The effect of near‐surface superparamagnetic material on electromagnetic measurements

Abstract

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.

Combined transient electromagnetic and magnetotelluric study of the southern Kenya rift valley

“…Possible strong 2-D or 3-D effects may explain the big contrast between the two curves. Nevertheless, this data set appears to be similar to the data that Buselli (Buselli, 1982; Figure 2) used to illustrate the superparamagnetism effect in TEM data. Laboratory studies have shown the effect to be a feature of intergranular magnetism, inversely proportional to grain size and temperature and concentrated at the surface (Buselli, 1982).…”

“…Nevertheless, this data set appears to be similar to the data that Buselli (Buselli, 1982; Figure 2) used to illustrate the superparamagnetism effect in TEM data. Laboratory studies have shown the effect to be a feature of intergranular magnetism, inversely proportional to grain size and temperature and concentrated at the surface (Buselli, 1982). At the surface, arid weathering products of lateritic soil (hydrated iron oxides) have superparamagnetism properties that reduce surface …”

UXO Target Detection and Discrimination with Electromagnetic Differential Illumination

“…For time-domain systems this characteristic decay of 1/t has been observed in archaeological prospecting (Colani and Aitken, 1996), time domain electromagnetic (TEM) surveys carried out over Laetril soils for mineral exploration (Buselli, 1982), and in UXO surveys on Kaho'olawe Island, Hawaii (Ware, 2001). …”

Characterization of marine near-surface sediments by electromagnetic profiling

“…Frequency dependence of the susceptibility signal can arise from superparamagnetic magnetite (Buselli, 1982;Dearing et al, 1996) or subsurface magnetization by secondary EM fields (Zhang and Oldenburg, 1997). However, currents in marine sediments will usually not exceed 1 A inducing a maximum secondary magnetic field <1 PT, which is at least two orders of magnitude lower than the primary magnetic field at this position.…”