Feasibility Study of Telluric Magnetic Field Frequency Selection Method Supported by Numerical Modeling and Karst Aquifer Case Studies

Abstract Driven by the magnetotelluric (MT), the telluric electrical field frequency selection method (TEFSM) and telluric magnetic field frequency selection method (TMFSM) measure the the horizontal components of the earth's telluric field at several frequencies along the direction of the survey line. The authors have already documented in previous study, the obtained anomalies applying the TEFSM are not free from the static shift effects. This study focuses on investigating the feasibility of the TMFSM in the context of a karst aquifer through two-dimensional forward simulations and field measurements. Utilizing the MT 2D forward modeling theory, we calculated the surface horizontal electric Ex and magnetic Hx field components along the profile in both telluric magnetic (TM) and in telluric electric (TE) polarization modes. Secondly, TEFSM was employed for groundwater exploration in the karst aquifer. Additionally, the field testing of audio-frequency magnetotelluric (AMT) was conducted using V8 electric acquisition system that minimized the interferences from anthropogenic noise sources. The simulation results indicated that the Ex component exhibited significantly high-value anomalies over high resistance bodies, whereas Ey and Hx components did not show clear anomalies. However, all three components displayed apparent anomalies for conductive bodies. Theoretically, TMFSM is feasible for exploring shallow conductive abnormal bodies, although the component Hx component is susceptible to external interference in practical applications. Therefore, designing specialized magnetic sensors with strong anti-interference capability and high accuracy is significant for achieving satisfactory results.