1D inversion of time-domain electromagnetic data with induced polarization effects for a sea-floor hydrothermal deposit

Masayuki Motoori, Lindsey J. Heagy, Gosuke Hoshino, Kunihito Yamamoto, Haruhisa Morozumi, Kunpei Nagase, Shingo Sugimoto

Seafloor hydrothermal deposits are polymetallic massive sulfide ore deposits formed by the precipitation of metal components contained in hot water ejected from the seafloor. The sea depth is 700- 2000m. Ore bodies extend hundreds of meters horizontally and tens of meters vertically. Ore bodies are exposed on the sea floor. Some lab-based petrophysics study indicates that resistivity and chargeability are diagnostic physical properties, even compared with seawater (3.0-3.5 S/m). Time-domain electromagnetic methods (TEM) are sensitive to variations in resistivity. WISTEM (Waseda integrated seafloor time-domain electromagnetic exploration) surveys have been conducted in several areas. Negative transients, which are due to induced polarization effects (IP), have been observed for data collected over known deposits. It is important to understand the system response to invert these data. The pressure vessel (PV), which contains the transmitter and receivers, can impact the data. We use numerical simulations to quantify these effects, and we develop a workflow for estimating a linear filter which captures the effects of the PV. This filter will then be used in subsequent simulations and inversions. Finally, we perform one-dimensional time-domain IP inversion of the field data. The estimated resistivity and IP parameters agree with physical property measurements from the area.