Low-Sidelobe Adaptive Asymmetry Beamforming for Controlled-Source Audio-Frequency Magnetotellurics Using Linear Array Artificial Field Sources

As an effective artificial field source electromagnetic detection technology in the frequency domain, the controlled-source audio-frequency magnetotelluric (CSAMT) method is widely used in resource exploration and environmental monitoring. However, traditional CSAMT systems primarily employ single electric dipole sources or non-adaptive array sources, which suffer from low radiation efficiency, uncontrollable directional patterns, and an insufficient signal-to-noise ratio (SNR) in complex electromagnetic environments. Additionally, the existence of electromagnetic weak zones and strict restrictions on transceiver distances limit the range of exploration and data accuracy. To address these issues, this study proposes an adaptive beamforming method for CSAMT based on linear array artificial field sources, replacing the single dipole source with parallel vibrator and coaxial vibrator linear array. The low-sidelobe pattern synthesis was achieved using the Taylor synthesis method, and the adaptive asymmetry beamforming was designed based on the maximum signal-to-noise ratio criterion. A GPS-based time synchronization system with a local high-precision clock backup was developed to ensure the synchronization of transmitters and receivers. At the same time, two phase control strategies (DPST and SPDT) were implemented for sine and square wave outputs, respectively. Numerical simulations and field experiments were conducted to verify the performance of the proposed system. The results demonstrate that, compared with traditional single dipole sources, the linear array artificial field sources significantly reduce the sidelobe level, enhance the energy density of the main lobe, and effectively suppress multipath interference. The adaptive beam scanning capability expands the exploration range from the traditional 60° to approximately 90°, eliminating the "zero zone" limitation. Meanwhile, the SNR is remarkably improved, and the exploration resolution and accuracy are enhanced. This study provides a reliable technical solution for CSAMT exploration in complex environments, promoting the application of array antenna technology in electromagnetic detection.