Step and impulse calculations from pulse-type electromagnetic data

Abstract

It has been shown by others working in the field of time-domain electromagnetic induction, that the late-time step response of the ground can be very useful for the detection of large, slowly decaying secondary fields. (Lamontagne, 1975; West et. al., 1984). A simple method is outlined here to calculate the step response from pulse-type time-domain EM data. Pulse-type systems are often described as impulse systems, but a measured impulse response cannot be used to derive the step response. These systems would better be described as hybrid systems ? lying somewhere between a pure impulse and pure step ? because their current termination is of short duration, but not instantaneous. By taking at least one measurement during the current turn-off time, and by ensuring that the turn-off is a very linear, controlled ramp, a simple step response calculation can be made. It can also be important to determine the impulse response of the ground since many interpretation schemes are based on this measurement (for example, Nabighian, 1979). It is shown that with a few additional calculations while the step response is being calculated, the impulse response can also be easily determined.