後藤 忠徳

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.

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。

The demand for groundwater resources has increased owing to global developments of urbanization, industry, and agriculture. There is thus a need for advanced geophysical techniques that can be used for accurate surveying of groundwater resources. Although electric sounding has been a standard technique in surveying, it is still difficult to specify the groundwater table and aquifer distribution accurately when considering only resistivity and induced polarization. The present paper aims to improve the accuracy by developing a variable-frequency-based electric sounding system that measures apparent resistivity in the frequency range of electrical current transmittance of 1-100 Hz at intervals of 1 Hz. Experiments using soil samples and an aquifer model based on a tank show that the coefficient of variation of resistivity (C-v) in the frequency range of 21-40 Hz was effective in detecting an aquifer because it was higher than coefficients in other frequency ranges. To verify the availability of this indicator, three field experiments with different geologic settings (i.e., plateau, coastal, and limestone aquifer fields located in the southwest and on the southern edge of Japan) were undertaken. Whereas resistivity distributions varied with the current frequency depending on the field, C-v distributions were consistent regardless of the frequency common to the three test fields. Although the resistivity characteristics did not indicate the existence of a groundwater table or aquifer, C-v for the frequency range of 21-40 Hz can be used to specify the locations of the table and aquifer and additionally the doline in the limestone aquifer field. An electrokinetically induced vibration of porous material was the most plausible mechanism that explains the large C-v in the specific frequency range. The effectiveness of using C-v and the developed variable-frequency-based electric sounding system were thus demonstrated by modeling and field experiments.

Seafloor hydrothermal deposits in the Okinawa Trough have been regarded as a modern analog of kuroko-type volcanogenic massive sulfide (VMS) deposits on land. VMS deposit is one of the primary producers of base metals (e.g. Cu, Pb, Zn) and precious metals (e.g. Au, Ag). However, owing to difficulties in accessing subseafloor samples/data without costly drilling operations, the spatial distribution of metal contents below the seafloor remains poorly constrained. We apply a combination of four spatial modeling methods: (1) principal component analysis; (2) k-means clustering; and (3, 4) conditional geostatistical simulations of turning bands and pluriGaussian. These modeling methods are based on the whole-rock geochemical data using inductively coupled plasma-quadruple mass spectrometry, together with lithologic log data obtained from onboard visual core descriptions, and X-ray diffraction analyses from the middle Okinawa Trough, Izena Hole, during the cruise CK1605 (Expedition 909) in 2016 by D/V Chikyu. The primary goal is to construct plausible 3D models for the contents of base metals and silver as well as lithotypes. The constructed models successfully map the configuration and zonation of subseafloor metal deposits with hydrothermal flow paths, which sheds light on hydrothermal circulation systems and metal accumulation mechanisms. This approach is shown to be effective for geologic and mineralization modeling and exploration of (sub)seafloor hydrothermal deposits.

Seafloor massive sulfide (SMS) deposits are typical of submarine mineral resources and generally rich in base metals (Cu, Pb, Zn); however, their distribution, configuration, and formation mechanism, especially sub-seafloor mineralization, remain poorly understood because of scant drilling and geophysical data. To address this problem, this study aims to identify and characterize mineralized zones in seafloor hydrothermal areas using limited metal content data from sparse drilling sites. We use principal component analysis to decrease the dimensionality of the content data. High metal content zones are delineated using principal component values by three geostatistical methods: (1) spatial estimation using ordinary kriging; (2) turning bands simulations (TBSIM); and (3) sequential Gaussian simulations. We selected an active seafloor vent area at 1570 m below sea level in the Okinawa Trough, southwest Japan, as a case study. Results from the three methods show two types of high metal content zones: One is around a sulfide mound, and the other is layered in association with lateral flow of hydrothermal fluids from the bottom of the mound. TBSIM is the most effective under scarce data conditions because the model yields the smallest cross-validation error, decreases the smoothing effect, and corresponds well to a conceptual deposit model that shows a stockwork below the sulfide mound. The results contribute to better understanding the formation mechanism of SMS deposits as well as constraining submarine metal reserves and mining.

Although solar activity may significantly impact the global environment and socioeconomic systems, the mechanisms for solar eruptions and the subsequent processes have not yet been fully understood. Thus, modern society supported by advanced information systems is at risk from severe space weather disturbances. Project for solar-terrestrial environment prediction (PSTEP) was launched to improve this situation through synergy between basic science research and operational forecast. The PSTEP is a nationwide research collaboration in Japan and was conducted from April 2015 to March 2020, supported by a Grant-in-Aid for Scientific Research on Innovative Areas from the Ministry of Education, Culture, Sports, Science and Technology of Japan. By this project, we sought to answer the fundamental questions concerning the solar-terrestrial environment and aimed to build a next-generation space weather forecast system to prepare for severe space weather disasters. The PSTEP consists of four research groups and proposal-based research units. It has made a significant progress in space weather research and operational forecasts, publishing over 500 refereed journal papers and organizing four international symposiums, various workshops and seminars, and summer school for graduate students at Rikubetsu in 2017. This paper is a summary report of the PSTEP and describes the major research achievements it produced.

