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 modeling 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.