Borehole EM and MMR Methods for Weak Conductors – A Project Review

Abstract

This paper reviews a 3-year research project targetted towards processing and interpretation methodologies for borehole EM methods in the search for weak electrical conductors. Weak conductors are best energised by current-channelling methods, which include both MMR, and inductive loops off-set from the hole collar. The following results have been achieved: Recognition of the importance of using current-channelling energisation of weak conductors; a sphalerite-rich conductor is frequently too poor a conductor to support an inductive EM response of vortex currents, but any conductor showing a conductivity contrast relative to host rock, can produce an EM response from currents channelled through the conductor from the conducting host rock.Recognition of the equivalence of MMR and long-offset TEM data, in terms of information and interpretation of current-channelling energisation of conductors.Comparison of numerical modelling techniques for inductive and current-channelling responses in TEM surveys using state-of-art software (MARCO and LEROI); identification of instabilities in multiple-plate modelling in LEROI.Development of an algorithm for modelling the three-component MMR or current-channelling EM response of multiple 3D conductors.Recognition of the importance of correct modelling of the MMR response of a layered earth as a precursor to stripping background and modelling of residual MMR anomalies.Development of a software package for processing three-component MMR field data (calibration, stripping of wire field and layered-earth background field) and for modelling the data in 3 components, and with multiple 3D conductive bodies.Development of a 3D rotatable screen viewer for an arbitrary number of boreholes, transmitters and dipping tabular conductors (incorporated in the above software package).Numerical modelling of an MMR phase response.A comparison of noise characteristics of different receivers and borehole EM probes.