Effect of transmitter turn-off time on transient soundings

Abstract

A general procedure for computing the effect of non-zero turn-off time on the transient electromagnetic response is presented which can be applied to forward and inverse calculation methods for any transmitter-receiver configuration. We consider in detail the case of a large transmitter loop which has a receiver coil located at the center of the loop (central induction or in-loop array). For a linear turn-off ramp of width t 0 , the voltage response is shown to be the voltage due to an ideal step turn-off averaged over windows of width t 0 . Thus the effect is similar to that obtained by using averaging windows in the receiver. In general when time zero is taken to be the end of the ramp, the apparent resistivity increases for a homogeneous half-space over a limited time range. For time zero taken to be the start of the ramp the apparent resistivity is affected in the opposite direction. The effect of the ramp increases with increasing t 0 and first-layer resistivity, is largest during the intermediate stage, and decreases with increasing time. It is shown that for a ramp turn-off, there is no effect in the early and late stages. For two-layered models with a resistive first layer ( ρ 1 > ρ 2 ), the apparent resistivity is increased in the intermediate stage. When the first layer is more conductive than the second layer ( ρ 1 < ρ 2 ) and the layer thickness is comparable or greater than the loop radius, similar results are obtained; however, when the layer is thin compared to the loop radius the apparent resistivity is initially decreased and then increases as time increases. Examples are presented which illustrate the strong influence of the geoelectrical section on the turn-off effect. Neglecting the turn-off ramp will affect data interpretation as shown by field examples; the influence is the greatest on near-surface layer parameters.