The use of Cole?Cole impedances to interpret the TEM response of layered earths

Abstract

Commonly used TEM (transient electromagnetic) sounding equipment such as Sirotem and EM-37, which measure the response of the earth 0.05?50 ms after source current turn-off, are usually considered to be high frequency devices whose voltage response should not be significantly affected by induced polarization effects. Thus inversion and forward modelling procedures usually assume real electrical resistivities. This implies that the Hankel transform of the time-domain layered earth impedance function, F(l), is strictly non-negative, a property which can be used to stabilize and speed up model calculations. Thus, in the case of horizontally layered structures, excited by coincident loop or in-loop configurations utilizing ramp function turn-off, the TEM voltage decay curve must be of one sign (by convention, positive). A more general result (Weidelt 1982) showed that the TEM decay curve had to be of one sign for the coincident loop response to any physically reasonable, frequency independent distribution of electrical resistivity and magnetic permeability.