.3.

Measurements of all four S-parameters were taken over the frequency range from 50 MHz

to 1.05 GHz for each material at 5 MHz intervals. This yielded 201 data points per plastic

sample. The time required typically did not exceed 15 min per plastic sample—including the

time required to store data on a floppy disk. The data are stored as nine numbers for each

4-4
frequency: four complex S-parameters (two numbers each) and the frequency (one additional

number).

All data are analyzed using a program^ written in BASIC. The program prompts the user to

input the following quantities: (1) the number of data points, (2) the length of the sample holder

in meters, (3) the length of the sample, (4) the distance from port 1 of the sample holder to the

sample, and (5) the distance from port 2 of the sample holder to the sample. The values given in

(2) to (5) above are used to rotate the electrical phases from reference planes located at the GR900 connectors to corresponding planes located at the and S22 ends of the sample located

inside the holder.

The program asks the user for a file name before retrieving the information from the disk.

Values for S21 and S 12 (after reference plane rotation) are displayed on the screen, as well as magnitude and phase differences. Since the system should be symmetrical after phase plane

rotation, these differences should be close to 0. A set of differences significantly greater than 0

indicates that one or more of the lengths entered in (2) to (5) above need adjustment, or that

various system calibrations have not been properly performed.

The user can request that either the and S21 products or the S22 and Sj2 products be

used in the calculations. We used both sets of data; accordingly, there are pairs of overlapping

curves for each of the four quantities shown in figures 3.4 and 3.5. The program calculates

values of the complex permittivity and permeability for the sample according to formulas given

in Appendix FV.B. At this point the program will, at the user’s option, print the results before

saving them on the disk. The stored values are used later to produce graphs.

B.3 Procedures for Determining EM Properties of Test Soils

Measurements of the permittivity and permeability of granular samples such as dry sand

and silicon carbide (SiC) sand mixtures are done with methods similar to those described earlier

in the section on plastic test targets. Two thin plastic washers are used to confine the material

inside the sample holder. The procedure we have adopted involves first inserting a Teflon washer

(14 mm diameter, 1.5 mm thickness) into one end of the beadless air line. A measured volume

of the sample is then poured in from the other end while gently tapping the body of the holder to

allow the sample to settle. A second 1.5 mm Teflon washer is inserted and pushed firmly into

place using a metal tube. The distance from the end of the holder to each of the Teflon washers

is then measured with a depth gauge and recorded. The GR-900 precision coaxial connectors are

reattached to the sample holder which is subsequently connected to the ANA (using GR900/SMA adaptors and precision coaxial cables). Figure 4.3 shows the measurement setup.

1 Appendix IV.A, attached.

4-5
Examples of the measurements made on dry sand and on a 60 percent/40 percent mixture of dry

sand and SiC are shown in figures 4.6 and 4.7.

Errors can be reduced by exercising care in sample preparation. The material between the

Teflon washers must be free of voids or air gaps. The washers must be machined so that they

present a flat face to the air on one side and to the particulate material on the other side. Washers

must be placed correctly in the holder with the plane of the washer perpendicular to the center

conductor. We tried to make the washers we used as thin as possible—yet with sufficient

material to maintain adequate rigidity. The washers are considered to be a part of the sample by

the analysis program; their use may introduce errors if the material under test is

electromagnetically very dissimilar. The method used for calculating permittivities and

permeabilities also requires that the sample be less than one.quarter of a wavelength long. This

limits the length of the sample which can be used or limits the frequency range which can be

analyzed for materials of high dielectric constant. Very lossy materials (for which no significant

transmission through the sample holder takes place) would also be difficult or impossible to

characterize. Finally, for solid samples, care must be taken to insure that the samples fit tightly

in the holder to minimize contact resistance.

C In Situ Methods

A number of investigators have, with varying degrees of success, attempted to develop

probe methods for determining the electromagnetic properties of soils. A report [4.31] outlines

the measurement technique and the construction details for a two-fi*equency, in situ probe which

can be inserted into holes bored into clay or loose soil. The instmment is reliable and easy to

use. It is powered by two 9 V batteries and is portable. Measurements at UHF frequencies

should be accurate to within 10 percent for conductivity and permittivity. Dalton et al. [4.32]

used a parallel electrode probe on the end of a parallel wire transmission line, with time-domain

reflectometer equipment for a read-out of soil moisture and electrical conductivity. The analysis

assumes that the permittivity and the conductivity are independent of frequency, that the soil has

a low loss tangent, and that the end of the parallel-electrode probe is a perfect open circuit at all

frequencies. There are conditions (such as in dry, salt-firee sand) for which the method might

give useful, qualitative information.

There are a number of other methods for determining the electromagnetic properties of

materials in situ [4.33-4.37]. More recently, Scott and Smith [4.38, 4.39] have shown how to use

monopoles and more complicated probes to measure the electromagnetic properties of granular

and liquid materials. Another approach, used in the petroleum industry, involves placing cavitybacked antennas on borehole instruments. With this technique, the transit-time and the

4-6
attenuation of the electromagnetic waves are measured—from which the complex permittivity

can be determined at a fixed frequency.

D Measuring the Electromagnetic Properties of Magnetic Materials

Many techniques for the measurement of the magnetic properties of materials are available

[4.40-4.42]. Most techniques for determining magnetic susceptibility involve placing the sample

in a weak uniform and time-varying source field (less than 4 x 10"^ A/m) which does not saturate

the sample, so the initial susceptibility obtained is independent of the magnetizing field and

hysteresis effects are avoided. Measurement techniques often involve balancing a Maxwell

inductance bridge with the sample inserted into one solenoidal inductance arm of the bridge

(figure 4.8). Another approach is to note the change in mutual reluctance between two coils set

up in a coaxial or orthogonal relation to each other when the magnetic sample material is placed

near the coils, calibrating the system with a standard of known susceptibility and normalizing

sample size to an equivalent half-space composed of material of identical susceptibility to the

sample’s. One paramagnetic standard often used for calibrating susceptibility bridge

measurements is ferrous ammonium sulfate Fe(NH4)2 ( 864)2 * 6H2O, with a relative molecular

mass of 392.15. The susceptibility of this salt is 32.6 x 10“^ x 4;r.

Figure 4.8 shows a typical susceptibility bridge which detects an inductance change in test

coil C2 by action of the coil’s field on a sample having unknown susceptibility. Coils Cj and C2 are solenoids carefully matched for inductance. Mj and M2 are identical variable inductors. In

operation, the bridge is balanced with no specimen in either coil through variable resistor R and

inductor M^. The specimen is then inserted into C2

(test coil) and the bridge balanced with

inductor M2 where the inductance adjustment is proportional to the susceptibility. Calibration of

the inductance M2 may be achieved by balancing the bridge with a test tube containing a known

weight of a paramagnetic compound such as ferrous ammonium sulfate of known susceptibility

in the coil C2

. Calibration is simply accomplished by balancing the bridge with inductor M2

when the test standard is placed in C2

. The test tube is then removed and bridge balanced with

variable inductor Mj. The same test tube with the same amount of the same standard substance

is again inserted into coil C2 and the bridge balanced with M2

. This process is repeated

throughout the inductance range of C2 so as to obtain a calibration curve referred to the known

standard. If ferrous ammonium sulfate is used, it should be kept in a fresh, sealed bottle since the

salt is slightly hygroscopic.

In these techniques, the sample is energized with a low-frequency field. This field must

have a low-enough frequency that no conductivity response of the sample will be observed. A

model which provides a general rule of thumb for the highest usable ac frequency is that of a

conducting permeable sphere in a uniform alternating magnetic field,

4-7
(4.14)

as shown in figure 4.9. Ward [4.43] has slightly rewritten (for time dependence) Wait’s

[4.44] original results for the in-phase M and out-of-phase N components of the induced dipole

moment of a sphere in a uniform alternating magnetic field when the wavelength in the external

host medium is much greater than the radius of the sphere ([Y\a\

1, where

7\ =

M-jN =

lfi2 (tan g - g) - (g - tan « -K tan g)

2/Z2 (tan a-a) + (a - tan a -i- tan a)

(4.15)

The in-phase and quadrature components of the induced dipole moment of a sphere are shown

plotted as a function of the response parameter of a sphere, 6 = (ft)<T2

/X2 )^^^ figure 4. 10. These components are shown in parametric fashion for = 1 (free space) to //2 /A^i =

(steel sphere).

In order that the conductivity response of the sample be small, figure 4.10 shows that

6 =(6)cT2//2 )^ should be «1. In the case ofpure magnetite, the permeability ^2 is

approximately 1.5 x 4;r x 10”^ H/m and the conductivity G2

is about 1.5 x 10"^ S/m. Thus, for a

2.5 cm diameter sphere and the condition that 0 = 0.1, we find that the maximum allowable

frequency is approximately 400 Hz. The maximum allowable frequency for samples that are considerably less conductive than pure magnetite may be higher while still avoiding sample

conductivity response. Wait’s work [4.45] can also be used to quantitatively assess detection

limits for both conductivity and permeability contrasts.

A susceptibility meter was used in measuring the susceptibility of a sample of sandmagnetite mix provided to NIST by Lee Anderson of BRDEC. The sample consists of silica

sand and magnetite of about 30 mesh size. The susceptibility meter is quite portable (0.5 kg with

dimensions of 190 x 80 x 30 mm, operating on one disposable 9 V battery).

The meter contains two coils placed orthogonally to each other in the detector head, which

is mounted in the bottom of the instrument case (circuit diagram shown in figure 4.1 1). In a

nonmagnetic environment the voltage induced in the receiver coil by the transmitter coil is zero. When a sample is brought near the coils, a voltage proportional to the magnetic susceptibility of

the sample is induced in the receiver coil. The received signal is detected by a phase-locked

amplifier and after rectification is sent to drive an analog panel meter, which is thermally

—3

compensated and directly calibrated for susceptibility. Field strengths are less than 4;r x 10

A/m at 1000 Hz so that with the phase sensitive receiver circuit the influence of electric

4-8
conductivity in most samples is usually eliminated. Calibration is usually done for a half-space

geometry, which is convenient when measurements are performed in the field. When laboratory

samples are measured, a multiplicative correction factor should be applied according to the soil

sample size. A chart indicating this half-space correction factor is given in figure 4.12 from

information provided by the manufacturer.

The silica sand-magnetite mixture’s susceptibility (~330 g

sample, ~50 mm diameter) was

measured and found to be approximately 0.175 uncorrected for sample size. Using a correction

factor of 2.0 from figure 4.12, the susceptibility of the 330 g sample is about 0.35. For mine

detection standards, such measurements can provide the practical property range limits for which

detection feasibility of various mine detection systems can be judged. Similar comments can be

made about other physical property contrast limits, such as complex permittivity (as a function of

frequency), density, and acoustic velocity.

E Recommendations and Conclusions

E.1 Laboratory Measurement System

Over the past two years NIST personnel have carefully analyzed all of the available

approaches for the accurate measurement of the electrical permittivity, magnetic permeability,

and conductivity of solid and granular materials. In addition to the methods described in this

chapter, we also spent much time investigating time-domain approaches to this problem [4.45-

4.50].

We have concluded that the time-domain approach, although potentially very accurate and

relatively inexpensive when compared to firequency-domain approaches, still requires a great

deal of developmental work to be practically useful to the Army. The major drawback at present

is the fact that these systems can be calibrated accurately only with the use of at least three

different standards, each consisting of a sample of material for which all of the electrical

properties are known over the frequency range of interest. Unfortunately, these required

standards have not been developed.

We have also concluded that the open-ended coaxial sample holder method described in

section rV.B.l still needs some additional work. Although Bussey [4.5] and Jesch [4.29] were

able to perform some fairly accurate measurements using this technique, there are still some

unanswered questions on the analysis computer program that they used to extract complex

permittivity from the measured data. In particular, there are some numerical constants in the

computer software which no one at NIST (including Bussey and Jesch) has been able to explain

or justify. As a result, we cannot recommend this approach to the Army at present.

4-9
The approach that NIST does recommend is the two-port technique using a vector

automatic network analyzer. Over the last two years, personnel at NIST have developed

considerable knowledge and expertise with this technique, and we feel that it is the best available

approach in terms of cost, adequate accuracy, and ease of use. The superiority of this approach is

further enhanced by the fact that the Army already possesses all of the required hardware, and by

the fact that the network analyzer manufacturer has recently introduced a comprehensive

software product that greatly increases the versatility, ease of use, and accuracy of the method.

Therefore, NIST has placed an order for this software package and, upon its arrival, will

configure and test the system at NIST. Then, NIST personnel will deliver this system to the

Army as soon as possible. The system will consist of the following components:

BRDEC’s existing

a. vector network analyzer system (300 kHz to 3 or 6 GHz);

b. computer controller system (to be delivered);

c. calibration standards kit (to be delivered);

d. all required software for system operation (to be delivered).

E.2 Field Measurement System

NIST has found no measurement system or approach which would meet the Army’s needs

fully for measurements in the field. The methods reviewed in section in.C all have serious

drawbacks; they are not sufficiently proven to yield accurate results over a broad range of

frequencies (100 MHz to 5 GHz) and a broad range of soil properties (for example, a dielectric

constant range of 2 to 25).

Therefore NIST has the following recommendation; the acquisition of a dielectric probe kit

that has recently been introduced by the same manufacturer as that mentioned above for the

network analyzer and computer controller. This kit includes all necessary software for operation

and data presentation. It is capable of measuring complex permittivity, in situ, with a dielectric

constant of 2 to 80 and for materials that are solid, liquid, or granular. Its specified operating

frequency range is 200 MHz to 20 GHz, and its specified uncertainty is typically ±5 percent

There are two major drawbacks to this approach. First, the system is not very portable

because it must be attached to the network analyzer/computer controller. However, if the latter is

placed on a wheeled cart, limited portability would be possible. The second drawback is that the

probe accuracy becomes seriously degraded for the combination of low dielectric constant and

low frequency. For example, from the manufacturer’s data sheets, the probe uncertainty for a

dielectric constant of 5 at a frequency of 2 GHz is about ±12 percent. Thus, it would be most

useful for the Army in measuring wet soils; its usefulness for performing field measurements of

dry soils is, at present, questionable.

4-10
Even with these shortcomings, NIST thinks that this probe will meet most of the Army’s

field measurement needs better than any other known approach. Also NIST thinks that the low

frequency limitation is caused by the small dimensions of the probe and could be remedied by

building a larger probe. As a subject for future work, we recommend that the Army pursue the

design of a larger probe.

NIST has ordered one of these probe kits for delivery to the Army along with the network

analyzer system. Before delivery, NIST will perform some laboratory tests on the probe in order

to verify its proper operation.

References (IV)

[4.1] von Hippel, Arthur R. Dielectric materials and applications. New York: John Wiley

& Sons. 438 p.; 1954.

[4.2] von Hippel, Arthur R. Dielectrics and waves. New York: John Wiley & Sons.

284 p.; 1954.

[4.3] Hill, N. E.; Vaughn, W. E.; Price, A. H.; Davies, M. Dielectric properties and

molecular behavior. London: Van Nostrand. 480 p.; 1969.

[4.4] Afsar, M. N.; Birch, J. R.; Clarke, R. N. The measurement of properties of materials.

Proc. IEEE; 74(1): 183-199; 1986.

[4.5] Bussey, H. E. Dielectric measurements in a shielded open circuit coaxial line. IEEE

Trans. Instrum. Meas.; IM-29(2): 120-124; 1980.

[4.6] Jones, R. N.; Bussey, H. E.; Little, W. E.; Metzker, R. F. Electrical characteristics of

com, wheat, and soya in the 1-200 MHz range; Nat. Bur. Stand. (U.S.) NBSIR 78-

897; 1978.

[4.7] Bimbaum, G.; Franeau, J. Measurement of the dielectric constant and loss of solids

and liquids by a cavity perturbation method. J. Appl. Phys.; 20: 817-818; 1949.

[4.8] Homer, F. Resonance methods of dielectric measurement at centimeter wavelengths.

J. IRE; 93(m): 55-57; 1946.

[4.9] Bethe, H. A.; Schwinger, J. Perturbation theory of resonant cavities; NDRC, Rpt.

D2-117; 1943.

[4.10] Cook, R. J. Microwave cavity methods, in high frequency dielectric measurements; J. Chamberlain and G. W. Chantry, ed., IPC Science & Technology Press, pp. 12-27;

1973.

[4.1 1] Bussey, H. E. Measurement of RF properties of materials: a survey. Proc. IEEE;

55: 1046-1053; 1967.

4-11
[4.12] Culshaw, W.; Anderson, M. V. Measurement of permittivity and dielectric loss with

a millimeter wave Fabry-Perot interferometer. Proc. IEEE; 109(B), Suppl. 23: 820-

826; 1962.

[4.13] Degenford, J.; Coleman, P. D. A quasi-optic perturbation technique for measuring

dielectric constants. Proc. IEEE; 54: 520-522; 1966.

[4.14] Degenford, J. A quasi-optic technique for measuring dielectric loss tangents. IEEE

Trans. Instrum. Meas.; IM-17: 413-417; 1968.

[4.15] Cullen, A. L.; Yu, P. K. The accurate measurement of permittivity by means of an

open resonator. Proc. R. Soc. A; 325: 493-509; 1971.

[4.16] Conbau, G.; Schwering, F. On the guided propagation of electromagnetic wave

beams. IRE Trans.; AP-9: 248-256; 1961.

[4.17] Cullen, A. L.; Nagenthiram, P.; Williams, A. D. Improvement in open resonator

permittivity measurement. Electron. Letters; Vol. 8: 577-579; 1972.

[4.18] Cullen, A. L.; Nagenthiram, P.; Williams, A. D. A variational approach to the theory

of the open resonator. Proc. R. Soc. A.; 329: 153-169; 1972.

[4.19] Kogelnik, H.; Li, T. Laser beams and resonators. Proc. IEEE; 54: 1312-1329; 1966.

[4.20] Cook, R. J.; Jones, R. G.; Rosenberg, C. R. Comparison of cavity and open resonator

measurements of permittivity and loss angle at 35 GHz. IEEE Trans. Instrum. Meas.;

IM-23: 438-442; 1974.

[4.21] Jones, R. G. Precise dielectric measurements at 35 GHz using an open microwave

resonator. Proc. IEEE.; 123(4): 284-290; 1976.

[4.22] Van Gemert, M. J. C. High-frequency time-domain methods in dielectric

spectroscopy. Phillips Res. Rpts; 28: 530-572; 1973.

[4.23] Gans, W. L.; Andrews, J. R. Time-domain automatic network analyzer for

measurement of rf and microwave components; Nat. Bur. Stand. (U.S.) Tech. Note

672; 1975 September. 165 p.; 1975.

[4.24] Nicolson, A. M.; Ross, G. F. Measurement of the intrinsic properties of materials by

time-domain techniques. IEEE Trans. Instrum. Meas.; IM-19 (4): 377-382; 1970.

[4.25] Peyrelasse, J.; Boned, C.; LePetit, J. P. Setting up of a time-domain spectroscopy

experiment. Application to the study of the dielectric relaxation of pentanol isomers.

J. Phys. Sci. Instr.; 14: 1002-1008; 1981.

[4.26] Nakamura, H.; Mashimo, S.; Wada, A. Precise and easy method of TDR to obtain

dielectric relaxation spectra in the GHz region. Japanese J. Appl. Phys.; 21(7): 1022-

1024; 1982.

[4.27] Suggett, A. Time-domain methods, dielectric and related molecular processes.

Chem. Soc. London; Vol. 1: 101-120; 1972.

4-12
[4.28] Bottreau, A. M.; Dutuit, Y.; Moreau, J. On a multiple reflection time domain method

in dielectric spectroscopy: Application to the study of some normal primary alcohols.

J. Chem. Phys.; 66(8): 3331-3336; 1977.

[4.29] Jesch, R. L. Dielectric measurements of five different soil textural types as functions

of frequency and moisture content; Nat. Bur. Stand. (U.S.) NBSIR 78-879; 1978.

[4.30] Measuring Dielectric Constant with the HP 85 10 Network Analyzer. Product Note

No. 8510-3, Hewlett-Packard Corp., Palo Alto, CA; August 1985.

[4.31] Caldecott, R.; et al. A radio frequency probe to measure solid electrical parameters;

Final Report 715616-4; Ohio State University ElectroScience Lab., Columbus, Ohio;

January 22, 1985.

[4.32] Dalton, F. N.; et al. Time-domain reflectometry: Simultaneous measurement of soil

water content and electrical conductivity with a single probe. Science; 224: 989-990;

June 1, 1984.

[4.33] King, R. W.; Smith, G. S. Antennas in matter; fundamentals, theory, and

applications. Cambridge, MA: M.I.T. Press; 1981.

[4.34] Smith, G. S.; King, R. W. P. Electric field probes in material media and their

application in EMC. IEEE Trans. Electromagn. Compat; EMC-17: 206-211; 1975.

[4.35] Smith, G. S.; King, R. W. P. Electric field probes in material media and their

application in EMC. IEEE Trans. Electromagn. Compat; EMC-18, p. 130; 1976.