Although the permeability of fractured rock mass is a fundamentally important property for the safe construction of civil and mining engineering structures such as tunnels, in situ characterization of permeability without resorting to hydraulic tests is difficult. For rapid, wide-area estimation, a method that can be conducted at a field-scale using geological and geophysical investigation data is proposed. The method is not based on conventional hydraulic test results. Instead, it combines the stochastic generation of fracture networks with the crack tensor theory. The most important parameter for this method is the fracture length distribution. Although the distribution parameters in the DFN model are assigned through sampling, a bias is generally experienced because of the limited sampling area. To improve the estimation of such parameters, in-situ electrical resistivity data and a symmetric self-consistent method are used to constrain the fracture length distribution. The proposed method is applied to the fractured crystalline rock mass of the Mizunami Underground Research Laboratory (URL) in the Tono area of central Japan. Its effectiveness and correctness are demonstrated through good correspondence of the derived effective permeability with the in-situ measured permeability.

The Atotsugawa fault is one of the most active faults in Japan, and the strain accumulation at the fault is considered to be caused by an aseismic shear zone in the fluid-rich lower crust. To identify the shear zone and investigate the origin of the aqueous fluid in the lower crust, we deployed a Network-MT survey in addition to a conventional wideband-MT survey around the fault and performed an inversion combining both the MT data sets. In the inversion, by modifying a conventional inversion algorism, we accurately represented kilometer-scale dipoles of the Network-MT measurement to provide constraints on the electrical resistivity structure. In the lower crust under the study area, there are localized conductive anomalies below the Atotsugawa fault, the Ushikubi fault, and the Takayama-Oppara fault zone. Comparing our electrical resistivity structure with the seismic velocity structure, we interpreted that the lower-crustal conductors are localized ductile shear zones with highly connected fluid. We considered that the localized ductile shear zones are responsible for the strain accumulation along the respective active faults. In addition, in the mantle wedge above the subducting Philippine Sea slab and its downward extension, a highly conductive portion is detected, which may be attributed to the fluid dehydrated from the Philippine Sea slab and/or the Pacific slab. The existence of the large conductive area supports the suggestion of previous seismic and geochemical studies that the fluid of the lower crust around the Atotsugawa fault originated from subducting slabs.

The magnetotelluric (MT) method has been used for visualizing subsurface resistivity structures and more recently for monitoring resistivity changes. However, electromagnetic data often include cultural noise, which can cause errors in the estimation of MT response functions and subsurface resistivity structure analysis. Frequency-domain independent component analysis (FDICA) offers advantages for MT data processing particularly because this method can extract hidden components in the observed data. These components can be decomposed into natural MT signals and cultural noise so that the noise effect in the recovered MT data is reduced. FDICA is applied to MT data acquired at the Kakioka Magnetic Observatory in Japan. The apparent resistivity and phase curves are obtained with small estimated errors between periods of 7 and 12,000 s, although the length of the time-series data is limited. The curves are smoother than those obtained using a conventional method. Various types of synthetic noise are added to the time series at Kakioka to test the noise-reduction performance of FDICA for MT data with high noise contamination. The results demonstrate that FDICA can be used to estimate MT response functions with high accuracy even under conditions in which more than half of the time series data are contaminated by noise.

Electromagnetic (EM) exploration techniques, as powerful and important geophysical tools, have been extensively used in researches ranging from tectonics and resource exploration to environmental and engineering studies. These tools have also proven their applicability to such emerging fields as ocean and airborne surveys. In this article, we quantitatively and qualitatively review advances and applications of EM-exploration studies in East Asia during the past 20 years. During these last two decades, the field of electromagnetic exploration has grown fast, as shown in the increase in the number of related published articles. These studies focus mostly on the development of system platforms (space, aerial, marine) and on data-processing technologies (inversion algorithms, noise reduction). However, most EM applications have been limited to professional geophysics activities. Along with advances in electronics and sensor technologies, EM-exploration instruments are likely to evolve into a modularized open-access system that will become available to more and more scientists at lower and lower costs.

Spatial gradients in the primary geomagnetic fields directly contribute to both the amplitudes and phases of inter-station transfer functions (IS-TFs). This suggests that, for the analysis of subsurface resistivity structures, IS-TFs should be carefully treated by checking the establishment of the plane-wave assumption. Geomagnetic time-series data include various and complicated characteristics and accordingly, time-frequency domain analysis is suitable for the discussion of spatial gradients of time-varying geomagnetic fields. However, such evaluations are complicated by the huge amount of information contained in the spectrograms from several stations. Therefore, we propose a Multi-Channel Nonnegative Matrix Factorization (MC-NMF) method that can decompose raw spectrograms into several components, allowing the spatial gradient of each geomagnetic temporal variation to be identified. We confirm that such components actually affect the estimation of IS-TFs using data acquired at the Kakioka and Memambetsu magnetic observatories in Japan. We derive the year-to-year changes in IS-TFs from each set of paired stations among Kakioka, Kanoya, and Memambetsu observatories. Although the IS-TFs should exhibit opposite polarities (a negative correlation) when the input and output observatories are swapped; surprisingly, some of them have "identical" polarities. The application of MC-NMF shows that the analyzed geomagnetic data include several components that have various spatial gradients. Although IS-TFs sometimes fail to give the expected implication regarding the spatial gradients of geomagnetic temporal variations, MC-NMF can verify whether the IS-TFs exhibit any spatial gradients. Thus, the use of IS-TFs with MC-NMF can yield better implications regarding subsurface resistivity information.

