Ground Penetrating Radar 2019, Volume 2, Issue 1, GPR-2-1-2,   https://doi.org/10.26376/GPR2019002

Discrimination of dispersive materials from radar signals using Q*

Chun An Tsai, Rebecca Ghent, Alexander Boivin, and Dylan Hickson 

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Abstract:  Using a combination of laboratory measurements and modeling results, we demonstrate the potential of distinguishing two dispersive materials by estimating quality factor (Q*) using radar signals at two different frequencies. Here, we report on new complex dielectric permittivity measurements of a pulp sample mainly composed of pyrite (25%) and quartz (55%) from a massive sulphide mine, which shows frequency-dependent permittivity, and of a calcium-rich montmorillonite sample (STx-1b) for comparison. We made these measurements using the coaxial transmission line technique. To understand the dispersion observed in both samples, we fitted the measured complex permittivities using the Cole-Cole model to obtain the relaxation times that best represent the dielectric losses. We chose montmorillonite as the “control” material because it readily absorbs water, which has well-known dielectric relaxation mechanisms, thus providing a means of testing whether the pulp sample relaxations could be distinguished from those caused by adsorbed water. Our inverted montmorillonite relaxation times show one interlayer-water relaxation and one free water relaxation, as expected for this clay structure. By contrast, the pyrite-quartz sample shows intrinsic dispersion that is independent of the influence of water. The measurements show that the two materials have opposite concavity in the attenuation v.s. frequency plot, which can be detected using Q* in principle. Using these results, we conducted a series of 3D Finite-Difference-Time-Domain (FDTD) simulations in a cross-hole setup to explore the effects of the observed dispersion on material detectability. We show that it is possible to distinguish intrinsically dispersive materials from those that are simply wet.

Keywords:  Ground Penetrating Radar (GPR); Dispersion; Complex permittivity measurement; Spectral decomposition; Quality factor.

Introduction

Ground penetrating radar (GPR) is a nondestructive measurement technique which uses the transmission or reflection of electromagnetic (EM) waves to locate targets, anomalies or interfaces beneath or within natural or artificial surfaces. One basic assumption of GPR surveys is that subsurface features return reflections that are replicas of the transmitted signal with lower amplitudes. This implies that electrical properties of materials are independent of frequency within the frequency range of GPR, which is often referred to as the “GPR plateau” [1]. While this assumption holds true for most materials in the frequency band of GPR operation, some materials, especially materials that contain water, have frequency-dependent dielectric permittivities. As the complex permittivity varies with frequency, both the velocity and attenuation of the EM waves also change. This type of dispersion is categorized as physical property dispersion [2]. Scattering from heterogeneities in the subsurface can cause frequency-dependent attenuation as well. The attenuation measured in the field is mainly the combination of intrinsic dispersion and scattering dispersion, and it is difficult to isolate the two. There are several parameters used to characterize frequency-dependent attenuation. Turner and Siggins [3] show that, similar to seismic wave analysis [4], we can use a constant Q* parameter to characterize materials with frequency-dependent attenuation in GPR surveys. Bradford [5] defines a more general dispersion parameter D that includes all frequency-dependent attenuation. One of the methods to extract Q* from radar signals is the spectral shift method [6]. As the signal propagates, the peak frequency shifts lower from the original source value, and the difference can be used to estimate Q* of the material.

In previous studies, water content was reported to be the major cause of dispersion. Therefore materials containing variable amounts of water, such as clay [7–10] and concrete [11–13] are of major interest in the study of frequency-dependent attenuation of radar waves. Our goal for this project was to identify a dispersion behaviour that is independent of the influence of water and determine whether the difference can be identified from radar signals [14]. Since clay is a typical soil material that can absorb a large amount of water, we chose a montmorillonite sample as a reference material to characterize the influence of water on its dielectric properties. We measured the complex permittivity of the montmorillonite at varying moisture levels, then fitted the data with a multi-pole Cole-Cole model to obtain the dielectric relaxations responsible for the dispersion. We also fitted the measurements of a pulp sample that is mainly composed of pyrite and quartz from the LaRonde massive sulphide mine that shows a different dispersive behaviour from the montmorillonite sample. Then, we show with numerical simulations that it is possible to distinguish between these two types of dispersive behaviours by comparing the Q* values at two frequencies. We believe that this technique expands the potential application of radar signals in material characterization.

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References

[1] J. L. Davis and A. P. Annan, “Ground-penetrating radar for high-resolution mapping of soil and rock stratigraphy,” Geophysical Prospecting, vol. 37, no. 5, pp. 531–551, July 1989, doi.org/10.1111/j.1365-2478.1989.tb02221.x.