[4.36] Smith, G. S.; Nordgard, J. D. Measurements of the electrical constitutive parameters

of materials using antennas. IEEE Trans. Ant Prop.; 33(7): 783-792; 1985.

[4.37] Scott, W. R. Dielectric spectroscopy using shielded open.circuited coaxial lines and

monopole antennas of general length. Ph.D. thesis, Georgia Inst. Tech., 228 p.; 1985.

[4.38] Scott, W. R. and Smith, G. S. Dielectric spectroscopy using monopole antennas of

general electrical length. IEEE Trans. Ant Prop.; AP-34(7): 919-929; July 1986.

[4.39] Smith, G. S. and Scott, W. R. A simple method for the in situ measurement of the

electrical properties of the ground. Paper JA02-3, pages 242-245, 1987 IEEE AP-S

International Symposium Digest, Virginia Tech. (IEEE Catalog No. CH2435-6/87);

June 15-19, 1987.

[4.40] Nagata, T. Rock magnetism. Tokyo: Manizen Press, 350 p.; 1961.

[4.41] Akimoto, S. Magnetic properties of ferromagnetic oxide minerals as a basis of rock

magnetism. Adv. Phys.; 6(288); 1957.

[4.42] Heiland, C. A. Geophysical exploration. Englewood Cliffs, New Jersey: Prentice

Hall, pp. 310-314; 1940.

[4.43] Ward, S. H. Electromagnetic theory for geophysical applications. Mining Geophys.;

II, p. 78; 1967.

4-13
[4.44] Wait, J. R. A conducting sphere in a time varying field. Geophy.; 16, p. 666; 1951.

[4.45] Fellner-Feldegg, H, The Measurement of Dielectrics in the Time Domain. J. Phys.

Chem.; 73 (3): 616-623; 1969.

[4.46] Cole, R. H. Evaluation of dielectric behavior by time domain spectroscopy. I.

Dielectric response by real time analysis. J. Phys. Chem.; 79 (14): 1459-1469; 1975.

[4.47] Cole, R. H. Evaluation of dielectric behavior by time domain spectroscopy, n.

Complex permittivity. J. Phys. Chem.; 79 (14), 1469-1474; 1975.

[4.48] Cole, R. H.; Mashimo, S.; Winsor, P., IV. Evaluation of dielectric behavior by time

domain spectroscopy. 3. Precision difference methods. J. Phys. Chem.; 84 (7): 786-

793; 1980.

[4.49] Bose, T. K.; Bottreau, A. M.; Chahine, R. Development of a dipole probe for the

study of dielectric properties of biological substances in radiofrequency and

microwave region with time-domain reflectometry. IEEE Trans. Instrum. Meas.; EM35 (1): 56-60; 1986.

[4.50] Cole, R H.; Berberian, J. G.; Mashimo, S; Chryssikos, G.; Bums, A.; Tombari, E.

Time domain reflection methods for dielectric measurements to 10 GHz. J. Appl.

Phys.; 66 (2): 793-802; 1989.

4-14
—4 14mm [-

APPROXIMATE FILL LEVEL

I

APPROXIMATE LOCATION

OF OPEN CIRCUIT, Ze= «

INPUT PLANE OF MEASUREMENT

14mm REFERENCE PLANE

Figure 4.1 Cross-sectional drawing of the coaxial transmission

line sample holder for the single-port measurement

scheme.

4-15
of

SiC-Sand

Mix

(at

600

MHz)

14.0

12.0

tan S (right scale) ®

10.0

vu

8.0

6.0

4.0

2.0

0.020

“

0.010

— 0.010

— 0.005

0.000

100

Percent SiC (by mass)

Figure 4.2 Electromagnetic properties (at 600 MHz) of a blend

of Sic and silica sand for various percentages (by

mass) of Sic.

4-16

Loss

Tangent
PORT 1 PNP PORT P

SPnPLE

SfinPLE

HOLDER

GRSOO/SnP PDfiPTER

PRECISION CUT

RG/58 LINE

Figure 4.3 Schematic of the ANA materials measurement system.

4-17
I

4 f

1 1

\

1 T

J r

0 0.2 0.4 0.S 0.8 1 1.2

FREQ (GHz)

MUr

0 0.2 0.4 0.6 0.8 1 1.2

FREQ (GHz)

Figure 4.4 Complex relative permittivity and permeability of

Teflon as measured with the ANA.

4-18
T T

1

EPSr

F

c 3 P-c

i:

E

U 2 -

rP

h

f»P

c

1 H

r

P

P

h

f- 0P0 0.2

tt

_J I 0.4 0.6

FREQ (GHz)

0.8

•i”1 4

1

d

j ~

-1

zi

s ^

1 1.2

3 C

1 1 T r T 3

0 0.2 0.4 0.6 0.8 1 1.2

FREQ (GHz)

Figure 4.5 Complex relative permittivity and permeability of

nylon as measured with the ANA.

4-19
MUr

Figure 4.6 Complex relative permittivity and permeability of

dry sand as measured with the ANA.

4-20
1

1

1

EPSr

MUr

Figure 4.7 Complex relative permittivity and permeability of a

60% dry sand and 40% (by weight) SiC mixture as

measured with the ANA.

4-21
Incut

Figure 4.8 Schematic diagram for a typical susceptibility

bridge.

4-22
H = Hoe-j«t

- ^

' -»

Figure 4.9 Conducting permeable sphere of radius a in a uniform

alternating magnetic field. The conductivity,

permeability, and permittivity of the host medium

are ^1/ respectively. The conductivity

permeability, and permittivity of the sphere are

02 , M-2 ^ ^2' respectively.

I

I 4-23
Figure 4.10 In-phase (M) and out-of-phase (N) components of the

induced dipole moment for a sphere in a uniform

alternating magnetic field for the case | ^T^a !<<1.

See reference [4.43].

4-24
Comp

coil

RC

j-o,!

100k

Figure 4.11 Circuit diagram for the susceptibility meter

4-25
HALFSPACE

MULTIPLICATIVE

FACTOR

Figure 4.12 Half-space multiplicative correction factor for

susceptibility measurements.

4-26
V Electromagnetic Properties of the Earth and

Options for Standard Media

A Typical Electromagnetic Properties of Real Soils

Knowledge of the electromagnetic properties of typical real soils is necessary before it is

possible to identify candidate methods for realizing soil standards. Several cases may be listed

and generally characterized in terms of conductivity and permittivity contrasts as representative

of the differing electrical environments likely to be encountered in either plastic or metallic mine

detection.

Case 1: Dry Beach Sands (Plastic Mine Detection)

One type of host soil environment is that in which either no pore water or very little pore

water (water saturation Sw <0.01) exists within a magnetically impermeable, unconsolidated,

and electrically resistive soil grain matrix having low cation exchange capacity. Such a situation

exists in a dry beach sand or dry gravel host and probably presents the most difficult plastic mine

detection test environment

The host soil conductivity in this case is generally low (<T// < 10“^ S/m) and the host

dielectric constant, £//, is low and probably less than the target (mine) dielectric constant

(ej/ = 2.70 when referred to dry sand consisting of clean quartz grains having diameters greater

than 0.03 mm but less than 0.60 mm where the water saturation, Sw, is 0 and the frequency is

300 MHz). If we take a nylon sphere as representative of our plastic mine target (where the

target conductivity, Gj, is much less than 10 S/m and the target dielectric constant Ej remains

fixed at 3.00), then = 0.90 and 0^1Gj =1.

Case 2: Wet, Fresh Water Beach Sands (Plastic Mine Detection)

This situation is characterized by moderately fresh (deionized) pure water saturation

(Sw = 0.14) in a magnetically impermeable, unconsolidated, and electrically resistive

(C7// <10 s/m) soil grain matrix of low cation exchange capacity. Due to increased water

saturation, the host soil dielectric constant at a typical UHF detection frequency of 3(X) MHz

would be approximately 10, leading to dielectric and conductivity ratios of = 3.3 and

C7///<Tj’ = 1 . Because of the higher permittivity contrast at the chosen operating frequency, this

case probably presents a less difficult detection test than Case 1.

5-1
Case 3: Wet Clay Soils and Wet Marine Beach Sands (Plastic Mine Detection)

The wet clay soil environment is characterized by highly brackish or saline pore water

saturation (Sw > 0.20) in a magnetically impermeable, unconsolidated, and electrically

conductive soil grain matrix of high cation exchange capacity. In such a case, both the measured

host soil conductivity and host soil dielectric constant at 300 MHz are relatively high

(0.1 S/m < O// < 1 S/m and£// = 20); this leads to significant dielectric and conductivity ratios

where e///ef = 6.7 and joj » 1 for a nylon spherical target. Such a situation is likely to be

that most commonly encountered; therefore, it is one which should be simulated in any

validation testing.

These electrical properties would also characterize a magnetically impermeable,

unconsolidated, and electrically resistive soil grain matrix of low cation exchange capacity

having highly saline pore waters, such as might be encountered in wet marine beach sands. All

other factors being equal, target visibility in terms of dielectric contrast is greatest for this case,

but because of the conductive flossy) nature of the host medium, test target detection at

increasing depths of burial for UHF operating frequencies is expected to be sharply limited.

Case 4: Magnetically Impermeable Host Soils (Metal Mine Detection)

Because the contrast in conductivity between a metallic mine target in either an electrically

resistive dry beach sand or an electrically conductive wet clay remains very high (for analysis

purposes, infinite), and because most metal mine detectors operate in the VLF region (affording

little attenuation for expected burial depths) and sense secondary magnetic fields due to induced

eddy currents within the buried mine, two test soils would seem to bracket those situations

commonly encountered in the field. These are a magnetically impermeable or magnetically

permeable host medium. The magnetically impermeable host soil may be simulated by quartz

sand grains having no magnetite-ilmenite, which are the common naturally occurring magnetic

minerals [5.1]. Hence, the host soil medium in this case would have a magnetic permeability

equal to that of firee space, Pq (4k x 10’^ H/m). Conductive losses in the host medium can be

controlled either by adding known amounts of saline pore water or aluminum powder.

Case 5: Magnetically Permeable Host Soils (Metal Mine Detection)

Attenuation rates for a plane electromagnetic wave in a uniform, magnetically permeable

earth are greater than in a uniform, magnetically impermeable earth—all other factors being

equal. In fact, the plane-wave skin depth (distance within the earth at which amplitudes of the

electric and magnetic fields are equal to Me = 0.3679 of their respective values at the surface) of

a magnetically permeable host soil decreases by (p^ where p^ is the relative permeability

and Pq is the permeability of free space or vacuum. The BRDEC already has a test lane

5-2
simulating this case that consists of a silica sand magnetite mixture (70 percent dry sand, 30

percent magnetite grains, -30 mesh size) whose relative magnetic permeability was measured

and found to be 1.35 [5.1]. Because not all metallic land mine detectors employ ground signal

removal techniques, a logical test procedure for evaluating differing systems would be to

simulate both Case 4 and 5 in the test lane environment. See table 5.1 (from [5.2]) for a list of

the magnetic susceptibility of naturally occurring rocks and minerals.

B Dielectric Mixing Rules

Soils are mixtures of minerals that are the weathered byproducts of rocks. Usually, natural

soils contain some water, so the way in which dielectric constants combine is important in

determining the bulk, or effective, dielectric constant of the soil at hand. The bulk dielectric

constant of any soil (whether natural or synthetic) determines the electromagnetic visibility of

buried land mines and, therefore, intrinsically affects the performance of any detection system.

Once a range of suitable dielectric background characteristics, relatable to conditions actually

encountered in the field, is accepted, mixing rules provide the easiest and most pragmatic

approach to specification of standard test soils.

Geyer [5.3] has noted that some mixing rules have little broadband applicability to soils

since they are based on systems where little or no conduction takes place. For example, when

one of the components in a composite dielectric is conductive, large (frequency-, temperature-,

and salinity-dependent) polarizations can occur that can be attributed to interfacial polarizations

at the boundaries of the conductive phase. A mixing rule that does not account for these

interfacial polarizations has little broadband applicability. Despite the fact that most mixing

rules fail in a broadband sense (since they do not account for all relaxation effects), it is

nonetheless useful to be able to predict the effective electromagnetic properties of soils of

different lithologies and porosities even over narrower frequency ranges—100 MHz to 2 GHz in

this case—that affect the signal levels measured by UHF land mine detection systems.

The purpose of a mixing rule is to allow estimation of the dielectric properties of a soil

(either test standard or soil encountered in the field) when given measurements of that soil’s

physical properties only. Its usefulness will be limited by its dependence on adjustable soil- and

frequency-specific parameters. An efficacious mixing rule, then, will provide a convenient

means for predicting a test soil’s dielectric behavior in microwave land mine performance testing

and in signal level determination. It will also provide a physically based mixing model that is

dependent on measurable soil parameters and can be used for subsequent development and

research.

5-3
Table 5.1

Magnetic susceptibility of rocks and minerals.

Magnetic minerals Susceptibility

Magnetite crystals 6.3 to 24.0

Magnetite 0.04 to 2.0

Ilmenite 0.03 to 0.14

Franklinite 0.036

Pyrrhotite 0.007 to 0.028

Specularite 0.003 to 0.004

Chromite 0.002

Major rock types

Basic effusive 0.001 to 0.004

Basic plutonics <0.0001 to <0.004

Granites and allied rocks <0.0001 to <0.001

Gneisses, schists, and slates <0.0001 to <0.001

Sedimentaries <0.0001 to <0.001

Specific rock types

Igneous rocks

Basalt 0.000 68 to 0.006 3

Diabase 0.000 078 to 0.004 2

Gabbro 0.000 44 to 0.004 1 Granite 0.000 03 to 0.002 7

Porphyry 0.000 023 to 0.000 5

Metamorphic rocks

Serpentine 0.000 25 to 0.014

Slate 0.000 039 to 0.003 0

Gneiss 0.000 01 to 0.002 0

Schist 0.000 026 to 0.000 24

Sedimentary rocks

Shale 0.000 04 to 0.000 5

Clay 0.000 2

Sandstone 0.000 005 to 0.000 017

Dolomite 0.000 000 9 to 0.000 014

Limestone 0.000 004

Iron ores and minerals

Siderite 0.000 1 to 0.003

Limonite 0.000 1 to 0.000 2

Hematite 0.000 04 to 0.000 1 Ankerite 0.000 02 to 0.000 1

5-4
Table 5.1

Magnetic susceptibility of rocks and minerals, continued.

Typical sulfide minerals

Arsenopyrite 0.000 005 to 0.000 2

Chalcopyrite 0.000 005 to 0.000 2

Chromite 0.000 005 to 0.000 2

Markasite 0.000 005 to 0.000 2

Pyrite

Diamagnetic minerals and rocks

0.000 005 to 0.000 2

Anhydrite -0.000 001 1 to -0.000 01

Quartz -0.000 001 1 to -0.000 001

Sylvite -0.000 000 9 to -0.000 001

Calcite -0.000 000 6 to -0.000 001

Rock salt -0.000 000 4 to -0.000 001

Generally, all materials can be classified into one of three dielectric groups. The first of

these, to which pure water and ice belong, is homogeneous substances. A second group, in

which ionic salts are dissolved in solution (usually water) is electrolytic solutions. The third

group, heterogeneous mixtures, includes that of wet, lossy soils or multicomponent (lossy or

nonlossy) media of interest here for test soil lanes. The merits of any chosen background

material (standard) for mine test lanes depend not only on correlation with actual field conditions

likely to be encountered but also on the ease with which its (complex) dielectric properties can be

predicted and controlled for the operational frequency range of interest.

B.1 Function-Theoretic Rules

In general, the average dielectric constant of a heterogeneous mixture consisting of two or

more substances is related to the dielectric constants of the individual substances, their volume

fractions, their spatial distributions, and their orientations relative to the direction of the incident

electric field vector [5.4]. In order to determine the functional dependence of the average

dielectric constant of a mixture on these variables, the average electric field within the mixture as a whole must be related to the electric fields within the inclusions [5.5]. The problem is that if

the inclusions are randomly dispersed throughout the host medium, it is not generally possible to

derive an exact solution for the fields within the inclusions since the mutual electromagnetic

interactions of the inclusions are dependent on their positions with respect to each other. Tinga

[5.6] gives a review of various approximations that have been proposed for solving the

interaction problem. These approximations vary from those that ignore short-range interactions

between inclusions (by restricting the validity of the dielectric mixing model to only those

5-5
mixtures characterized by a low concentration of inclusions) to relations that account for firstorder inclusion interactions by the solution of Maxwell’s equations and appropriate boundary

conditions [5.6, 5.7]. In all cases, the dimensions of the inclusions are much smaller than the

propagation wavelength in the host medium. Bottcher [5.8] gives a comprehensive review of

dielectric mixing models that includes both empirical (or semi-empirical) formulations for

specific mixtures as well as theoretical models developed for highly specialized media that

contain either ellipsoidal particle inclusions (spheres, disks, needles) or confocal ellipsoidal

spheroids.

B.2 Generalized Heuristic Rules for Soil Mixtures

Natural soils are mixtures of matrix minerals, air, and water. As noted in [5.3], water in the

pore spaces of surface soils consists of two phases. One of these phases is the bulk (free) pore

water. The other is adsorbed water that is adjacent to the matrix grain surfaces and usually

several molecular layers thick. This adsorbed water is called surface (or bound) water. The

saturation at which the dielectric constant becomes less sensitive to water saturation is entirely a

function of the amount of surface water in soil pore spaces; this explains the dependence of the

dielectric constant on soil texture types (particle size and shape distributions) as well as on the

intrinsic porosity of the soil mixture. Of course, the porosity is simply related to the bulk

density, and the degree of conductive loss responsible for electromagnetic wave attenuation is

dependent on the salinity of the pore water which, in turn, depends quite naturally on both the

matrix cation exchange capacity and the soil temperature. The fact that the electrical properties

of water-saturated soils are frequency dependent is also not physically surprising since we expect

polarization, that is, the orientation of polar molecules (molecules with asymmetric charge

distributions), to occur in an applied electric field. The mobility of any polar molecule (such as water) will always depend on the time rate of variation of that field.

Bound water adsorbed to a solid surface has a dielectric response significantly different

from that of free water. This is so because the molecular mobility of the water molecules has

been reduced by physical bonding to the matrix grain surfaces [5.9, 5.10, 5.1 1]. Most of the

measurement evidence to date seems to indicate that the restricted mobility causes a reduction of

the static dielectric constant of water adsorbed to the surface from 80 for free water to about 6 for

sorbed water [5.10] which is greater than an order-of-magnitude reduction. Furthermore,

surface-bound water exhibits a relaxation frequency of about 10"* Hz instead of 100 GHz, which

is seven orders of magnitude lower than that of bulk water [5.3, 5.12].

Such observations have led McCafferty and Zettlemayer [5.1 1] to suggest that dielectric

dispersion in the VLF region in systems composed of a solid and adsorbed water may principally

be due to relaxation of the adsorbed water. However, an equally plausible explanation is that

5-6
conductivity inhomogeneities in a soil matrix are responsible for dielectric dispersion and it is

not the rotational mobility of water molecules in the surface water that is important, but rather

the conductivity of the surface layer [5.13, 5.14].

Thus the most general form of a heuristic, predictive dielectric mixing rule that might be

used for soil specifications would separate the volumetric percentages of bound and free water.

The soil mixture is then described electrically as a four-component system whose constituent

components are linearly combined in terms of the respective volume fractions as

^so'd ~ ^m^m ^bw^bw ^ (5.1)

where v represents volume fraction; the subscripts m, a, bw, andfw represent soil matrix, air,

bound water, and free water, and a is a constant

Here we recognize that

4) = total porosity = (p„ - p* )/p^ = 1 - Pi,/p„

where and p^ denote the matrix (solid material) and bulk (air, water, material mixture)

density, respectively, and

S'v = ''bw + V/U-. (5 2)

The two terms involving bound and free water are often combined into a single term with

an empirically determined multiplicative factor of the free water permittivity in the soil [5.15].

That is,

+ V - Sw''e^, (5.3)

where the exponent v of total water saturation Sw is empirically derived.

Substituting eqs (5.2) and (5.3) into (5.1) and recognizing that = 1 yields the following

relation in terms of porosity and water saturation:

eZu = (1 - ^)£“ + 0 - Sw + Sw^e%, (5.4)

In terms of the bulk and soil matrix densities, eq (5.4) may be written

4a = l +—(em-l) + (S'v‘'£^-Sw), (5.5)

Pm

where pbjpm -'^mhb is the soil matrix volume fraction.

5-7
For a = 1, eq (5.5) is known as a linear model; for a = 1/2 as a refractive model (since

is the refractive index); and for a = 1/3 as the cubic model.

The results of over 500 measurements made by Dobson et al. [5.15], on five differing soil

types at nine frequencies (from 4.0 GHz to 18 GHz) and for volumetric moistures ranging

from 0.01 to total saturation gave values of v between 1.0 and 1.16 and suggested a refractive

model (a = 1/2) as most suited for soil-water mixtures. This was observed earlier by Shutko and

Reutov [5.16]. Hence Sw^ = Sw and

= 1 + -^ - 1) + Sw(ef - 1) . (5.6)

Pm

Combining the effects of bound and free water into one term (eq (5.3)) reduces the implicit

dependence of on soil type (5.4); this is not surprising since clays are expected to have more

bound water than, say, sand grains. In general, however, e^ou and ^ in eq (5.6) are complex

quantities, since attenuative loss due to electrolytic conduction is fundamentally implicit in

natural soils and therefore must be incorporated into test lane soils for realistic performance

testing. In fact, the loss tangent of soil pore water, which depends on salinity (and temperature),

is essentially an in situ parameter and cannot be ascertained correctly by pore fluid extraction.

