Some Positive thoughts about Negative TEM Responses

Abstract

Negative responses recorded during moving in-loop TEM surveys, where only positive data are expected, are a regular occurrence in TEM surveys. They are however generally poorly understood and little reported. In most cases, such negatives are ascribed to induced polarisation (IP) effects, but we suggest that there is at least one other cause of negative TEM response, one which is associated with very resistive terranes containing little if any polarisable material. This paper presents some examples of this negative resistive response where there is little or no polarisable material present and where a source other than IP should be sought. A series of tests have been proposed to try and isolate the cause.