Noise Reduction for Down-hole Three-component TEM probes

Abstract

Three-component downhole TEM probes can provide a unique service for mineral exploration surveys. Apart from assisting with target recognition the individual vectors can be used to resolve rotational ambiguities associated with filament inversion routines. However noise levels in the crosshole components are often considered to be extreme compared to data obtained with the more common axial probes. Substantial improvements in data quality are unlikely in view of the physical limitations relating to probe construction. In particular core materials are relatively ineffective in the short crosshole sections and consequently there is no substantial gain in effective physical area. In addition background noise levels in the horizontal EM field may be 5 times larger than for the vertical field. Consequently digital filters and advanced numerical techniques are required to improve data quality in post survey processing. Deconvolution involving exponential basis functions can provide substantial improvements in crosshole data assuming consistent decay curves scaled for variations in coupling. Apart from reductions in noise the verification of a uniform decay can eliminate the possibility of spurious axial anomalies associated with an extreme core response. More complex scaling factors are required for complex conductors with several iterations to establish the extent of coupling for each source.