A comparative study for the source depth estimation of very low frequency electromagnetic (VLF-EM) signals

In general, a quantitative interpretation of geophysical data yields information on the nature of subsurface structures of geologic interest besides depth of the source and the associated physical property contrast However, all geophysical interpretation invariably incurs an inherent ambiguity including VLF-EM method. Techniques such as Karous-Hjelt (K-H) current density, Hilbert transform etc. are in vogue to estimate the depth to the source that are either qualitative or semi quantitative. Here, we present a comparative analysis of the depth derived from VLF-EM signals by different methods over a uranium rich basement fractures of Raigarh District, Chhattisgarh, India. The obtained results are based on techniques that are not very common in VLF-EM data interpretation such as Euler deconvolution (ED), Hartley spectral analysis and analytical signal approach of denoised in-phase component realized by Empirical Ensemble Mode Decomposition (EEMD). The estimated depth from ED and Hartley spectral analysis range 10-62 m and 12-40 m respectively are compared with K-H pseudo section, Hilbert transform and drilling depth. Overall, the results of the aforesaid techniques have shown satisfactory comparison wherein the Hilbert transform of EEMD de-noised in-phase component of traverse T1 (35 m), Hartely power spectrum of the principal profile PPQ (40 m) and the radially averaged power spectrum (38.8 m and 40 m) are close to the drilling depth. Therefore, the results obtained by methods presented emphasize that these techniques are equally applicable to VLF-EM signals for estimation of depth to subsurface conductors. (C) 2019 Elsevier B.V. All rights reserved.