The analysis of ZTEM data across the Humble magnetic anomaly, Alaska

Abstract

ZTEM data acquired across the Humble magnetic anomaly of almost 30 000 nT were analysed for the presence of a magnetic gradient response and the effects from elevated magnetic susceptibilities. Mag3D inversion of the magnetic data indicates magnetic susceptibility values as high as 2.0 (SI).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.ZTEM responses were forward modelled with a 3D algorithm developed at the University of British Columbia Geophysical Inversion Facility (UBC-GIF) that takes into account electric conductivities σ and magnetic susceptibilities κ, in order to assess the impact of the elevated κ−values derived from the Mag3D inversion. Computing the ZTEM response for these κ-values combined with resistive half-spaces indicates that the response amplitudes and shapes strongly depend on the background resistivities. Ignoring the elevated κ-values during an inversion can result in patterns that resemble crop circles.The approximate conductivity structure of the survey area was derived with a UBC-GIF 3D ZTEM inversion, which models κ = 0. Forward-model results of these conductivities combined with the elevated κ-values derived from the Mag3D inversion indicate that the conductivities are underestimated with the κ = 0 assumption. For an environment such as Humble, with deep-seated zones of elevated κ-values, the shallow inverted conductivity structure appears to be reliable, but the deeper structure should be interpreted with caution.