Although seafloor massive sulfide (SMS) deposits are crucially important metal resources that contain high-grade metals such as copper, lead, and zinc, their internal structures and generation mechanisms remain unclear. This study obtained detailed near-seafloor images of electrical resistivity in a hydrothermal field off Okinawa, southwestern Japan, using deep-towed marine electrical resistivity tomography. The image clarified a semi-layered resistivity structure, interpreted as SMS deposits exposed on the seafloor, and another deep-seated SMS layer at about 40-m depth below the seafloor. The images reinforce our inference of a new mechanism of SMS evolution: Upwelling hydrothermal fluid is trapped under less-permeable cap rock. The deeper embedded SMS accumulates there. Then hydrothermal fluids expelled on the seafloor form exposed SMS deposits.

Unlike in coastal and sedimentary basins, regional-scale exploration of groundwater resources using only geophysical methods is costlier in consolidated rocks such as volcanic rocks and crystalline basement complexes in Africa because of the highly heterogeneous structure of aquifers. Therefore, advanced analysis of remotely sensed images and an accurate assessment of groundwater resources are crucial before carrying out a geophysical prospecting survey. This study proposed a joint analysis of satellite images from optical sensors and synthetic aperture radar (SAR) which aimed to enhance potential mapping accuracy of groundwater resources in crystalline rock areas in a semiarid region. The backscattering coefficient of the SAR data analysis effectively detected the zones of relatively high weathering degree and thus having thick permeable regolith. In addition, a modified clay index calculated from the four band reflectances of the optical sensor imagered, near infrared, and two shortwave infrared bandswas applied to discriminate clay-rich zones from high vegetation activity zones. The clay-rich zones detected corresponded with the highly weathered zones estimated from the small SAR backscattering coefficients. The zones also corresponded with a large density of faults and lineaments and furthermore were verified by high potential yields from groundwater wells. The thickness of weathered zones was likely to increase with a decreasing backscattering coefficient and higher modified clay index values. Conversely, large backscattering coefficients in the narrow zones along the major lineaments from large volumetric scattering because of high vegetation activity, as confirmed by the large vegetation index values, suggested that high moisture content was retained in the soils. In fact, the potential yields of the groundwater wells tended to increase near the lineaments. Accordingly, shallow groundwater occurrence is plausible in those zones.

Fracture distribution in bedrock has a great influence on the groundwater flow, whose characteristics should be understood for resource exploration and deep geological repository and so on. Previous studies have been attempting to grasp the hydraulic characteristics on a local scale using boreholes and so on. However, it is considered to be difficult to grasp the hydraulic permeability without drillings. In this study, we tried to estimate the permeability of rocks in deep underground from physical property values like resistivity and Young's modulus in order to grasp macroscopic hydraulic characteristics of rock including fracture. The validity of the present estimation method was examined, by comparing this estimated value with the measured value. The resistivity and Young's modulus of exposed granitic rock mass at depth of 300-500m were measured in the underground research gallery at the Mizunami Underground Research Laboratory, Japan Atomic Energy Agency in 2017. We estimated the permeability using a rock physics model. The permeability was also measured in 2015 along the same gallery, therefore compared with the estimated permeability from our geophysical measurements.

An excavation damaged zone (EDZ) develops under an excavation-induced redistributed stress in a fractured rock mass. Natural fractures further develop and new fractures are formed due to a tunnel excavation. As one of the intrinsic properties of a fractured rock mass, seismic velocity such as P-wave velocity will change when a tunnel is excavated in fractured rock mass. To our knowledge, there is limited research on seismic velocity change due to tunneling in a fractured rock mass. A novel numerical study of seismic velocity (such as P-wave velocity) change due to a tunnel excavation in a fractured rock mass is carried out in this paper. The stress in a fractured rock mass will redistribute after a tunnel is excavated. Fracture apertures change accordingly. Using discrete fracture network (DFN) modeling approach, fractures are generated in a rock mass. Fracture lengths, orientations, apertures, etc. are mathematically represented. Based on empirical equations, fracture apertures under the in-situ stress (before and after a tunnel excavation) can be calculated. To verify our proposed method, a case study is carried out. It shows that our proposed method can be used to estimate seismic velocity change due to a tunnel excavation in a fractured rock mass.

