In physical models that evaluate the stability of hillslope surface soil masses, soil depth contributes directly to slope stability due to its weight and volume, as well as control of groundwater movement. Electrical resistivity imaging (ERI), a noninvasive geophysical technique, is a recently introduced method used to detect geological structures in landslide-prone areas. However, this method has not been well studied as a tool to detect the depth of the surface soil layer. Questions remain about the reliability of ERI, especially in surface zones of mountainous areas. Here, we present a case study of the use of ERI to detect the surface soil thickness of hillslopes in granitic and slate watersheds in central Japan. Like invasive penetration tests and geotechnical surveys using boreholes, ERI appears to be suitable for detecting soil-bedrock interfaces, due to the high contrast of resistivity values between surface soil and bedrock layers that we found in all of the watersheds. However, ER subsurface values vary over a relatively wide range, as well as from site to site. ERI also failed to clearly identify the soil-bedrock interface at some points along our measurement line. By referring to hydrological properties of bedrocks observed in a previous study, we presume that differences in the water retention characteristics of weathered granitic bedrock are a major factor in the variation in bedrock ER values that we observed.
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