Design and validation of inert homemade explosive simulants ...Keywords: Improvised explosives, Homemade explosives, Ground Penetrating Radar, Simulant, Training aide 1. INTRODUCTION

Design and validation of inert homemade explosive simulantsfor Ground Penetrating Radar

Brian W. VanderGaasta, John E. McFeeb, Kevin L. Russella, and Anthony A. Fausta

aDefence R&D Canada – Suffield Research Centre, Medicine Hat, AB, CanadabMcFysics Consulting, Medicine Hat, AB, Canada

ABSTRACT

The Canadian Armed Forces (CAF) identified a requirement for inert simulants to act as improvised, or home-made, explosives (IEs) when training on, or evaluating, ground penetrating radar (GPR) systems commonly usedin the detection of buried landmines and improvised explosive devices (IEDs). In response, Defence R&D Canada(DRDC) initiated a project to develop IE simulant formulations using commonly available inert materials. Thesesimulants are intended to approximate the expected GPR response of common ammonium nitrate-based IEs, inparticular ammonium nitrate/fuel oil (ANFO) and ammonium nitrate/aluminum (ANAl). The complex permit-tivity over the range of electromagnetic frequencies relevant to standard GPR systems was measured for bulkquantities of these three IEs that had been fabricated at DRDC Suffield Research Centre. Following these mea-surements, published literature was examined to find benign materials with both a similar complex permittivity,as well as other physical properties deemed desirable — such as low-toxicity, thermal stability, and commercialavailability — in order to select candidates for subsequent simulant formulation. Suitable simulant formulationswere identified for ANFO, with resulting complex permittivities measured to be within acceptable limits of targetvalues. These IE formulations will now undergo end-user trials with CAF operators in order to confirm theirutility. Investigations into ANAl simulants continues. This progress report outlines the development program,simulant design, and current validation results.

Keywords: Improvised explosives, Homemade explosives, Ground Penetrating Radar, Simulant, Training aide

1. INTRODUCTION

The Canadian Armed Forces (CAF) identified a requirement for inert simulants to act as improvised, or home-made, explosives (IEs) when training on, or evaluating, ground penetrating radar (GPR) systems commonlyused in the detection of buried landmines and improvised explosive devices (IEDs). During routine training,where the logistical burden of acquiring, handling, and disposing of IEs is undesirable, GPR operators were attimes tested against surrogate IEDs consisting of containers filled with locally available materials. Unfortunately,such ad hoc solutions can generate non-representative GPR signals, which in turn may lead to poorer operatortraining and detection performance against the targets of interest.

In response, Defence R&D Canada (DRDC) initiated a project to develop IE simulant formulations usingcommonly available inert materials. These simulants are intended to approximate the expected GPR responseof common ammonium nitrate-based IEs, in particular ammonium nitrate/fuel oil (ANFO) and ammoniumnitrate/aluminum (ANAl).

This progress report outlines the development program, simulant design, and current validation results.

Other author information: (Send correspondence to B.W.V.)B.W.V., K.L.R, A.A.F.: Email: [Brian.VanderGaast|Kevin.Russell|Anthony.Faust]@drdc-rddc.gc.ca; Telephone: +1 403544 4011 x.[5273|4746|5362]; Fax: +1 403 544 4704J.E.M. contributed to this work while an employee of Defence R&D Canada: Email: [email protected]

c© Her Majesty the Queen in Right of Canada, as represented by the Minister of National Defence, 2015. Li-censed material is licensed pursuit to the Creative Commons Attribution-Noncommercial-NoDerivative Works 3.0Unported License (CC-BY-NC-ND). Provisions of Section 5 of the above noted license apply to the licensed material.

Detection and Sensing of Mines, Explosive Objects, and Obscured Targets XX, edited by Steven S. Bishop, Jason C. Isaacs, Proc. of SPIE Vol. 9454, 945412

© 2015 SPIE · CCC code: 0277-786X/15/$18 · doi: 10.1117/12.2175586

Proc. of SPIE Vol. 9454 945412-1

DRDC-RDDC-2015-P085

2. GROUND PENETRATING RADAR

Ground penetrating radar is a well-known electromagnetic (EM) technique that has found wide use as a sub-surface analysis tool in such diverse applications as archaeological surveying, concrete inspections, ice thicknessmeasurements, forensic investigations, and, of course, explosive hazard avoidance, to name only a few.

