Comparison of response characteristics between the electromagnetic method of “Earth-ionosphere” mode and traditional magnetotellurics

As an emerging electromagnetic exploration method, the electromagnetic method of "Earth-ionosphere" mode utilises a fixed high-power long-source antenna to emit extremely low frequency electromagnetic waves. Signals can be received with a large source–receiver offset to perform resource exploration. As a novel exploration method, it is necessary to compare the results with traditional electromagnetic exploration methods to verify its feasibility and analyse its response characteristics. We first introduce the basic principle of the electromagnetic method of "Earth-ionosphere" mode. Then, one-dimensional forward simulations are developed to determine the similarities and differences between the response of this electromagnetic method in the near, far, and waveguide zones with that from the magnetotellurics. The responses of the three-dimensional model are simulated using the edge-based vector finite element method. The forward simulation responses show that the electromagnetic method of "Earth-ionosphere" mode has a significant abnormal response to geological targets, which is similar to other electromagnetic exploration methods. At large source–receiver offsets, the tensor response of the proposed electromagnetic method is consistent with the magnetotellurics. At small source–receiver offsets, there is a near-field response that is similar to the controlled source audio-frequency magnetotellurics. There are some differences between the scalar response and the tensor response when the antennas have certain azimuth angles. Finally, we present the field measured data for the electromagnetic method of "Earth-ionosphere" mode and compare the field measured data with the magnetotelluric data. We deepen our understanding of the electromagnetic method of "Earth-ionosphere" mode by comparing the results with the traditional magnetotellurics, which provides guidance for further development.