A comparison of electromagnetic exploration systems

Abstract

Electromagnetic (EM) exploration systems fall into three distinct classes, (1) frequency domain, (2) impulse response, and (3) step response, and have overall frequency responses that approximate [Formula: see text], [Formula: see text], and [Formula: see text]. To examine further these three classes, the transfer function of a frequency‐domain system, and the step and impulse response functions of ideal time‐domain EM systems (TEM), are derived in terms of a single set of target‐specific parameters. The inductive time constants of practical exploration targets extend over three orders of magnitude, imposing severe constraints on practical exploration systems. Field data demonstrate that the impulse TEM response from a conductive half‐space may show a 100‐fold spatial variability at early times (≲1 ms) and then will decrease to a two‐fold variability at late times ( ⩾4 ms). This differential variability influences the ability of an EM system to detect an exploration target. For a half‐space where the impulse response varies as [Formula: see text], an inductive target of time constant τ exhibits the greatest contrast to the half‐space at a delay time of kτ for an impulse‐response system, and at [Formula: see text] for a step‐response system.