GPR finds its utility in its ability to penetrate soils and other obscuring materials in order to interrogate buriedobjects for size, shape, and depth. Simplistically, EM waves are transmitted into the ground where they interactand reflect off the different material boundaries, based on differences in their electromagnetic properties. Thesereflected waves are subsequently measured and analyzed. While the general concept is simple to understand, thetechnique is theoretically rich and more detailed descriptions are available elsewhere.1

GPR has been used in explosive hazard avoidance roles since at least the 1970s.2 First employed as a vehicle-mounted sensor for in-route landmine detection,3,4 advances in the technology have allowed for the developmentof hand-held versions,5 where it is often paired with an electromagnetic induction-based (EMI) sensor.

For military explosive hazard detection applications, an operating frequency band of 200 MHz to 7 GHzis typical, being determined as a trade-off between soil penetration and spatial resolution. In this frequencyrange, which is itself a loose subset of the microwave band (0.3 – 300 GHz), the electromagnetic property whichdominates the response of a material to a transmitted EM pulse is the complex permittivity.

2.1 Complex Permittivity

The frequency-dependent complex permittivity, ε(ω), in a linear medium is defined as

ε(ω) = ε′(ω)− iε

′′(ω), (1)

where i =√−1 and ω = 2πν is the angular frequency associated with the frequency of propagation of the

electromagnetic wave in the medium ν. In the remainder of the paper, the frequency dependence of ε(ω) will besuppressed and it will be written simply as ε.

The real component of ε, ε′

= Re(ε), governs propagation through the medium, and the imaginary component,ε′′

= Im(ε), accounts for loss (absorption of the incident electromagnetic wave).

In materials where conductivity loss is also a concern, Eqn. 1 can be written in the more familiar form,

ε = ε′− i(ε

′′+σ

ω), (2)

= ε′

[1− i(ωε

′′+ σ

ωε′)

], (3)

= ε′[1− i tan δ] , (4)

= εrε0 [1− i tan δ] , (5)

where εr = ε′/ε0 is the dimensionless real component of the relative permittivity, ε0 is the permittivity of free

space, and δ is the skin depth related to attenuation losses in a material.

The frequency-dependent loss tangent, tan δ, includes both dielectric damping and conductivity loss, whichare indistinguishable from one another,

tan δ =

(ωε

′′+ σ

ωε′

). (6)


The importance of εr and tan δ to GPR can be seen by examining the index of refraction n of a medium,

n =c

v=

√ε

ε0µr, (7)

where c is the speed of light in vacuum, v is the speed of the electromagnetic wave in the medium, and µr isthe relative permeability in the medium, which may in general be a complex quantity6 ∗. For most of the IEs ofinterest, µr = 1 and Eqn. 7 reduces to,

n =

√ε

ε0(8)

Many IE materials have low losses (tan δ 1), in which case n can be approximated as a real quantity givenby,

n =√εr. (9)

IEs which involve mixtures of metals may have significant electrical conductivity, making losses significant.The refractive index then becomes a complex quantity given by Eqn. 8.

Assume an electromagnetic plane wave propagates in a medium of refractive index n1 and is traveling normalto the boundary between that medium and a second one having refractive index n2. The scalar reflectioncoefficient, R, which is the fraction of the incident wave energy that is reflected, is given by8 (independent of theincident polarization),

R =

∣∣∣∣n1 − n2n1 + n2

∣∣∣∣2 . (10)

Thus, at the frequencies of interest, the reflection of electromagnetic waves at a boundary is a function of therelative permittivities of the materials on either side of the boundary.

According to Eqn. 10, detection of targets by GPR depends on the contrast between the permittivities ofthe target components, and those of the background materials in which the target is embedded. For buriedIEDs and land mines, the background material is soil and the target components can consist of metal, wood,plastic, rubber and explosives. When making a surrogate device, nonexplosive components can be copied usingthe actual materials. Explosive components require the one-for-one substitution of the explosive material withan appropriate explosive simulant, which is the focus of this paper.

Let us assume a simple geometry of a buried IED, comprising an IE-filled container with a top and bottomsurface, which is illuminated by an above-ground, downward-looking GPR. The refractive index, and hence(roughly) the square root of the relative permittivity, determines the fraction of the initial EM wave that isreflected from the top surface of the IED, the refraction angle(s) at that surface, and the reflected fraction atthe bottom surface of the IED. The two-way transit time of the EM wave through the IE is a function of therelative permittivity, which may have an effect on interpreting GPR signatures of IEDs.

The loss tangent, through the skin depth, determines the attenuation of the wave propagating through theexplosive to the bottom surface and its attenuation along the return path through explosive after reflection fromthe bottom surface.

