Impact of Gypsum on Electromagnetic Properties of Desert Soils

Radar remote sensing of soil requires an understanding about the electromagnetic properties of soils. Propagation velocities and attenuation rates at ground-penetrating radar frequencies (0.25-4 GHz) were measured as a function of soil moisture content for soils from Iraq and Afghanistan. Soil samples in the study include two with and two without gypsum (CaSO4·2H2O) as a major mineral component. When measured at 100°C, volumetric moisture content of gypsum-rich soils ranged from 12% to 24%. In addition to the high moisture content, the propagation velocities were higher than expected, and attenuation rates were lower than expected for soils with moisture contents in this high range. The apparently anomalous relationship between high moisture content and low attenuation rate is explained by the presence and characteristics of gypsum in the soil. Radar signals are not affected by the chemically bound water molecules in gypsum which dehydrates at 100°C. These results show that soil mineralogy is critically important to the interpretation of dielectric properties.