後藤 忠徳

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.

地下比抵抗構造は地下流体の分布や移動を議論する際に有益な情報であるが，流体の粘性の違いが地層の比抵抗に与える影響については，十分には議論されていない。本研究では実験室において，高粘性の泥水を堆積物試料に浸透させ，泥水飽和度と試料比抵抗の関係を調査した。試料として，砂礫混じりの川砂および細粒の砂を用いた。室内実験の結果，泥水使用時の比抵抗は，水道水を用いた時と比べて，低飽和域で高い値を示し，飽和係数も大きくなった。その要因について定性的な議論を行ったところ，泥水の非ニュートン流体（ビンガム流体）的性質により，泥水の毛管現象がほとんど起きなかった事が主な要因であると推測された。高粘性流体が探査対象の場合は，室内実験を通じて，流体の特性や飽和度と比抵抗の関係を調査しておく必要性が示唆された。