Feasibility study of telluric magnetic field frequency selection method in groundwater exploration

Abstract

Over the years, scholars have made significant contributions in the development of various electric pulse of a natural field methods, collectively referred to as frequency selection method (FSM). The methods still lacking the satisfactory exploratory theoretical studies for interpreting the results effectively. In a previous study, authors concluded that results obtained at several frequencies with both telluric electrical field frequency selection method (TEFSM) and telluric magnetic field frequency selection method (TMFSM) are affected by static shift effects. The present study is dedicated to the feasibility of the methods in groundwater exploration of karst aquifer applying two-dimensional forward simulations and field measurements. In the first stage, utilizing the magnetotellurics (MT) 2D forward modeling theory, authors computed the surface horizontal electric E_x and magnetic H_x field components along the survey line in both telluric magnetic (TM) and telluric electric (TE) polarization modes. Secondly, TEFSM was used where the measured curves of electric field at different frequencies were found well synchronized in depicting the abnormal position. Further, the field testing of audio-frequency magnetotellurics (AMT) conducted with V8 electric acquisition system considerably suppressed the interferences from anthropogenic noise sources. The synchronicity of magnetic field component curves at different frequencies was worse than that of electric field component. The simulation results indicated that the component E_x exhibited significantly high-value anomalies over high resistance bodies, whereas E_y and H_x components did not show clear anomalies. All three components displayed apparent anomalies for conductive bodies. Theoretically, TMFSM is a feasible approach for exploring shallow conductive abnormal bodies, although the component H_x is susceptible to external interference in practical applications. Therefore, designing specialized magnetic sensors with strong anti-interference capability and high-accuracy is a significant step towards achieving the satisfactory results.