Short note: Sensor response and resolution in downhole TEM data

Abstract

Downhole TEM probes can be constructed using several different types of core material. These cores are designed to concentrate the lines of magnetic flux associated with currents induced in a conductive target. In axial probes the resulting signal levels are greatly enhanced and anomalies associated with major targets are readily detected. However for crosshole sensors the advantages are not so evident. In particular the aspect ratio is greatly reduced in probes of reasonable diameter and flux densities can not be greatly improved. Variations in permeability as a function of frequency present a more serious problem. Data obtained at early times may become seriously distorted in any cored probes and deconvolution techniques may be required to compensate for permeability transfer functions. In these circumstances air-cored probes may be preferred but final response remains limited by self inductance.