Mapping the near-surface geoelectrical structure of the Mottled Zone using the very low frequency–electromagnetic method

Abstract

Mottled Zones (MZ) in south Levant regions show discrete, varied-dimensional rock complex exposures and many unexplored areas in the vicinity of the Dead Sea Transform Fault (DSF). The research area, located south of the Daba–Siwaqa Mottled Zone complex in Jordan, was surveyed using the very low-frequency electromagnetic method (VLF-EM) for the purpose of resolving the geoelectrical setting of MZ rocks, the associated structural features, and the underlying sedimentary layers. The VLF-EM tipper data acquired along five profiles was analyzed using Fraser, Karous-Hjelt, and apparent resistivity linear filters. The quantitative interpretation was conducted using a 2D inversion of the VLF-EM tipper data, and a proposed synthetic model was tested and analyzed at MZ exposure. The majority of MZ rocks are likely to be located in a region of negative Fraser values (≥ −10.0%) and negative Karous-Hjelt values (≥ −8.0%). The apparent resistivity map exhibits MZ rocks at high values (1000–2500 Ω·m).The derived resistivity models recovered from the 2D inversion of the VLF-EM data showed a relatively resistive MZ rock body immersed in relatively less resistive (conductive) sedimentary layers composed of clayey marl and marl. The known and inferred locations of MZ rocks derived from the 2D resistivity section are compatible with the locations proposed by the filtering techniques. Furthermore, the 2D resistivity sections showed a stressed underlying sequence of sedimentary rocks demonstrating sub-vertical contacts and possible displacement along fault planes. A synthetic 2D resistivity model revealed a wedge-shaped formation of 38.0 m thick MZ rocks that can be resolved from the underlying sedimentary layers.