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The development of an instrument that enables transient electromagnetic (TEM) measurements to be made to voltage levels of 1 μV/A and less has enabled the detection of an anomalous transient response in some areas with lateritic soil cover. This anomalous transient causes apparent resistivity values derived from the measured transient decay to decrease at late delay times in areas where the known geology indicates the values should increase with delay time toward the resistivity value of the basement. The main cause of the anomalous transient has been identified as the response of superparamagnetic material in the lateritic soil cover. Both field and laboratory measurements of the voltage M induced by this transient, show a [Formula: see text] time dependence. This is the same behavior reported previously for magnetic viscosity over a longer time scale. Measurements of magnetic susceptibility of material separated magnetically from soil samples taken at areas where a residual response is measured, show that over a wide temperature range (from −196°C to 590°C) the susceptibility increases with temperature, confirming the presence of superparamagnetic particles. The anomalous transient response is localized to within 3 m of the transmitter loop; it is consequently detected only by loop configurations where the receiver loop is in proximity to the transmitter loop. The effects caused by the presence of a superparamagnetic response within 3 m of the transmitter loop apply to all electromagnetic methods, whether the measurements are made in the time or frequency domain.

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Coincident-loop TEM sounding data are often presented by plotting the half-space apparent conductivity as a function of delay time. A new algorithm generates an improved presentation that plots the apparent conductivity as a function of depth. The resulting data may be further processed to sharpen or "spike" the smoothly varying apparent-conductivity/ depth curves in an attempt to better represent the rapid changes in conductivity that often exist in the earth. The algorithm described involves an approximation, but is simple, easy to use, and computationally efficient.