Abstract

A newly developed, combined penetrometer–moisture probe (CPMP), together with two conventional geophysical methods, electrical resistivity imaging (ERI) and ground-penetrating radar (GPR), was used to determine soil depth, soil type, and water distribution of a forested slope underlain by granitic bedrock. Profiles of volumetric water content (θ) measured using the CPMP accurately depicted alternations of sandy, loamy, and gravelly layers above the water table. In most cases soil–bedrock interfaces were successfully detected by ERI because of the high contrast between the electrical resistivity of dry surface soil and lightly weathered bedrock. In some areas, the soil–bedrock interface was obscured by the presence of pore water or by heavily weathered bedrock. Although a good match was obtained between θ, as directly measured using the CPMP, and θ estimated from electrical resistivity (ρ), as measured with ERI, the CPMP was more accurate than ERI for detecting heterogeneous θ distributions caused by alternations of sandy, loamy, and gravelly soil layers. Using the CPMP and ERI together is effective in quantifying soil depth, soil material, and water content distribution on a forested slope. Although soil thickness, distribution, and groundwater depth were not successfully detected using GPR on the forested slope studied, it may be possible to achieve quantitative estimates of the soil thickness and water table by calibrating GPR data with CPMP data obtained from the same locality at the same time.

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