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 audio frequency telluric electricity method (TEFM), however there are still ongoing debates on the involving 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 pseudosection of apparent resistivity (𝜌 𝑠 ) and horizontal electric field component (𝐸 𝑦 ) in Transverse Magnetic field (TM) mode are obtained by forward calculation. As a result, the static shift in 𝜌 𝑠 is observed over the near-surface inhomogeneities, as documented in 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 pseudosections of 𝐸 𝑦 also showed static shift characteristics at the horizontal position above the anomaly, referred to as "noodles phenomenon". The FSM results obtained from case studies related to the groundwater and low resistivity clay-filled karst body identification. The ∆𝑉 section curves and pseudo-section showed a significant low potential, and a "noodles phenomenon" respectively, above the low resistive anomalous body. These abnormal characteristics of ∆𝑉 are the basis for delineating the horizontal position of groundwater aquifer 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 that the static shift feature of surface 𝐸 𝑦 component can be utilized to explore near-surface geological bodies such as clay-filled or water-filled cavities.Keywords：Profile curve; Pseudo-sections; noodles phenomenon; Geophysics; Frequency selection method of telluric current (FSM); Groundwater; Magnetotelluric sounding (MT)

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