Very low frequency electromagnetic (VLF-EM) method as a tool for fault investigation: examples from İzmir, Türkiye

The investigation of faults in active tectonic regions is of great importance in geosciences, and geophysical methods play a key role in their detection. In particular, determining subsurface characteristics related to faults and identifying palaeoseismological trench locations using geophysical methods are topics of significant research interest to researchers. The very low frequency electromagnetic (VLF-EM) method is one of the geophysical techniques employed to investigate such problems. This study presents the results of simulation and field investigations using the VLF-EM method for fault detection. The well-established Fraser and Karous–Hjelt filters, which are widely used and effective for fault detection, were applied to the VLF-EM data. Moreover, the in-phase and quadrature components of both synthetic and field data sets were inverted to obtain the subsurface resistivity distribution. This enabled the interpretation of shallow subsurface resistivity distribution in relation to fault characteristics. A simulation was carried out to demonstrate the importance of selecting an appropriate initial value for the inversion algorithm. In addition, denoising of the VLF-EM data was investigated using the noise-assisted multivariate empirical mode decomposition technique, which yields more accurate inversion results by effectively eliminating noise. The intrinsic mode functions obtained from the decomposition process were used to identify both the noise and the trend within the VLF-EM data. The performance of the denoising procedure prior to inversion is presented for four field datasets acquired around the city of İzmir. The inversion results of the field datasets revealed that lateral resistivity variations can be traced in the two-dimensional sections and are associated with near-surface faults. The identified faults extend to the surface and provide crucial information on the presence of active faulting in the study areas. Thus, the efficiency of the VLF-EM method for fault investigation has been demonstrated.