Buried explosive threat scenarios exhibit a high degree of variability. For example, there are many possiblegeometries and electromagnetic properties of soil may vary substantially due to variations in water content andsoil properties, local soil inhomogeneity and the structure of the mine or IED, including voids and casings. Thusit is difficult to predetermine the requirements for how closely the surrogate electromagnetic properties should

∗Strictly speaking, this is the magnitude of the phase velocity. A group index can also be defined by replacing v withthe magnitude of the group velocity. The refractive index and group index are equal for a non-dispersive medium.7


match those of the IE. In practice, choices must be verified by testing the potential surrogates in a realistic IEDagainst an appropriate GPR in different appropriate soil types.

When selecting materials to be simulants for the IE materials in GPR surrogate devices, the goal then is tomatch as closely as possible both relative permittivity and the loss tangent of the true material.

3. EXPERIMENTAL DESIGN

3.1 Relative Permittivity for Dielectric Materials

As discussed previously, GPR requires spatial changes in the complex permittivity to detect targets. Since thecomplex permittivities of IEs are in general unknown, these values first needed to be measured before suitablesimulants could be developed.

Dielectric materials research by von Hippel9 in the 1950s established the baseline for measuring the complexpermittivity for unknown materials. Further work in the 1970s by Nicolson and Ross10 established a time-domainmeasurement technique, calculating the complex permittivity by use of scattering parameters; the s-parameters.The s-parameters describe the electromagnetic behavior of an electric circuit, specifying the transmission and re-flection characteristics (Fig. 1). S11 and S22 specify the reflection quantities, S21 and S12 specify the transmissionquantities.

Figure 1. S-parameter diagram for an EM wave traveling within the coaxial waveguide through a sample material ofthickness d and characteristic impedance Zs. The incident wave is travelling from left to right along the horizontal x-axis.The left-hand boundary of the sample is at x = 0 and the right-hand boundary is at x = d.

The EM wave equations can be solved at the boundary conditions (x ≈ 0, x ≈ d, and x ≈ ∞), such that thes-parameters can be used to derive reflection and transmission coefficients:10

S11(ω) = ([1− T 2]Γ)/(1− T 2Γ2) (11)

S21(ω) = ([1− Γ2]T )/(1− T 2Γ2) (12)

Γ =Zs − ZoZs + Zo

=

(√µrε0ε− 1

)/

(√µrε0ε

+ 1

)(13)

T = e−i(ω/c)d√µrε/ε0 (14)

where T is the transmission coefficient, Γ is the reflection coefficient and Z0 is the characteristic impedance inthe reference medium (reference impedance). When air is used as the reference medium, Z0 is approximatelyequal to the impedance of free space (≈ 377Ω). S11 and S21 can be used to derive T and Γ through Eqns. 11and 12, which are then used to calculate the complex permittivity through Eqns. 13 and 14.

Following extraction of the real and imaginary components of the complex permittivity, the relative permit-tivity and loss tangent may be calculated using Eqn. 5 as εr = Re (ε) /ε0 and tan δ = −Im (ε) /Re (ε).


(a) Experimental setup used for complex permittivitymeasurements.

(b) Sample holders for the 3000T coaxial waveguide

Figure 2. Setup of the s-parameter measurements, includes the Damaskos 3000T coaxial waveguide and the Agilent PNA-Xvector network analyser (left), and Clamshell/Soilcell sample holders (right).

3.2 Equipment and Laboratory Setup

A vector network analyzer (VNA) and an EM propagation system were used to measure the s-parameters,where the VNA is used to drive the system though a pre-determined frequency range and the EM propagationmechanism is used to guide the EM energy though the sample material.

A variety of EM propagation systems are available including free space, open-ended coaxial probe, andwaveguide transmission line. The free space technique is best for large, thin, parallel-faced materials. Theopen-ended coaxial probe technique is best for high relative permittivity liquids and semi-solids with no air voidswithin the material. The waveguide transmission line method is best for materials with low relative permittivityand can be used for liquids, solids and granular materials of varying thicknesses, and so was the chosen method.

The waveguide transmission line method involves placing the material inside a portion of an enclosed trans-mission line, which is usually a section of a rectangular or coaxial waveguide. In this work, a coaxial waveguidewas selected for the IE measurements as it covers a broader frequency range as compared to a rectangularwaveguide.

The 3000T coaxial waveguide kit shown in Fig. 2(a) was acquired from Damaskos Inc., PA, USA. The kitincludes a liquid/granular sample holder (soilcell) and a solid sample holder (clamshell), both shown in Fig. 2(b).Paired with a Agilent PNA-X vector network analyzer, the s-parameters could then be measured. A sweepfrequency range of 375 MHz – 3 GHz was selected as it spans the wide spectral range which is of most interestto GPR systems used in IED and landmine detection. Higher frequencies, although also of some interest, wouldrequire enhanced machining tolerances for the coaxial waveguide.