Exploring for groundwater in crystalline rocks in semiarid areas is a challenge because of their complex hydrogeology and low potential yields. An integrated approach was applied in central western Mozambique, in an area covered by Precambrian crystalline basement rocks. The approach combined a digital elevation model (DEM), remote sensing, and a ground-based geophysical survey. The aim was to identify groundwater zones with high potential and to identify geological structures controlling that potential. Lineaments were extracted from the DEM that had been enhanced using an adaptive-tilt, multi-directional, shading technique and a non-filtering technique to characterize the regional fracture system. The shallowness and amount of stored groundwater in the fracture zones was assessed using vegetation indices derived from Landsat 8 OLI images. Then, 14 transient electromagnetic (TEM) survey profiles were taken in different geological settings across continuous lineaments that were considered to be aligned along inferred faults. In the central lineament zones, the TEM soundings gave resistivity values of less than 300 Omega m at a depth of 20-80 m. The values varied with location. Conversely, values greater than 400 Omega m were observed at the sites away from the central zones. This contrast is probably caused by the differences in permeability and degree of weathering along the fractured zones. These differences could be key factors in determining groundwater occurrence. By integrating five water-related factors (lineament density, slope, geology, vegetation index, and proximity to lineaments), high groundwater potential zones were located in the vicinity of the lineaments. In these zones, vegetation remains active regardless of the season.

On 29 June 2015, a small phreatic eruption occurred at Hakone volcano, Central Japan, forming several vents in the Owakudani geothermal area on the northern slope of the central cones. Intense earthquake swarm activity and geodetic signals corresponding to the 2015 eruption were also observed within the Hakone caldera. To complement these observations and to characterise the shallow resistivity structure of Hakone caldera, we carried out a threedimensional inversion of magnetotelluric measurement data acquired at 64 sites across the region. We utilised an unstructured tetrahedral mesh for the inversion code of the edge-based finite element method to account for the steep topography of the region during the inversion process. The main features of the best-fit three-dimensional model are a bell-shaped conductor, the bottom of which shows good agreement with the upper limit of seismicity, beneath the central cones and the Owakudani geothermal area, and several buried bowl-shaped conductive zones beneath the Gora and Kojiri areas. We infer that the main bell-shaped conductor represents a hydrothermally altered zone that acts as a cap or seal to resist the upwelling of volcanic fluids. Enhanced volcanic activity may cause volcanic fluids to pass through the resistive body surrounded by the altered zone and thus promote brittle failure within the resistive body. The overlapping locations of the bowl-shaped conductors, the buried caldera structures and the presence of sodium-chloride-rich hot springs indicate that the conductors represent porous media saturated by high-salinity hot spring waters. The linear clusters of earthquake swarms beneath the Kojiri area may indicate several weak zones that formed due to these structural contrasts.

Data processing techniques are often used to estimate the noise-free response of marine controlled-source electromagnetic (CSEM) data and magnetotelluric transfer functions. We have implemented a new CSEM data processing scheme that uses a robust method based on independent component analysis (ICA) to extract interpretable datasets from noisy marine CSEM data. We applied the data processing scheme to signals from a new CSEM observation system comprising a remotely operated vehicle (ROV) and an ocean bottom electromagnetometer (OBEM). These datasets were obtained around the Iheya North hydrothermal field, Okinawa Trough, Japan. The observation system allows a small-scaleCSEMsurvey to be conducted in areas of steep topography, such as hydrothermal fields, because the ROV can deploy the OBEM at the exact observation site. The results show that the coherent and environment noise that exists in the raw time series is reduced sufficiently by ICA processing. It makes interpretation of the resulting electric field data possible. The results also show that the processed data has a higher signal-to-noise ratio in the middle-to-high-frequency band than the data without ICA. The normalised spectrum, obtained by normalising the observed data from the hydrothermal area, indicates that a conductive anomaly exists in the near-offset area around the OBEM. We apply 2D inversion to the electric field data and find that a low resistivity body exists beneath the OBEM and 50 m offset from the OBEM. This resistivity structure is consistent with images taken by the ROV that show characteristic organisms in hydrothermal seepage around the OBEM site.

Shiretokoiozan volcano in northern Japan is well known for its eruptions, which eject huge amounts of molten sulfur. Watanabe (1940) reported details of the 1936 eruption, but its mechanisms, and how and where the huge amount of sulfur is produced and pushed out remain unknown. The aim of this study is to elucidate the near-surface underground structure of this area and the mechanisms of the molten sulfur eruption. We implemented aerial photographic observations, geological surveys, hot spring analysis, Self-Potential survey and DC resistivity surveys at the western flank of Mt. Shiretokoiozan. The geology of this area is mostly composed of hydrothermally altered boulders, gravels, sand, and clay. Some areas of fumaroles are covered by sulfur cement. Chemical analyses revealed that SW and Cl are rich in hot water, which imply an area with upwelling hot water/gas below the surface. Results of DC resistivity surveys conducted at several sites show extremely low resistivity, suggesting an aquifer several meters below the surface. Compiling this evidence, we infer a possible mechanism of molten sulfur eruption: the sulfur has been produced and stored in an aquifer located at the eastern hill from Crater I for several decades by chemical reactions of volcanic gases; it gushes out when volcanic activity becomes high. (C) 2017 The Authors. Published by Elsevier B.V.

