Design of Depth-Focused Electromagnetic Transmitting Scheme Based on MFSPWM Method.

Aiming at the frequency-domain electromagnetic method (FEM) detection system, a depth-focused transmitting scheme based on multifrequency sinusoidal pulsewidth modulation (MFSPWM) method is proposed to solve the problems of low exploration efficiency in conventional single-frequency transmitting scheme and low vertical resolution in multifrequency pseudorandom (MFPR) transmitting scheme. Depending on the target depth range and required measurement accuracy, a depth-focused waveform is designed to focus the excitation field energy on a narrow frequency band. In this article, the principle of the MFSPWM method is theoretically analyzed and the expression of depth-focused waveform is derived by Fourier analysis. Then, the parameter optimization of the main frequencies is performed for the desired spectrum. Furthermore, simulations and experimental tests were carried out to verify the feasibility and effectiveness of the proposed scheme. The results show that the proposed scheme has better spectral features with multiple closely spaced frequencies and less undesired harmonics, which ensures high vertical resolution for the specific depth target. After parameter optimization, the peak-to-peak value of the current waveform was reduced by about 40% and the effective signal of the high-frequency component was increased by about 20%. The forward simulation and field test results show that the proposed scheme has higher detection accuracy and interference resistance for the specific depth target compared to the MFPR scheme.