Abstract

Investigations in many parts of the world have indicated that resistivity of saprolite (a chloritization zone at the base of the weathering profile) depends uniquely on the underlying rock type. A study of electrical properties of the weathered layer was undertaken near Yala in the Nyanzian-Kavirondian greenstone belt in western Kenya. Resistivity soundings were systematically carried out in a 24 X 40 km area. Saprolite formed over basalt had the lowest resistivity (14 Omega .m), followed by andesite (27 Omega .m) and rhyolite (67 Omega .m). Statistically meaningful data sets conclusively show, for the first time, that saprolite resistivity increases with the silica content of parent volcanic rocks. Plutonic and sedimentary rocks had higher average resistivities (granite 135 Omega .m, mudstone 79 Omega .m, grit 213 Omega .m). Thickness of the weathered layer varied between 10 and 40 m, with the largest values observed over grit (sandstone). Compared with other tropical regions, the saprolite layer in western Kenya appears relatively thin, possibly because of the relatively rapid tectonic uplift of the area which leads to fast erosion. As observed in earlier studies, resistivity and electromagnetic surveys can increase the speed and reliability of geologic mapping in tropical terrains.

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