The Bayonnaise knoll, an active submarine volcano belonging to an actively rifted part of the Izu-Bonin volcanic arc, exhibits hydrothermal ore deposits on its caldera floor in a region known as the Hakurei Sulfide Deposit (HSD) area. We observed the HSD area using high-resolution acoustic observation equipment consisting of multibeam echo sounder (MBES), sidescan sonar (SSS), and sub-bottom profiler (SBP) systems, on the AUV Urashima. We used visual and acoustic results to examine the consistency of the HSD area extent and to consider possibilities of other ore areas within the caldera. The resultant high-resolution acoustic imageries suggest expansion of the HSD area to the northeastern caldera wall and the southwestern sub-seafloor of the caldera floor. The SBP data show a thick sediment layer on the western part of the caldera floor where many high-backscattering signals were observed. Small chimney-like features were acoustically observed in the HSD area and also at the central cone and along the rim of the caldera. However, most are remnant features of ancient volcanic activity of the knoll, and thus may not indicate current hydrothermal deposits. Acoustic investigations such as this, along with appropriate interpretation, are very useful to determine the detailed distribution of ore on the seafloor and at the shallow subsurface, and should be an effective tool for regional site surveying before seabed mineral mining.

Abrupt changes of geomagnetic field can make large induced electric field and resultant electric current on the earth, which is called as geomagnetically induced current (GIC). It can yield damages to pipelines, cables, and other architectures. For understanding the phenomena and future risks of GIC, it is necessary to evaluate how the sub-surface electrical conductivity structure is important for the GIC because the heterogeneous conductivity structure in the crust and mantle affects the induced electrical current locally. The hazard prediction based on the homogeneous earth may result in the underestimation. Here, I introduce possible cases of geomagnetically induced electric field (GIE) on seafloor and near coastal areas, based on numerical forward simulations on one-, two-, and three-dimensional (1-D, 2-D, and 3-D) earth's structure including the sea layer. On the 1-D case, I show the possible amplitude of GIE on the seafloor, far from the coastal area. The second case study comes from 2-D forward simulation, in which the straightly elongated coastal line is assumed, and various sub-surface and sub-seafloor conductivity structures are imposed. The numerical results suggest that the amplitude of GIE on land becomes more than two times larger than that of the homogeneous earth without the sea layer. The width of land zone with larger GIE is about 20 km from the coast. In forward modeling with a simplified 3-D bathymetry, land electric field near the bay area increases with about ten times larger than that of the inland one. The seafloor GIE near the peninsula area also indicates about four times larger value than that of the other area at the same water depth. These phenomena can be explained by the boundary charge along the coastal area. I conclude that 3-D earth's conductivity structure including the realistic bathymetry and sub-surface and sub-seafloor structures should be essential and focused for the hazard assessment of GIC.

We present a Hamiltonian particle method (HPM) with a staggered particle technique for simulating seismic wave propagation. In the conventional HPM, physical variables, such as particle displacement and stress, are defined at the center, i.e., at the same position, of each particle. As most seismic simulations using finite difference methods (FDM) are practiced with staggered grid techniques, we know the staggered alignment of space variables could improve the numerical accuracy. In the present study, we hypothesized that staggered technique could improve the numerical accuracy also in the HPM and tested the hypothesis. First, we conducted a plane wave analysis for the HPM with the staggered particles in order to verify the validity of our strategy. The comparison of grid dispersion in our strategy with that in the conventional one suggests that the accuracy would be improved dramatically by use of the staggered technique. It is also observed that the dispersion of waves is dependent on the propagation direction due to the difference in the average spacing of the neighboring two particles for the same parameters, as is usually observed in FDM with a rotated staggered grid. Next, we compared the results from the conventional Lamb's problem using our HPM with those from an analytical approach in order to demonstrate the effectiveness of the staggered particle technique. Our results showed better agreement with the analytical solutions than those from HPM without the staggered particles. We conclude that the staggered particle technique would be a method to improve the calculation accuracy in the simulation of seismic wave propagation.

A Hamiltonian particle method (HPM), which is one of the mesh-free methods, can simulate seismic wavefields for models including surface topography in a simple manner. Numerical error caused by a curved free surface or by particles not aligned with the surface is not obvious in HPM. In general, the accommodation of irregular free surfaces requires more grids or particles in a minimum wavelength for achieving sufficient accuracy in the simulation. We tested the accuracy of HPM with staggered particles for simulating seismic-wave propagation including the surface topography, and we established the relationship between desired accuracy and spatial resolution. We conducted numerical simulations for models with a planar free surface aligned with the regular particle alignment and a dipping free surface. Our accuracy tests revealed that the numerical error strongly depends on the dipping angle of the slope. We concluded that about 25 particles in a minimum wavelength are required to calculate Rayleigh waves propagating along the irregular topography with good accuracy. Finally, we simulated Rayleigh wave propagation along irregular topography using a layered model with a hill. HPM can reproduce not only surface-wave propagation but also the reflected and refracted waves. Our numerical results were in good agreement with those from a finite-element method. Our investigations indicated that HPM could be a solution to simulate Rayleigh waves in the presence of complex surface topography.

