Conductivity-depth transformation of slingram transient electromagnetic data

Abstract

Approximate conductivity-depth transformation has become popular for presentation of transient electromagnetic (TEM) profile data. The majority of published schemes apply only to data acquired using the coincident-loop and in-loop configurations. TEM surveys for mineral exploration or groundwater studies, however, sometimes employ the 'slingram' configuration, in which the receiver is located at a fixed distance outside the transmitter loop. Accordingly, a conductivity-depth transformation, based on existing in-loop and fixed-loop schemes, has been developed for slingram impulse-response TEM data. Apparent conductivity for the slingram configuration is almost always dual-valued. Nonetheless, it is possible to select a unique apparent conductivity for the purpose of depth transformation, since only one apparent conductivity at each delay time is compatible with downward diffusion of current. The unique apparent conductivities selected in this manner may not, however, be suitable for time-depth transformation, as they do not always define a smooth conductivity versus time curve. In particular, apparent conductivity is least reliable as an indicator of ground conductivity for measured voltages close to the maximum (or minimum) possible half-space response for the given system geometry and delay time. Such values are automatically edited from the apparent conductivity versus time curve. Finally, the depth assigned at each remaining delay time is the depth to the physical current maximum in a half-space with conductivity equal to the apparent conductivity at that time. The conductivity-depth transformation yields a valid qualitative portrayal of the actual variation of conductivity with depth when applied to theoretical slingram data for layered-earth models. Conductivity-depth pseudo-sections generated from field data show good correspondence to known geology, and may be generated in a fraction of the time required for inversion.