Inversion of induced polarization parameters from time-domain electromagnetic data using the Debye Decomposition model

Time-domain electromagnetics (TEM) is a valuable tool for exploring seafloor hydrothermal deposits, which are characterized by variations in resistivity and chargeability. Several surveys using the Waseda Integrated Seafloor Time-domain EM (WISTEM) system have been conducted. Negative transients, which are attributed to induced polarization (IP) effects, have been observed in data collected over a known deposit in the Okinawa Trough (2018). The Cole–Cole type model is a common parameterization of complex resistivity due to IP effects; however, it implicitly assumes that the data are sensitive to a wide range of frequencies. TEM data have limited frequency content, governed by the measurement time range and the diffusive nature of the fields. Using a synthetic example, I show that TEM inversion using the Cole-Cole model suffers from significant instability when the frequency content of the data is insufficient to constrain all four Cole-Cole parameters. The Debye Decomposition model provides a way to overcome this challenge by allowing explicit selection of the relaxation time band over which the influence of chargeability in the data behaves distinctly from the influence of resistivity. In this thesis, I introduce a workflow for inverting TEM data using a Debye Decomposition model. I demonstrate how to set the relaxation time band when inverting TEM data. The Spectral IP (SIP) response of a material describes how its resistivity varies as a function of frequency, over a wide frequency range. I then compare the model recovered from the TEM data with a model obtained by inverting SIP data from the target. When inverting SIP data, I use the same Debye Decomposition model as was used for the TEM data. By using the same relaxation-time band for both inversions, I ensure that the IP parameters recovered from each are comparable; I refer to this as the reconciling process. Using a synthetic example, I demonstrate that the inversion results obtained from the TEM data are consistent with the reconciled true model obtained from the SIP response. Finally, I present a field application and show that the method recovers IP parameters that are consistent with expected physical property values for seafloor hydrothermal deposits.