Inversion of downhole TEM data using circular current filaments

Abstract

A circular current filament in free space constitutes a useful idealisation of late-time secondary current flow in a geological conductor. The mathematical simplicity of the current filament model permits efficient inversion of down-hole TEM (DHEM) logs via iterative adjustment of a starting guess. The application of current filament modelling in exploration is illustrated using data from a shallow hole intersecting a mineralised quartz vein. The DHEM log at 1.2 msec is closely replicated by combining the effects of two filaments centred up-dip, one encircling the hole and one away from the hole; these are interpreted to represent a single distributed current system pierced by the hole. Inversion of the 5.8 msec DHEM log suggests that this current system contracts and migrates up-dip. Inductive coupling between the vein and the regolith zone is hypothesised as the cause of the upward migration. The current system dominating the DHEM response is small in diameter and located near the hole; the down-hole survey therefore provides no indication of the better mineralisation later intersected down-dip.