A synthetic study investigating induced polarization effects on time-domain electromagnetic data for Sea-floor hydrothermal deposit.
The Japan Organization for Metals and Energy Security (JOGMEC) has conducted a comprehensive evaluation of its efforts to date in developing seafloor hydrothermal deposits. The second comprehensive evaluation reported the identification of a mineral resource potential of 51.805 million tons in the Okinawa and Izu/Ogasawara regions. There are important physical properties to consider for exploration using electromagnetic methods: (1) the depth of the sea is 700- 2000m, (2) sea water is conductive (3-3.5 S/m), (3) ore bodies are estimated to extend hundreds of meters horizontally and tens of meters vertically, and (4) ore bodies are exposed on the sea floor. JOGMEC’s exploration strategy includes SONAR, geophysics, and rock sampling surveys. Geophysics data are collected with the aim of estimating the spatial distribution of ore bodies. Some petrophysics indicate that resistivity and chargeability are diagnostic. Time-domain electromagnetic methods (TEM) are sensitive to variance of resistivity. WISTEM (Waseda integrated seafloor time-domain electromagnetic exploration) surveys have been conducted in several areas. Some “fixed type” measurements are collected where the WISTEM landed on the seafloor. In these type of measurement negative transients, which are due to the induced polarization effect (IP), have been observed for data collected over known deposits. IP effects are particularly relevant when chargeable materials, namely sulfides are present. Therefore, estimating the IP parameters, such as the chargeability and time-constant are motivation for this project. To perform inversions, it is essential to consider the geometry, source-receiver configuration, current waveform and other system parameters that impact the response The WISTEM system utilizes a 3.5 m size loop; equipment like the transmitter and the receivers are placed in the center of the loop. It has been reported that the pressure vessel that contains this equipment may affect the data. To study the impacts of the pressure vessel on the EM data, we conduct a synthetic study that includes a conductive, permeable vessel in the marine EM survey. We used both SimPeg and empymod for forward simulations.
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