Unambiguous apparent conductivity for fixed-loop transient electromagnetic data

Abstract

We derive an unambiguous apparent conductivity from total magnetic field TEM data for fixed-loop geometry. For single-component fixed-loop TEM measurements, apparent conductivity is either dual-valued or undefined. The ambiguity or non-existence is particularly evident for readings taken outside the transmitter loop, both for step and impulse response data. Therefore, computing apparent conductivity from single-component fixed-loop TEM data can be problematic, especially at intermediate delay times. However, if multi-component fixed-loop magnetic field data is available, an unambiguous apparent conductivity can be derived from |B(t)| at all times, except in the inductive limit. Impulse response measurements can be time-weighted and summed to yield ?quasi-|B|? data. Apparent conductivity derived from quasi-|B| amplitudes is dual-valued, but usually only one of the alternatives is geologically plausible. Computing apparent conductivity from |B| or quasi-|B| amplitudes expedites generation of conductivity-depth sections from fixed-loop TEM. A field data example with multi-component SQUID data shows significant improvement in the conductivity-depth section when |B(t)| is transformed rather than the vertical component, Bz, alone; the lateral extent of a conductive target is under-estimated in the Bz CDI.