We apply a Hamiltonian particle method, one of the particle methods, to simulate seismic wave propagation in a cracked medium. In the particle method, traction free boundaries can be readily implemented and the spatial resolution can be chosen in an arbitrary manner. Utilisation of the method enables us to simulate seismic wave propagation in a cracked medium and to estimate effective elastic properties derived from the wave phenomena. These features of the particle method bring some advantages of numerical efficiencies (e.g. calculation time, computational memory) and the reduction of time for pre-processing.We describe first our strategy for the introduction of free surfaces inside a rock mass, i.e. cracks, and to refine the spatial resolution in an efficient way. We then model a 2D cracked medium which contains randomly distributed, randomly oriented, rectilinear, dry and non-intersecting cracks, and simulate the seismic wave propagation of P- and SV-plane waves through the region. We change the crack density in the cracked region and determine the effective velocity in the region. Our results show good agreement with the modified self-consistent theory, one of the effective medium theories. Finally, we investigate the influence of the ratio of crack length to particle spacing on the calculated effective velocities. The effective velocity obtained becomes almost constant when the ratio of crack length to particle spacing is more than similar to 20. Based on this result, we propose to use more than 20 particles per crack length.

Sonic logging has been widely used for many years to acquire physical properties of formations in the vicinity of borehole penetrating hydrocarbon reservoirs. For acquiring physical properties of fluid in the formation, Biot physics or poroelastic analysis would be the best method. Among the available technologies, quality factors based on the Biot's equation could be used to estimate fluid viscosity from the matrix-fluid friction. Although earth materials are attenuating any elastic waves travelling through, Biot physics does not consider the effect of solid friction in the matrix. In the present study, The solid friction is included in our model using a filter for frequency-independent quality factor (constant-Q). Adding the solid friction we investigate if we take the effect of the viscous attenuation from the acquired quality factor, Based of the result, we then try to estimate the locations of fluid contact using both compressional and shear attenuation factors. The results of our numerical simulations indicate the difference of these factors could be a good indicator of fluid contacts even in attenuating media.

For the estimation of the propagation of fractures hydraulically induced, in-situ stress, the strike of preexisting fractures, and the viscosity of injected fluid should be taken into consideration. We performed a series of simulations for hydraulic fracturing in naturally fractured rock by using a 2D flow-coupled DEM code to examine the influence of the strike of natural fractures around a borehole on the propagation direction of fractures, and the influence of injection fluid viscosity on fluid intrusion. In this study, a low viscosity fluid and a high viscosity fluid are used, and the fracture intersects with a single natural fracture with three different angles of strike. When the angle of strike is low, the orientation of induced fracture does not converge in the maximum stress direction. Our results indicate: (i) the rectilinearity in the fracture propagation direction could be loosened as the angle between the fracture and principal stress orientations, and (ii) the use of viscous fluid for hydraulic fracturing could minimize the influence of pre-existing fractures.

Scaling is a key phenomenon for understanding of subsurface or sub-seafloor hydrothermal circulation. Our goal in this study is to evaluate the importance of both chemical kinetic and hydrodynamic effects on silica deposition. For qualitative and quantitative discussion, we estimate the amount and the distribution of silica deposition with these two processes based on the numerical simulation, and compare with the data from a laboratory or a field experiment. We solve the fluid, temperature and the dissolved silica concentration field by using the lattice Boltzmann method. From our simulation results, it is found that the effects of the hydrodynamic process are very important to reproduce the growth of scale qualitatively, whereas the simple chemical kinetic deposition could not sufficiently contribute to the real silica deposition. It is, therefore, necessary to emphasize the hydrodynamic effect should be take into account for reproducing silica scaling.

In this study, we developed a code to deal with two kinds of three-dimensional (3D) inversion schemes for the analysis of self-potential data (SP). One is for the transient SP data under saturated condition, and the other is for the SP data under steady state with saturated and unsaturated condition. The inversion program for the former is tested with the synthetic transient SP distribution and pumping data. Our inversion can estimate both specific storage and hydraulic conductivity structure, simultaneously. The inversion result of the hydraulic conductivity from the transient SP data has high resolution comparing to the estimated result from the SP data under the steady state. The latter is developed to evaluate the effect of the unsaturated zone on the inverted image. Our inversion results show that the unsaturated zone would become the cause of some artifacts in the image by the inversion without consideration of unsaturated zone, and the consideration of unsaturated zone is essential for accurate estimation of hydraulic structures.

We evaluated three-dimensional (3D) resistivity structure using marine controlled-source electromagnetic (mCSEM) data obtained around hydrothermally active Iheya north area, offOkinawa, Japan. To handle complex submarine topography around the target, we discretized Maxwell's equations using a particle-based method. This method has an advantage over the other methods in terms of numerical model can be flexibly and easily formed with arbitrary topography shape even in 3D modeling. We developed a new mCSEM inversion based on this particle-based forward simulation. Applying our new inversion scheme to the mCSEM data acquired in Iheya north area, we revealed a conductive anomaly under the subsurface in the northern area, which cannot be discovered by the seafloor camera observations. This conductive anomaly suggests the existence of buried hydrothermal fluid chamber or ore deposits (massive sulphide). We conclude that mCSEM method has potential to evaluate sub-seafloor resistivity structure around hydrothermal area.

