Depth-Focused Waveform Based on SHEPWM Method for Ground-Airborne Frequency-Domain Electromagnetic Survey

Abstract

The ground-airborne frequency-domain electromagnetic (GAFDEM) survey is a mixture of ground and airborne electromagnetic methods with a source on the ground and a magnetic field receiver in the air. The technique probes different depths of the earth by transmitting waveforms at different frequencies. Therefore, multiple closely spaced frequencies are needed when surveying a specific target depth in detail. However, in the existing GAFDEM systems using a square wave or a 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</sup> pseudorandom wave, the ground source needs to transmit more sets of waveforms with different fundamental frequencies in more flights to improve the vertical resolution for the same observed line, which will seriously reduce the exploration efficiency. In this paper, we design a depth-focused waveform to solve this problem based on a new full-cycle asymmetrical selective harmonic elimination pulse width modulation (SHEPWM) method. In the SHEPWM method, we obtain a desired spectrum by directly controlling the switching angles of a binary sequence with an artificial neural network algorithm. In our study, we first provide the basic theory and method for waveform design. Then, we realize the depth-focused waveform and pseudorandom waveform via simulation. Next, we employ the depth-focused waveform into practice and compare it with the 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</sup> pseudorandom wave in a GAFDEM field survey. The test shows that the survey result with a depth-focused waveform has a higher vertical resolution and efficiency.