In addition to the 3000T EM propagation system, Damaskos Inc. also provided a software package containingdata reduction routines that convert s-parameters into complex permittivity. This data reduction makes assump-tions regarding the sample, including being isotropic and having a relative permeability µr = 1 (non-magnetic).With the s-parameters measured by the VNA, the data reductions only require sample thickness and sampleholder size to calculate the complex permittivity.

During the measurement procedure, the VNA applies an electric field to the coaxial waveguide and throughthe sample contained within. The VNA begins the measurement process at the minimum output level of -25 dBm(3 µW) and based on the parameters measured, increases the applied field to a maximum of 20 dBm (100 mW).This prevents any overloaded current and excess heat being applied to the sample, thereby reducing the risk ofexplosive sample ignition. To further reduce the risk of occurrence of a safety incident, standoff protection wasachieved by controlling the VNA remotely via a fiber-optic link. Additionally, the VNA was connected to the


Figure 3. Experimental procedure for a TRL, two port calibration. A calibration plane is selected along the transmissionline, then four measurements are made in reference to this plane. This procedure establishes the baseline s-parameters,used in the system calibration.

coaxial waveguide using 3 m-long RF cables in order to maximize the standoff distance between the VNA andthe IE, while minimizing cable error †. Lastly, a fragmentation shield was placed between the VNA and the IEto provide additional protection.

3.3 Coaxial Waveguide Calibration

The Agilent PNA-X has two ports that send and receive the driven source signal within the selected frequencyrange, measuring not only the signal transmission from port to port but also isolating the energy that is reflectedback to the source, which is required for s-parameterization.

As the frequency of operation increases into the microwave band, magnitudes of the imaginary componentsincrease relative to those at lower frequencies as radiation loss, dielectric loss, and capacitive coupling becomemore predominate. To compensate for this, a calibration measurement was first taken in order to normalizesubsequent data analysis against systematic errors caused by imperfections in the test setup (internal connections,cables, coaxial transmission line, etc.).

For a two port system, such as the 3000T coaxial transmission line arrangement, calibration is achieved bythe transmission-reflection-line (TRL) method (Fig. 3). A position along the transmission line is selected asthe calibration plane. All components on one side of the plane are considered to belong to port one and allcomponents on the other side belong to port two. Four separate measurements are then made. The first two aremade with each port shorted in turn (reflection). Next, the ports are connected and a “through” measurement ismade (transmission). Lastly, a measurement is made with a straight section of waveguide (line). This procedureestablishes the baseline s-parameters, used in the system calibration. The selection of the line is importantsince its length is frequency dependent. Typically the line is selected such that there is maximum phase shift of−20 to −160 while the source signal is swept through the selected frequency range. The corresponding linelength selected for 375 MHz – 3 GHz was 4.450 cm.

Care must be taken to control errors and drift caused by environmental conditions (temperature, humidity,pressure). These errors can be minimized by adopting good measurement practices, such as visually inspect-ing all connectors for dirt or damage, minimizing physical movement of the test port cables, and minimizingenvironmental changes after a calibration.

†The calibration process normalizes losses and the use of extensively long cables (>10 m) will decrease the sensitivityof the measurement.


(a) Liquid/granular sample holder (soilcell). (b) Solid sample holder (clamshell).

Figure 4. Aluminum coaxial waveguide sample holders with sample dimensions.

3.4 Measurement Methodology

The 3000T coaxial waveguide has two measurement containers: a soilcell for liquid/granular materials, and aclamshell for solids. The experimental procedure is similar for both containers with the exception of the soilcellcalibration. Fig. 4(a) and Fig. 4(b) illustrate the sample setup for the soilcell and clamshell sample holders. Inaddition to sample height, the sample weight is recorded to track the density of the sample, if applicable.

The general procedure for the permittivity measurement involved several steps. First, the VNA/Coaxialwaveguide system and measurement container were calibrated using the TRL method. Next, port 2 of thecoaxial waveguide was removed from the top of the measurement container and the required sample was addedto the container. The sample offset was measured (see Fig. 4(a), Fig. 4(b)). Port 2 of the coaxial waveguide wasthen connected to the measurement container and a wait period of 10 minutes was imposed to allow the setupto thermally stabilize. The s-parameter measurement was then initiated. After the measurement was complete,the measurement container was removed from the coaxial waveguide. The sample was then removed from thecontainer which was cleaned of all sample residue to allow re-use of the container. To minimize statistical errors,each permittivity measurement was conducted a minimum of three times, with the reported values being anaverage of the measurements.

