Tada-nori Goto

Understanding the distribution of physical properties around shallow subducting plate interfaces, where both destructive and "slow" earthquakes occur due to rapid and slower fault slips, respectively, presents a major scientific and disaster mitigation challenge. Pore water is a key factor in understanding the different slip mechanisms and their spatial relationships; however, its distribution remains understudied. In this study, based on marine magnetotelluric survey in Hyuga-nada, southwestern Japan, we identified distinct resistive and conductive anomalies along the plate interface. These anomalies correspond to areas of scarce pore fluid and high concentration area of pore fluids sourced from subducting seamounts (Kyushu-Palau Ridge), respectively. The wet area corresponds to the slow slip area, whereas the dry and transition areas correspond to areas of fast fault slip. These findings provide clear observational evidence that pore fluid distribution correlates with fault rupture behavior.

Blurred resistivity boundaries resulting from smoothness-regularized inversions of electrical resistivity tomography (ERT) data can lead to inaccurate interpretations of sharp boundary structures. To address this issue, various ERT inversion algorithms have introduced localized adjustments (localized discontinuities) in the regularization operator at positions where sharp boundaries are anticipated. Current approaches rely on prior information about sharp boundary locations, obtained from complementary geophysical, geological, and drilling data, to determine the positions and weights for these regularization adjustments. However, such prior information is frequently insufficient, limiting the application of localized regularization adjustments. Accordingly, we developed a sharp boundary inversion (SBI) algorithm using the Akaike Bayesian Information Criterion (ABIC) that determines the optimal positions and weights for localized regularization adjustments by testing various configurations and selecting the one that minimizes ABIC. A synthetic modeling study demonstrated that the SBI algorithm correctly delineated the sharp boundaries of a conductor. Its application to field data demonstrated that it delineated the sharp boundaries of a utility tunnel, and the size and horizontal position of the recovered tunnel were consistent with the estimated dimensions from the blueprint. As it does not rely heavily on prior information, the SBI algorithm can be applied to a wide range of geophysical survey data, even when prior knowledge of sharp boundary locations is limited.

Abstract Near-seafloor concentrated gas hydrates (GHs) containing large amounts of methane have been identified at various gas chimney sites. Although understanding the spatial distribution of GHs is fundamental for assessing their dissociation impact on aggravating global warming and resource potential, the spatial distribution of GHs within gas chimneys remains unclear. Here, we estimate the subseafloor distribution of GHs at a gas chimney site in the Japan Sea using marine electrical resistivity tomography data. The resulting two-dimensional subseafloor resistivity structure shows high anomalies (10–100 Ωm) within seismically inferred gas chimneys. As the resistivity anomalies are aligned with high amplitude seismic reflections and core positions recovering GHs, we interpret the resistivity anomalies are near-seafloor concentrated GH deposits. We also detect various distribution patterns of the high resistivity anomalies including 100-m wide and 40-m thick anomaly near the seafloor and 500-m wide anomaly buried 50 m below the seafloor, suggesting that GHs are heterogeneously distributed. Therefore, considering such heterogeneous GH distribution within gas chimneys is critical for in-depth assessments of GH environmental impacts and energy resources.

Abstract The physical properties of seafloor massive sulfides are crucial for interpreting sub-seafloor images from geophysical surveys, shedding light on the evolution of seafloor mineral deposits. While some studies have explored the relationship between electrical properties and the volume of conductive minerals in rocks from seafloor massive sulfide deposits, they primarily focused on artificial samples, leaving the characteristics of natural samples less understood. Moreover, there has been no comprehensive study detailing the general characteristics of electrical properties, particularly chargeability and relaxation time, in relation to the volumetric fraction of sulfides in rocks from massive sulfide mounds in typical hydrothermal areas. In this study, we employed complex conductivity measurements, elemental concentration analysis, and mineral content identification on to rock samples from the active hydrothermal zones of the Okinawa Trough in Japan. The complex conductivity observed was remarkably high, with a pronounced imaginary component and a broad frequency range. This is attributed to induced polarization extending beyond our measurement range. The rock samples were rich in conductive sulfide minerals such as pyrite, chalcopyrite, and galena. Using the Cole–Cole rock physics model, we established a correlation between rock chargeability and relaxation time coefficient with the volume fraction of conductive sulfide minerals, which deviated from previous findings. The intensity of induced polarization was notably higher than anticipated in earlier studies using artificial samples. Furthermore, we observed a distinct positive correlation between the coefficient of relaxation time and the increase in sulfide volume, likely due to the geometric characteristics of the sulfide minerals. Our findings suggest that rocks in massive sulfide mounds may generally construct sulfide clusters that lengthen the conductive path of the electrical carrier. Graphical Abstract

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.

Subsurface resistivity structure is useful for discussion about distributions and movements of underground fluids; however, the effect by fluid viscosity on the resistivity of strata has not been sufficiently discussed. In this study, we infiltrated highly-viscous mud fluid into sediment samples in the laboratory, then investigated the relationship between mud fluid saturation and the resistivity of sample. River-bed sand, mixed with gravel and fine sand, was used in this study. As a result, the mud fluid samples indicate resistive feature at low saturated condition, higher than the samples filled by tap water, and have the larger saturation exponent. Based on qualitative discussion, the behavior of non-Newtonian fluid (Bingham fluid), which restricts the capillarity of mud fluid, seems to be the major cause. It is suggested that pre-survey investigations of characteristics of fluid and the relationship between saturation and resistivity through laboratory experiments are required, when high-viscosity fluid is the survey target.