Application and comparison of very low frequency electromagnetic and electrical resistivity techniques to investigate a Karstic Region: A case study of EL Hajeb Municipality, Morocco

Abstract

Geophysical methods are widely used to determine subsurface karst structures' depth because of their risks. However, their qualitative or quantitative interpretations involve a degree of uncertainty. Consequently, several methods are used to improve the credibility of drawn conclusions. In this study, we used two well-known geophysical methods, namely Very Low Frequency Electromagnetic (VLF-EM) and electrical resistivity methods, to characterize the observed or suspected sinkholes in an area of the municipality of El-Hajeb, Morocco, where no previous research had been conducted. This built-up zone is characterized by soluble carbonate rocks and sinkholes, which qualifies it as a high-risk area. We also compared four interpretation methods to estimate the depth of anomalies from VLF-EM data: Derived Euler Deconvolution (DED) and Extended Euler Deconvolution (EED) which, to authors' knowledge, have not been applied before, Karous-Hjelt (K-H) pseudosections, and Classic Euler Deconvolution (CED). Electrical resistivity data were used to confirm the VLF-EM results. The findings indicated that some conductive anomalies, manifested by decreased real component and apparent resistivity, are in good agreement with shallow sinkholes and depressions observed in the study area. In addition, other conductive anomalies indicate hidden sinkholes. The estimated depths of an observed sinkhole from K-H pseudosections, CED, DED, and by electrical resistivity tomography are comparable. The finding of this study revealed the effectiveness of the used geophysical methods to identify karst features. It also demonstrated the efficacy of the VLF-EM data processing tools, particularly CED and DED, to estimate the depth of these features, with the exception of the EED, which unexpectedly provided large depth values.