Coda-Q is a stochastic parameter reflecting the heterogeneities of medium that seismic waves travel through. We confirmed that coda-Q would vary with the stress loaded to an elastic medium using numerical simulations of seismic wave propagation. When the stress is loaded, cracks in the crust could either close or newly open. The closure and opening of the cracks are not random but depending on the magnitude and the direction of the stress and the crack aspect ratio. The cracks in the medium after loading stress could be aligned in a specific orientation, and elastic wave velocity field would become anisotropic due to the alignment of specific crack orientations. Elastic wave velocity is in general faster along the direction corresponding with the crack orientation while slower along the perpendicular direction. In the numerical simulation, the effect of anisotropy in elastic wave velocity field due to the selective closure and opening of the cracks is calculated using a 2-D finite difference method assuming elastic wave velocity to be a function of the magnitude of loaded stress. The coda-Q calculated from seismic waves simulated for a model varies when the averaged normal stress changes. Our simulation indicated that the sensitivity of coda-Q(-1), that is the reciprocal of the coda-Q, would be 1.0 x 10(-2) (1.0 MPa-1) against the magnitude of the confining pressure and 1.0 x 10(-3) (1.0 deg(-1)) against the direction of principal stress. We would like to conclude that coda-Q, a stochastic parameter reflecting heterogeneities of subsurface medium, could become a quantitative state indicator of the stress field of the medium where seismic waves propagate through. Spatiotemporal variation of coda-Q reflects change in the stress field in the crust.

Reflection coefficients and arrival times, together with seismic velocities, are significantly important for possible evaluation of reservoir properties in exploration seismology. Reflectivity inversion is one of the robust inverse techniques used to estimate layer properties by minimising misfit error between seismic data and model. On the other hand, the layer-stripping method produces subsurface images via a top-down procedure so that a given layer is modelled after all the upper layers have been inverted. In this paper, we have combined these two methods to develop a new random layer-stripping scheme which first determines the reflectivity series via a random-search algorithm and then estimates P-wave velocities. The first step can be viewed as a variant of sparse spiking deconvolution, and the second step is accomplished by considering empirical relations between density and P-wave velocity. The method has been successfully applied to Marmousi synthetic data to examine dipping reflectors and velocity gradients, and it has been found to be quite reliable for analysing complex structures. A comparison with minimum entropy deconvolution showed that our inversion algorithm gives better results in detecting the amplitudes and arrival times of seismic reflection events.

The failure of brittle materials, for example glasses and rock masses, is commonly observed to be discontinuous. It is, however, difficult to simulate these phenomena by use of conventional numerical simulation methods, for example the finite difference method or the finite element method, because of the presence of computational grids or elements artificially introduced before the simulation. It is, therefore, important for research on such discontinuous failures in science and engineering to analyze the phenomena seamlessly. This study deals with the coupled simulation of elastic wave propagation and failure phenomena by use of a moving particle semi-implicit (MPS) method. It is simple to model the objects of analysis because no grid or lattice structure is necessary. In addition, lack of a grid or lattice structure makes it simple to simulate large deformations and failure phenomena at the same time. We first compare analytical and MPS solutions by use of Lamb's problem with different offset distances, material properties, and source frequencies. Our results show that analytical and numerical seismograms are in good agreement with each other for 20 particles in a minimum wavelength. Finally, we focus our attention on the Hopkinson effect as an example of failure induced by elastic wave propagation. In the application of the MPS, the algorithm is basically the same as in the previous calculation except for the introduction of a failure criterion. The failure criterion applied in this study is that particle connectivity must be disconnected when the distance between the particles exceeds a failure threshold. We applied the developed algorithm to a suspended specimen that was modeled as a long bar consisting of thousands of particles. A compressional wave in the bar is generated by an abrupt pressure change on one edge. The compressional wave propagates along the interior of the specimen and is visualized clearly. At the other end of the bar, the spalling of the bar is reproduced numerically, and a broken piece of the bar is formed and falls away from the main body of the bar. Consequently, these results show that the MPS method effectively reproduces wave propagation and failure phenomena at the same time.

Coda-Q is a stochastic parameter depending on the heterogeneities of a medium in which seismic waves travel through. Coda-Q is calculated from attenuation ratio of a coda wave. Seismic waves are attenuated by three causes i.e., geometric spreading, scattering and intrinsic attenuation. The coda-Q is affected by both scattering attenuation and intrinsic attenuation. In this study we examine the coda-Q change due to the change in crack condition in an inelastic model. We simply assume an inelastic medium which has uniform background intrinsic attenuation and cracks which scatter a seismic wave. The cracks make scattering attenuation. We change the condition of the cracks (i.e., density and orientation angle of the cracks), and observe the change in the coda-Q. Through numerical experiments, it is revealed that the change in the crack condition can be detected from the coda-Q if the intrinsic attenuation is relatively small. We conclude that temporal change in a state of cracks during the hydraulic fracturing for shale gas, stress accumulation process for earthquake, etc. can be detected through coda-Q, given the intrinsic attenuation is relatively small and constant.