3.5 Control Samples

To validate the 3000T coaxial waveguide experimental apparatus, control materials were selected such that themeasured relative permittivity could be compared with documented values. These material standards wereselected from Dielectric Materials and Applications,9 which contains an extensive list of materials and corre-sponding relative permittivities, from the low MHz region up to 10 GHz. Materials chosen for the validationwill act as a set of control samples to ensure the measurement technique is valid. These materials also representthe range of relative permittivities and material states relevant to the coaxial waveguide kit. Selected materialsinclude distilled water, propanol, kerosene, paraffin wax, and TNT.

Two methods of TNT forming were evaluated, cast and flake. The cast TNT was a machined sample that fitinto the clamshell sample holder, similar to paraffin wax samples. The flake TNT was packed into the soilcellsample holder, similar to the AN and IE samples. The density of the flake TNT was 0.79 g/cm3, and that of thecast sample was 1.57 g/cm3. As anticipated, the permittivity of the cast TNT was higher than the permittivityof the flake TNT. The TNT stated in von Hippel9 is most likely a cast TNT variant as it matches the permittivityof our cast TNT samples.

Tables 1 and 2 present the relative permittivity, εr, and loss tangents, tan δ, of material standards in thefrequency range of interest, 0.375 – 3.0 GHz. The values measured closely match the corresponding literaturereference and strengthen the confidence in the 3000T coaxial waveguide approach.


Table 1. Relative permittivities and loss tangents for material standards from Reference 9, measured at 25 C.

Material Property 0.1 GHz 0.3 GHz 3.0 GHz 10.0 GHz Note

Distilled Water εr 78 77.5 76.7 55 [9, p. 361]tan δ × 104 50 160 1570 5400

Propanol εr 19 16 3.7 2.3 [9, p. 362]tan δ × 104 2000 4200 6700 900

Kerosene εr 2.09 [9, p. 365]tan δ × 104 45

Jet Fuel a εr 2.12 2.09 [9, p. 365]tan δ × 104 12 68

Paraffin Wax εr 2.25 2.25 2.25 2.25 [9, p. 358]tan δ × 104 1.4 1 2 2.5

Trinitrotoluene εr 2.89 2.89 2.86 2.82 [9, p. 315](TNT) tan δ × 104 28 39 18 14

a Kerosene and jet fuel are chemically very similar. Jet fuel values at 0.3 GHz should be areasonable approximation to that of kerosene at the same frequency.

Table 2. Measured relative permittivities and loss tangents of material standards with 3000T coaxialwaveguide, measured at 23 C.


Distilled Water εr 78.4 78.0 77.4 76.7tan δ × 104 194 493 977 1470

Propanol εr 15.0 8.4 5.3 4.5tan δ × 104 4320 8129 8691 7547

Kerosene εr 2.09 2.09 2.09 2.09tan δ × 104 < 50 < 50 < 50 < 50 a

Paraffin Wax εr 2.25 2.25 2.25 2.24tan δ × 104 < 50 < 50 < 50 < 50

Trinitrotoluene εr 2.89 2.87 2.85 2.84(Cast TNT) tan δ × 104 < 50 < 50 < 50 < 50

Trinitrotoluene εr 1.77 1.77 1.77 1.77(Flake TNT) tan δ × 104 < 50 < 50 < 50 < 50a Loss tangents measured below 50×104 are stated as < 50 × 104 as this is the minimum

accuracy of the coaxial waveguide data reduction for low loss materials.Table 3. IE samples used for relative permittivity measurements.

Material Density (+/- 0.02 g/cm3)

Ammonium Nitrate 0.71 – 1.11Ammonium Nitrate Fuel Oil 1.13Ammonium Nitrate Aluminum 0.96

4. IE MEASUREMENT RESULTS

Measurements of AN, ANFO, and ANAl were complicated by the need for uniform compaction in the sampleholders, as the density of the sample will affect the resultant relative permittivity.11 While the varying densityand resulting permittivity of each IE sample could be tracked and used to determine the corresponding linear-lawapproximation, for the purpose of this investigation the density only needed to match that of IEs found in use.Using the guidance of local IE subject matter experts, the density of each sample was controlled using a layeredcompaction technique, where each IE sample was loaded into the sample holder in three thinner layers of uniformthickness. The sample was then compacted by hand after each layer was added, and the resultant density wasmeasured and recorded. Analysis showed that the variability of the density of the IE samples was on average±0.02 g/cm

3.


