Enhanced Magnetotelluric Responses of Deep Resistive Anomalies Beneath Sedimentary Cover from Deep Underground Observations

Abstract

The identification and delineation of subsurface resistive anomalies beneath sedimentary cover remain challenging for the magnetotelluric (MT) method. In this study, we compared the attenuation behavior of electromagnetic (EM) fields and their response characteristics under surface and/or subsurface observation configurations using both synthetic and field data. The synthetic results show that the conductive cover layer substantially suppresses the EM responses of the subsurface high-resistivity body, resulting in only weak relative response perturbations at surface stations. In contrast, the relative response amplitudes recorded at underground stations are enhanced, indicating that underground EM observations offer advantages in resolving subsurface resistive anomalies. The results further reveal distinct attenuation behaviors of the electric and magnetic fields, with the electric field being more sensitive to variations in the bulk conductivity of the sedimentary layer. Furthermore, one-dimensional Bayesian probabilistic inversions of both synthetic and field datasets indicate that underground observations provide more robust and reliable estimates of the resistivity structure. These findings suggest that further exploration of deep underground observations has considerable potential for detecting and characterizing weak EM responses from resistive targets in sedimentary environments, while also improving the reliability of inversion results and reducing their uncertainty.