Analysing frequency-domain EM data for highly conductive targets

Abstract

The sensitivity of RESOLVE data to highly conductive targets is examined. Modelling results from synthetic data across horizontal plates suggest that RESOLVE data detect highly conductive structures, but don?t resolve their conductances and time-constants if those values exceed 500 S and 8 ms, respectively. A RESOLVE data set flown initially for kimberlite exploration has been re-analysed for nickel sulphides. Since the latter are highly conductive, the standard processing products such as apparent resistivity grids, deemed useful for mapping kimberlites, did not provide the optimum resolution for data analysis. The survey area is characterized by strong magnetic and EM responses due to the presence of banded-iron formations and pyrrhotite-rich massive sulphides. In order to determine the strongest conductors, time-constants and apparent conductivities were derived from the RESOLVE survey data. Due to the strong magnetic response, correction of the RESOLVE data for magnetic permeability resulted in better-resolved time-constants. This was achieved by inverting the data for the conductivity s and magnetic permeability mr of a layered-earth, followed by the forward modelling of the layer conductivities with mr=1. A comparison of independently derived time-constants and apparent conductances suggests that reliable values do not exceed 0.5 ms and 200 S, respectively. Conductors with values above 200 S / 0.5 ms are detectable, but not resolvable.