Table 4. Measured relative permittivities and loss tangents of the IE samples using the 3000T coaxial waveguide.


Ammonium Nitrate a εr 2.36 – 3.70 2.36 – 3.63 2.35 – 3.61 2.35 – 3.60tan δ × 104 <50 – 425 <50 – 300 <50 – 200 <50 – 150 b

Ammonium Nitrate εr 3.67 3.63 3.59 3.57Fuel Oil tan δ × 104 175 125 60 <50

Ammonium Nitrate εr 9.57 9.39 9.11 8.83Aluminum tan δ × 104 340 470 675 900

a Range due to differing physical forms of commercially available AN.b Loss tangents measured below 50×104 are stated as < 50× 104 as this is the minimum accuracy

of the coaxial waveguide data reduction for low loss materials.

The density of the AN and the two IE materials were found to be similar (Table 3) which was expected sincethe AN makes up a large portion of each IE. The fuel oil additive increases the density slightly, and Al decreasesthe density. As expected, AN formulations with larger particle sizes and hence higher void fractions have smallerdensities. Of the three IE samples evaluated, AN had the largest compaction factor at 15%. Given such a largechange in density, care must be taken to present consistent sample characteristics for each measurement. Usingthe layered compaction method, each sample measured will have uniform density.

Table 4 presents the measured relative permittivities and loss tangents of AN and the two IE materials. TheANFO relative permittivity was measured to be approximately 3.6. The diesel portion in the ANFO pulls thecombined permittivity down, but the low percentage of diesel leaves AN as the dominant component of thepermittivity. The ratio of aluminum to AN in the ANAl IE is also small, however the Al has a substantiallydifferent relative permittivity, driving the relative permittivity of the ANAl up to approximately 9.

The behaviour of the loss tangent also differs for the two IE samples. The loss tangent increases with frequencyfor ANAl and decreases for ANFO, within the selected frequency range. This difference can be attributed to thealuminum in the ANAl, since the loss tangent of the AN sample also decreases with frequency, similar to theANFO sample. When selecting simulant material, the loss tangent behavior may become a critical factor.

5. SIMULANTS

5.1 Military Explosives and Simulants

The complex permittivities of military explosives in the microwave region have been known for some time andsuitable GPR simulants exist. Table 5 displays relative permittivities, εr, and loss tangents, tan δ, of somecommon military high explosives, as well as some possible simulants in the frequency range of interest for GPR.Matching is made easier by the fact that, being far from the resonance regions of most materials, both εr andtan δ are fairly flat over this frequency range, the former more so than the latter.

The relative permittivities are close in value to those of many soil types,9 although the loss tangents are inmost cases lower. Thus GPR return signals are more heavily influenced by explosive device construction (airgaps, case material and thickness) than by the military explosive fill. The same argument applies to the simulantfilling, provided a pathological material (air, soil, metal) is not chosen. Regardless, the listed simulants providea fairly good match to the electromagnetic properties of the explosives. Only a small number of candidates forsimulants have been listed, but there are many more. It should be noted that beeswax is commonly used as aGPR simulant for TNT.

The εr and tan δ of IE materials tend to be higher than those for military explosives, thus simulants formilitary explosives would not be optimal simulants for these IEs.

5.2 Improvised Explosives

5.2.1 ANFO

There appear to be a number of possible simulant materials for ANFO, as εr ≈ 3.6 and the flat frequency responsecan be fairly easily matched. Table 6 presents εr and tan δ for a selection of possible simulants for ANFO.


Table 5. Relative permittivities and loss tangents for high explosives and possible simulant materials. Material temperatureof 20-25 C is assumed.


Military Explosives

TNT (trinitrotoluene) εr 2.89 2.86 2.82 [9, p. 315]tan δ × 104 39 18 14

Comp B εr 3.20 3.20 3.20 [12], 0.3–3.0 GHztan δ × 104 35 20 29 [13], 10 GHz

Possible Military Explosive Simulants

Bees wax εr 2.35 [12]tan δ × 104 50

RTV silicone rubber 3110 εr 3.15 3.11 3.10 3.05 [13]a tan δ × 104 160 150 180 290

Paraffin wax εrb 2.25 2.24 [9, p. 358]

tan δ × 104 2.0 2.1E-resin c εr 2.43 [12]

tan δ × 104 6Plastic c εr 2.67 2.91 [12]

tan δ × 104 285 784Nylon c εr 3.08 3.06 3.02 [12]

tan δ × 104 138 140 120

a Filling used in ITOP Standard Test Targets. Permittivity values intended to closely approximate those ofTNT.14

b Value of 2.25 is constant down to 100 Hz.c E-resin, plastic, and nylon are common landmine case materials.

