The effect of highly magnetic material on ZTEM data

Abstract

ZTEM data acquired across the Humble magnetic anomaly of almost 30,000 nT were analyzed for the presence of a magnetic gradient response and the effects from elevated magnetic susceptibilities. The response of moving the receiver coil through the magnetic-field gradient peaks at 0.01 Hz and drops off strongly with frequency. Lacking information about the field strength at the base station precludes the comparison of amplitudes between computed gradient responses and the survey data, but the comparison of response shapes suggests that the gradient responses are too small to have a noticeable effect on the survey data. The 3D inversion of the magnetic survey data indicates magnetic susceptibility values as high as 2.0 (SI). Forward-modeling the ZTEM response for these Ƙ–values combined with resistive half-spaces indicates that the response amplitudes and shapes strongly depend on the background resistivities. Ignoring the elevated Ƙ–values during an inversion can result in the underestimation of conductivities and other artifacts, such as the mapping of patterns that resemble crop circles. For an environment such as Humble, with deep-seated zones of elevated Ƙ–values, the shallow inverted conductivity structure appears to be reliable, but the deeper structure should be interpreted with caution.