These complicating facts will be addressed below.

B.2. 1 Dry Soil Mixture

For a dry soil mixture, eq (5.6) simplifies to

+ (5 - 7) Pm

since Sw = 0. Or, in terms of the porosity ^ of the soil mixture

41,1 = (1 - + <t>4£ (5.8)

or

4Jll = (^-<l>)4J^ix + it>,

since =1 — jO. In this case, the soil mixture will have a real dielectric constant (or exhibit

no conductive losses). Therefore, essentially no electromagnetic attenuation will be imposed on

field behavior by the background soil medium in the frequency range of interest and detection

systems operating at higher relative frequencies would be expected to perform best, all other

5-8
factors being equal [5.3]. Examples of dielectric, nonlossy test soils are dry sands (Si02 or SiC).

Frequency-domain measurements of the dielectric constant on these soils have been reported

previously [5.3].

B.2.2 Lossy, Fluid-Saturated Soils

For lossy, fluid-saturated soils the dielectric mixing rule becomes more complicated since

both the bulk soil mixture and the pore water become electrically lossy and attenuate

electromagnetic fields. In this case the electromagnetic fields of a mine detection system

operating at a higher relative frequency would suffer greater attenuation. If the effective skin

depth is too small relative to the burial depth of a buried mine, the detection system would

perform more poorly than a system operating at a lower frequency. There are two cases to

consider, one where background soils are only partially water saturated and the other where soils

are fully saturated.

B22.1 Partially Saturated Soil Mixtures

The complex dielectric constant of the soil may be written

= (1 - + Sw)el{^ + (5-9)

where

and

.1/2 _

'matrix =v^matrix (nonlossy),

(nonlossy),

(lossy).

^^water J ^water )1 > f" _ ^water

water “

'water

(5.10)

= WsoU - j£'i>uf^ (lossy),

= [£'soU (1 - ; tan dsoii )f^, tan 8,^ =

Use of the half-angle relation

tan5 =

2 tan S/2

1 - tan^ 5/2

5-9
allows a complex dielectric constant e = e\l - y tan 5) to be written as

V?

1 - tan —

2 )

l-ytan—

V

V y

Use of eqs (5.11) and (5.10) in (5.9) then yields

/ 5 ..\

r

1 - tan

2 ^soU

\

\II2

a

2 J

= (l-^)Ve^ix +(^-Sw)

2 J

(5.11)

Sw^Je^water

l-tan^-^w^

2

y/2

)

1 -ytan- 'water

Separating real and imaginary parts yields

l-tan^^j

n1/2

water

l_tan2iw2>S!L

2

(5.12)

and

V

^C/lf / fLi/tan''-'

2 _

water tan

'water

lS ; V'

1 -tan^-^

2 j

l_tan2.^!!:a!sr

2

y/2

y

(5.13)

Both eqs (5.12) and (5.13) must be satisfied simultaneously. tan^^^,/ and e'rnatrix known firom measurements. The total porosity (j> and water and air saturations, Sw, and (j>-Sw

may be determined by using a value for the water salinity S which is based on the cation

exchange capacity of the matrix soil type. The dielectric constant s'^ater of Pore water is

computed firom eqs (2.17) and (2.20). The loss tangent of the pore water is then calculated from

eq (2.21). Substituting these values and the measured and tan 7/2 into eq (5.13) allows

the determination of Sw from which the total porosity is computed (eq (5.12)).

B2.2 2 Saturated Soil Mixtures

A more expedient approach is to relate the complex dielectric constant of the soil mixture

to the water-filled porosity <j>^ so that we may write

el'ol = (1 - 4'w)£mlrix + (5 - 14)

5-10
Use of eqs (5.10) and (5.11) now yields

and

'yj^soil

/ \l/2

2 )

matrix + 0wV^water

V 2 y

^soil 1^ 'soil 0wV^water tan-

'water

f c \l/2

2 / 2 )

\l/2

(5.15)

(5.16)

Again, tan and ^re known from measurements. A value for the salinity S of

the pore water is then taken. The real part of the pore water permittivity E'y^ater temperature T,

°C, is then computed from

e'^ater = <.o(2’) - 0.1556-4.13 x 10"^S + 1.58 x 10‘®5^ (5.17)

where

ewo(2') = 88.045 - 0.41477 + 6.295 x lO'^T^. (5.18)

Next the loss tangent of the pore water is calculated from

tan

[e^aier]"' [5-66 + 2.65 x 10‘^S - 4.5 x 10"* 5^] . (5.19)

This initial value of tan obtained above is substituted into eq (5.15) to determine (/)^. A corrected value of tan5^^g;./2 is determined from eq (5.16), which is then used to determine a

new value for the water salinity and porosity until they converge to constant values. This

approach is one commonly used in geophysics and reservoir engineering to determine both the

water-filled porosity and pore water salinity of rocks from in situ dielectric measurements. For

our purposes here, these are the fundamental parameters that allow us to predict, from a mixing

relationship, how attenuative a background soil is. The important point is that the complex

permittivity (both lossless and lossy parts) of the background test medium be the same in

performance evaluations of mine detector systems operating at the same frequency. While

various synthetic substances can be thought of that can simulate the real field situation of watersaturated soils in terms of the complex permittivity at a single frequency, there are few synthetics

which, in combination, can simulate the dielectric behavior of water- saturated soils over a broad

range in frequency.

5-11
B.3 Application to Granular Soil Mixtures

As an application to dielectric property prediction of soil mixtures, consider a two-phase

mixture of air and dry sand, where sand consists of washed quartz grains that have diameters

ranging from 0.027 and 0.60 mm. Recalling eq (5.7) we write

pi/2 =1 + P^rpi/2 _-|x

^soil ^ y^matrix

Pm

For this mixture, Geyer [5.3] and Jesch [5.17] have reported a measured mean relative

dielectric constant, e'soih of 2.702 at 300 MHz and for a mean bulk density, p^, of 1.540 g/cm^.

Standard deviations for the dielectric constant and mean bulk density were 0.0486 and 0.022

g/cm^, respectively. Bussey [5.18, 5.19] has reported e' of silica equal to 3.822, measured at

9 GHz with the use of cavity resonators. Von Hippel [5.20] has reported e' of (fused) silica at

25°C and at 300 MHz equal to 3.78. Von Hippel’s results show fused silica essentially

dispersionless over the frequency range of 100 Hz to 25 GHz, with tanSsmca = 0.5 x 10“^ at

300 MHz. Clark [5.21] and Morey [5.22] give the matrix density of pure silica, p^„, to be

2.203 g/cm^

In summary, use of the two-phase refractive dielectric mixing rule yields a predicted

dielectric constant

^sod, predicted ” ~ 1)]^ “ 2.756.

This result differs by 2.1 percent from observed measurements using an open-ended

transmission line with a vector automatic network analyzer. This difference is probably due to

impurities in the quartz sand, which would affect both p^ and Sf^atrix'

B.4 NIST Measurements of Electromagnetic Soil Properties

B.4. 1 Natural Soils

A large number of dielectric measurements on soils have been made by NIST over the past

decade. Jesch [5.17], using frequency-domain open-circuit coaxial transmission line techniques

(described in detail in Chapter FV), has made dielectric measurements of five different natural

soil types. The textural types were sand, sandy loam, silt loam, clay loam, and clay. Sand is

defined as washed quartz grains having diameters from 0.027 mm to 0.60 mm, whereas silt is

washed quartz grains having diameters between 0.0033 mm and 0.021 mm. The clay used was

inorganic (kaolin or bentonite) with plate diameters less than 0.0033 mm.

The soils used by Jesch [5.17] were constructed as follows:

5-12
Sand texture

Sandy loam

Silt loam

Clay loam

Clay

9.5 parts sand, 0.5 parts clay, 0.3 parts silt;

6.4 parts sand, 3.6 parts clay, 2.5 parts silt;

2 parts sand, 8 parts clay, 6.8 parts silt;

3.2 parts sand, 6.8 parts clay, 3.3 parts silt; and

2 parts sand, 8 parts clay, 2 parts silt. These soil types, which are classified according to particle size and distribution, are

illustrated in figures 5.1 and 5.2.

The natural soil measurements for the above five soil types were accomplished for moisture

levels ranging from 0 to 48 percent and for test frequencies firom 300 to 9300 MHz. A small but

representative sample of the results is shown in tables 5.2, 5.3, 5.4, 5.5, and 5.6. Litde dispersion

is observed for all soils when Sw = 0. However, as water saturation increases, greater dispersion

in e' occurs, particularly as the matrix particle diameters decrease. From previous discussion,

this is expected, since the ratio of surface water to bulk pore water dramatically increases by

several orders of magnitude.

B.4.2 Synthetic Soils

A further extensive study of permittivity measurements has recently been accomplished at

NIST for synthetic soil materials that might be used in land mine validation testing. These

measurements were also performed in the frequency domain, using open-circuit coaxial

transmission-line techniques. Data were collected for wetted sands and various silicon carbide

(60-grit) and sand mixtures, as well as for silicon carbide and aluminum powder mixtures, over

the firequency range of 50 to 750 MHz. A brief summary of the results is given here.

Initially, the coaxial line was filled with tap water at 20°C in order to verify the

experimental procedure and calculations. Calculated relative permittivity and loss tangent from

50 MHz to 1.20 GHz are shown in figure 5.3, validating laboratory procedures.

Dielectric dispersion curves for wetted sand samples (water saturations varying from 4

percent to 14 percent) are given in figures 5.4, 5.5, and 5.6. The dielectric constant predictably

increases as water saturation increases, with dispersion evident principally at frequencies less

than 150 MHz. In addition, the relative permittivities at 600 MHz of a 5 cm section of various

water-saturated sands were measured as a function of drying time at approximately 20°C. These

results, illustrated in figure 5.7, show that host soil relative permittivities can vary by as much as

a factor of 2 over a 72 h drying time. Hence, if wetted soils are used for test beds, the host

relative permittivity should be measured at the time of validation testing so as to properly

compare different detection systems.

Various mixtures of silicon carbide and sand were also examined and the results are given

in figures 5.8, 5.9, 5.10, 5.11, and 5.12. The loss tangent increases slightly with increasing

5-13
Table 5.2

Experimental results of dielectric measurements on sand as a function of water saturation

Sw and frequency /[5.17].

Sand

/

Sw

£

= 0

/ Sw = 0.04

£'

Sw = 0.14

e'

(MHz) (Mean) (Std. Dev.) (Mean) (Std. Dev.) (Mean) (Std. Dev.)

300 2.702 0.0486 4.225 0.168 10.458 0.413

500 2.694 0.0492 4.079 0.162 10.641 0.415

1000 2.693 0.0515 4.019 0.165 11.179 0.349

2000 2.695 0.0515 3.940 0.142 9.951 0.334

4000 2.656 0.0419 3.946 0.148 10.544 0.441

9300 2.737 0.0660 3.373 0.198 10.179 1.140

Table 5.3

Experimental results of dielectric measurements on sandy loam as a function of water

saturation Sw and frequency/[5.17].

Sandy loam

/

Sw = 0

e'

Sw = 0.06

e'

5w = 0.24

e'

(MHz) (Mean) (Std. Dev.) (Mean) (Std. Dev.) (Mean) (Std. Dev.)

300 2.650 0.040 5.967 0.076 24.527 0.326

500 2.642 0.047 5.407 0.062 24.328 0.185

1000 2.660 0.079 4.909 0.028 22.705 0.625

2000 2.680 0.102 4.408 0.035 21.156 1.072

4000 2.691 0.125 4.301 0.019 15.549 1.560

9300 2.663 0.027 3.813 0.084 9.884 2.400

frequency as opposed to the case of wetted sands, where the loss tangent either remained

constant or decreased with frequency. Provided these silicon carbide mixtures have little

conductive (lossy) material in them, almost no dispersion is evident. Therefore, by controlling

the sand-silicon carbide mix, we may simulate a host medium whose effective permittivity

ranges from 3 to 13 but remains constant over the 50 to 750 MHz range (for any given mix) for

plastic land mine validation tests.

The addition of conductive aluminum powder to silicon carbide yielded expected

dispersion, as seen in figure 5.13. In fact, addition of even 5 percent aluminum powder (by

mass) caused dispersion to be evident. Hence, if conductive constituents such as aluminum

5-14
Table 5.4

Experimental results of dielectric measurements on silt loam as a function of water

saturation Sw and frequency/[5.17].

Silt loam

/

Sw ; e

= 0

t Sw = 0.05

e'

Sw = 0.10

(MHz) (Mean) (Std. Dev.) (Mean) (Std. Dev.) (Mean) (Std. Dev.)

300 2.340 0.049 4.453 0.097 9.151 0.186

500 2.321 0.051 4.038 0.083 8.083 0.182

1000 2.318 0.059 3.692 0.078 7.458 0.343

2000 2.343 0.085 3.391 0.080 5.573 0.157

4000 2.325 0.044 3.319 0.108 5.275 0.048

9300 2.408 0.094 3.170 0.092 4.065 0.268

Table 5.5

Experimental results of dielectric measurements on clay loam as a function of water

saturation Sw and frequency/ [5.17].

Clay loam

/

Sw

e

= 0

/ Sw = 0.06

e'

Sw = 0.12

£'

(MHz) (Mean) (Std. Dev.) (Mean) (Std. Dev.) (Mean) (Std. Dev.)

300 2.775 0.024 5.667 8.719 0.183 0.186

500 2.760 0.027 5.108 0.168 8.083 0.182

1000 0.037 4.649 0.101 7.363 0.223

2000 2.771 0.051 4.151 0.065 5.702 0.018

4000 2.758 0.048 4.024 0.156 5.682 0.005

9300 2.708 0.076 3.826 0.122 4.657 0.418

powder are added to host test lanes, the dispersion characteristics of the lossy test media should

be known a priori so that detection systems operating at different frequencies may be compared

properly (just as for wetted soils).

C Loose Matrix Synthetic Soiis for Narrowband or Fixed Frequency Test Lanes

for Nonmetaiiic Buried Objects

The point of the discussion on polarization mechanisms and relaxation processes is to

provide insight into the dielectric frequency dependence. Because the dielectric frequency

dependence for most soil materials is generally flat from 1 MHz to 1 GHz, there is valid

5-15
Table 5.6

Experimental results of dielectric measurements on clay as a function of water saturation

Sw and frequency /[5.17].

Clay

/

Sw

e

= 0

/ = 0.06

e'

Sw = 0.12

e'

(MHz) (Mean) (Std. Dev.) (Mean) (Std. Dev.) (Mean) (Std. Dev.)

300 2.717 0.052 6.030 6.034 10.676 0.204

500 2.703 0.053 5.399 0.015 9.402 0.186

1000 2.721 0.065 4.952 0.058 9.871 0.807

2000 2.721 0.068 4.187 0.005 6.294 0.124

4000 2.732 0.105 4.289 0.085 5.642 0.823

9300 2.677 0.052 3.754 0.053 5.185 0.950

Table 5.7

Preliminary set of standard background media for plastic mine detector validation testing.

Background medium Dielectric constant, e' Loss tangent, olcoe'

Dry silica sand 2.8 ± 0.05 0.01

Mixture: 40% SiC,

60% dry sand 5.0 ±0.2 0.013 ±0.002

Mixture: 80% SiC,

20% dry sand 10.0 ±0.4 0.018 ±0.003

justification for specifying synthetic test soils since their constituent electromagnetic properties

may be more controllable. A preliminary suggested set of standard background media is given in

table 5.7 for plastic mine detector validation testing.

In the natural soil environment encountered in the field, both conduction and dielectric

mechanisms influence mine detector signals. The common factor affecting both conduction and

dielectric mechanisms is the water saturation, Sw. Thus, in standard background test media it is

critical to control and/or to know water saturations. As a matter of test procedure, mine lanes for

UHF detectors should not only be enclosed in an electromagnetically anechoic environment (to

avoid spurious noise reflections off metallic objects), but also one which is shielded from the

varying influences of outside weather that affect Sw.

5-16
D Loose Matrix Natural Soils for Test Lanes for Metallic Buried Objects and

Broadband Systems

The use of synthetic soils for test lanes for single-frequency metal land mine detectors may

also be justified in terms of greater control of constituent properties governing detector signals.

Since these detectors generally operate at a frequency around 2500 Hz, there are usually no

significant attenuation problems in sensing metallic mine signals at burial depths that may be as

great as 1 m, even for a relatively conductive (CT// = 0.1 S/m) soil or for that which is

magnetically permeable (/t// = 1.4^o)- Suggested background media for single-frequency metal

mine detectors are given in table 5.8. Because the conductivity contrast between a metallic mine

and almost any conceivable natural soil is so high (effectively infinite for analysis purposes),

requirements on testing environments for metal mine detectors are much less stringent. That is,

test lanes for VLF or ELF single-firequency metal mine detectors need not be in an anechoic

environment nor do they need to be shielded from the influences of outside weather.

Signals measured by both single-frequency plastic mine UHF and metal mine VLF

detectors are sensitive only to the volume of the buried land mine. In order to gain shape

information (and therefore minimize false alarms due to ground clutter), future detectors must be

developed that are broadband in design and measurement—a fact well recognized in radar work.

As such development proceeds, the sensitivity to complex permittivity contrasts (mine visibility),

affected by dielectric relaxation processes, becomes relevant in understanding detector

performance and in specifying optimal system bandwidth.

The use of synthetic or natural soils in test lanes for broadband system validation should be

approached with caution. Natural soils often depart drastically from Debye behavior (single

relaxation dielectric dispersion) at frequencies less than 1 MHz. Furthermore, broadband

dispersive characteristics of synthetic soils (in the ELF or VLF frequency range) may not

compare to that of natural soils commonly encountered by the field soldier.

Thus, although it would be desirable to have background test material that is

electromagnetically analogous to those commonly encountered in the field, it is equally desirable

to have test soils that can be accurately specified, and whose properties can be closely controlled

and measured. On account of these two requirements, it is expedient to examine in more detail

the dielectric characteristics of water saturated soils with a view toward understanding those

parameters that can be controlled and are responsible for governing electromagnetic field

behavior. At the very least, such an examination should provide insight into how natural soils

might be used in performance testing of broadband mine detection systems or in predicting how

they may differ from the suggested set of standard background media given in tables 5.7 and 5.8.

5-17
Table 5.8

Preliminary set of standard background media for metal detector validation testing.

Background medium

Relative magnetic

permeability,

Dry sand 1.00

Mixture: Dry sand with

30% magnetite 1.35

E NIST Recommendations and Conclusions

Considering the choice of a test range configuration, whether to use actual soils,

simulations formulated with liquids or loose granular materials, or rigid-matrix “brick” soil

simulations, any one of these possibilities could be used for target-based testing. Our

recommendation is to use the rigid-matrix brick approach because it is the most versatile and

allows the tightest control of all test range parameters. Also, several of the proposed tests

described in chapter VII could not be performed well, if at all, with the other approaches. Those

tests are explicitly identified in chapter VII. What follows in this chapter is a discussion of the

advantages and disadvantages of the different approaches for test range soil media mentioned

above. (It should be noted here that NIST’s work on developing the technology required for the

rigid-matrix brick approach is not yet complete and will be reported to BRDEC separately.)

E.1 Natural Soil Test Range

Although utilizing natural soils in the test range would be the best approach in terms of

approximating realistic test conditions, there are many disadvantages to this scheme.

(a) Water!Soluble Salt Content Variation. The complex relative permittivity of soils,

expressed as e' — is heavily dependent on the soil’s water and soluble salt

content. The magnitude of the permittivity of dry soils |e' - is usually in the

range of 2 to 3, even with soluble salts present, but if the soil contains water, the

magnitude of the permittivity can rise to 20 or greater, depending on the water content

and the frequency of the EM excitation. This is due primarily to the fact that the

permittivity of pure water is about 80. Also, the soil’s attenuation of EM energy,

directly proportional to the imaginary part e'' of its permittivity, is a strong function

of water/salt content. The attenuation of most dry soils is in the range of very nearly

zero to about 1 dB/cm, depending on the soil type and the frequency. Adding water

to the soil can cause this attenuation to rise as high as 40 dB/cm or greater in the

5-18
extreme case of water saturation, high salt content, and high frequency. Much of this

high loss is caused by the highly conductive electrolytes formed by the addition of

water to the salts present in the soil.

More than any other variable, the permittivity and attenuation of soils is

controlled by their water/soluble salt content. Therefore, it is crucial to control the

value of this variable if accurate, repeatable detector tests are desired. Unfortunately,

the use of natural soils makes this control practically impossible. Water evaporation

and changes in relative humidity will cause the soil’s water content to vary

continuously. Even if a means were developed to continuously monitor the

permittivity and attenuation of the test range soil, the interpretation of comparative

detector test results would be extremely difficult. Also, attempts to both control and

continuously monitor the permittivity and attenuation of natural soils in a test range

would undoubtedly be time-consuming and expensive.

