Transient surface impedance (TranSIM) measurements using discrete lightning for electromagnetic mapping at audio frequencies

Abstract

The paper describes the first field measurements using recently developed TranSIM instrumentation. The method is based on registering horizontal components of electric and magnetic field of distant lightning strokes (sferics). In fact it is an expansion of classical MT method in the VLF frequency range of 3 to 30 kHz thus providing information about conductivity distribution at the depths of approximately 50 to 300 meters. Measurements are conducted atop basalt body in the vicinity of Ipswich (Queensland) over 16 sites on 2 parallel profiles. From 20 to 50 sferics were registered at every site. Data was processed using short time Fourier transform obtaining spectrograms of electric and magnetic field. Apparent resistivity was calculated and frequency pseudo sections are presented. The apparent resistivity sections were matched with geological scheme of the site. Highly resistive zones correlate with high thickness of a basalt layer and conductive zones show where the underlying basalt sandstone is closest to the surface.