Frequency-domain analysis for pulse current sources in transient electromagnetic method
X. Zhu, Z. Fu, X. Su and S. Qin
Journal name: Near Surface Geophysics
Issue: Vol 15, No 2, April 2017 pp. 155 - 162
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The transient electromagnetic method has a wide range of depth of exploration in the ground. The transient-electromagnetic-method-based shallow sounding is becoming popular in engineering geophysics such as grounding grid measurement, pipeline detection, and mine exploration. It is useful for shallow sounding to raise the transmitting frequency, improve the spectral resolution, and enhance the power of withstanding all kinds of near-surface noise. Therefore, a well-designed transmitter current waveform should not only induce more effective geoelectric information at a low energy cost but also prevent external interferences during transient electromagnetic method prospecting. For this purpose, the frequency-domain characteristics of single-pulse waveforms and multipulse sequences are analysed, on the basis of three fundamental waveforms—rectangular, triangular, and half-sine shapes. The energy efficiency ratio, high-frequency energy percentage, and frequency resolution are introduced to evaluate the performance of the transmitter current source, in a numerical or graphical manner. Furthermore, six window functions are used to modulate the rectangular pulse sequence. The analysis and simulation results prove that the pulse current source modulated by the Blackman window is advantageous in terms of energy efficiency, high-frequency content, and frequency resolution. In addition, the Blackman modulating sequence has many blank frequency bands (also called identifiable bands) where the noise is completely exposed and easy to remove. This work is useful for the design, optimisation, and selection of transmitter current waveforms for different applications, especially for near-surface geophysical prospecting.