Clay mapping using electromagnetic methods

Abstract

Conductive water-saturated clays occur in weathered layers, fracture zones, and unconsolidated sediments. Generally, clays are not as conductive as massive sulphides or graphitic sediments, but their resistivity is less than that of fresh igneous and metamorphic rocks. Clay conductors can be detected by modern ground and airborne electromagnetic (EM) systems. A fully developed weathering layer consists of several horizons (from bottom to top): zone of fractured rock, saprolite, leached zone, mottled zone, and duricrust. Electrical properties of the weathered layer depend on the type of clay present and its water content. The most conductive horizon is saprolite; duricrust is the most resistive. Saprolite developed over mafic and ultramafic rocks is more conductive than that formed over felsic rocks, hence the results of resistivity or EM surveys can be used for pseudogeological mapping. Examples from the Itapicurú greenstone belt in Bahia, Brazil, a crystalline area near Nantes, France, and the Rice Lake greenstone belt in Manitoba, Canada, demonstrate that mafic and ultramafic rocks can be identified by airborne EM surveys. Often, as in the three areas shown, mafic rocks are not magnetic. In tropical areas, outcrops are scant and mapping of soil colour differences often results in some formations being missed or incorrectly outlined. In regions of temperate elimate (e.g. France), a thin cover of Quaternary sediments makes mapping of the underlying lithological units difficult. In areas covered by glacial sediments (e.g. Canada), the continuity of formations may not be obvious from existing outcrops. The extent of weathered layers in countries of temperate and cold climate is larger than previously thought. Conductive saprolite may also be preserved in a sedimentary sequence below basal sediments. Such a situation is demonstrated in borehole resistivity data from Bells Corners, Ontario, Canada. Resistivity and EM measurements can be used to map the extent and thickness of Quaternary clays. Examples of such measurements are given from the Sennecey test site, near Dijon, France.