Effect of tropical weathering on electrical and electromagnetic measurements

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Abstract
Electrical properties of the weathered layer in tropical regions of Brazil were investigated by means of resistivity soundings, airborne, and ground electromagnetic measurements. Five case histories illustrate how changes of climate, lithology, and geomorphology affect geophysical measurements. In humid and subhumid tropical regions (annual rainfall over 650 mm) the weathered layer is between 10 and 80 m thick and moderately conductive. Results from one region (Minas Gerais) indicate that excessive depth of weathering and leaching of massive sulfides, rather than the conductivity of overburden, present the greatest obstacle to effective use of airborne EM methods in mineral exploration. Seasonal variations of precipitation cause changes in soil resistivity, but such changes are not apparent in the underlying weathered layer. In semiarid and temperate regions of Brazil, the weathered layer is 10 to 20 m thick and regional airborne EM surveys are an efficient exploration tool. In all regions, the degree of weathering depends upon lithology and, in several areas, anomaly patterns obtained from airborne EM surveys correlate well with the surface geologic map. However, when comapring electrical properties of similar rock types among regions of the same climatic type, a considerable variation is observed. It seems that also geomorphology plays an important role in weathering. A careful interpretation of airborne EM data is necessary to distinguish anomalies caused by the weathered layer from those due to underlying conductors. Highly conductive, saline alluvia, which cause strong EM anomalies in Australia, were encountered (sporadically) in only one region of Brazil, the semiarid Valley of Curaçá, Bahia.

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