The comparative analysis of the Ex and Hz fields sensitivity generated by electric dipole sources

Abstract In the controlled source audio-frequency magnetotelluric method, orthogonal electric and magnetic fields are commonly measured to determine the Cagniard apparent resistivity. However, in the near-field zone, the Cagniard resistivity is severely distorted, which is unrelated to underground structures. The Ex and Hz amplitudes in a homogeneous half-space monotonically vary in resistivity, and a numerical algorithm could achieve high-precision apparent resistivity without distortion for all frequencies. On this basis, the main focus of this investigation is on the comparative analysis of the sensitivity for the Exfield, Hzfield, and Cagniard apparent resistivity to conductive and resistivity targets via synthetic models. The achieved results confirm that the Ex field could exhibit a more enhanced sensitivity for the resistive objects, whereas the Hz field could more effectively identify the conductive target. Besides, the static effect often distorts the electromagnetic data, which rigorously influences their application. The influence of the static effect on both the Exand Hzfields is also examined in detail. The apparent resistivity based on the Exfield and Cagniard apparent resistivity is significantly affected by the static effect, which can mask deep anomalous blocks. However, the apparent resistivity based on the Hz field is almost unaffected by the static effect. Finally, a more efficient observation approach is provided for both the insulating and conductive targets.