Deep transient electromagnetic soundings with a grounded source over near-surface conductors

Abstract

Three-dimensional (3-D) transient electromagnetic (TEM) responses are assessed for near-surface conductors beneath and remote from a grounded-wire transmitter. When a conductor is located close to the transmitter the time derivative of the magnetic field (vertical component), after early times, is shifted in amplitude from that of the layered host. Depending on the location of the transmitter, the response that is observed can be shifted above or shifted below the layered-host response. The conductor's response does not vanish at late times and is important for large transmitter-receiver separations. When the conductor is near to a receiver that is remote from the transmitter, the conductor is detectable only at early times. During these times the conductor can produce responses with sign reversals and other behaviour not characteristic of a layered earth. At later times the responses are more or less that of the layered half-space. An exception to this is when the receiver is located over the edge or close to the edge of the conductor. In this case the conductor produces a response at later time which is like that of the transmitter close to the conductor. The response is shifted in amplitude compared to the layered-earth response. If the transmitter is located close to a near-surface conductor it distorts layered-earth interpretations of the deeper geoelectric section. However, the effect of the conductor can be removed to a large degree by scaling the amplitude of the layered-earth curves that are used to fit the data by an arbitrary constant. This scaling can be determined in the inversion process and allows for acceptable data fits. Alternatively, prior to inversion, the data can be rescaled so that the area beneath the decay curve equals the DC magnetic field of the transmitter. This type of scaling also allows for acceptable data fits. In either situation the layered-earth interpretation then provides an accurate estimate of the deeper geoelectric section. Nevertheless, the interpreted layering in the near surface is still biased by conductors in the near surface.