Time‐constant analysis of frequency‐domain EM data

Abstract

Summary Deriving time-constants from EM data allows a fast analysis of survey data, but historically time-constants were only derived from transient EM data. The relationship between time-constant and conductance is examined for plate and sphere models for frequency-domain EM data of the RESOLVE system. The results confirm published relationships between the two parameters and indicate that time-constants are underestimated for targets with elevated magnetic permeabilities. Further, the plate model results suggest that RESOLVE data detect highly conductive structures, but don’t resolve their conductances and timeconstants if those values exceed 500 S and 8 ms, respectively. A RESOLVE data set flown initially for kimberlite exploration has been re-analyzed 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 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 σ and magnetic permeability µr of a layeredearth, followed by the forward modeling of the layer conductivities with µr=1. A comparison of independently derived time-constants and apparent conductances suggests that reliable values derived from the survey data do not exceed 0.4 ms and 200 S, respectively. Conductors with values above 200 S / 0.4 ms are detectable, but not resolvable.