The benefit of combining downhole with surface IP

Abstract

Resistivity and induced polarisation data are very useful for defining lithological boundaries, shear zones (often with conductive and chargeable graphite), and sulphide alteration zones. The useful depth for interpretation depends upon the magnitude of the measured voltages and the effective current penetration, which in turn are determined most often by surface conditions at the electrode locations and subsurface electrical structure. Downhole resistivity and IP measurements are relatively quick to acquire, and can add significantly to the depth information of the surrounding surface survey. Bottom-of-hole to surface surveying can be performed without any specialised downhole IP equipment. Examples over shear zones in southern Mali demonstrate the added information from inverting surface data with data from bottom-of-hole current injection with surface receiver electrodes.