(b) Water Gradient Variation. The use of natural soils would also make it very difficult

to control the gradient of water content as a function of soil depth. Gravity and

evaporation will cause the soil’s water content to increase with increasing depth under

most conditions. (An exception, of course, would be if the soil were completely

saturated, as in a swamp.) The main point here is that it would be impossible to

control this gradient, and thus, the soil’s permittivity as a function of soil depth.

Although it could be argued that water content gradients certainly exist “in the field,”

it may be desirable in the future to analyze detector test results with the assumption

that the soil permittivity is uniform throughout the test range soil volume. In any

event, the most versatile test range configuration would allow the control of this

variable.

(c) Soil Density Variation. The use of natural soils in a detector test range would

inevitably lead to variations in the soil density, and therefore, in its permittivity.

Compaction over time would occur and the localized soil density would be changed

whenever the range was disturbed, say, by walking on the soil or by burying a new

target. Although the permittivity changes would not be as great as in the case of

water content, they still could be on the order of tens of percent. As in the water

content case, it would be very difficult and expensive to both control and monitor the

soil density.

(d) Soil Density Gradient Variation. Similar to the water content gradient problem, the

effects of gravity cause a soil density gradient problem, with the density of the soil

increasing with increasing depth. Again, there may be analysis considerations that

5-19
would make it desirable to be working with test range soils that are uniform in

density.

(e) Soil Reproducibility. Finding and/or preparing natural soils that would be suitable for

use in a detector test range would be very difficult. Variations in the distributions of

grain size, grain shape, and mineral content would almost certainly occur, leading to

nonuniformities in the soil’s permittivity and attenuation. This problem would also

make it difficult for any other laboratories to duplicate the Army’s test range.

However, this reproducibility problem does not, in itself, preclude the use of natural

soils. It simply increases the testing uncertainties.

(f) Target Position and Orientation Variations. The use of natural soils would allow for

wide variations in the repeatibility of target placement. Small variations would

probably occur even if the targets were never moved. Removing and reburying

targets would cause still larger variations.

(g) Soil Surface Variations. The amount of EM energy entering the soil from the air

above, along with its spatial distribution is a strong function of the air-soil interface

profile. Variations in this profile would almost certainly occur over time with the use

of natural soils, making it very difficult to ensure that each detector is being tested

under the exact same conditions.

(h) Soil Contamination Variations. Over time, it would be very difficult to see to it that

no contaminating materials were ever introduced into a natural soil test range. Even

more difficult would be trying to identify any observed variations in test results as being caused by range contamination, rather than by variations in one of the other

variables previously mentioned.

E.2 Loose Granular Material Test Range

Another approach to configuring the detector test range is the use of mixtures of loose,

granular materials in place of natural soils. The advantage to this approach, as compared to using

natural soils, is that the sources of parameter variation described in (a), (b), and (e), above, could

be eliminated. Mixtures of different granular materials could be formulated which would possess

the same permittivities and attenuations as those occurring in natural soils. No water would be

required because its effects would be included in the mixture. Also, there would be no

reproducibility problem because the EM properties of the mixtures would presumably be

constant as long as the ratios of constituent materials were held constant. Therefore, we believe

that using loose granular materials in place of natural soils for a test range would be a better

approach.

5-20
However, similar to the case of natural soils, there are many disadvantages to this scheme

as well. The parameter variation problems listed above in (c), (d), and (f) through (h), would still

exist In addition, two new problems would be created. First, the permittivity and attenuation of

natural soils, especially with water present, is a strong and generally nonlinear function of

frequency. Although it is possible to formulate mixtures of dry, granular materials that would

simulate the EM properties of various natural soils, this simulation would be accurate only over a narrow band of frequencies. We know of no mixtures which would simulate water-containing

natural soils over the frequency band of, say, 100 MHz to 5 GHz, a useful band for detectors.

Thus, different mixtures would have to be formulated for testing detectors that possess different

operating frequencies. Also, the testing of broadband detectors, for example, swept-frequency or

pulse-type detectors, would necessarily have to be done under unrealistic conditions.

Second, there would be problems in terms of temperature dependence of the permittivities

of the loose granular mixtures. The variation in permittivity for damp or wet natural soils, as a

function of temperature, is of the same order as the variation as a function of frequency.

Therefore, the same problems as those posed by frequency variation, mentioned above, would

exist

E.3 Liquid Material Test Range

Still another approach to configuring the detector test range is to simulate natural soil media

with a liquid phase material, such as an emulsion of oil and water, or oil and salt water. If water

were one of the main constituents of the liquid mixture, then this method would be superior to

using either natural soils or loose granular materials. Smith and Scott [5.23] are presently doing

some work on this approach at the Georgia Institute of Technology. Essentially, their reported

work consists of mixing mineral oil and water or salt water into emulsions that possess

electromagnetic properties that closely approximate those of natural soils. As part of their

approach, they have incorporated frequency scaling in order to reduce the necessary sizes of the

range and test components.

All of the disadvantages stated above, in principle, would not exist with this approach.

However, there would be some restrictions and disadvantages. First, it would not be possible to

simulate water or soil density gradients. Second, it would be very difficult, if not impossible, to

incorporate surface profile variations into the range. Third, the benefits of frequency scaling

would not be applicable for mine detector testing because it is not feasible for the Army to scale

detectors, that is, reduce their size and increase their operating frequency proportionately. And

last, there could be a problem with stability of the properties of the liquid mixture. Emulsions

tend to separate, generally, on a time scale of minutes to weeks, so a means would be necessary

for re-emulsifying the mixture periodically. Depending on the size and configuration of the test

5-21
range, this problem could easily become practically overwhelming. There are other potential

problems with this approach, such as cost and ease of detector testing, but we should not rule out

this approach.

E.4 Rigid-Matrix Brick Test Range

The last test range approach to be considered here is one using rigid-matrix bricks as a

natural soil simulation. This is the approach that we recommend because it is the most versatile

and allows the tightest control of all conceivable test range parameters.

This approach is quite similar to the liquid emulsion approach, discussed above, in the

sense that both methods involve the use of water or salt water solutions as one of the material

constituents. Thus, the dominating effects of water/salt water on the permittivity of natural soils

could be accurately and faithfully reproduced, with none of the parameter control problems that

would exist with the use of natural or loose granular material simulations.

The principal advantages of using rigid-matrix bricks, as opposed to liquid emulsions, are

as follows: First, although bricks can be constructed with negligible property gradients, it also

would be possible to build gradient simulations into the bricks if so desired. That is, by

constructing the bricks in layers, with slightly different permittivities in each layer, naturally

occurring water or soil density gradients could be simulated. Second, irregular surface profiles

could be built into the top layer of bricks allowing the testing of detectors under conditions

where the air-earth interface is not a flat plane. The built-in surface irregularities would remain

constant, thus ensuring repeatable detector tests. Third, target position and orientation variations

would be held to a minimum because the targets could be embedded in the bricks. Fourth, a

wider variation in permittivity contrast between the targets and the soil medium would be

possible because targets could be formulated from the same materials used to make the bricks.

And last, the range would be very versatile because changing its testing configuration would only

involve moving a small number of bricks.

The disadvantages of this approach are few. As with the case of liquid emulsion

approaches, insufficient work has been done on this approach to identify and quantify the real

disadvantages. However, potential problem areas can be identified. First, like the liquid

emulsion approach, this brick approach could be quite expensive to implement due to brick

material costs and brick manufacturing costs. We cannot estimate these costs until our brick

development work is complete. Second, there is a potential problem with unwanted

electromagnetic wave reflections from brick interfaces and from the boundaries of the range.

(Reflections from the range boundaries are a problem that affects all four of the approaches

discussed here and must be dealt with in the actual range design.) The measurement and analysis

of brick-interface reflections is one of the tasks for future work by NIST. A third potential

5-22
problem is ease of range operation. Repeated moving of the bricks must be minimized because

the process would be time consuming and arduous, and it would reduce the operating life of the

range due to damage and contamination. In this regard, NIST has also proposed that a computercontrolled detector-positioning system be employed on the range in order to eliminate the need

for personnel to walk on it.

References (V)

[5.1] Geyer, R.G. Magnetic susceptibility measurements for mine detection: Some general

comments. NIST Report, U.S. Army Belvoir Research Development and

Engineering Center, Fort Belvoir, VA, 10 p.; 1987.

[5.2] Heiland, C.A. Geophysical Exploration. Englewood Cliffs, New Jersey: Prentice

Hall, pp. 310-314; 1940.

[5.3] Geyer, R.G. Experimental parameters for land mine detection performance

evaluations. NIST Report, U.S. Army Belvoir Research Development and

Engineering Center, Fort Belvoir, VA, (Proj. 7232457), 100 p.; November 1987.

[5.4] Ulaby, F.T.; Moore, R.K.; Fung, A.K. Microwave remote sensing, Vol. HI: From

theory to applications. London, England: Addison-Wesley Publishing Co.; 1982.

[5.5] Hasted, J.B. Liquid water: Dielectric properties. Chapter 7 Water—

comprehensive treatise, I, The physics and chemistry of water. New York: Plenum

Press; 1972.

[5.6] Tinga, W.R. Generalized approach to multiphase dielectric mixture theory. J. Appl.

Phys. 44: 3897-3902; 1973.

[5.7] Ho, W.W.; Love, A.W.; VanMelle, M.J. Measurements of the dielectric properties of

sea-water at 1.43 GHz. NASA Contractor Report CR-2458. NASA Langley

Research Center, Langley, VA; 1974.

[5.8] Bottcher, C.J.F. Theory of electric polarization. Amsterdam: Elsevier, 1952.

[5.9] Kurosaki, S. The dielectric behavior of water sorbed in silica gel. J. Phys. Chem. 58:

320-324; 1954.

[5.10] Bockris, J.; Gileady, E.; Huller, K. Dielectric relaxation in the electric double layer.

J. Chem. Physics. 44: 1445-1456; 1966.

[5.1 1] McCafferty, E.; Zettlemayer, A.C. Adsorption of water vapor on discussions, Vol.

52: 239-254; 1971.

[5.12] Hasted, J.B. The dielectric properties of water in Progress in Dielectrics, Vol. 3., J.B.

Birks and J. Hart, eds. New York: John Wiley & Sons; 1961, 101-149.

5-23
[5.13] Nair, N.K.; Thorp, J.M. Dielectric behavior of water sorped in silica gels, Part 1,

commercial silica gels and the elimination of dielectric hysteresis. Trans. Faraday

Soc. 61:962-974; 1965.

[5.14] Kenyon, W.E. Texture effects on megahertz dielectric properties of calcite rock

samples. J. Appl. Phys. 55: 3151-3159; 1984.

[5.15] Dobson, M.C.; Ulaby, F.T.; Hallikainen, M.T.; El-Rayes, M.A. Microwave dielectric

behavior of wet soil. Part II: Dielectric mixing models. IEEE Trans. Geosci. Remote

Sensing. GE-23(1): 35-46; 1985.

[5.16] Shutko, A.M.; Reutov, E.M. Mixture formulas applied in estimation of dielectric and

radiative characteristics of soils and grounds at microwave frequencies, IEEE Trans.

Geosci. Remote Sensing. GE-20: 29-32; 1982.

[5.17] Jesch, R.L. Dielectric measurements of five different soil textural types as functions

of frequency and moisture content. Nat. Bur. Stand. (U.S.) NBSIR 78-896, 26 p.;

1978.

[5.18] Bussey, H.E. International comparison of complex permittivity measurement at 9

GHz. IEEE Trans. Instrum. Meas. IM-23(3): 235-239; 1974.

[5.19] Bussey, H.E.; Gray, J.E.; Bamberger, E.C.; Rushton, E.; Russel, G.; Petley, B.W.;

Morris, D. International comparison of dielectric measurements. IEEE Trans.

Instrum. Meas. IM-13: 305-311; 1964.

[5.20] Von Hippel, A.R. Dielectric materials and applications. New York: The Technology

Press of M.I.T. and John Wiley & Sons, 438 p.; 1954.

[5.21] Clark, S.P. Handbook of physical constants. Geol. Soc. Amer. Memoir 97, 587 p.;

1966.

[5.22] Morey, L. Properties of glass. New York: Reinhold Press; 1954.

[5.23] Smith, G.S.; Scott, W.R., Jr. The use of emulsions to represent dielectric materials in

electromagnetic scale models. IEEE Trans. Ant. Prop., vol. 38, No. 3: 323-334;

March, 1990.

5-24
Particle

Diameter

(mm) 2.0

1.0

0.5

0.25

0.1

0.05

0.002

0.000 _°

10° 10^ 10^ 10° 10° io’°

USDA

International

System

Very Coarse

Sand Coarse

Sand Coarse Sand

Fine Sand

Fine Sand

Fine

Sand Very Fine

Sand

Silt Silt

Clay Clay

Approximate Surlace

Area, cm^ g’'

\

\

\

\

\

\

\

\

\

\

\

10

° 10^ 10

° 10° 10^ 10° 10°

Figure 5.1 Particle properties for different soil types.

5-25
% Clay

Figure 5.2 Particle distribution for different soil types.

5-26
TAP WATER

Figure 5.3 Measured permittivity and loss tangent vs frequency

for tap water at 20 °C.

5-27
SAND

Figure 5.4 Measured permittivity and loss tangent vs frequency

for sand with 4% water.

5-28
LOSS

TANGENT

SAND

Figure 5.5 Measured permittivity and loss tangent vs frequency

for sand with 8% water.

5-29
SAND

Figure 5.6 Measured permittivity and loss tangent vs freguency

for sand with 14% water.

5-30
PERMITTIVITY OF 5 CM SECTION OF WET SAND AS

Figure 5.7 Measured permittivity at 600 MHz vs drying time at

2 0‘’C for various sand and water mixtures.

5-31
SILICON CARBIDE & SAND MDGDRE

Figure 5.8 Measured permittivity and loss tangent vs frequency

for 100% silicon carbide.

5-32
SILICON CARBIDE & SAND MDCTURE

90% SiC 10% Sand

Figure 5.9 Measured permittivity and loss tangent vs frequency

for 90% silicon carbide 10% sand mixture.

5-33
SILICON CARBIDE & SAND MIXTURE

Figure 5.10 Measured permittivity and loss tangent vs frequency

for 80% silicon carbide 20% sand mixture.

5-34
SILICON CARBIDE & SAND MDOURE

Figure 5.11 Measured permittivity and loss tangent vs frequency

for 70% silicon carbide 30% sand mixture.

5-35
LOSS

TANGENT

Figure

SILICXJN CARBIDE & SAND MIXTURE

. 12 Measured permittivity and loss tangent vs frequency

for 100% sand.

5-36
LOSS

TANGENT

SILICON CARBIDE & ALUMINUM MIXTURE

Figure 5.13 Measured permittivity and loss tangent vs frequency

for various silicon carbide and aluminum powder

mixtures

5-37

VI Target Standards

A introduction

“Standard” targets should have certain desirable characteristics—such as stability over

time, reproducibility, and ease of fabrication. Unfortunately, some approaches to standardization

involve particularly high costs; this tends to limit the use of good standards to a few well

equipped laboratories. We distinguish two classes of standards:

(1) standards based on some well established theory or on known, immutable physical

properties, and

(2) artifact standards which are reproducible and stable but for which a good theoretical

model does not yet exist.

Where possible, we specify that testing of mine detectors be carried out with metallic and

nonmetallic targets fabricated from pure or well analyzed materials. We also suggest that targets

have simple cylindrical or spherical shapes. When, for purposes of realistic modeling, other

target shapes and components must be used, at least the primary standards should be cylindrical

or (preferably) spherical. Good theoretical models for the scattering of electromagnetic waves

are available for these simple shapes. Also, the detection of spherical targets does not depend on

target orientation. Furthermore, we suggest that the primary standard targets be molded or

machined of single component materials without voids or simulated firing mechanisms. This can

aid in further reducing the number of variables. Oxidation, water adsorption, and any surface or

dimensional changes of the targets should be carefully monitored or entirely avoided if possible.

B Metallic Standards

Testing metal detectors has traditionally involved checking whether they can succeed in

finding an ordinary U.S. one-cent coin buried in various soils. This nicely illustrates one of the

pitfalls of using artifact standards. The Government started minting copper-clad zinc pennies in

October 1982. The weight of the coin decreased from approximately 3.1 g to approximately

2.55 g. To our knowledge, the copper alloy used is chosen on the basis of its durability—its

conductivity is not controlled. For careful work, we should avoid using ordinary coins as standard targets.

Objective testing of metal detectors can best be carried out with a set of known, repeatable

metallic targets. Low frequency metal detectors transmit in the kilohertz range and sense eddy

currents induced in buried metallic objects. The magnitude of these eddy currents varies as the

-1/2 power of the conductivity of the target material and is directly proportional to target

6-1
volume for simple shapes such as cylinders or spheres [6.1]. It is important, when describing

how these targets are made or copied, to specify the conductivity of the material. Certain

materials, such as brass or aluminum alloys, exhibit very different conductivities depending on

their composition and heat treatment. Highly purified elemental metals, especially those with the

highest conductivity, show very little variation in conductivity from lot to lot. A further

advantage is that their conductivity varies only slightly with changes in room temperature so this

effect can be neglected when testing metal detectors. Accordingly, suitable standard test objects

should have high conductivity. In increasing order of conductivity, this means gold, copper, or

silver. We chose copper since it is the least expensive. “Five nines” purity electro-refined

copper is readily available. The American Society of Metallurgists (ASM) of Metals Park, Ohio,

lists the properties of various grades of wrought copper in the 8th edition of the Metals

Handbook [6.2]. Oxygen-free, electro-refined copper has a conductivity of 5.977 x 10"^ S/m at

20°C.

Copper corrodes easily. We spray our copper test objects with a thin coating of spray

lacquer. If considerable wear and handling are likely to take place, a hard chromium plating over

a nickel plating would be appropriate for these targets.

A set of seven copper cylindrical test objects was provided to BRDEC in July 1986. These

copper test objects were all machined firom right-circular cylinder bar stock on a precision lathe

at NIST. Electro-refined copper is commercially available. The weights and other specifications

are given in table 6.1.

The smaller objects are suitable for calibrating the response of metal detectors (those

operating at frequencies in the kilohertz range) to objects as small as firing pins. The other

objects in this set were provided to test the response of low-frequency metal detectors to

significantly larger amounts of metal. Large metal objects (such as unexploded ordnance) must

often be located using mine detectors optimized for very small metal objects. Eddy currents die

out slowly for the case of the large copper targets. Tests of target detectibility with the 32 g and

larger cylinders should be performed with due regard for the long time constants involved (on the

order of one-half second or more).

In addition to the copper cylinders, which we consider the primary targets for testing

purposes, we have fabricated a small set of stainless steel pins. We consider the pins to be

secondary standards, useful for simulating the properties of firing pins in mines. Because of the

difficulty of specifying the composition of stainless steel, results using these objects may show

greater variability than those using the copper cylinders. The pins should, however, prove useful

in tests of the detectors for plastic mines where the orientation of such pins may be important.

These pins can simply be cut from existing right-circular cylinder bar stock, also commercially

available. Specifications for the pins are given in table 6.2.

6-2
Table 6.1

Copper test objects.

Material: Copper, electro-refined, 99.999% pure

Shape: Right-circular cylinders, diameter twice the height

Density: 8.96 g/cm^

Mass (g) Diameter (mm) Height (mm)

1 6.58 3.29

2 8.28 4.14

4 10.44 5.22

8 13.15 6.58

32 20.87 10.44

128 33.14 16.57

512 52.60 26.30

All American Iron and Steel Institute (AISI) type 300 series steels are chromium-nickel

austenitic steels in grades having up to 30% chromium and up to 20% nickel. Type 304 contains

the following percentages:

Cr 18 to 20 %

Ni 8 to 12%

C 0.08 % (max)

Mn 2 % (max)

Si 1 % (max)

The composition range is based on ladle analysis for standard AISI Type 304 as of 1959. It

is not a specification. It is a classification which has been agreed upon and used by producers in

the United States for grades which have attained an arbitrary annual minimum tonnage [6.2]. We

do not recommend 304 stainless steel for the standard targets, although it is useful for secondary

standards and for estimating the performance of mine detectors in locating objects such as firing

pins.

C Nonmetallic Standards

NIST fabricated Teflon and nylon spheres as calibration targets for nonmetallic mine

detectors and delivered these to BRDEC in July 1986. The masses and diameters for these

objects are listed in table 6.3.

6-3
Table 6.2

Stainless steel test objects.

Material: Type 304 stainless steel

Shape: Cylindrical pins or narrow rods

Density: Approximately 7.9 g/cm^

Mass (g) Diameter (mm} Height (mm

)

1.56 3.175 25.4

0.39 1.5875 25.4

0.096 0.7938 25.4

Table 6.3

Spherical plastic test objects.

Material: Nylon Material: Teflon

Shape: Spherical Shape: Spherical

Density: 1.15 g/cm^ Density: l.lg/crr?

