VLF-EM data conversion into apparent resistivity using the Hilbert transform and its application for shallow fault investigation at Pinggirsari, Bandung Regency, Indonesia

Abstract

This research presented the ability of the Hilbert transformation of The Very Low Frequency-Electromagnetic (VLF-EM) data to investigate the presence of faults. VLF-EM data was collected in the Pinggirsari village area, Arjasari District, Bandung Regency, West Java, Indonesia. Through the Hilbert transformation, the VLF-EM data was converted into apparent resistivity data and then inverted into a resistivity cross-section. For comparison, qualitative interpretation was also conducted using both the Fraser and the Karous-Hjelt filters. Interpretation results using both the Fraser and Karous-Hjelt filters indicate the presence of faults through tilt-angle curve peaks and apparent current density anomalies. The resistivity cross-section resulting from the Occam inversion of the apparent resistivity data, derived from the Hilbert transformation, accurately delineated fault positions. Resistivity changes around 700–800 m indicated fault proximity to a depth of approximately 150 m, with a good match to the geological map. Within a distance of 0–700 m, low resistivity values were associated with the Malabar tuff, while >700 m, higher resistivity values were associated with the Waringin-Bedil andesite. The Hilbert transform helps in clarifying VLF data interpretation for precise fault detection through resistivity parameters.