Table 6. Relative permittivities and loss tangents for possible simulants for ANFO. A material temperature of 20-25 C isassumed.


Sulfur (sublimed) εr 3.62 3.58 [9, p. 302]tan δ × 104 0.4 1.50

Borosilicate glass εr 4.05 4.05 [9, p. 309]tan δ × 104 10.6 15.5

Neoprene compound εr 4.24 4.0 4.0 [9, p. 353]tan δ × 104 636 339 261

Bakelite BM-120 εr 3.70 3.68 [9, p. 315]tan δ × 104 438 410

Potassium Bromide (KBr) εr 4.90 4.90 [9, p. 301]tan δ × 104 <1 2.3

Proc. of SPIE Vol. 9454 945412-10

Table 7. Measured relative permittivities and loss tangents for some ANFO simulant candidates. Nominal materialtemperature was 20 C.


IE

ANFO εr 3.67 3.63 3.59 3.57tan δ × 104 175 125 60 <50 a

Possible Simulants

Simulant 1 εr 3.79 3.72 3.65 3.56tan δ × 104 690 630 530 430

Simulant 2 εr 3.12 – 3.71 3.09 – 3.38 3.04 – 3.64 3.00 – 3.59tan δ × 104 290 – 360 280 – 350 270 – 335 250 – 320

Simulant 3 εr 3.17 3.16 3.14 3.12tan δ × 104 50 85 140 200

Simulant 4 εr 3.04 3.00 2.95 2.90tan δ × 104 450 420 350 280

a Loss tangents measured below 50×104 are stated as < 50× 104 as this is the minimumaccuracy of the coaxial waveguide data reduction for low loss materials.

A challenge arose in acquiring promising materials listed in von Hippel. While an extensive reference, it waspublished in 1954 and many of the materials are now difficult to come by. Available materials included neoprene,paraffin wax, epoxy, silicone rubber, and urethane resin.

Mixtures of these and other materials were then developed guided by theories that predict the electromagneticproperties of mixtures, based on relative permittivities and loss tangents of the components. Unfortunately thisestimation is not a simple sum of the respective quantities for the individual materials. Numerous theories andempirical equations have been developed for solids, often based on the shape of the particles of each component.Spheres, oblate spheroids (which can approximate discs, plates and lamellae) and prolate spheroids (which canapproximate needles and cylinders) are common particle shapes for which theories exist.15 Other formulae existfor solid/liquid mixtures.16 Yet more equations for various particle shapes may be found in the literature.15,17–19

A detailed discussion is beyond the scope of this report.

Table 7 displays εr and tan δ of selected IE simulant candidates. Simulants 1 and 2 closely match thepermittivity of ANFO. Both materials have slightly larger loss tangents but match the frequency dependence,decreasing with higher frequencies.

5.2.2 ANAl

Identifying suitable simulant candidates for ANAl has been challenging as there are few readily available materialsthat have a permittivity near 9 in the frequency range of interest. Noting that AN has a measured permittivityεr ≈ 3.6, it appears that aluminum is the major contributor to the properties of ANAl. A simulant thatcontains aluminum may be the best way to match the characteristics of ANAl. At the time of this publication,investigations to identify practical ANAl simulants continue.

6. CONCLUSION

The complex permittivity over the range of electromagnetic frequencies relevant to standard GPR systems wasmeasured for bulk quantities of AN, ANFO, and ANAl that had been fabricated at DRDC Suffield ResearchCentre. Following these measurements, published literature was examined to find benign materials with both asimilar complex permittivity, as well as other physical properties deemed desirable in order to select candidatesfor subsequent simulant formulation.

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Suitable simulant formulations were identified for ANFO, with resulting complex permittivities measuredto be within acceptable limits of target values. Candidate simulants for ANAl have been identified and seempromising, but further study is required. This work is ongoing.

Once the ANAl simulants investigation is complete, the most promising IE simulant formulations will undergoend-user trials with CAF operators in order to confirm their utility.

ACKNOWLEDGMENTS

The authors wish to thank Louis Gagne for preparation of the IE materials used during the dielectric measure-ments, for repeatedly packing and removing the IEs from the sample cells before and after measurements. Theauthors also thank Cristian Mosquera for his support and advice conducting the dielectric measurements.