Relative dielectric constant: 3.0 Relative dielectric constant: 2.1

Mass(g) Diameter (mm) Mass(g) Diameter (mm)

36.8 39.0

72.0 49.2

144.0 62.0

288.0 78.2

576.0 98.5

68.4 39.0

136.8 49.2

273.6 62.0

547.2 78.2

1094.4 98.5

Teflon and nylon were chosen for these reasons:

(1) nylon and Teflon are relatively impervious to moisture as well as to common alkaline,

acidic, or salt components likely to be found in ordinary soils;

(2) these materials are readily available commercially in shapes and sizes suitable for

further machining; and

(3) their relative dielectric constants (2.1 and 3.0) nicely bracket known values for many

common explosives.

We decided to systematically vary the parameter of target volume rather than target mass,

specifying five different sizes in each of the two materials. The volumes increase by an

approximate factor of 2 for each step up in size.

6-4
Quantity

Table 6.4

Cylindrical plastic test objects.

Diameter (in mm) Height (in mm

Material

2 162 25.4 Nylon

2 152.4 50.8 Nylon

2 304.8 76.2 Nylon

2 76.2 25.4 Teflon

2 152.4 50.8 Teflon

2 304.8 76.2 Teflon

Discussions about the procedures for plastic mine detectors have focussed on two

additional points: (1) the need for still larger test objects, and (2) whether the shape of the test

objects could be made to conform more closely to that of known, real mines. We think that

small spheres are most likely to give the best, most repeatable results. Spherical targets do not

require careful alignment with the surface of the ground—only the depth matters. Spherical

targets have a well defined radar cross section which does not depend on target orientation. They

are also preferred on theoretical grounds, since the EM scattering cross section is well known

even when the illuminating wavelength is much smaller than the diameter of the sphere [6.3]. A

theoretical paper by Hill [6.4] outlines a method for calculating the scattering cross section for

buried spheres, cylinders, and rectangular parallelopipeds when the dielectric contrast between

the target material and the background medium is low.

We have fabricated both nylon and Teflon cylindrical targets of various sizes for use as secondary standards. They are similar to the dummy mines of Type I, class B, sizes 1, 2, and 3

[6.5]. However, they do not have air voids or replicas of firing mechanisms installed. All of the

plastic targets, both spherical and cylindrical, were machined on a precision lathe at NIST.

Numerical control of the lathe was required for the spherical shapes. All were cut from rightcircular cylinder bar stock which is commercially available. Quantities delivered to the Army in

August, 1988, and dimensions for these mine replicas are given in table 6.4.

D Relationship of Standards to Realistic Targets

D.1 Metallic Objects Relationships

For low-ffequency treasure finders and metallic mine detectors, the tests with the

cylindrical copper targets are more than adequate to establish detector sensitivity and other

performance requirements. The response of low-sensitivity equipment (operating in the kilohertz

range) shows little if any dependence on target orientation even for targets which are highly

6-5
acicular (needlelike). The situation is very different for equipment operating at microwave

frequencies, where polarization effects become important. When testing the ability of plastic

object detectors to locate, say, metallic firing pins, target orientation must be carefully controlled

for the resulting data to be useful and repeatable. In particular, the targets listed in table 6.2 are

likely to exhibit very different radar cross sections when placed parallel or perpendicular to the

E-field of an operating AN/PRS-7 plastic object detector.

D.2 Nonmetallic Object Relationships

The materials from which these targets were machined were chosen to approximate the

range of dielectric constants likely to be encountered in real mines. These targets (perfectly

spherical or perfectly cylindrical) scatter electromagnetic waves in a very predictable fashion.

An analytic solution for spherical objects is well known [6.3, 6.6]. Good approximations are available for the scattered fields from cylindrical objects [6.4]. The targets we have provided

cover the range from anti-tank to smaller than anti-personnel. However, these standards do not

mimic all features of real mines. Typical mines and mine replicas contain air voids, firing

mechanisms, pull rings, and other accessories which significandy modify the radar cross section.

BRDEC has made arrangements for the manufacture of a number of mine-like targets to address

this issue [6.5]. Such targets cannot simultaneously satisfy two divergent requirements: (a)

complete properties of real mines, and (b) precision in detector performance measurement and

test repeatability. Realistic mine replicas possess radar cross sections which vary with changes

in temperature and barometric pressure. Their nonuniform filler separates into layers or even

deteriorates over time. Air voids (and light-weight foam inserts to simulate voids) distort or change shape permanently. Nevertheless, they are interesting test objects which certainly look

like mines; it should be possible to use them as secondary or tertiary standards so long as their

radar cross sections are frequently compared with those of the primary standards.

The detection of buried, nonmetallic objects is a difficult task [6.7]. Their typically lowcontrast ratio in sandy soil or loam leads to their becoming nearly invisible to electromagnetic

waves. Because of their small size, carefully made anti-personnel mines (those without air voids

or metal pins) are often completely undetectable with present-day equipment.

E NIST Recommendations and Conclusions

Accurate testing of treasure finders and mine detectors is notoriously difficult When

performance tests are carried out, the degree to which meaningful, repeatable data can be

obtained is critically dependent on the targets being used. At UHF and higher frequencies, target

orientation must also be carefully established unless the targets are spherical. What follows are

6-6
NIST’s recommendations for target standards that should be used to test metal detectors and

nonmetaUic object detectors.

E.1 Targets for Metal Detector Testing

It is difficult or impossible to obtain consistent results with mine replicas for coins. The

coated pure copper standards that NIST has provided to the Army should be used as primary

standard targets for the testing of low-frequency metal detectors; their permeability and

conductivity are known very accurately and their masses and volumes have been carefully

controlled. For more realistic target detection testing, the Type 304 stainless steel cylinders that

have also been provided to the Army should be used. Although the permeability and

conductivity are not known to the accuracy of the copper standards, they should prove

sufficiently accurate and repeatable for all but the most demanding tests.

E.2 Targets for NonmetaUic Object Detector Testing

Sets of standard spherical and/or cylindrical nylon and Teflon targets should be utilized for

the most accurate tests of nonmetaUic object detectors. The spherical standards should be used

whenever errors due to target orientation must be avoided in the tests. The cylindrical standards,

being somewhat more reaUstic in shape, should suffice for the majority of the tests as described

in chapter Vn. The principal reason that NIST recommends the use of these targets for most of

the tests is that they are homogeneous, have weU known constituent electromagnetic properties,

and, most important for theoretical modeling puiposes, have known scattered electromagnetic

field signatures. As is stated in chapter Vn, these standards do not preclude using more realistic

mine replicas as targets for other tests.

For a more robust testing strategy, NIST recommends fabricating targets using the same

rigid-matrix technology as is being studied for soU standards. The results of these studies on

rigid-matrix technology, being done under contract by NIST for BRDEC, are not yet available

and wiU be reported in a second document. With little additional effort, standard targets can be

molded with known electromagnetic properties that can cover a wide range of values. Thus, for

example, targets for a set of tests to measure permittivity contrast ratio could be constructed with

incrementally varying permittivity. Using the nylon or Teflon targets, on the other hand, would

require varying the permittivity of the soil standards which would be more difficult and

expensive as well as less realistic. In addition, using this technology would allow the fabrication

of extremely realistic mine replica targets with such features as air gaps, inhomogeneities

(explosives with a plastic shell, for example), and firing pins.

6-7
References (VI)

[6.1] Wait, J.R. A conducting sphere in a time-varying magnetic field. Geophysics; 16(4):

666-672; 1951.

[6.2] Metals Handbook, 8th Edition, American Society of Metallurgists; 1980.

[6.3] Mie, G. Beitrage Zur Optik Triiber Medien, Speziell Kolloidaler Metallosungen.

Annalen der Physik 25, p. 377; 1908.

[6.4] Hill, D. A. Electromagnetic scattering by buried objects of low contrast IEEE Trans.

Geosci. Remote Sensing GE-26, pp. 195-203; 1988.

[6.5] Brown, D.; Tanner, John D.; Paca, Francis B. Mine detector targets: Metallic and

nonmetallic, generic set, development efforts. VSE Corp. Report ASD/0047-

87/35RD; October 1987.

[6.6] Van deHulst, H. C. Light scattering by small particles. N.Y.: J. Wiley & Sons, 1957.

[6.7] Pear, Robert. Returning Afghan refugees face peri] of millions of land mines. NY

Times; 13 August, 1988.

6-8
VII standard Measures of Effectiveness

A Introduction

This chapter contains NIST’s recommended strategy for testing and comparing portable,

electromagnetic-based mine detectors. The next section contains a description of this strategy

and our reasons for recommending it. Section VII.C contains detailed instructions on how to

perform a recommended core set of tests, and section Vn.D contains general descriptions for an

advanced set of tests. And last, section VII.E contains a recommended approach for

comparatively rating each detector tested.

B Test Strategy

The test strategy that we recommend is predominantly a target-based strategy, as opposed

to a detector-based. (See Chapter EH, p. 3-1, for NIST’s definitions of target-based and detector

based.) The method proposed, which we call the “eye-chart” method, is patterned after the

physician’s eye chart. This chart consists of rows of letters or other characters, with the

characters of each row smaller than those of the previous row. The physician’s test consists of

having the patient read the characters from a fixed distance until they are too small for the patient

to see. The physician can thus determine the patient’s visual acuity.

In a like manner, by carefully selecting a set of targets and embedding them in a soil or

soil-like medium, we can create a test range wherein each succeeding target is more difficult to

detect than the last in some sense. For a particular detector test, we would simply determine how

many of the targets the detector could find from the complete set when the detector is operated

according to the manufacturer’s instructions. Then, by conducting tests on an ensemble of

different target sets, each one designed to test for a different parameter sensitivity, we could

determine an overall “score” for each detector. This in turn, would allow us to comparatively

rate each manufacturer’s detector(s) in order to determine which one exhibits the “best”

performance.

Probably the best way to illustrate this target-based approach to detector testing is with an

example. Let us assume we wish to perform a “target depth” test with which we want to

determine the detector’s ability to find identical targets buried at different depths in the

surrounding soil medium. The test range configuration for this test would consist of a number of

identical targets, buried at increasing depths and suitably separated from each other in order to

assure target isolation. All other variables, such as target rotational orientation, target £^, and

Gf, and soil medium and <7^, would be held constant. Then, the detector under test would be

7-1
operated according to the manufacturer’s instructions in an attempt to have it detect each of the

targets in the set. The raw “score” for the detector for this test would simply be the number of

targets detected.

The resolution of this test is a function of the number of buried targets just as the resolution

of the physician’s eye chart is a function of the number of rows of characters. The range of

difficulty for this test is a function of the maximum depth at which a target is buried, just as the

range of difficulty for the physician’s eye chart is determined by the size of the characters in the

bottom row. The absolute accuracy of this test is a function of our ability to control all of the

pertinent target/soil parameters, but here it should be noted that the relative accuracy of the test,

for purposes of comparing two or more detectors, is a function only of the stability in time of the

pertinent target/soil parameters. Their absolute values are only of secondary importance.

There are many advantages to this method. First is its simplicity. Since each test is

designed to observe the detector’s sensitivity to only one parameter at a time, then analysis of

each detector’s weak and strong points will be relatively easy.

Second, this approach is relatively easy. It does not require the Army to perform difficult,

expensive, and time consuming detector-based tests. Although NIST recommends a few

detector-based tests in this report, none of them requires performing any measurements on the

detector itself. NIST believes that it is not necessary to know such details about the detector as

its electromagnetic antenna field pattern, its bandwidth, or its signal-to-noise ratio, in order to

determine its ability to detect mines. In general, the design and modeling of detectors should be

left to industry whenever possible, and the Army, as the end user of detectors, should be

concerned only with their functionality. The test strategy presented in this report reflects this

philosophy.

The third advantage of this strategy is its generality. Practically any portable EM-based

mine detector can be tested using this strategy. Any test strategy that requires development of

new tests for each new detector technology that evolves is inferior.

A fourth advantage of this strategy is its realism. Rather than an approach which uses a set

of detector-based parameters to predict the detector’s ability to find mines, this approach

involves the actual quantitative measurement of a detector’s ability to find mines under varying

conditions. No mathematical modeling, predictions, or extrapolations are necessary.

The last advantage of this strategy is its versatility. NIST has made a number of

recommendations in this report concerning optimal target standards (nylon and Teflon spheres

and cylinders) and soil standards (rigid-matrix bricks). However, this test strategy will also work

using practically any type of target soil. The strategy, itself, imposes no restrictions on the actual

test range design chosen by the Army.

7-2
C Recommended Core Tests

Contained in this section are detailed descriptions of seven core tests that we recommend

the Army implement as soon as is practical. NIST has chosen these particular tests as a core set

because they are relatively easy to implement and because they offer a substantial challenge to

any portable, EM-based mine detector. The next section contains less-detailed descriptions of

seven more tests that the Army may choose to implement in the future. However, we

recommend that these seven core tests be implemented first. With the experience gained from

performing these core tests over a period of time, NIST believes that the Army will be in a much

better position to decide what additional tests may be required and what the test designs should

include.

In the detailed descriptions of each recommended test that follow, we have chosen

numerical values for each of the relevant variables that we believe are realistic. However, these

values should be considered to be only examples; the final choice of such parameters as the

number of targets and the range of variables for each test should be decided by Army personnel,

using their superior knowledge of actual field conditions that are desired for simulation. For each

test, five elements are presented; the objective of the test, the test’s requirements, test restrictions,

a description of the recommended test technique, and test variations that the Army may wish to

try.

In order to implement these tests, any one of the test range configurations and target sets

discussed in chapters V and VI can be used. However, we note under “Restrictions” those range

configurations that would make that particular test extremely difficult.

We recommend that, at minimum, two different soil media be used for each test in the case

of nonmetallic detectors, and one soil medium in the case of kilohertz range metallic detectors.

The two different media for nonmetallic detectors should have relative permittivities (at

500 MHz, 23°C) of (2.7 -y0.02) (dry sand approximation) and (10 -yi2) (wet clay/loam

approximation). The relative permeability for these two media should be (1 -yO). The relative

permeability for the metallic detector medium should be approximately 1.35, the value of the

sand/magnetite mixture now present in the Army’s existing test range at Fort Belvoir. The

successful detection of targets in these media we believe would be a minimally robust indication

that the detector could be expected to perform well in the field. Of course, the range could

always be expanded at a later date to provide testing capabilities for other soil types. (NIST’s

recommendations on how to create these soil simulations in a rigid-matrix brick formulation will

be delivered to the Army in a separate report.)

Before describing these tests in detail, we must mention a few other caveats here. First, we

recommend that each of the detectors be operated exactly according to the manufacturer’s

instructions. If the instructions are faulty, we believe that this fact should be part of the test; it

7-3
should be the manufacturer’s responsibility to provide instructions for its use that optimize its

performance. Also, we must make the assumption that the detection of a target by a detector is

very clear cut; it either detects a target or it doesn’t and there is no gray area allowed. If there is

gray area, then we recommend that the Army insist to the manufacturer that it be removed. A

third caveat concerns target separation in the test range. It is important to know that nearby

targets are not interfering with a particular test in spite of the fact that nothing is known about the

detector’s radiated EM field pattern. To alleviate this problem we recommend that the Sensor

Active Envelope Test be performed first. The results of this test will indicate what the minimum

target spacing should be. To begin testing, and with no a priori knowledge of required target

separation, we arbitrarily recommend a spacing of 1 m. If the Sensor Active Envelope Test

indicates that this distance is too small then the targets can be spread out. The last caveat

concerns choice of targets for each test Except for the Target Shape Test and the Target

Orientation Test, we recommend that, at minimum, both the nylon and the Teflon spheres,

described in chapter VI, be used for the reasons stated there. If the Army wishes, however,

practically any targets [7.1] may be used in addition to these. Restrictions, if any, are listed for

each test. (In our forthcoming report on rigid-matrix brick formulations, we will specify a

method for making targets with variable permittivities, suitable for use in the Target-Earth

Contrast Tests.)

In the test discriptions which follow, procedures are given for nonmetallic-target tests using

the dry sand and wet clayAoam media approximations, as well as metallic-target tests using the

sand/magnetite medium approximation. NIST considers this to be minimal set of required tests. As a first step toward increasing the robustness of the testing strategy, we recommend that the

nonmetallic-target tests be repeated in the sand/magnetite medium approximation, and the

metallic-media approximations. Regardless of which medium is used, the testing procedure and

scoring system for each target set remains unchanged.

C.1 Sensor Active Envelope Tests

(1) Objective: To determine the size of the area beneath the sensor in which targets are

detected.

(2) Requirements: Bury a target, separated from any other targets in the surface plane by

enough distance to ensure target isolation. The target burial depth should be 0 m, as measured from the top of the target, and this test need only be performed in the “dry

sand” medium.

(3) Restrictions: This is the only test for which a “score” is not explicitly recorded.

Rather, it is necessary to record the perpendicular distances between the center of the

target and the center of the sensor at which target detection fails.

7-4
(4) Recommended Techniques:

a. Dry Sand: We recommend the use of two spherical targets for this test, one of

Teflon and one of nylon, each with a diameter of 15.24 cm. Bury each of the

targets to a depth of 0 m measured from the top of the target such that there are no other targets within 3 m. For each of the targets, move the detector sensor

along a straight line such that the center of the sensor moves over the center of

the target. The height of the sensor must be at the optimal height recommended

by the manufacturer, and must be kept constant above the surface of the earth.

Also, the rotational orientation of the detector must be recorded and kept

constant. Verify that the detector detects the target. (If not, since this is the

easiest of all challenges for the detector, the test is complete and the detector’s

score is 0.) Then, move the detector sensor along another straight line that is

parallel to but displaced by 3 cm from the first line. Again, verify detection.

Continue this test along displaced, parallel sensor paths until the displacement

reaches 1.5 m. Record the displacement distances for which detection occurs,

stops, and reoccurs. (Detection may stop at a given distance due to a null in the

radiated field, and then reoccur when the target appears in a side lobe of the

radiation.) Now, rotate the sensor about the vertical axis by an angle of 30°

clockwise and repeat the above scans, again recording the distances. Continue

this rotation in 30° steps until the sensor has been rotated a full 360°. Now, a

chart may be sketched showing the sensor active envelope(s). For all

subsequent tests for this detector, adequate target isolation, when required, can

be achieved based on these test results. (If the Army wishes, they may also

devise a numerical score for this test, similar to the scores obtained for all the

other tests. NIST personnel do not have enough information to specify this

score, but it is clear that a sensor active envelope that is too small would be

disadvantageous, as would be one that were not centered under the physical

sensor.)

b. Wet ClaylLoam: Same as that for dry sand.

c. MagnetitelSand: We recommend the use of two right-circular cylinder metallic

targets for this test, one of pure copper and one of Type 304 stainless steel. The

height of the copper target should be 4.14 mm and its diameter should be

8.28 mm. The height of the stainless steel target should be 25.4 mm, and its

diameter, 1.5875 mm. (NIST has supplied the Army with samples of the

“standard” and “realistic” targets under a previous contract; they are described

in chapter VI.) Each of these targets should be buried in an upright orientation

7-5
(flat sides parallel to the earth surface) at a depth of 0 m measured from the top

of each target. Then, the test should be performed as described above for the

plastic targets.

(5) Variations: This test may be conducted with a Teflon sphere, nylon or Teflon

cylinder, or with a more realistic target such as one of the mine replicas supplied to

the Army by others. Also, this test can be repeated for different detector sensor

heights above the earth in order to estimate a three-dimensional sensor active

envelope.

C.2 Target Burial Depth Tests

(1) Objective: To determine a detector’s ability to detect targets at increasing burial

depths in the surrounding soil.

(2) Requirements: Bury a set of identical targets at increasing depths in the medium,

separated from each other in the surface plane by enough distance to ensure target

isolation. The parameters that must be held constant for this test are target and soil

constitutive EM properties, and target orientation if the chosen targets are not

spherical in shape.

(3) Restrictions: None.

(4) Recommended Techniques:

a. Dry Sand: We recommend the use of two target sets for this test, the first set

consisting of nine Teflon spheres with a diameter of 7.62 cm, and the second set

consisting of nine nylon spheres with the same diameter. The targets should be

buried at depths of 0, 2.5, 5, 7.5, 10, 12.5, 15, 20, and 25 cm, measured from the

top of the targets. Each target should be positioned at least 1 m from a range

boundary or any other target, or at a greater distance if the results of the Sensor

Active Envelope Test so indicate. Operating the detector according to the

manufacturer’s instructions, count and record the number of targets detected for

each set. Two scores will result, each with a range of 0 to 9, 9 indicating that all

targets were detected in the set.

b. Wet ClaylLoam: Same as that for dry sand, above.

c. Magnetite!Sand: For the burial depth testing of metallic detectors we

recommend two sets of targets. The “standard” set consists of pure copper

right-circular cylinders with a height of 4.14 mm and a diameter of 8.28 mm.

the “realistic” set consists of Type 304 stainless steel right-circular cylinders

with a height of 25.4 mm and a diameter of 1.5875 mm. (NIST has supplied the

Army with samples of these “standard” and “realistic” targets under a previous

7-6
contract; they are described in Chapter VI.) The targets should be buried in an

upright orientation (flat sides parallel to the earth surface) at the same depths,

measured from the top of the targets, as those stated above for the plastic

targets. The testing method is also the same as for the plastic targets; again, the

possible scores for each target set will range from 0 to 9, with 9 signifying that

all targets were detected.

