石須 慶一

Deep-sea massive sulfide deposits formed by hydrothermal fluid circulation are potential metal resources. They can exist not only as mound manifestations on the seafloor (seafloor massive sulfides) but also as embedded anomalies buried beneath the seafloor (embedded massive sulfides). The distribution of embedded massive sulfides is largely unknown, despite their expected high economic value. Recent drilling surveys have revealed a complex model suggesting embedded massive sulfides coexist beneath seafloor massive sulfides. In the coexisting case, geophysical methods are required to distinguish and map both seafloor and embedded massive sulfides for accurate resource estimation. Marine controlled-source electromagnetic (CSEM) methods are useful for mapping massive sulfides as they exhibit higher electrical conductivity compared to the surrounding host rock. However, CSEM applications capable of distinguishing and mapping both massive sulfides are lacking. We employ a towed electric dipole transmitter with two types of receivers: stationary ocean bottom electric (OBE) and short-offset towed receivers. This combination utilizes differences in sensitivity: the towed receiver data are sensitive to seafloor massive sulfides and the stationary OBE receiver data are sensitive to embedded massive sulfides. Our synthetic data example demonstrates that the combined inversion of towed and OBE data can recover resistivities and positions of both massive sulfides more accurately than the existing inversion methods using individual applications. We perform the combined inversion of measured CSEM data obtained from the middle Okinawa Trough. The inversion models demonstrate that a combined inversion can map the location and shape of embedded massive sulfides identified during drilling more accurately than the inversion of individual datasets.

Coastal freshwater provides a water source for more than one billion people living in coastal regions. For sustainable groundwater management in coastal areas, an understanding of freshwater distribution is necessary. Freshwater distribution in a coastal area can extend across the shoreline and into the offshore region. Offshore-onshore mapping of freshwater helps us to gain a comprehensive understanding of the freshwater distribution in coastal areas. Resistivity imaging using electromagnetic methods has been used to reveal the freshwater distribution in coastal areas because electrical resistivity in these settings is primarily controlled by porosity and porewater salinity. We have considered a controlled-source electromagnetic (CSEM) method for offshore-onshore resistivity imaging of freshwater at a depth range of 0–500 m below the seafloor. Our CSEM method is novel in considering an array of onshore-offshore electromagnetic receivers with onshore electric dipole transmitters. We have conducted a feasibility study to investigate the ability of the CSEM method for offshore-onshore resistivity imaging of freshwater in a coastal area. The test results indicate that the method could image the resistivity distribution of freshwater located at a depth of 500 m below the seafloor. Our model study also indicates that the offshore-onshore CSEM method can detect offshore aquifers up to 5 km from the shoreline. These numerical test results imply that our CSEM method is a promising technique for offshore-onshore resistivity imaging of freshwater in coastal areas.