The penetration depth of GPR is determined by antenna frequency and the electrical conductivity of the earthen materials being profiled (Daniels, 2004). Soils having high electrical conductivity rapidly attenuate radar energy, restrict penetration depths, and severely limit the effectiveness of GPR. The electrical conductivity of soils increases with increasing water, clay and soluble salt contents.
Electrical conductivity is directly related to the amount, distribution, chemical composition, and phase (liquid, solid, or gas) of the soil water (McNeill, 1980). At a given frequency, the attenuation of electromagnetic energy increases with increasing moisture contents (Daniels, 2004). The lack of adequate data on soil moisture and the high spatial and temporal variations in the degree of soil wetness within most soil map units precluded the use of moisture content in the preparation of GPR soil suitability maps. As a consequence, properties selected to prepare these maps principally reflect variations in the clay and soluble salt contents of soils. These properties include clay content and mineralogy, electrical conductivity, sodium absorption ratio, and calcium carbonate and calcium sulfate contents.
Clays have greater surface areas and can hold more water than the silt and sand fractions at moderate and higher water tensions. Because of their high adsorptive capacity for water and exchangeable cations, clays produce high attenuation losses (Daniels, 2004). As a consequence, the penetration depth of GPR is inversely related to clay content. While soils with more than 35 percent clay are restrictive, soils with less than 10 percent clay are generally favorable to deep penetration with GPR.
Soils contain various proportions of different clay minerals (e.g., members of kaolin, mica, chlorite, vermiculite, smectite groups). The size, surface area, cation-exchange capacity (CEC), and water holding capacity of clay minerals vary greatly. Variations in electrical conductivity are attributed to differences in the CEC associated with different clay minerals (Saarenketo, 1998). Electrical conductivity increases with increasing CEC (Saarenketo, 1998). Soils with clay fractions dominated by high cation exchange capacity clays (e.g., smectitic and vermiculitic soil mineralogy classes) are more attenuating to GPR than soils with an equivalent percentage of low cation exchange capacity clays (e.g., kaolinitic, gibbsitic, and halloysitic soil mineralogy classes). Soils classified as kaolinitic, gibbsitic, and halloysitic characteristically have low cation-exchange capacity and low base saturation. As a general rule, for soils with comparable clay and moisture contents, greater depths of penetration can be achieved in highly weathered soils of tropical and subtropical regions that have kandic or oxic horizons than in soils of temperate regions that have argillic horizons. Compared with argillic horizons, kandic and oxic horizons have greater concentrations of low activity clays (Soil Survey Staff, 1999).
Electrical conductivity is directly related to the concentration of dissolved salts in the soil solution, as well as the type of exchangeable cations and the degree of dissociation of the salts on soil particles (Soil Survey Staff, 1993). The concentration of salts in the soil solution is dependent upon the degree of water-filled porosity, the soil texture, and the minerals found in soils. In semi-arid and arid regions, soluble salts and exchangeable sodium accumulate in the upper part of some soil profiles. These salts produce high attenuation losses that restrict penetration depths (Doolittle and Collins, 1995). Because of their high electrical conductivity, saline (saturated extract electrical conductivity ≥ 4 mmhos cm -1) and sodic (sodium absorption ratio ≥ 13) soils are considered unsuited to GPR.
Calcareous and gypsiferous soils are characterized by layers with secondary accumulations of calcium carbonate and calcium sulfate, respectively. These soils mainly occur in base-rich, alkaline environments in semi-arid and arid regions. High concentrations of calcium carbonate and/or calcium sulfate imply less intense leaching, prevalence of other soluble salts, greater quantities of inherited minerals from parent rock, and accumulations of specific mineral products of weathering (Jackson, 1959). Grant and Schultz (1994) observed a reduction in the depth of GPR signal penetration in soils that have high concentrations of calcium carbonate.