(5) Variations: The Army may wish to perform these tests with other targets in addition

to those recommended.

C.3 Target Volume Tests

(1) Objective: To determine the detector’s ability to detect targets of decreasing size

(volume.)

(2) Requirements: Bury a set of targets with differing volumes at identical depths in the

medium, separated from each other in the surface plane by enough distance to ensure

target isolation. The parameters that must be held constant for these tests are target

and soil constitutive EM properties, target burial depth, and target orientation.

(3) Restrictions: None.

(4) Recommended Techniques:

a. Dry Sand: We recommend the use of two target sets for this test, the first set

consisting of six Teflon spheres, and the second set, consisting of six nylon

spheres. In both cases, their diameters should be 2.54, 5.08, 7.62, 10.16, 12.7,

and 15.24 cm. The targets should all be buried at a depth of 10 cm, measured

from the top of each target. Each target should be positioned at least 1 m from a

range boundary or any other target, this distance, again, depending on the results

of the Sensor Active Envelope Test. Operating the detector according to the

manufacturer’s instructions, count and record the number of targets detected for

each set. Two scores will result, with a range of 0 to 6 for both the Teflon

targets and the nylon targets.

b. Wet ClaylLoam: Same as that for dry sand.

c. MagnetitelSand: For the target volume testing of metallic detectors we

recommend two sets of targets. The “standard” set consists of pure copper

right-circular cylinders, and the “realistic” set consists of Type 304 stainless

steel right-circular cylinders. Five copper targets (coated with a thin layer of

lacquer to resist corrosion) are recommended, with dimensions as follows: (1)

h = 3.29 mm, d = 6.58 mm; (2) h = 4.14 mm, d = 8.28 mm; (3) h = 5.22 mm,

d = 10.44 mm; (4) h = 6.58 mm, d = 13.15 mm; and (5) h = 10.44 mm.

7-7
d = 20.87 mm. All of these cylinders have a diameter-to-height ratio of 2:1, and

their weights are 1, 2, 4, 8, and 32 g. Three stainless steel targets are recommended, all with a height of 25.4 mm (1 in). Their diameters and weights

are as follows: (1) d = 0.7938 mm, w = 0.096 g; (2) d = 1.5875 mm, w = 0.39 g;

and (3) d = 3.175 mm, w= 1.56 g. (These dimensions were chosen because

they are realistic and available, as mentioned in chapter VI.) The same burial

and testing procedures should be followed as those for the dry sand case

described above, except that the cylinders should be buried in an upright

orientation with their flat sides parallel to the earth surface.

(5) Variations: The Army may choose different target sets in addition to the ones

recommended by NIST.

C.4 Target-Earth Contrast Tests

(1) Objective: To determine the detector’s ability to detect targets as the permittivity of

the target approaches that of the surrounding soil.

(2) Requirements: Bury a set of spherical targets with increasing permittivities (but

otherwise identical) in the medium, separated from each other in the surface plane by

enough distance to ensure target isolation. The target-to-medium permittivity ratios

should be centered around 1, with an equal number of targets possessing

permittivities above and below this value. The parameters that must be held constant

for these tests are the soil constitutive EM properties, target burial depth, and target

orientation.

(3) Restrictions: We recommend that this test only be undertaken for the case of dry

sand. All available evidence indicates that there is never a contrast ratio problem,

either with permittivity or permeability, if the soil is wet or contains magnetic

material. Also, NIST recommends that the targets be constructed from the rigidmatrix formulations of polyester and glass microballoons, as described in our

forthcoming report. We know of no other simple way to manufacture targets with the

required permittivities.

(4) Recommended Techniques:

a. Dry Sand: We recommend the use of one target set for this test, consisting of

six spheres, each with a diameter of 7.62 cm. The target-to-medium

permittivity ratios should be 0.9, 0.94, 0.98, 1.02, 1.06, and 1.1. With the

assumption that the magnitude of the permittivity of dry sand is 2.7, then the

target permittivity magnitudes should be 2.43, 2.54, 2.65, 2.75, 2.86, and 2.97,

and the imaginary part of the permittivity, £/', should be on the order of 0.02 to

7-8
match the loss characteristics of dry sand. The targets should all be buried at a

depth of 10 cm, measured from the top of each target. Each target should be

positioned at least 1 m from a range boundary or any other target, again,

depending on the outcome of the Sensor Active Envelope Test. Operating the

detector according to the manufacturer’s instructions, count and record the

number of targets detected. The resulting score will have a range of 0 to 6, with

6 being perfect.

(5) Variations: The Army may wish to perform this test with other target volumes,

shapes, or burial depths, in addition to the ones recommended by NIST.

C.5 Target Shape Tests

(1) Objective: To determine the detector’s ability to detect targets with varying shapes.

(2) Requirements: Bury a set of targets with varying shapes (but otherwise identical) in

the medium, separated from each other in the surface plane by enough distance to

ensure target isolation. The parameters that must be held constant for these tests are

target burial depth, target volume, and target orientation.

(3) Restrictions: None

(4) Recommended Techniques:

a. Dry Sand: We recommend the use of two target sets for this test, the first set

consisting of five Teflon right-circular cylinders, and the second set consisting

of five nylon right-circular cylinders. For both sets, all should possess equal

volumes, and diameters of 2.54, 5.08, 7.62, 10.16, and 12.7 cm, respectively.

Since we wish to have a diameter-to-height ratio of 3: 1 for the middle target in

each set, then, in order to maintain equal volumes for all targets, their respective

heights must be 22.86, 5.71, 2.54, 1.43, and 0.91 cm. The targets should all be

buried at a depth of 10 cm, measured from the top of each target. Each target

should be buried with its flat sides as parallel as possible to the air-earth

interface, and be positioned at least 1 m from a range boundary or any other

target, again, depending on the results of the Sensor Active Envelope Test.

Operating the detector according to the manufacturer’s instructions, count and

record the number of targets detected. The resulting scores for each set will

have a range of 0 to 5, with 5 being perfect.

b. Wet Clay/Loam: Same as that for dry sand.

c. Magnetite!Sand: We recommend the use of one target set for this test,

consisting of five lacquer-coated pure copper right-circular cylinders, all with

equal volumes, and with diameters of 3, 6, 8.28, 10, and 13 mm. Since we wish

7-9
to have a diameter-to-height ratio of 2: 1 for the middle target in the set, then, in

order to maintain equal volumes for all targets, their respective heights must be

31.54, 7.88, 4.14, 2.84, and 1.68 mm. The same burial and testing procedures

should be followed as those for the dry sand case described above.

(5) Variations: The Army may choose to perform these tests with targets of different

shapes or volumes than those recommended by NIST. (These shapes were chosen

because, as mentioned in chapter VI, their scattered fields are computable.)

C.6 Target Orientation Tests

(1) Objective: To determine the detector’s ability to detect targets with varying

orientations in the soil.

(2) Requirements: Bury a set of identical targets (nonspherical) in the medium with

varying orientations with respect to the air-earth interface plane. The targets should

be separated from each other in the surface plane by enough distance to ensure target

isolation. The parameters that must be held constant for these tests are target and soil

constitutive EM properties, target burial depth, and target volume.

(3) Restrictions: None

(4) Recommended Techniques:

a. Dry Sand: We recommend the use of two target sets for this test, the first

consisting of five Teflon right-circular cylinders, and the second consisting of

five nylon right-circular cylinders. All should possess a diameter of 7.62 cm

and a height of 2.54 cm. The targets should be buried such that the vector

normal to the cylinders’ flat surface subtends angles of 10°, 20°, 30°, 40°, and

50°, respectively, with respect to the vector normal to the air-earth interface

plane. The targets should all be buried at a depth of 10 cm, measured from the

uppermost point of each target, and should be positioned at least 1 m from a

range boundary or any other target, again, depending on the results of the

Sensor Active Envelope Test. Operating the detector according to the

manufacturer’s instructions, count and record the number of targets detected.

The resulting scores for each set will have a range of 0 to 5, with 5 being

perfect.

b. Wet ClaylLoam: Same as that for dry sand.

c. Magnetite/Sand: For the target orientation testing of metallic detectors we

recommend the use of one target set, consisting of five lacquer-coated pure

copper right-circular cylinders, all with a diameter of 3 mm and a height of

31.54 mm. The targets should be buried such that the vector normal to the

7-10
cylinders’ flat surface subtends angles of 0°, 22.5°, 45°, 67.5°, and 90°,

respectively, with respect to the vector normal to the air-earth interface plane.

The same burial and testing procedures should be followed as those for the dry

sand case described above.

(5) Variations: In addition to the targets recommended by NIST as a minimum set, the

Army may wish to repeat this test with other targets of differing shape and/or

constitutive EM properties.

C.7 Sensor Height Tests

(1) Objective: To determine the heights above the air-earth interface plane at which the

detector sensor functions properly.

(2) Requirements: Bury a target, separated from any other targets in the surface plane by

enough distance to ensure target isolation. The parameters that must be held constant

for this test are target and soil constitutive EM properties, target burial depth, shape,

volume, and orientation.

(3)

. Restrictions: This test is recommended only for hand-held detectors where normal

operation would be expected to involve variations in the height of the sensor above

the earth.

(4) Recommended Techniques:

a. Dry Sand: We recommend the use of two spherical targets for this test, one of

Teflon and one of nylon, each with a diameter of 7.62 cm. The targets should

be buried to a depth of 10 cm, measured from the top of each target. Each target

should be positioned at least 1 m from a range boundary or any other target,

again, depending on the results of the Sensor Active Envelope Test. Move the

detector sensor along a straight line such that the center of the sensor moves

over the center of the target. The height of the sensor must be at the optimal

height recommended by the manufacturer, and must be kept constant above the

surface of the earth. Verify that the detector detects the target. (If not, this test

is complete and the detector’s score for this test is zero.) Then, repeat the

straight-line scan for increasing sensor heights of from 0.5 cm above the earth to

the height at which detection ceases, in step sizes of 0.5 cm. Record the heights

at which detection both commences and ceases, and then compute the height

range for which detection is successful. Compute the score for the detector as

follows: If the height range is less than or equal to 0.5 cm, the score is 0; if the

range is between 0.5 cm and 1.0 cm, the score is T, if the range is between 1 cm

and 2 cm, the score is 2; if the range is between 2 cm and 4 cm, the score is 3; if

7-11
the range is between 4 cm and 8 cm, the score is 4; and if the range is greater

than 8 cm, the score is 5. Thus, a score of 5 is considered perfect.

b. Wet ClaylLoam: Same as that for dry sand, above.

c. MagnetitelSand: We recommend the use of two right-circular metallic

cylinders for this test, one or pure copper and the other of Type 304 stainless

steel. The copper target should have a height of 4.14 mm and a diameter of 8.28

mm, while the stainless steel target should have a height of 25.4 mm and a diameter of 1.5875 mm. Both should be buried in an upright orientation at a depth from the top of each target of 10 cm. Then, follow the procedure

described for the dry sand/plastic targets, above.

(5) Variations: The Army may wish to perform these tests using additional targets to the

ones NIST recommends. Also different target burial depths may be desired. And

possibly, the Army may want to combine this test with the Sensor Active Envelope

Test (Vn.C.l) in order to measure an estimate for the three-dimensional sensor active

area.

D Future Tests

In this section we describe, in general terms, a set of advanced tests which the Army may

choose to develop and implement at a later date. For various reasons, stated in the test

descriptions, they would be more difficult to implement than the core set of tests. Therefore,

NIST recommends that the Army should develop the core set first, gain some months of

experience in performing the core set, make modifications as necessary, and then consider adding

more advanced tests.

D.1 Sensor Velocity Test

In order to determine the target detection sensitivity to variations in the velocity of the

detector sensor, it may be desirable to develop a test for this. The test would consist of moving

the sensor along a straight line over a buried target at varying velocities and determining the

minimum and maximum velocities for which successful detection occurs. A raw score could

then be derived as a function of the range of velocity for which the detector performs

successfully. Detectors with a very narrow sensor velocity range would receive a lower score

than those that exhibited a wider range. The reason that this test is more difficult to implement is

that it would require an overhead positioning system that would be capable of moving the sensor

over the earth at variable and known velocities.

7-12
D.2 Sensor Tiit Angle Tests

The sensor tilt angle test would involve moving the detector sensor along a straight line

over a buried target with the sensor rotated to increasing angles about its horizontal axes. At

minimum, the sensor would be rotated about both the x and y horizontal axes separately. The

angle at which the detector no longer detects the target would then be recorded, indicating the

detector’s sensitivity to sensor tilt. There would almost certainly be a relationship between the

results of this test and the Sensor Active Envelope and Sensor Height tests, all of which should

yield a good estimate of the sensor’s radiated EM beam pattern. This test would be more

difficult to implement because it would also require an overhead positioning system.

D.3 Target Resolution Test

The objective of this test would be to measure the minimum distance between two buried

targets for which the detector successfully detects the presence of each target, individually. The

two targets need not be the same. This test would be designed to test for the possibility that, for

example, a large anti-vehicle mine were buried close to a smaller anti-personnel mine. This twotarget test would be more difficult to implement because of its spatial complexity and number of

degrees of freedom.

D.4 Surface Clutter Tests

These tests would involve placing objects at or embedded in the air-earth surface above a

buried target in order to test for the detector’s ability to “see through” the surface object clutter

and successfully detect the buried target. These objects could be, for example, pieces of rock,

wood or other vegetation, and/or metal fragments, and they could be either uniform or random in

both size and positioning. These tests also could involve simply varying the surface profile of

the earth. The reasons that these tests are more difficult are that, first, there are practically an

infinite number of possibilities for configuring the clutter, and, second, it would be difficult to

maintain the exact configuration of the clutter over time. The use of the rigid-matrix brick

approach, recommended by NIST in chapter V, would eliminate the latter difficulty because it

would be possible to either glue or partially embed the clutter objects in one or more bricks.

D.5 Volume Clutter Tests

These tests would be very similar to the Surface Clutter Tests; the major difference is that

the clutter object(s) would be completely buried in the medium in close proximity to the target.

Again, such objects could be rocks, wood or other vegetation, and/or metal fragments.

7-13
D.6 Earth Stratification Test

These tests would be designed to measure the detector’s ability to find targets when the

constitutive EM properties of the soil are not constant functions of soil depth. This type of test

may be considered a special case of the Volume Clutter Test. As one example of this type of

test, a target could be buried in dry sand (relative dielectric constant = 2.70) at a depth of 10 cm,

and in which a 1 cm layer of wet soil (relative dielectric constant = 10) is located between the

air-earth surface and the target, say at a depth of 5 cm. The detector would then be operated

according to the manufacturer’s instructions to see if it could detect the target. The reasons that

these tests are more difficult are, first, like all of the clutter tests, the number of possible tests is

practically infinite, and second, this test is probably possible only if dry, granular soil simulations

or rigid-matrix bricks are used for the soil medium.

D.7 False Alarm Tests

All of the tests presented so far have been some sort of measure of a detector’s ability to

detect targets. It may also be important to the Army to measure a detector’s ability to avoid

detecting innocuous objects that are buried in the earth. These tests would involve burying such

objects as rocks, wood or other vegetation, and/or metal fragments, with no real targets present,

to measure the detector’s ability to differentiate between these objects and real targets. These

tests would be more difficult to implement, again, because of the enormous number of innocuous

objects that would need to be considered.

E Test Result Analysis

In this section a NIST-recommended method is presented for reducing the raw scores from

each test performed into an overall score. This overall score for each detector will then allow the

Army to rate competing detectors. It is important for the Army to understand that the soil and

target standards, the core test instmctions, and the following data reduction method,

recommended by NIST, are as objective as possible. That is to say, if our recommendations are followed closely, then the score that each detector receives should be based on sufficiently

accurate and precise measurements, and that score should be quite repeatable over time.

However, the analysis of the test results is necessarily a subjective task that only the Army

can perform. As described below, we have attempted to build this subjectivity into the data

reduction method in the form of weighting factors. In the process of the Army assigning

numerical values to these weighting factors they will automatically be factoring in the relative

importance to the Army of the results of each test. As an example of this process, the Army may

decide that since a particular detector’s ability to detect targets is strongly inhibited by the fact

that the sensor height must be maintained within too small a range (say ±3 mm), it must assign a

7-14
lower overall score, even though this detector otherwise out-performs all other candidates. By a

judicious choice of weighting factor values, this decision by the Army can be incorporated into

the scoring method. NIST personnel, with their primary area of expertise lying in the area of

electromagnetic measurements, are not as qualified as Army personnel to assign values to these

subjective weighting factors.

By design, the raw scores that result from each of the core tests described in Vn.C are in

the form of positive integer numbers. If numerical weighting factors are assigned to each test

then the raw score total for each detector may be expressed as N

RST = '^WiSi, (7-1)

t=l

where RST is the raw score total, Wi is the weighting factor for the iih test, is the positive

integer score for that test, and N is the total number of tests performed. This raw score total may

be normalized to cover the range 0 to 100 by the following,

1VST = --!^£w^.5,- (7-2)

where NST is the normalized score total, and

= (7

-3)

t=l

where are the maximum scores possible for each of the N tests.

As an example, assume that the tests described in VII.C.2 through Vn.C.7 (dry and and wet

clay/loam only) were performed on a detector with the following results.

Test No.

Dry Sand

Result

Perfect

Score

Weighting

Factor

ClaylLoam

Result

Perfect

Score

Weighting

Factor

C.2. 7 9 10 5 9 10

C3. 5 6 5 4 6 5

C4. 4 6 8 N/A N/A N/A

C.5. 5 5 3 5 5 3

C.6. 4 5 3 5 5 3

C.7. 4 5 7 3 5 7

N, the total number of tests is 1 1, and,

RST =70 + 25 + 32 + 15 + 12 + 28 + 50 + 20 + 15 + 15 +21

= 303.

7-15
RSTj^j^, the maximum possible raw score, is

RSTj^ax =90 + 30 + 48+ 15 + 15 + 35 + 90 + 30+ 15+ 15 + 35 = 418.

Therefore, NST, the normalized score for this detector, is

100 = —(303) = 72.5.

418

This algorithm is open-ended and may be used for any number of tests, including non-EM

based tests such as for ease of operation, detector mass, and battery life. It will always yield a score of between 0 and 100 so competing detectors will be compared on an equal footing as long

as the same tests are performed on each detector in the same manner.

Another point that needs to be mentioned about this algorithm is that, in general, neither the

tests themselves nor the weighting factors are required to be constants or linear functions; they

may each be nonlinear. For example, the Target Depth Test, described in C.2, is inherently

nonlinear because the increase in depth, from one target to the next, is not constant. Thus, there

is not a linear relationship between a detector’s numerical score for this test and the maximum

depth at which it can detect targets. Similarly, the Army may wish to make one or more

weighting factors a nonlinear function of the numerical score achieved. NIST’s

recommendation, for future test development, is for the Army to build desired nonlinearities into

the tests, themselves, and to try to keep the weighting factors constant whenever possible.

References (VII)

[7.1] Brown, D.; Tanner, J. D.; Paca, F. B. Mine Detector Targets: Metallic and

Nonmetallic, Generic Set, Development Efforts. Report No. VSE/ASD/0047-

87/35RD, VSE Corporation, Alexandria, VA; 1987.

7-16
VIII Archival Record Requirements

As a part of this study, the Army has asked NIST to identify those data that should be

permanently recorded for each test. We recommend that for each individual test the following

items be recorded, on paper, into an electronic data base with backup, or both.

(1) The title of the test.

(2) The test date.

(3) The test start time.

(4) The test finish time.

(5) The test start ambient temperature to ±0.5 °C.

(6) The test finish ambient temperature to ±0.5°C.

(7) The test start ambient relative humidity to ±5.0%.

(8) The test finish ambient relative humidity to ±5.0%.

(9) The test start barometric pressure to ±50 Pa.

(10) The test finish barometric pressure to ±50 Pa.

(11) The identification number(s) of the target(s) used.

(12) The identification number(s) of the soil medium or (media) used.

(13) The exact position of each target in the range (x, y, z coordinates.)

(14) The target(s) relative permittivity to ±5% (real) and ±10% (imaginary) from either

NIST-supplied data (Teflon and nylon) or measured data.

(15) The target(s) relative permeability to ±5% (real) and ±10% (imaginary) fi-om either

NIST-supplied data (Teflon and nylon) or measured data.

(16) The soil medium (or media) relative permittivity to ±5% (real) and ±10% (imaginary)

from measured data.

(17) The soil medium (or media) relative permeability to ±5% (real) and ±10%

(imaginary) from measured data.

(18) All measured depths, angles, velocities, and so on, as appropriate for each test. (These variables must be kept as constant as possible.)

(19) The numerical values measured from the Sensor Active Envelope Test, both detection

start and stop distances and their associated sensor rotation angles.

(20) The integer numerical score for the DUT for that test

(21) The identity of the person(s) conducting the test

In addition to this recorded data for each individual test, it is recommended that the Army

keep a continuing record of test range data (Range Log). This log should be used to record all of

the pertinent history of the range including the following.

