Abstract

The electromagnetic conductors of interest are often overlain by a partially conducting overburden layer which may be uniform or varying in conductance (conductivity-thickness product) and which may or may not be in galvanic contact with the target conductor below. Laboratory scale-model experiments for the Slingram configuration are conducted to simulate such realistic conditions. Results generally indicate the following. (1) The overburden with uniform conductance and inductive coupling with the massive sulfide vein-type orebody attenuates the response of the target and causes a clockwise rotation of the anomaly index diagram (AID) related to its conductance. The better the conductor, the more serious would be the error in estimating its conductance. The depth index curves in the AID shift toward greater depth-to-the-top of the conductor. (2) A conductively coupled overburden distinctly improves the response of the target as expected according to the concept of negative electromagnetic screening. There is a counterclockwise rotation of the AID accompanied by higher sensitivity in in-phase as well as out-of-phase components simultaneously. (3) A laterally inhomogeneous overburden layer produces diagnostic response profiles.

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