Focusing frequency and significance of multi-frequency very low-frequency electromagnetic measurement in delineating near-surface conducting structures
S.P. Sharma and V.C. Baranwal
Journal name: Near Surface Geophysics
Issue: Vol 9, No 5, October 2011 pp. 435 - 447
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The very low-frequency (VLF) electromagnetic method utilizes primary signals (field) transmitted from worldwide distant transmitters located in coastal areas. These transmitters are meant for long distance marine communication. VLF transmitters operate at a low communication frequency band (between 5–30 kHz) and the transmitted signal travels a long distance. Transmitted signals penetrate the Earth’s subsurface and produce electromagnetic induction in the subsurface even several thousands of kilometres away from the transmitters. The VLF method is quite simple and frequently used in the delineation of near-surface conducting structures of various practical applications. Several conducting structures lying along a measured profile with different conductivities can be properly induced at distinct frequencies that yield the maximum response. Therefore, such conductors may not be identified or resolved well using single frequency VLF measurement. A 2D numerical modelling study was carried out over a wide frequency range (1–500 kHz) to find the frequency that produces the maximum response for a given conductor. Results show that a particular frequency (focusing frequency) produces the maximum (peak) response for a conductor. When the measuring frequency either increases or decreases with respect to the focusing frequency, then the peak response always decreases. The focusing frequency remains almost similar with an increase in target depth and host resistivity. An increase in the overburden conductivity shows a decline in the focusing frequency. Two or more targets of different conductivity present in the subsurface yield peak responses at corresponding focusing frequencies. This shows that they will be resolved well at corresponding focusing frequencies. In such circumstances, inversion using single frequency VLF data yields inaccurate results. However, the use of multi-frequency VLF data yields better results. Inversion of multi-frequency VLF data is presented to show the efficacy of the approach. A field measurement is also presented and the effectiveness of multi-frequency VLF measurement is highlighted. Since the numerical modelling study is performed over a broad frequency range covering the VLF and radiomagnetotelluric signal, the focusing study is valid for radiomagnetotelluric applications as well.