[2]  A. P. Annan, “Transmission dispersion and GPR,” Journal of Environmental and Engineering Geophysics, vol. 1, no. B, pp. 125–136, January 1996, doi.org/10.4133/jeeg1.b.125.

[3]  G. Turner and A. F. Siggins, “Constant Q attenuation of subsurface radar pulses,” Geophysics, vol. 59, no. 8, pp. 1192–1200, August 1994, doi.org/10.1190/1.1443677.

[4]  K. Aki and B. Chouet, “Origin of coda waves: Source, attenuation, and scattering effects,” Journal of Geophysical Research, vol. 80, no. 23, pp. 3322–3342, August 1975, doi.org/10.1029/jb080i023p03322.

[5]  J. H. Bradford, “Frequency-dependent attenuation analysis of ground-penetrating radar data,” Geophysics, vol. 72, no. 3, pp. J7–J16, May 2007, doi.org/10.1190/1.2710183.

[6]  Y. Quan and J. M. Harris, “Seismic attenuation tomography using the frequency shift method,” Geophysics, vol. 62, no. 3, pp. 895–905, May 1997, doi.org/10.1190/1.1444197.

[7]  R. Calvet, “Dielectric properties of montmorillonites saturated by bivalent cations,” Clays and Clay Minerals, vol. 23, no. 4, pp. 257–265, September 1975, doi.org/10.1346/ccmn.1975.0230401.

[8]  G. Sposito and R. Prost, “Structure of water adsorbed on smectites,” Chemical Reviews, vol. 82, no. 6, pp. 553–573, December 1982, doi.org/10.1021/cr00052a001.

[9]  G. R. Olhoeft, “Electrical properties from 10-3 to 10+9 HZ–physics and chemistry,” in AIP Conference Proceedings. AIP, March 1987, doi.org/10.1063/1.36399.

[10]  T. Ishida, T. Makino, and C. Wang, “Dielectric-relaxation spectroscopy of kaolinite, montmorillonite, allophane, and imogolite under moist conditions,” Clays and Clay Minerals, vol. 48, no. 1, pp. 75–84, January 2000, doi.org/10.1346/ccmn.2000.0480110.

[11]  A. Robert, “Dielectric permittivity of concrete between 50 mhz and 1 ghz and GPR measurements for building materials evaluation,” Journal of Applied Geophysics, vol. 40, no. 1-3, pp. 89–94, October 1998, doi.org/10.1016/s0926-9851(98)00009-3. 

[12]  G. Klysz, J.-P. Balayssac, and S. Laurens, “Spectral analysis of radar surface waves for non-destructive evaluation of cover concrete,” NDT & E International, vol. 37, no. 3, pp. 221–227, April 2004, doi.org/10.1016/j.ndteint.2003.09.006.

[13]  W. Lai, T. Kind, and H. Wiggenhauser, “Frequency-dependent dispersion of high-frequency ground penetrating radar wave in concrete,” NDT & E International, vol. 44, no. 3, pp. 267–273, May 2011, doi.org/10.1016/j.ndteint.2010.12.004.

[14]  J. H. Bradford, “Frequency dependent attenuation of GPR data as a tool for material property characterization: A review and new developments,” in 2011 6th International Workshop on Advanced Ground Penetrating Radar (IWAGPR), IEEE, June 2011, doi.org/10.1109/iwagpr.2011.5963870.

[15]  P. J. W. Debye, Polar molecules. Chemical Catalog Company, Incorporated, 1929.

[16]  K. S. Cole and R. H. Cole, “Dispersion and absorption in dielectrics i. alternating current characteristics,” The Journal of Chemical Physics, vol. 9, no. 4, pp. 341–351, April 1941, doi.org/10.1063/1.1750906.

[17]  S. Havriliak and S. Negami, “A complex plane representation of dielectric and mechanical relaxation processes in some polymers,” Polymer, vol. 8, pp. 161–210, January 1967, doi.org/10.1016/0032-3861(67)90021-3.

[18]  M. Wollensack, J. Hoffmann, J. Ruefenacht, and M. Zeier, “VNA tools II: S-parameter uncertainty calculation,” in 79th ARFTG Microwave Measurement Conference. IEEE, June 2012, doi.org/10.1109/arftg79.2012.6291183.

[19]  D. K. Rytting, “Network analyzer accuracy overview,” in 58th ARFTG Conference Digest. IEEE, November 2001, doi.org/10.1109/arftg.2001.327486.

