Frequency‐domain analysis for pulse current sources in transient electromagnetic method

Abstract

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.investigated the time-domain responses excited by different current waveforms and the applicability to the geological targets based on a loop model in free space (Liu 1998;Chen, Lin and Zhang 2012). The half-sine waveform is employed for deep sounding in the GEOTEM system, MEGATEM system, and HELITEM system of the CGG Company (Smith and Annan 2000; Chen, Hodges and Miles 2015). The AeroTEM system of GEOTECH uses a triangular pulse as the transmitter current waveform (Boyko and Paterson 2003). To maximise the response of an anisotropic layer, Tripp (1996, 1997) created a sinusoidal damped current and injected the optimised array current into the earth through the distributed poles. The SkyTEM is of dual-moment capability to map both shallow and deep geology (Sørensen and Auken 2004). As a new version of the SkyTEM, the Mini-SkyTEM has been developed with a smaller transmitter loop and is dedicated to identifying geological structures and heterogeneities (Refsgaard et al. 2014). CCG also launched its innovative MULTIPULSE system with a highpower pulse (half-sine wave) for large exploration depths and a lower power pulse (square wave) for near-surface resolution (Chen et al. 2015).

Research on cross‐well pseudorandom electromagnetic detection method and extraction of response characteristics

“…[17][18][19] Moreover, this method can measure the total field while supplying power, which overcomes the shortcoming of the time-domain electromagnetic method, in which the received signal is weak due to secondary field measurement, but increasing power supply current contradicts the need for lighter equipment. 20,21 The practical results in recent years have proved that the pseudorandom frequency-domain electromagnetic method has achieved large depth detection under the premise of ensuring high precision and efficiency. Good results have been achieved in geophysical prospecting of deep-seated metallic minerals, conventional oil and gas, shale gas, engineering geology, coal mine water disasters, and other areas.…”

“…Compared with other frequency‐domain electromagnetic methods, it has the advantages of high speed, high precision, and large detection depth 17‐19 . Moreover, this method can measure the total field while supplying power, which overcomes the shortcoming of the time‐domain electromagnetic method, in which the received signal is weak due to secondary field measurement, but increasing power supply current contradicts the need for lighter equipment 20,21 . The practical results in recent years have proved that the pseudorandom frequency‐domain electromagnetic method has achieved large depth detection under the premise of ensuring high precision and efficiency.…”