REFERENCES

[1] Daniels, D., [Ground Penetrating Radar ], Institute of Electrical Engineers, London, United Kingdom, 2 ed.(2004).

[2] Cubic Corporation, “Microwave system senses plastic mines,” Machine Design 51, 44 (1979).

[3] McFee, J. E. and Carruthers, A., “A multisensor mine detector for peacekeeping - Improved LandmineDetector Concept (ILDC),” in Detection and Remediation Technologies for Mines and Mine-like Targets ,Dubey, A., Barnard, R., Lowe, C., and McFee, J. E., eds., Proc. SPIE 2765, 233–248 (1996).

[4] Faust, A. A., Chesney, R. H., Das, Y., McFee, J. E., and Russell, K. L., “Canadian teleoperated landminedetection systems. Part I: The Improved Landmine Detection Project,” International Journal of SystemsScience 36, 511–528 (July 2005).

[5] Ngan, P., “Data fusion technique for handheld standoff mine detection system (HSTAMIDS),” Proc.SPIE 3392, 1150–1160 (1998).

[6] Lorrain, P. and Corson, D., [Electromagnetic Fields and Waves ], W. H. Freeman, San Francisco (1970).

[7] Born, M. and Wolf, E., [Principles of Optics, 7th edition ], Cambridge University Press, Cambridge (1999).

[8] Jackson, J. D., [Classical Electrodynamics ], John Wiley, New York (1975).

[9] von Hippel, A., [Dielectric Materials and Applications 2nd Edition ], Artech House, London (1954).

[10] Nicolson, A. and Ross, G., “Measurement of the intrinsic properties of materials by time-domain techniques,”IEEE Instrumentation and Measurement, IM-19(4), 377–382 (1970).

[11] Duque, A. L. H., Perry, W. L., and Anderson-Cook, C. M., “Complex microwave permittivity of secondaryhigh explosives,” Propellants, Explosives, Pyrotechnics 39(2), 275–283 (2014).

[12] Sai, B., Morrow, I., and van Genderen, P., “Limits of detection of buried landmines based on local echocontrasts,” in 28th European Microwave Conference , Ligthart, L. P., ed. (October 1998).

[13] VSE Corporation, “Simulant mines (SIM) overview and technical data.” Specification Sheet 14126 fromContract N00174-98-U-0005 (July 1998). From Table 4.1-1.

[14] Doheny, R., Burke, S., Cresci, R., Ngan, P., Walls, R., and Chernoff, J., “Handheld standoff mine detectionsystem (HSTAMIDS) field evaluation in namibia,” in Proceedings of SPIE , 6217, 62172K (July 2006).

[15] Sheen, J., Hong, Z.-W., Liu, W., Mao, W.-L., and Chen, C.-A., “Study of dielectric constants of binarycomposites at microwave frequency by mixture laws derived from three basic particle shapes,” EuropeanPolymer Journal 45, 1316–1329 (2009).

[16] Sihvola, A., Nyfors, E., and Tiuri, M., “Mixing formulae and experimental results for the dielectric constantof snow,” Journal of Glaciology 31(108), 163–170 (1985).

[17] Polder, D. and Van Santen, J. H., “The effective permeability of mixtures of solids,” PhysicaXII(5), 257–271(1946).

[18] Sheen, J., Hong, Z.-W., Mao, W.-L., Liu, W., and Chen, C.-A., “Microwave measurements of dielectricconstants by mixture equations,” in IEEE Antennas and Propagation Society Symposium (APSURSI) ,(July 2010).

[19] Kim, D.-W., Park, B., Chung, J.-H., and Hong, K. S., “Mixture behavior and microwave dielectric propertiesin the low-fired tio2 - cuo system,” Japan Journal of applied Physics 39, 2696–2700 (2000).

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[20] Daniels, D. J., “An assessment of the fundamental performance of GPR against buried landmines,” Proc.SPIE 6553, 65530G–1–65530G–15 (2007).

[21] Takahashi, K., Igel, J., Preetz, H., and Sato, M., “Influence of heterogeneous soils and clutter on theperformance of ground-penetrating radar for landmine detection,” Geoscience and Remote Sensing, IEEETransactions on 52, 3464–3472 (June 2014).

[22] Charankar, N., Landmine Detection Architectures And Their Implementation on FPGA, Master’s thesis,Department of Electrical and Computer Engineering, McMaster University of Mumbai, Mumbai, India(2006).

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Design and validation of inert homemade explosive simulants ...Keywords: Improvised explosives, Homemade explosives, Ground Penetrating Radar, Simulant, Training aide 1. INTRODUCTION