A Novel Optimization Method for SHEPWM-Based Depth-Focused Electromagnetic Transmitting System

The towed frequency-domain electromagnetic (TFEM) detection system is an effective tool for detecting underground anomalous targets. To improve detection efficiency and measurement accuracy for specific depth targets, the depth-focused waveform with multiple probing frequencies closely spaced in the spectrum was introduced. However, the transmitting coil impedance severely suppresses the transmission energy of the excitation signal, which will reduce the transmission efficiency and signal-to-noise ratio (SNR). To solve this problem, a novel optimization method for the traditional depth-focused transmitting system is proposed to simultaneously improve the transmission energy of multiple probing frequencies in the effective band. First, the traditional depth-focused waveform is optimized with the objectives of matching the desired spectrum and maximizing the transmission energy by the nonlinear programming approach, which provides closer spectral energy between the probing frequencies and less undesired harmonic pollution. Then, we design the corresponding broadband resonant compensator based on an improved LC series resonant circuit to simultaneously enhance multiple spectral amplitudes in the effective band. Furthermore, the enhancement effect of compensator parameters on the depth-focused waveforms is analyzed, and the design principle of compensator parameters is provided. Finally, the feasibility of the proposed optimization method is verified by simulation and laboratory experiments and the actual limitation of the novel scheme is discussed in detail.