Downhole Induced-polarization Detection of Ore Bodies: Conductive Overburden Effects

Abstract

Highly c.onducting .overburderi layers inhibit the surface detecti.on .of .ore b.odies by induced p.olarizati.on (lP) measurements. The excitati.on and resp.onse .of an IP s.ource .obtained by means .of undergr.ound (d.ownh.ole) electr.odes is investigated utilizing a simple m.odel: the .overburden is represented by a unif.orm surface layer and the .ore b.ody by a small sphere. The shielding .of the s.ource from the surface is shewn. Quantitative estimates .of the improvement .obtained by siting the electr.odes bel.ow the .overburden-h.ost-rock interface are .obtained. It is expected that similar impr.ovement will be .obtained with larger .ore b.odies. It is shewn that the impr.ovement applies t.o time-d.omain and frequency-d.omain measurements. A feature .of the results is the presence .of a 'blind' z.one just beneath the interface. The .optimizati.on .of electrode p.ositi.ons in a particular ge.ometry is examined.