Determination of Attenuation and Penetration Depth of Microwaves in Soil

Abstract

Abstract When probing soil with microwaves to locate water, seepage paths, buried metal objects, or other subsurface features, knowledge of two fundamental properties is required. The first, relative dielectric constant, was the subject of an earlier paper. The second, attenuation coefficient, which is used in determining microwave penetration depth, is the subject of this paper. The laboratory testing method in the L band frequency range (∼ 1 GHz) used in this paper is based on open wire transmission line theory and is called OWL II. Using this method on a remolded beach sand, a clayey silt, and a Kaolinite clay at varying water contents (up to 20%) and densities it was found that the measurement technique is straightforward, convenient, rapid, and appears well suited to the evaluation of remolded soils in the laboratory; that attenuation for the three soils was in the range of 1 to 12 nepers/m; that attenuation increases with increasing volumetric water content; that attenuation is slightly lower in the clay soil than in the sand or silt; and that the effective penetration depths calculated with a simple model with these dielectric constant and attenuation data are in good keeping with those determined empirically in the field.