[20]  A. M. Nicolson and G. F. Ross, “Measurement of the intrinsic properties of materials by time-domain techniques,” IEEE Transactions on Instrumentation and Measurement, vol. 19, no. 4, pp. 377–382, November 1970, doi.org/10.1109/tim.1970.4313932.

[21]  W. Weir, “Automatic measurement of complex dielectric constant and permeability at microwave frequencies,” Proceedings of the IEEE, vol. 62, no. 1, pp. 33–36, January 1974, doi.org/10.1109/proc.1974.9382.

[22]  A. H. Boughriet, C. Legrand, and A. Chapoton, “Noniterative stable transmission/reflection method for low-loss material complex permittivity determination,” IEEE Transactions on Microwave Theory and Techniques, vol. 45, no. 1, pp. 52–57, February 1997, doi.org/10.1109/22.552032.

[23]  P. L. Hall and D. M. Astill, “Adsorption of water by homoionic exchange forms of wyoming montmorillonite (SWy-1),” Clays and Clay Minerals, vol. 37, no. 4, pp. 355–363, August 1989, doi.org/10.1346/ccmn.1989.0370409.

[24]  P. Bala, B. K. Samantaray, and S. K. Srivastava, “Dehydration transformation in ca-montmorillonite,” Bulletin of Materials Science, vol. 23, no. 1, pp. 61–67, February 2000, doi.org/10.1007/bf02708614. 

[25]  A. Kuligiewicz and A. Derkowski, “Tightly bound water in smectites,” American Mineralogist, vol. 102, no. 5, pp. 1073–1090, May 2017, doi.org/10.2138/am-2017-5918.

[26]  G. R. Olhoeft, “Low-frequency electrical properties,” Geophysics, vol. 50, no. 12, pp. 2492–2503, December 1985, doi.org/10.1190/1.1441880.

[27]  D. E. Stillman, “Frequency and temperature dependence in electromagnetic properties of martian analog materials,” Ph.D. dissertation, Colorado School of Mines, Golden, Colo., 2006.

[28]  D. Stillman and G. Olhoeft, “Frequency and temperature dependence in electromagnetic properties of martian analog minerals,” Journal of Geophysical Research, vol. 113, no. E9, September 2008, doi.org/10.1029/2007je002977.

[29]  A. Benedetto, “Water content evaluation in unsaturated soil using GPR signal analysis in the frequency domain,” Journal of Applied Geophysics, vol. 71, no. 1, pp. 26–35, May 2010, doi.org/10.1016/j.jappgeo.2010.03.001.

[30]  M. Newville, T. Stensitzki, D. B. Allen, and A. Ingargiola, “Lmfit: Non-linear least-square minimization and curve-fitting for python,” 2014, doi.org/10.5281/zenodo.11813.

[31]  C. Warren, A. Giannopoulos, and I. Giannakis, “gprMax: Open source software to simulate electromagnetic wave propagation for ground penetrating radar,” Computer Physics Communications, vol. 209, pp. 163–170, December 2016, doi.org/10.1016/j.cpc.2016.08.020.

[32]  K. Yee, “Numerical solution of initial boundary value problems involving maxwell’s equations in isotropic media,” IEEE Transactions on Antennas and Propagation, vol. 14, no. 3, pp. 302–307, May 1966, doi.org/10.1109/tap.1966.1138693.

[33]  I. Giannakis, A. Giannopoulos, and N. Davidson, “Incorporating dispersive electrical properties in FDTD GPR models using a general cole-cole dispersion function,” in 2012 14th International Conference on Ground Penetrating Radar (GPR). IEEE, June 2012, doi.org/10.1109/icgpr.2012.6254866.

[34]  M. Loewer, J. Igel, and N. Wagner, “Spectral decomposition of soil electrical and dielectric losses and prediction of in situ GPR performance,” IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, vol. 9, no. 1, pp. 212–220, January 2016, doi.org/10.1109/jstars.2015.2424152.

[35]  F. Uddin, “Clays, nanoclays, and montmorillonite minerals,” Metallurgical and Materials Transactions A, vol. 39, no. 12, pp. 2804–2814, September 2008, doi.org/10.1007/s11661-008-9603-5.

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Unrestricted use, distribution, and reproduction in any medium of this article is permitted, provided the original article is properly cited.   Please cite this article as follows: C. A. Tsai, R. Ghent, A. Boivin, and D. Hickson, "Discrimination of dispersive materials from radar signals using Q*,"  Ground Penetrating Radar, Volume 2, Issue 1, Article ID GPR-2-1-2, March 2019, pp. 26-50, doi.org/10.26376/GPR2019002.

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