8-1
(1) Dates and times of range usage.

(2) Identities of all persons using the range at those times.

(3) Dates and times of range parameter measurements, such as soil medium relative

permittivity and permeability. (We recommend that they be measured and recorded

at least monthly.)

(4) Numerical results of those measurements and measurement technique(s) and

system(s) used, including relevant software.

(5) Identity of person(s) performing the range measurements.

(6) The dates, times, and exact nature of all modifications to the range.

(7) The identity of the person(s) performing those modifications.

(8) The dates, times, and exact nature of any accidents that occur which could

conceivably change the range parameters.

(9) The dates, times, and exact nature of any electromagnetic interference (EMI)

problems that are observed. (The sources of these problems should be located and

eliminated as soon as possible.)

(10) Any other remarks that the range user believes are appropriate to maintaining the

integri ty of the range.

These are all of the data that NIST has been able to identify, both for individual tests and

for overall range integrity. As experience is gained in utilizing the range, the Army may discover

that there are other variables or facts that they wish to record. Space should be allowed, either on

paper forms or within electronic data base fields, for additional data to be recorded in the future.

8-2
IX Conclusion

The first section of this chapter contains a brief summary of the key conclusions that NIST

has drawn firom our work for the Army from January 1, 1985 to December 31, 1990. In the

second section, we present NIST’s recommendations for the future work required in order for the

Army to realize an actual, functional mine detector test range.

A Key Conclusions

There were three key tasks for which the U.S. Army Belvoir Research, Development, and

Engineering Center requested NIST’s assistance. Drawing upon NIST’s expertise in

electromagnetic measurements and metrology, they asked us to perform the following tasks:

1. Specify “standard” materials and configurations that the Army could use for test

targets and background soil media for the laboratory testing of portable,

electromagnetic-based mine detectors. The key objective was to specify target and

soil standards that possess constitutive electromagnetic properties as close in values to

actual mines and soil backgrounds as possible, and whose properties are well defined

and stable over time.

2. Specify measurement methods for constitutive electromagnetic properties of the

materials specified above, and measurement systems that the Army can use for

periodically verifying the numerical values of those properties. The approximate

accuracy of these measurements requested by the Army was ±10%.

3. Specify measurement methods for determining the performance effectiveness of

portable, electromagnetic-based mine detectors in the laboratory.

Chapters V and VI contain our recommendations for soil standards and target standards,

respectively. For the reasons stated there, we have recommended the use of Teflon and nylon

spheres for standard targets which simulate plastic mines, and copper and stainless steel rightcircular cylinders for standard targets which simulate mines containing metal. A small number

of each of these standards has been fabricated at NIST and delivered to the Army. We have also

recommended the use of rigid-matrix bricks, presently being designed by NIST and an outside

contractor, as the optimum approach for realizing soil standards. For the reasons stated in

chapter V, we believe that the rigid-matrix bricks will prove superior to natural soils, loose,

granular mixture soil simulations, or liquid soil simulations. (The results of NIST’s design

efforts for rigid-matrix bricks will be presented to the Army in a separate report.)

Chapter HI contains our recommendations for measurement methods and systems which

the Army can use for periodically measuring the electromagnetic properties of relevant target and

9-1
soil materials. The methods and systems finally chosen by NIST possess state-of-the-art

accuracy, and are commercially available, versatile, and easy to operate. These systems will be

assembled and tested at NIST, and delivered to the Army upon manufacturer’s delivery of the

ordered equipment.

Chapter VI contains NIST’s recommendations for a portable, electromagnetic-based mine

detector test strategy. Briefly, this “eye-chart” strategy consists of designing objective tests that

test for detector sensitivity to only one critical parameter at a time. For each test, all the critical

parameters except the tested one are held as constant as possible. In the test range, each

parameter is varied over a range of “easy” to “difficult” and the detector is scored on how many

targets in each test it can detect. We have described in as much detail as possible seven “core”

tests which clearly illustrate the test strategy, and which we recommend be implemented first.

We then present, in less detail, seven “advanced” tests which we recommend that the Army

implement later, after gaining experience from actual performance of the first set. Also presented

in this chapter is a simple, objective algorithm which can be used to provide the Army an overall

performance effectiveness rating for each tested detector.

B Recommendations for Future Work

With the information supplied in this report, the report on the rigid-matrix brick design, and

the measurement systems, we believe that the Army’s next effort should be devoted to actually

implementing our recommendations for a mine detector test range. Practically all of the

remaining work to be done to complete this range lies outside the primary area of expertise of

NIST personnel. However, we present here a listing of the remaining tasks that we believe must

be accomplished to realize this goal. This listing represents a phased approach with the objective

of verifying the efficacy of each step before continuing on to the next step. In this manner, the

maximum cost-effectiveness should be realized.

1. The Army should provide a temperature-controlled, relative humidity-controlled, and

enclosed space for the construction of the first test range. We recommend that this

first range be dedicated to the testing of mine detectors for plastic objects in a dry

sand medium. Our reasons for this choice are that it provides a very difficult

challenge for mine detectors and that it should be the easiest of the ranges to

implement.

2. The Army should pursue the actual design of this range, using outside contractors as

necessary. This design must include further studies on the optimal methods and

configurations for the manufacture of rigid-matrix bricks, their placement and

manipulation in the range, and the required range boundaries to minimize

electromagnetic wave reflections. For this range, we recommend that the rigid-matrix

9-2
brick depth be at least 80 cm, and that the surface area be large enough to allow the

positioning of at least 10 targets, each positioned at least 1 m from range side

boundaries and from each other. As part of this design, we recommend that actual

radiated electromagnetic field tests be performed on ensembles of bricks in order to

determine the manufacturing methods and configuration which minimize unwanted

wave reflections from brick surfaces. Also as part of this design process,

manufacturing methods should be developed for embedding standard and other

targets into the bricks. ^

3. Based on the actual design plans produced in Step 2, above, the Army should

construct this range with the assistance of outside contractors, as necessary.

4. The Army should pursue the design of an accurate, three-dimensional, computercontrolled positioning system for installation above the rigid-matrix brick test range.

Attaching detectors under test to this positioning system will allow much more

accurate control of the detector motion during tests than if the detectors were to be

human-operated. We recommend that the same computer system as that delivered to

the Army as part of the measurement system be used for this automated positioning

function.

5. Upon completion of the first test range construction, the Army should implement the

seven “core” tests described in chapter VII of this report. It is anticipated that some

experimentation will be required in order to optimize the range and step size of each

of the critical measurement parameters.

6. After gaining experience with this range for a satisfactory time (perhaps one year), the

Army should consider the design and construction of several other ranges, each one

designed around a different set of realistic soil electromagnetic constitutive

properties. We recommend that future test ranges should be designed and built, based

on the information provided by NIST in this and the brick report, for the simulations

of damp soils, salty wet soils, and magnetic soils as a minimum set.

9-3

X Glossary of Terms

The following is a selected alphabetized listing of terms used in this report. Whenever

possible, the definitions have been taken from References [10.1] or [10.2].

clutter. Any deviation from geometric or material uniformity in the constitutive properties of

the earth. Clutter will cause undesired scattering of radiated electromagnetic energy and

may be caused by (1) an irregular (non-plane) air-earth surface profile or (2) any

variations in the constitutive properties of the earth at or below the surface,

complex dielectric constant. The complex permittivity of a physical medium in ratio to the

permittivity of free space [10.1].

complex conductivity. For isotropic media, at a particular point, and for a particular frequency,

the ratio of the complex amplitude of the total electric current density to the complex

amplitude of the electric field strength. Note: The electric field strength and total current

density are both expressed as phasors, with the latter composed of the conduction current

density plus the displacement current density [10.1].

complex permeability. For isotropic media, the ratio of the magnetic flux density to the

magnetic field intensity. In anisotropic media, complex permeability becomes a matrix

[ 10. 1 ].

complex permittivity. For isotropic media the ratio of the electric flux density to the electric

field, in which the displacement current density represents the total current density.

Note: This term is used to describe both the conductive and dielectric properties of a

medium [10.1].

conductivity (material). A factor such that the conduction-current density is equal to the

electric-field intensity in the material multiplied by the conductivity. Note: In the

general case it is a complex tensor quantity [10.1].

constitutive relations (radio wave propagation). Constraints imposed by the medium on the

relationships between electric and magnetic field intensity vectors and their respective

flux density vectors [10.1].

dielectric constant That property which determines the electrostatic energy stored per unit

volume for unit potential gradient [10.1].

dielectric loss tangent. A measure of the energy loss in a dielectric medium. It is expressed as tan5 and is the power loss in the medium divided by its reactive power at a sinusoidal

voltage of specified frequency.

magnetic susceptibility (isotropic medium). The (unitless) ratio of the magnetization to the

magnetic field intensity.

10-1
mine. An encased explosive designed to destroy enemy personnel or equipment [10.2].

mine detector (electromagnetic). A device for the indication of the presence of mines that

senses that presence by means of electromagnetic fields,

permeability. A general term used to express various relationships between magnetic flux

density and magnetic field intensity. These relationships are either (1) absolute

permeability, that in general is the quotient of a change in magnetic flux density divided

by the corresponding change in magnetic field intensity: or (2) relative permeability,

which is the ratio of the absolute permeability to the permeability of a vacuum,

permittivity. A general term used to express various relationships between electric flux density

and electric field intensity. These relationships are either (1) absolute permittivity, that in

general is the quotient of a change in electric flux density divided by the corresponding

change in electric field intensity: or (2) relative permittivity, which is the ratio of the

absolute permittivity to the permittivity of a vacuum,

sensor (test, measurement and diagnostic equipment). A transducer that converts a parameter

at a test point to a form suitable for measurement by the test equipment [10.1].

standard (test, measurement and diagnostic equipment). A laboratory type device which is

used to maintain continuity of value in the units of measurement by periodic comparison

with a higher echelon or national standards. They may be used to calibrate a standard of

lesser accuracy or to calibrate test and measurement equipment directly [10.1].

target (radar). (A) Specifically, an object of radar search or tracking; (B) Broadly, any

discrete object which reflects energy back to the radar [10.1].

transducer. A device that is actuated by power from one system and supplies power in any

other form to a second system [10.2].

References (X)

[10.1] Jay, F., Editor in Chief. IEEE Standard Dictionary of Electrical and Electronics

Terms. ANSI/IEEE Std. 100-1984, Third Edition; 1984.

[10.2] Webster’s Seventh New Collegiate Dictionary. G. & C. Merriam Company,

Springfield, MA; 1967.

10-2
Appendix IV.A BASIC Program to Print Out Values of n and e

110! Last Revision August 7, 1987 11:21

115! This program calculates the relative mu and epsilon of a

116! material from

120 ! S-parameter results obtained from the HP8510 network analyzer.

125!Written by Eric J. Vanzura (303) 497-5752 6/26/1987

130 GOSUB Init

135 GOSUB Load_matrix

140 GOSUB Calc_rotation

145 GOSUB Calc_dielectric

150 GOSUB Print_results

155 GOSUB Save_results

160 GOTO End

165!

170! ////////////////////////////////////////////////////////////

175!

180 Init:

185 OUTPUT KBD USING *'#, K" ; "SCRATCH KEY"&CHR$ (255) &CHR$ (69)

190 DEG

195 OPTION BASE 1

200 INTEGER I , J , Use_sll , Datacount , Bad_number

205 REAL Samplelength, LI , L2 , Rotl , Rot2 , Lairline , Test

210 DIM Test$[160]

215 Datacount=201

220 Samplelength=. 095 !METERS

225 Lairline=. 0994

230 L2=.002

235 Ll=Lairline-Samplelength-L2

240 GOSUB Init_expt

245 ALLOCATE A ( Datacount , 9 )

, B ( Datacount , 2 )LamO ( Datacount )

Prompt$ [40], Id$ [40]

250 ALLOCATE OPTION Sll (Datacount) , S12 (Datacount) , S21 (Datacount)

S22 (Datacount)

255 ALLOCATE OPTION Garni (Datacount ), Gam2 (Datacount )

Gam (Datacount) , K (Datacount) , Lam (Datacount) , T (Datacount)

260 ALLOCATE OPTION Mu (Datacount) , Eps (Datacount

265*

270 COM /Files/ Sourcedisk$ [ 20 ]

, Diskout$ [ 20 ], Diskdrive$ [ 20 ]

Filename$ [ 10]

275 COM /Bugs/ INTEGER Bugl , Bug2 , Bug3 , Printer , Printer_on

280 Printer=701

285 Printer_on=0

290 Prty=VAL(SYSTEM$ ("SYSTEM PRIORITY" )) +1

295 DISP CHR$(129);" DO YOU WANT TO USE (S11,S21) DATA OR

(S22,S12) DATA? ";

300 DISP CHR$(128)

305 ON KEY 4 LABEL "S11,S21 DATA" , Prty GOSUB Use_sll_yes

310 ON KEY 5 LABEL "S22,S12 DATA" , Prty GOSUB Use_sll_no

315 Done=0

320 LOOP

325 EXIT IF Done

330 END LOOP

335 RETURN

340 !

4A-1
355 Init_expt:!

360 Enter_datacount: OUTPUT KBD USING , K” ; Datacount

365 LINPUT "ENTER number of frequencies in datafile” , Test$

370 CALL Test_real (Test , Test$ , 51 .

, 401 . , Bad_number)

375 IF Bad_number THEN GOTO Enter_datacount

380 Datacount=INT(Test)

385 Enter_airline:

390 OUTPUT KBD USING ”# , K” ;Lairline

395 LINPUT "ENTER length of airline (meters) ”, Test

$

400 CALL Test_real (Test,Test$, . 01, . 5, Bad_number)

405 IF Bad_number THEN GOTO Enter_airline

410 Lairline=Test

415 Enter_sample:

420 OUTPUT KBD USING "# , K” ;Samplelength

425 LINPUT "ENTER sample length (meters) ”, Test$

430 CALL Test_real (Test, Test$, . 001, Lairline, Bad_number)

435 IF Bad_number THEN GOTO Enter_sample

440 Samplelength=Test

445 Enter_ll:!

450 OUTPUT KBD USING "#,K”;L1

455 LINPUT "ENTER distance from Port 1 interface to

sample (meters) ” ,Test$

460 CALL Test_real (Test ,Test$ , 0 .

, . 5 , Bad_number)

465 IF Bad_number THEN GOTO Enter_ll

470 Ll=Test

475 Enter_12:!

480 OUTPUT KBD USING ”#,K";L2

485 LINPUT "ENTER distance from Port 2 interface to

sample (meters ) ” , Test$

486 CALL Test_real (Test,Test$, 0. , . 5,Bad_number)

490 IF Bad_number THEN GOTO Enter_12

495 L2=Test

500 RETURN

505 ! 510 ! ////////////////////////////////////////////////////////////

515 ! 520 Use_sll_yes:OFF KEY

525 Use_sll=l

530 Done=l

535 RETURN

540 ! 5451 ////////////////////////////////////////////////////////////

550 1 555 Use_sll_no:OFF KEY

560 Use_sll=0

565 Done=l

570 RETURN

575 I 58 0 1 ////////////////////////////////////////////////////////////

585 ! 590 Load_matrix:

595 Prompt$="8510 S-PARAMETER DATA”

600 CALL Select_disk( Prompts)

605 CALL Enterfilename ("CAT” , Prompts)

610 IF FilenameS=*"' THEN

4A-2
615 CAT Diskdrive$

620 CALL Enterfilename ( "ABORT” , Prompt$)

625 IF Filename$="" THEN

630 BEEP

635 DISP "PROGRAM ABORTED"

640 STOP

645 END IF

650 END IF

655 DISP CHR$(129);" GETTING DATA FROM DISK ";CHR$(128)

660 ASSIGN ^Datapath TO Filename$&Diskdrive$

665 ENTER @ Datapath ; A (*

670 ASSIGN 0Datapath TO * 675 FOR 1=1 TO Datacount

680*

685*

690*

695*

700 NEXT I

705 IF Printer_on THEN PRINTER IS Printer

710 PRINT "FREQ Sll S21 S22

S12"

715 FOR 1=1 TO Datacount STEP INT (Datacount/ 10)

720 PRINT USING "2D. 2D, 4X, 4 (2 (SD. 3D) , 2X) " ;A (I , 1) , Sll (I)

S21(I) ,S22 (I) ,S12 (I)

725 NEXT I

730 PRINTER IS 1

735 RETURN

740 ! 745 1 ////////////////////////////////////////////////////////////

750 ! 755 Calc_rotation:

760 DISP CHR$(129);" CALCULATING PHASE ROTATIONS L1=";L1;" ";

CHR$(128)

765 FOR 1=1 TO Datacount

770 LamO(I)=.29979/A(I,l) IMETERS

775 Rotl=360*Ll/Lam0 (I)

780 Rot2=360*L2/Lam0 (I)

785*

790*

795 S11(I)=S11(I) * (Phasel‘2)

800 S21(I)=S21(I) * (Phasel*Phase2)

805 S12 (I) =S12 (I) * (Phasel*Phase2)

810 S22 (I)=S22 (I) * (Phase2‘2)

815 NEXT I

820 DISP " Sll AND S22 SHOULD NOW BE VERY CLOSE TO EQUAL ";

CHR$ (128)

825 IF Printer_on THEN PRINTER IS Printer

830 PRINT "FREQ (GHz) Sll S22

S11-S22 ANGLE (S11-S22)

835 FOR 1=1 TO Datacount STEP INT (Datacount/30)

840*

845 NEXT I

846 PRINT " Sll AND S22 SHOULD NOW BE VERY CLOSE TO EQUAL ";

CHR$ (128)

850 PRINTER IS 1

4A-3
855 RETURN

860 ! 865 ! ////////////////////////////////////////////////////////////

870 1 875 Calc_dielectric: ! 880 Cd:

885 DISP CHR$(129);" CALCULATING COMPLEX MU AND EPSILON

CHR$(128)

890 FOR 1=1 TO Datacount

895 IF Use_sll THEN

900 K(I)=(S11(I) “2-S21(I) ^2+l)/(2*Sll(I)

)

905 ELSE

910 K(I) = (S22(I) *2-S12(I) ‘ 2 + 1) / (2*S22 (I)

)

915 END IF

920*

925*

930 IF ABS(Gaml(I) )<1 THEN

935 IF ABS(Gam2(I))<l THEN

940 BEEP

945 DISP "GAM1(";I;”) AND GAM2(";I;”) ARE BOTH LESS

THAN 1 !?"

950 PAUSE

955 ELSE

960 Gam(I)=Gaml(I)

965 END IF

970 ELSE

975 IF ABS(Gam2(I) )>1 THEN

980 BEEP

985 DISP "GAMl AND GAM2(”;I;") ARE BOTH

GREATER THAN 1 1?"

990 PAUSE

995 ELSE

1000 Gam ( I

) =Gam2 ( I

)

1005 END IF

1010 END IF

1015 IF Use_sll THEN

1020 T(I)=(Sll(I)+S21(I)-Gam(I) )/ (1- (Sll (I) +S21 (I) )*Gam(I)

)

1025 ELSE

1030 T(I)=(S22 (I)+S12 (I) -Gam (I) )/ (1-(S22 (I)+S12 (I) ) *Gam(I)

)

1035 END IF

1040*

1045 Mu(I)=LamO (I) * (l+Gam(I) )/ (Lam(I) * (1-Gam (I) )

)

1050 Eps(I)=LamO(I) *2/ (Lam (I) “2*Mu(I)

)

1055 NEXT I

1060 RETURN

1065!

1070! ////////////////////////////////////////////////////////////

1075!

1080 Print_results :

1085 Prty=VAL(SYSTEM$( "SYSTEM PRIORITY" )) +1

1090 DISP CHR$(129);" DO YOU WANT A PRINTOUT OF MU AND

EPSILON DATA? ";CHR$(128)

1095 ON KEY 5 LABEL "PRINT TO SCREEN", Prty GOSUB Prnt_to_screen

1100 ON KEY 7 LABEL "PRINT TO PRINTER" , Prty GOSUB Prnt_to_printer

1105 ON KEY 6 LABEL "NO" , Prty GOSUB Done_return

4A-4
1110 Done=0

1115 LOOP

1120 EXIT IF Done

1125 END LOOP

1130 OFF KEY

1135 RETURN

1140!

