Effects of base frequency, duty cycle, and waveform repetition on TEM responses: Insights from models of a deep-buried conductor

In transient electromagnetic (TEM) methods, the recorded data for both the magnetic field and its time derivative are influenced by the waveforms of the transmitting current. These waveforms are characterized by parameters such as the durations of the turn-on, steady, and turn-off stages, as well as the base frequency, duty cycle, and waveform repetition. The full-waveform effects encompass all counteracting effects observed in TEM responses that arise from using a waveform other than a step-off or step-on waveform. To address these complexities, a 3D forward-modeling solver was developed for TEM methods, capable of calculating TEM responses while considering realistic waveforms with considerations for base frequency, duty cycle, and waveform repetition. The effects of these parameters on the magnetic field and its time derivative were investigated through numerical experiments using models involving deep-buried horizontal and vertical conductors. The results indicated that as the waveform period increases, there is a significant improvement in the detection and discrimination capability of the magnetic field for perfect conductors with a conductivity of 1000 S/m or higher in the models presented in this study. However, the improvement in the time derivative of the magnetic field is minimal. Interestingly, the improvement in both the magnetic field and its time derivative does not show a consistent trend as the conductor conductivity increases. In addition, a novel equivalence phenomenon was uncovered in both the magnetic field and its time derivative data. The results also suggested that as the number of waveform repetitions increases or the duty cycle decreases, the discrepancies between the recorded TEM responses at the two off-time stages within a single period gradually diminish. However, the influence of the full waveform on the TEM responses becomes more pronounced with an increasing number of waveform repetitions or a decrease in the duty cycle, especially for the magnetic field.