New Insights into the Anomaly Genesis of the Frequency Selection Method: Supported by Numerical Modeling and Case Studies

Abstract

The frequency selection method (FSM) is the further development of the audio frequency telluric electricity method (TEFM). However, debate continues regarding the mechanisms leading to anomaly genesis. Therefore, the present study intends to explore this using 2D forward modeling of magnetotelluric (MT) sounding and practical applications of FSM on three Chinese case studies in karst and granitic settings. In the first stage, the profile curves and pseudo-section of apparent resistivity (${\rho }_{s}$) and horizontal electric field component ${(E}_{y})$ in MT mode are obtained by forward calculation. As a result, the static shift in ${\rho }_{s}$ is observed over the near-surface inhomogeneities, as documented in the literature. Additionally, the profile curves of ${E}_{y}$ showed an obvious static shift in the rectangular coordinate system (i.e., the curve rises with the increase in frequency) which is a well-known phenomenon. The pseudo-sections of ${E}_{y}$ also showed static shift characteristics at the horizontal position above the anomaly, referred to as the “noodles phenomenon.” The FSM results were obtained from case studies related to groundwater and low-resistivity clay-filled karst body identification. The $\Delta V$ section curves and pseudo-section showed a significant low potential, and a “noodles phenomenon,” respectively, above the low-resistivity anomalous body. These abnormal characteristics of $\Delta V$ are the basis for delineating the horizontal position of the groundwater aquifer by applying FSM. It is concluded that the anomaly of FSM is the reflection of the static shift in MT, and hence, the FSM can be categorized as a “static shift method.” Therefore, this inspired us to investigate whether the static shift feature of the surface ${E}_{y}$ component could be utilized to explore near-surface geological bodies such as clay-filled or water-filled cavities.