11451 ////////////////////////////////////////////////////////////

11501

1155 Prnt_to_screen: OFF KEY

1160 PRINTER IS 1

1165 GOSUB Prnt_results

1170 RETURN

11751

11801 /////////////////////////////////////////////////////////////

11851

1190 Prnt_to_printer:OFF KEY

1195 PRINTER IS Printer

1200 GOSUB Prnt_results

1205 RETURN

1210 ! 12151 /////////////////////////////////////////////////////////////

12201

1225 Prnt_results:OFF KEY

1230 PRINT *'Freq (GHz) Mu Epsilon"

1235 IF Printer_on THEN

1240 FOR 1=1 TO Datacount

1245 PRINT USING "2D. 4D, 2 (4X, 2 (S3D. 4D, 2X) ) " ;A (I , 1)

Mu(I) ,Eps(I)

1250 NEXT I

1255 ELSE

1260 FOR 1=1 TO Datacount STEP INT (Datacount/20)

1265 PRINT USING "2D. 4D, 2 (4X, 2 (S3D. 4D, 2X) ) " ; A (I , 1)

MU(I) ,Eps(I)

1270 NEXT I

1275 END IF

1280 PRINTER IS 1

1285 Done=l

1290 RETURN

12951

1300! ////////////////////////////////////////////////////////////

13051

1310 Save_results:

1315 FOR 1=1 TO Datacount

1320 B(I,1)=A(I,1)

1325 NEXT I

1330 Prty=VAL( SYSTEMS ("SYSTEM PRIORITY" )) +1

1335 DISP "SAVE MU (PERMEABILTY) RESULTS?"

1340 ON KEY 5 LABEL "YES",Prty GOSUB Save_mu

1345 ON KEY 6 LABEL "NO",Prty GOSUB Done_return

1350 Done=0

1355 LOOP

1360 EXIT IF Done

1365 END LOOP

1370 OFF KEY

4A-5
1375 DISP "SAVE EPSILON (PERMITTIVITY) RESULTS?"

1380 ON KEY 5 LABEL "YES",Prty GOSUB Save_eps

1385 ON KEY 6 LABEL "NO",Prty GOSUB Done_return

1390 Done=0

1395 LOOP

1400 EXIT IF Done

1405 END LOOP

1410 OFF KEY

1415 RETURN

1420!

1425! ////////////////////////////////////////////////////////////

1430!

1435 Save_mu:OFF KEY

1440 Prompt$="REAL PART OF MU RESULTS"

1445 CALL Select_disk (Prompt$)

1450 IF Diskdrive$="NO DISK" THEN GOTO Save_im_mu

1455 Id$=""

1460 CALL Enter_id(Id$, Prompt$)

1465 IF Id$="" THEN GOTO Save_im_mu

1470 CALL Enterfilename("ABORT",Prompt$)

1475 IF Filename$="" THEN GOTO Save_im_mu

1480 FOR 1=1 TO Datacount

1485*

1490 NEXT I

1495 CALL Save_f ile (B(*) , Datacount , Id$)

1500 PRINT "REAL PART OF MU SAVED"

1505 Save_im_mu:

1510 Prompt$=" IMAGINARY PART OF MU RESULTS"

1515 !CALL Select_disk(Prompt$)

1520 IF Diskdrive$="NO DISK" THEN GOTO Save_mu_rtn

1525 !ld$=""

1530 ! CALL Enter_id (Id $, Prompt$)

1535 IF Id$="" THEN GOTO Save_mu_rtn

1540 CALL Enterfilename ( "ABORT" , Prompt$)

1545 IF Filename$="" THEN GOTO Save_mu_rtn

1550 FOR 1=1 TO Datacount

1555*

1560 NEXT I

1565 CALL Save_file(B(*) , Datacount, Id$)

1570 PRINT "IMAGINARY PART OF MU SAVED"

1575 S ave_mu_rtn : Done=

1580 RETURN

1585!

1590! ////////////////////////////////////////////////////////////

1595!

1600 Save_eps:OFF KEY

1605 Prompt$="REAL PART OF EPSILON RESULTS"

1610 !CALL Select_disk( Prompt$)

1615 IF Diskdrive$="NO DISK" THEN GOTO Save_im_eps

1620 !Id$=""

1625 !CALL Enter_id (Id$ , Prompt$)

1630 IF Id$="" THEN GOTO Save_im_eps

1635 CALL Enterfilename ( "ABORT" , Prompt$)

1640 IF Filename$="" THEN GOTO Save_im_eps

1645 FOR 1=1 TO Datacount

4A-6
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NEXT I

CALL Save_file (B(*) , Datacount , Id$)

PRINT "REAL PART OF EPSILON SAVED"

Save_im_eps :

Prompt$=" IMAGINARY PART OF EPSILON RESULTS"

!CALL Select_disk( Prompt $)

IF Diskdrive$="NO DISK" THEN GOTO Save_ep_rtn

!Id$=""

!CALL Enter_id (Id$ , Prompt$)

IF Id$="" THEN GOTO Save_ep_rtn

CALL Enterf ilename ( "ABORT" , Prompt$

IF Filename$="" THEN GOTO Save_ep_rtn

FOR 1=1 TO Datacount

NEXT I

CALL Save_f ile (B (*) , Datacount , Id$)

PRINT "IMAGINARY PART OF EPSILON SAVED"

Save_ep_rtn : Done=l

RETURN

1750!

1755! ////////////////////////////////////////////////////////////

1760!

1765 Done_return:

1770

1775

1780!

1785!

1790!

1795 End:!

Done=l

RETURN

////////////////////////////////////////////////////////////

1796

1800

1805!

1810!

1815!

1820

1825

DISP "Program finished.

END

Have a nice day."

************************************************************

SUB Enterfilename (Ac$ , OPTIONAL Prompt$)

Enterfilename: ! 1830 COM /Files/ Sourcedisk$ , Diskout$ , Diskdrive$

1835 INTEGER I, Ascii num

1840 DIM Test$[160]

1845 SELECT NPAR

1850 CASE 1

1855 DISP " ENTER the FILE NAME It •

f 1860 CASE 2

1865 DISP " ENTER the FILE NAME for ";Prompt$

1870 END SELECT

1875 SELECT Ac$

1880 CASE "CAT"

1885 DISP " ... (ENTER alone to CAT) ";

1890 CASE "ABORT"

1895 DISP " ... (ENTER alone to ABORT) ";

1900 CASE "VALID"

1905 END SELECT

1910 LINPUT Test$

1915 Test$=TRIM$ (Test$)

4A-7
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IF LEN(Test$)=0 THEN

SELECT Ac$

CASE "VALID"

DISP "You MUST enter the FILE NAME now."

BEEP

WAIT 1.8

GOTO Enterfilename

CASE "ABORT", "CAT"

GOTO Abortline

CASE ELSE

DISP "Ac$=";Ac$;" in SUB Enterf ilename"

BEEP

PAUSE

END SELECT

END IF

IF LEN(Test$) >10 THEN

BEEP

DISP "ERROR in NAME ENTRY—up to 10 chars, you have

DISP LEN(Test$) ;" " WAIT 1.8

OUTPUT 2 USING "# ,K,K" ; "?#" ;Test$

GOTO Enterf ilename

END IF

Filename$=Test$

FOR 1=1 TO LEN(Filename$)

Ascii_num=NUM(Filename$ [I]

)

SELECT Ascii_num

CASE 65 TO 90,95,97 TO 122,48 TO 57

! Allowed characters

CASE ELSE

BEEP

DISP "ERROR in NAME ENTRY—ILLEGAL CHARACTERS,

TRY AGAIN."

WAIT 1.8

OUTPUT 2 USING "#, K, K" Filename$

GOTO Enterfilename

END SELECT

NEXT I

SUBEXIT

Abortline: Filename$="

SUBEXIT

SUBEND

I

j ************************************************************

I SUB Select_disk (OPTIONAL Prompt$)

Select_disk:OFF KEY

COM /Files/ Sourcedisk$ , Diskout$ , Diskdrive$ , Filenames

INTEGER Prty

Prty=VAL( SYSTEMS ("SYSTEM PRIORITY") )+l

IF NPAR=1 THEN

DISP " SELECT DISK DRIVE for Prompts ... NO

DISK to abort. " ELSE

DISP " SELECT DISK DRIVE .... NO DISK to abort. "

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END IF

ON KEY 0 LABEL "NO DISK" , Prty GOSUB No_disk

ON KEY 1 LABEL "9133H VO ‘Vl",Prty GOSUB Hard9133h0

ON KEY ll,Prty GOSUB Hard9133hl

ON KEY 2 LABEL "9133H Floppy" , Prty GOSUB Floppy9133h

ON KEY 3 LABEL "9133XV Hard",Prty GOSUB Hard9133xv

ON KEY 4 LABEL "9133XV Floppy", Prty GOSUB Floppy9133x

IF Sys_id$[l,4]o"S300" THEN

ON KEY 5 LABEL "LEFT Internal" , Prty GOSUB Left_internal

ON KEY 6 LABEL "RIGHT Internal" , Prty GOSUB Right_internal

END IF

ON KEY 7 LABEL "9125 Floppy", Prty GOSUB Floppy9125

ON KEY 8 LABEL "9122 Left",Prty GOSUB Floppy91221

ON KEY 9 LABEL "9122 Right", Prty GOSUB Floppy9122r

LOOP

EXIT IF Done

END LOOP

SUBEXIT

Left_internal : Diskdrive$=" :HP9153 ,700 , 0"

GOTO Diskselected

Right_internal : Diskdrive$=" : INTERNAL, 4 , 0"

GOTO Diskselected

Hard9133xv: Diskdrive$=" :HP9133 , 700 , 0"

GOTO Diskselected

Floppy9133x: Diskdrive$=" :HP9133 , 702 , 0"

GOTO Diskselected

Hard9133h0 : Diskdrive$=" :, 700 , 0 , 0"

GOTO Diskselected

Hard9133hl : Diskdrive$=" :, 700 , 0 , 1"

GOTO Diskselected

Floppy9133h: Diskdrive$=" ;, 700 , 1"

GOTO Diskselected

Floppy9122r : Diskdrive$=" : ,707 , 1"

GOTO Diskselected

Floppy91221 : Diskdrive$=" ;

, 707 , 0"

GOTO Diskselected

Floppy9125 : Diskdrive$=" : ,704 , 0"

GOTO Diskselected

No_disk: Diskdrive$="NO DISK"

Diskselected: OFF KEY

Done=l

RETURN

SUBEND

I

• ***********************************************************8

I SUB Enter_id (Id$ , OPTIONAL Return_test$)

Enter_id:

I• !LAST REVISION 30/SEPT/86

OPTION BASE 1

COM /Bugs/ INTEGER Bugl , Bug2 , Bug3 , Printer , Printer_on

\ DIM Test$[160]

INTEGER N

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N=LEN(Id$)

Test$=Id$

SELECT Id$

CASE

! OUTPUT NOTHING

CASE ELSE

OUTPUT 2 USING "K,#";Test$

END SELECT

SELECT NPAR

CASE 1 I NO Return_test$ given

DISP CHR$ ( 129) ; "Please ENTER a description (<= 40 chrs).";

DISP CHR$(128)

CASE ELSE

DISP CHR$ ( 129 ); "Please ENTER a description (<= 40 chrs)

DISP CHR$ (128)

SELECT Return_test$

CASE Id$

DISP " for THIS ID"?

CASE "ABORT"

DISP " CLR LN/ ENTER to ABORT."

CASE ELSE

DISP " for " ;Return_test$

?

END SELECT

END SELECT

LINPUT Test$

DISP "" Test$=TRIM$ (Test$)

N=LEN (Tests

)

SELECT N

CASE >40

DISP "Length of data_id$ too long. You entered "?N;

DISP " characters. Try again."

BEEP

WAIT 1.5

IF NPAR=2 THEN

IF Id$oReturn_test$ THEN

OUTPUT 2 USING "#,K";Test$

END IF

END IF

GOTO Enter_id

CASE =0

IF NPAR>1 THEN

IF Return_test$="ABORT" THEN

Id$=Test$ !=""

SUBEXIT

END IF

END IF

DISP "You must ENTER SOMETHING or you'll ";

DISP "never get out of this."

BEEP 1000, .3

WAIT 1.8

GOTO Enter_id

CASE ELSE

! Everything ok

END SELECT

4A-10
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Id$=Test$

SUBEND

I•

I ***************************************

I• SUB Save_file(T_f (*), INTEGER Datacount , Id$

Save_file: ! COM /Files/ Sourcedisk$ , Diskout$ , Diskdrive$ , Filenames

ON ERROR CALL Errortrap

Diskspace=INT ( (3500+ (Datacount*16) )/256)+l

CREATE BOAT Filename$&Diskdrive$ , Diskspace , 256

CREATE ASCII Filename$&Diskdrive$ , Diskspace*2

ASSIGN ^Datapath TO Filename$&Diskdrive$

OUTPUT ^Datapath ; "N"

OUTPUT 0Datapath ; TRIMS (Id$)

OUTPUT 0 Datapath ; Datacount

OUTPUT 0Datapath;Datacount

OUTPUT 0Datapath;T_f (*)

ASSIGN 0Datapath TO * OFF ERROR

SUBEND

****************************************

SUB Errortrap

Errortrap: !Trap disk errors here

COM /Files/ SourcediskS , DiskoutS r DiskdriveS , Filenames

DIM FileS[20] ,TestS[160] ,WhatS[20] ,AcS[5]

BEEP 400, .6

SELECT ERRN

CASE 54

DISP "DUPLICATE FILE NAME: Filenames ; DISP " PURGE old one? (Y/N)»;

LINPUT WhatS

SELECT WhatS [1,1]

CASE

PURGE FilenameS&DiskdriveS

CASE ELSE

AcS="VALID”

PromptS=”

CALL Enterfilename (AcS)

END SELECT

CASE 52,53

DISP "Improper FILE NAME ENTER NEW FILE NAME"

OUTPUT 2 USING "# , K, K" ; ;FilenameS

LINPUT Filenames

FilenameS=TRIMS (Filenames)

CASE 56

DISP "FILE: "; Filenames ; " is not on this disk,

please insert";

DISP " correct disk"

PAUSE

CASE 64

DISP "This disk is full, PLEASE insert clean disk"

PAUSE

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CASE 56

DISP "DATA INPUT disk must be in drivel!

DISP "...CONTINUE when ready."

PAUSE

CASE 12 , 12,16

DISP Diskdrive$;

DISP " is not available, type correct";

DISP " unit specifier (ie. ' :

, 707 , 0

' )

. "

OUTPUT 2 USING "K, #" ; Diskdrive$

LINPUT Diskdrive$

CASE 80

DISP "CHECK DISK drive door!"

PAUSE

CASE ELSE

DISP ERRM$;" * CONTINUE' when fixed"

PAUSE

END SELECT

DISP CHR$(12)

SUBEXIT

SUBEND

I•

I ********************************************

( SUB Test_real (Test, Test$, Low, High, INTEGER Bad_number)

Test_real :

Bad_number=0

ON ERROR GOSUB Trap_bad_number

IF Bad_number THEN RETURN

Test=VAL(TRIM$ (Test$)

)

OFF ERROR

SELECT Test

CASE <Low

BEEP 1000, .3

DISP " Number entered is TOO LOW. ";

DISP " LOWEST allowable number is ";Low

WAIT 2.1

Bad_number=l

CASE >High

BEEP 1000, .3

DISP " Number entered is TOO HIGH. ";

DISP " HIGHEST allowable number is ";High

WAIT 2.1

Bad_number=l

CASE ELSE

Bad_number=0

! Number within limits

END SELECT

SUBEXIT

////////////////////////////////////////////////////////////

I Trap_bad_number :

SELECT ERRN

CASE 15,32

DISP CHR$ (129) ; "What you ENTERED is not a number! Try again.

4A-12
3280

3285

3290

3295

3300

3305

3310

3315

3320

3325

3330

3335

DISP CHR$(128)

Bad_number=

WAIT 1.7

LINPUT "Please ENTER the number you wish",Test$

CASE ELSE

DISP ERRN,ERRM$

BEEP 850, .5

Bad_number=l

PAUSE

END SELECT

RETURN

SUBEND

4A-13
i

. (t

w s

’ %I

*

•'

'

; ,‘v#-7-

' r- ikV v :

)«

T.-V:.*-'

'

t.‘ i' - f'

y*.. ' .£ ' ^

' 'C*' a

“I . ;:,' *”•

:';i ‘..^ /i» , 7..,

* ' '

i'.hv i" ft-v r-* '

•,'r.

'.•• : X

•

' ^;-, Pi‘MT

^S’ '^’c;

i

. ^ *

•

.r. y. V * ‘fr %-. r:

c ; / . /

'

, :" » . -

;v i-v ,, _ ::•'' Ha ''7 \

••A'

;i, irs 1

'"'E

Xp

' '-i; '

.1 fc* 7^7 *J It’ li*-<1

^

V7, ,:; A if'.

b: •' af.,:- -^P' ^

, .*, '’ '

' '

i;:. ^•‘•^'7 ' .

• 7 . . •Pi W .

'

. ,

’

7 i. 45,

, '>.^' 3 V “ •?

^ • at

'^:fV •V#

3 ..., ti ^

'’ 7,. -’. A

' /7-.V' a : .'

'^ 3 ^ KP} a..;l .,7 IT -4' .H 1•'^4 ^

' '

, • ' „

"

.

.

' 7

.-'c/ifH

'

4'.\&

' T'v;

TVv.

.

' ' •

.s . ^ ’1 ' • ;'v ‘:i‘ '•i ,'

: ; /' .

' ’ ; '.''’w/ia!®'?*;

' .^,' 7 ;

;: " '

a: ' ., "

:

:"''

'

-'/'•r

p'

v.^

'

4> W

.: %;••• •, • lif

t,:.a ' //,7V//V/4

l.-a4 "'SA!SK*«?Sa"'.,47-"77':S .a^^

:4>'N
Appendix IV.B: Automatic Network Analyzer System

for Measuring e and ji

The ANA approach is based on measuring reflection and transmission 52i from and

through a section of material-filled air line. These S parameters are then mathematically inverted

to determine e and fi. The analysis is straightforward and is repeated here for convenience. We

follow the notation used in [3.30].

In terms of the normal plane-wave reflection (F) and transmission (T) coefficients, 5^1 and

S21 are given by

(4B.1)

and

where

Z + Zo’

(4B.2)

and

Inverting eq (4B.1) yields

r=K±(K'^-i), (4B.3)

where

(4B.4)

and

[5ii(6))-S2i(Q))]-r

l-[Sii(«B) + 52i(tU)]r‘

We find that

4B-1
^rl^r = li-r>

(4B.5)

and

It follows that

and

1 .In

JcQd \T )_

IXr = iyx)^'^.

(4B.6)

(4B.7)

4B-2
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(5-90)

U.S. DEPARTMENT OF COMMERCE

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

BIBLIOGRAPHIC DATA SHEET

1. PUBUCATION OR REPORT NUMBER

NISTIR 3982

2. PERFORMING ORGANIZATION REPORT NUMBER

B91-0305

3. PUBUCATION DATE

October 1991

4. TITLE AND SUBTITLE

Quantifying Standard Performance of Electromagnetic-Based Mine Detectors

5. AUTHOR(S)

William L. Cans, Richard G. Geyer, and Wilfred K. Klemperer

6. PERFORMING ORGANIZATION (IF JOINT OR OTHER THAN NIST, SEE INSTRUCTIONS)

U.S. DEPARTMENT OF COMMERCE

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

BOULDER. COLORADO 80303-3328

7. CONTRACT/GRANT NUMBER

8. TYPE OF REPORT AND PERIOD COVERED

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (STREET. CITY, STATE, ZIP)

U.S. Army Belvoir Research, Development, and Engineering Center

Ft. Belvoir, VA 22060-5606

10. SUPPLEMENTARY NOTES

11. ABSTRACT (A 200-WORD OR LESS FACTUAL SUMMARY OF MOST SIGNIFICANT INFORMATION. IF DOCUMENT INCLUDES A SIGNIFICANT BIBUOGRAPHY OR

LITERATURE SURVEY, MENTION IT HERE.)

This is a final report to sponsor on work performed by National Institute of Standards and Technology (NIST)

personnel from January 1, 1985 to December 31, 1990. An overview of the theory of the electromagnetic

properties of soils is presented along with a brief review of existing technologies for the detection of buried

objects using electromagnetics. The critical electromagnetic performance factors for portable EM mine

detectors that NIST has identified are presented, along with a discussion of measurement systems for

measuring the constitutive properties of soil and mine-like materials. Recommendations are then presented

for a measurement system configuration that should meet most of the Army's requirements. A recommended

mine detector testing strategy is then presented along with a set of instructions for specific tests and an

algorithm for comparatively scoring the performance of detectors. The tests and the scoring algorithm are as

specific and as detailed as is possible at this stage of development. Last, a section is included that contains

NIST's recommendations for the test data that should be archived.

12. KEY WORDS (6 TO 12 ENTRIES; ALPHABETICAL ORDER; CAPITALIZE ONLY PROPER NAMES; AND SEPARATE KEY WORDS BY SEMICOLONS)

buried object; constitutive properties; conductivity; dielectric constant; dielectric loss; electromagnetic

detection; mine; mine detector; permeability; permittivity; remote sensing; sensor; target

13. AVAILABILITY X

UNUMITED 184

FOR OFFICIAL DISTRIBUTION. DO NOT RELEASE TO NATIONAL TECHNICAL INFORMATION SERVICE (NTIS).

ORDER FROM SUPERINTENDENT OF DOCUMENTS, U.S. GOVERNMENT PRINTING OFFICE,

WASHINGTON, DC 20402.

15. PRICE

AO 9

JL_ ORDER FROM NATIONAL TECHNICAL INFORMATION SERVICE (NTIS), SPRINGRELD, VA 22161.

14. NUMBER OF PRINTED PAGES

ELECTRONIC FORM
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