AbstractThe electrical properties of the weathered layer were investigated by means of Geonics VLF‐EM 16/16R equipment in two areas of the Andhra Pradesh State. The resistivity and thickness of the weathered layer were found to be variable even over a small survey area. In the area underlain by Precambrian granite‐gneiss, most of the recorded VLF‐EM anomalies were caused by variations in the resistivity of the weathered layer. Changes in thickness were well reflected in the VLF‐EMR curves. The second area was underlain by Cretaceous basalts and dolerites.Quantitative interpretation of the VLF‐EMR data with a simple one‐dimensional model yielded considerable detail about the weathered layer. For the granitic area, a prior estimate of at least one resistivity parameter of the ground is required. If this is not already available, a limited amount of direct‐current resistivity surveying can provide the required information. A study of the EMR data from the basaltic area revealed the presence of a thin, highly conductive layer between the weathered layer and the bedrock. The parameters of this layer were found to be variable, making it necessary to use a set of diagrams for quantitative interpretation. Due to the presence of this highly conductive layer, the EMR data contain little information on the bedrock resistivity.Our field studies suggest that the VLF‐EMR method can be used as a fast and inexpensive tool for mapping of the weathered layer in tropical regions with hard rock geology. Such mapping is of considerable importance because the weathered layer is an important source
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