Broadband (ULF-VLF) surface impedance measurements using MIMDAS

Abstract

Electromagnetic radiation from distant, discrete, ground-to-cloud lightning return-strokes can be recorded as atmospherics (spherics), using both electric and magnetic field antennas. The Earth-ionosphere waveguide filters this radiation into the ULF-VLF radio bands from 3 Hz to 30 kHz with a notch centred at approximately 2 kHz. The relationship between the orthogonal horizontal electric and magnetic field signatures at the receiver is directly related to the surface impedance of the Earth at that point. Transforming the transient fields to the frequency domain allows the calculation of the surface impedance over the frequency range determined by the sample rate and record length. From this, resistivity versus depth models of the Earth can be determined. This paper demonstrates that a discrete spheric can be considered as a broadband vertically polarised plane-wave source suitable for magnetotelluric (MT) style geophysical surveying, and that the data collected could be used to deduce the near-surface resistivity profile. The results from measurements made with MIM Exploration’s proprietary distributed acquisition system (MIMDAS) are presented. Multi-station data were acquired in a telluric profiling mode (without a magnetic field reference) to demonstrate spatial coherence and correlation with conventional MIMDAS MT pseudosections and IP inversion models. The advantages of the technique include the broad band of measurement and rapid data-acquisition time for reliable statistics. Valid data were collected between 24 kHz and less than 200 Hz and while stacking improves data integrity, useful information may be extracted from less than 5 ms of a 6 s record, corresponding to a single strong spheric. This technique has potential value in near-surface geotechnical applications and as an aid in removing statics from conventional continuous-MT survey data.