Self-potential (SP) method is one of geophysical methods expected useful to estimate the property of groundwater flow. The effect of permeability anomaly on SP profile has sometimes been discussed, but neither the feasibility of SP measurement to estimate the permeability structure nor the resolution of inversion result of SP profile has been well considered. In this study, we developed a two-dimensional inversion code for the estimation of permeability structure from synthetic SP profile on the surface of predefined permeability structure to evaluate the resolution of our SP inversion. Four models including the permeability anomalies located in the center of slope model are used for the evaluation of the performance of our inversion. A priori data of the distribution of streaming current coefficient, electrical conductivity and the flux volume at the discharge and recharge are given to our inversion. The horizontal zone with high permeability and the vertical zone with low permeability can be reconstructed with our inversion properly. However, the horizontal low permeable and the vertical high permeable zone cannot be imaged clearly. The regional groundwater flow pattern around the permeability anomaly has the great effect on the SP pattern on the surface. As our conclusion, the consideration of flow pattern around the permeability anomaly and effect on the SP profile are necessary for the accurate inversion of SP data.

This study investigates the effectiveness and the applicability of full waveform inversion (FWI) method to estimate underwater sound velocity structures. We use a frequency domain full waveform inversion method in this study. We use an adjoint-state method for the calculation of the gradient in an iterative inversion based on a preconditioned conjugate gradient method. We first apply the FWI method to a synthetic dataset that is simulated for a sound velocity structure. The results of our inversion are then compared with those from a conventional ray-based traveltime inversion (TTI) method to evaluate the effectiveness of the method. The results show that the full waveform inversion method could provide more precise image with higher resolution than the ray-based method. The FWI method is also applied to a field dataset acquired by a vertical-cable-seismic (VCS) data acquisition experiment in Lake Biwa. In spite of very limited raypath condition using only direct arrival wave, the full waveform inversion method could reproduce a horizontally stratified velocity structure whose vertical profile showed the existence of a seasonal thermocline in the lake that was to be confirmed by temperature measurements after the VCS experiment. We conclude that the FWI method could be the key success factor for the higher resolution at estimation of underwater sound velocity structure. © 2013 SEG.

Measured electromagnetic fields in marine controlled source electromagnetic (CSEM) method always have some uncertainties to some extent. One is the structure in the subsurface and the other is the source signature that could have uncertainties due to the environmental conditions around the source. We hypothesized that the perturbation in the source waveform would be estimated using the backpropagation of anomalous electric fields and tested the hypothesis. We first developed an electromagnetic full waveform inversion that could deal with both the conductivity of the subsurface and the source waveform simultaneously. We compared the simultaneous inversion with a conventional inversion for a synthetic data. The synthetic example shows that even if the amplitude of an initial source waveform is erroneously underestimated with 5%smaller than true source waveform, we could estimate the source waveform employing this simultaneous inversion algorithm. We also find that the obtained conductivity structure of the subsurface from the simultaneous inversion is appropriate than that from the conventional inversion. From numerical results, we conclude that it is realizable to estimate the unknown perturbation of the source parameters.

It is well known that the hydraulic fracturing is a tool commonly used for stimulating hydrocarbon reservoirs and that the orientation and the propagation length of fractures created by hydraulic pressure influenced in by in-situ stress field. It is, however, difficult to predict the behavior of fracture propagation from boreholes in a medium under regional stress due to a lack of numerical schemes to simulate rock failures. In order to solve this problem of hydraulic fracturing, we have developed a program to simulate fracture propagation from a borehole due to increasing fluid pressure using an extended finite difference method (X-FEM), which deals with any fractures independent from grid or mesh for the numerical simulation. Numerical simulations are conducted for a 2D elastic medium having a borehole and a fracture. We first confirmed that our program could simulate the stress distribution whose local stress field near the borehole showed some deviated orientation from the regional stress field. We then confirmed that the tendency of fracture propagations to be a function of fluid pressure to induce the extension of fracture. The orientation of the fracture propagation converges to that of the principal stress. However, the higher the fluid pressure is, the smaller the curvature of fracture trace becomes. We would like to conclude that the orientation of maximum in-situ principal stress and the fluid pressure for fracturing is a major parameters to control the propagation of fractures due to increasing fluid pressure.

We propose to use an approximately vertical bipole electric current towed by a ship as a source for a Magnetometric Resistivity (MMR) method. This proposal requires the precise positioning of the bottom electrode for the bipole source, and our newly developed MMR system achieved this. We conducted an MMR experiment in the central Mariana Trough, and we obtained data using two different methods along a survey line: one method towed a bipole source transmitting continuously along the survey line, and the other used a conventional vertical bipole source transmitting at several stationary transmission stations along the survey line. We found that the towed bipole source tilted from the vertical by an angle of 8 degrees at the maximum during the MMR experiment. We compared the results from the two methods to evaluate the towed bipole source method. Our results indicate that the tilted bipole source approximates well with the vertical bipole source at the mid-point between the surface and the bottom electrodes. Since the towed bipole source method requires much less survey time and the results show a higher spatial resolution, it is a powerful tool for MMR experiments to image a shallow oceanic crustal resistivity structure efficiently.