Excitation process under the ramp-step waveform of inductive source-induced polarization method

Abstract

Most previous studies explain the induced polarization (IP) effects in transient electromagnetic (TEM) data using an idealized but unrealizable step-waveform transmitter current. However, the ramp-step waveform, which is commonly applied in TEM measurement, has been given less attention. To explore the effects of the switch-off time, we have compared the IP responses induced by two waveforms: the step and the ramp step. We apply a wire-filament circuit composed of a Debye model and an inductor to identify the differences in the aspect of the energy transfer process. Furthermore, we extend the analysis to illustrate the IP effects in a frozen-soil zone, metallic sulfide ore, and graphite ore and to analyze the relationship between the switch-off time, IP effects, and the polarization parameters. The results indicate that the primary and secondary fields act as excitation sources of the polarization field. In the step waveform case, the excitation source of the polarization field is the secondary field. As the switch-off time increases, the contribution rate of the primary field gradually increases, especially in the high-resistivity media. The finding provides a new understanding of the excitation process of the IP effects and indicates that source contributions are variable in different situations. Moreover, a longer switch-off time weakens the IP effects severely, and in the high-resistivity, high-polarizable media, the IP effects are more sensitive to the switch-off time. Therefore, a suitable switch-off time should be chosen based on the properties of the polarizable media, such as resistivity and time constant. To detect a relatively high-resistivity, high-polarizable body, the switch-off time should be as short as possible. Nevertheless, to detect a relatively low-resistivity polarizable body, the IP effects are fairly insensitive to the switch-off time, so the transmitter waveform can easily meet the requirements.