A novel phased-array transmitting source in controlled-source audio-frequency magnetotellurics

In traditional controlled-source audio-frequency magnetotellurics (CSAMT), the orthogonal magnetic and electric field components in the far-field region are recorded to calculate the apparent resistivity. However, as the distance between the receiver and the transmitter increases, the electromagnetic signal weakens considerably. Moreover, as the location of the transmitter is fixed, the location of the far-field region also is fixed, which limits the exploration range. In this study, a novel phased-array transmitting source was proposed to overcome this limitation. In traditional CSAMT, a grounded single-dipole long-line source is used to transmit the electromagnetic wave. This source has a low gain and no directionality, resulting in a low signal-to-noise ratio (SNR) in the area of interest (AoI). Conversely, by using a phased-array source, beam control and stable directional radiation will be achieved so that the electromagnetic energy should be capable of effectively focusing within the AoI. Without increasing the transmitting power, the SNR should be conveniently enhanced and the size of the AoI can be greatly increased. In this paper, the characteristics of the radiation field of a single-dipole long-line source and its application in traditional CSAMT are analyzed. The design principle and the method of implementation of a novel phased-array source are provided. The feasibility and effectiveness of the proposed source are verified by simulations and experiments, and a prototype is realized.