Maxwell–Wagner relaxation in common minerals and a desert soil at low water contents

Abstract

Penetration of 100- to 1000-MHz ground-penetrating radar (GPR) signals is virtually non-existent in arid and desert soils despite their low water content and moderate conductivity, the latter of which cannot explain the loss. Under the hypothesis that strong dielectric relaxation supplements DC conductivity to cause high intrinsic attenuation rates, we compared the complex permittivity of a desert soil sample with that of controlled samples of quartz, feldspars, calcite, coarse and crystallite gypsum, kaolinite and montmorillonite. The soil had 80% quartz, 10% feldspars and 10% gypsum by weight, with the latter composed of crystallites and crustations. All samples had 4–7% volumetric water content. We measured permittivity most accurately from 1.6 MHz to 4 GHz with Fourier Transform time domain reflectometry, and used grain sizes less than 53 μm. All samples show low-frequency dispersion with the soil, gypsum crystallites and montmorillonite having the strongest below 100 MHz, the highest attenuation rates, and conductivity values unable to account for these rates. The soil rate exceeded 100 dB m− 1 by 1 GHz. Through modeling we find that a broadened relaxation centered from 2 to 16 MHz sufficiently supplements losses caused by conductivity and free water relaxation to account for loss rates in all our samples, and accounts for low-frequency dispersion below 1 GHz. We interpret the relaxation to be of the Maxwell–Wagner (MW) type because of the 2- to 16-MHz values, relaxation broadening, the lack of salt, clay and magnetic minerals, and insufficient surface area to support adsorbed water. The likely MW dipolar soil inclusions within the predominantly quartz matrix were gypsum particles coated with water containing ions dissolved from the gypsum, and the conducting water layers themselves. The inclusions for the monomineralic soils were likely ionized partially or completely water-filled interstices, and partially filled galleries for the montmorillonite. The low water content may be necessary to help isolate these inclusions. For our common, low conductivity minerals, the MW contributions to attenuation rates are significant above 10 MHz, whereas they are significant above about 100 MHz for the more conductive minerals and soil.