We proposed a procedure to estimate Love wave phase velocities based on rotational motions obtained from a triangular array of ordinal three-component seismometers. Rotational motions depends on only SH wavefiled, including SH and Love waves. Thus, phase velocities of the rotational motions of microtremors are expected to be same as ones of Love waves. Spatial derivatives to calculate rotational motions were approximated from the first-order Taylor series expansion of observed array records. Synthetic validation test shows that the rotational motions estimated from both of the SH-wave field synthetic microtremors and the full wave (P, SV and SH) synthetic ones agree very well. This agreement indicates that this approach well estimates rotational motions from the observed microtremor records. We calculated rotational motions of a triangular array configured with combinations of the three stations in the double triangular array of 6 stations (without the center point of the array). Phase velocities were obtained from the array rotational motions by using SPAC method. The obtained phase velocities agree well with the theoretical Love wave phase velocities. Expected amplitudes of rotational motions observed on the array of common size were estimated to be enough large for the typical microtremor observation system.

Thanks to the recent development, distributed acoustic sensing (DAS), which utilizes optical fibers deployed in a well as permanent multi-channel receivers continuously distributing along the wellbore, has attracted geoscientists’ interests as a potential alternative to conventional point sensor such as geophone or hydrophone for a long-term surface and subsurface monitoring. Geological Carbon Dioxide Storage Technology Research Association conducted a large-scale field experiment of vertical seismic profile (VSP) survey using DAS technology in August 2017. A producing well was selected as the observation well for the 2D walkaway DAS-VSP, which total depth was more than 4,400 m in total depth and has 45 degree of wellbore deviation. We also recorded background noise during nighttime when active shooting by vibroseis trucks was suspended, in order to make the best use of the valuable opportunity of evaluating DAS technology. The main purpose of the passive monitoring is to evaluate DAS background noise level, but we were also motivated to evaluate whether DAS measurement was capable of recording small and natural earthquake. During the passive recording period, an anomalous event was observed, and the comparison of recorded waveform and other noise events concluded that the anomalous event is caused by earthquake event. Referring to the earthquake table compiled by Japan Meteorological Agency, it can be identified that the event relates to M2.3 earthquake occurred at Kamo, Niigata, on 27th Aug. 4:15 (Japan Standard Time). We also compiled wave records by high sensitivity borehole seismic monitoring network from nearby the observation well. Considering the noise floor of DAS measurement and comparing Hi-net data of other earthquake events, it was concluded that only the event has larger amplitude than DAS noise floor and the amplitude of other earthquakes are less than DAS detectable level. Although there are still several issues to be overcome in earthquake monitoring by DAS technology, it must be emphasized that small earthquake can be detectable by DAS that has several advantages over conventional sensors.

Estimating the underground water distribution by means of geophysical explorations (e.g., seismic and electromagnetic explorations) is essential to develop fluid resources (e.g., geothermal fluid). To improve the accuracy of data that were obtained from geophysical explorations, we should experimentally investigate the physical properties experimentally. In this study, we conduct a fluid-flow test on a rock core sample to elucidate the relationship between complex resistivity, elastic wave, and water saturation of the geothermal reservoir rock. The reservoir rock core sample (andesite) contains an initial macro crack, and we further induced thermal cracks in the rock core sample. The fluid-flow test was conducted using this sample (10.5% of porosity) under a confining pressure of 20 MPa and a temperature of 25 °C. In this test, initially, the sample was filled with nitrogen gas under 20 MPa of confining pressure. The gas is observed to emulate the superheated steam that is observed in geothermal fields. Further, brine (1wt%-KCl, 1.75 S/m), which emulates the artificial recharge into the reservoir, is injected into the sample. During the test, water saturation, permeability, complex resistivity (in the frequency range of 10^－2–10⁵ Hz), and elastic wave (P-wave, 250 kHz) are measured. As a result, complex resistivity was observed to dramatically decrease from 10⁴ to 10² Ωm owing to brine injection. After starting the brine injection, complex resistivity decreased continuously with the increasing water saturation. The relationship between complex resistivity and water saturation exhibited almost no frequency dependence. P-wave velocity almost remained constant against the increasing water saturation, whereas the amplitude of P-wave decreased continuously with the increasing water saturation. These results indicate that complex resistivity varies with minor changes in water saturation, whereas P-wave velocity does not depict any variation. In other words, complex resistivity can be potentially used to monitor the changes in water saturation in geothermal reservoirs. Based on the changes in complex resistivity, P-wave velocity, and amplitude against water saturation, the two−phase (nitrogen−brine) fluid flow form of in the sample is suggested to be divided into two stages.

3D velocity structure model for evaluating strong earthquake motion is constructed from geophysical and geological data such as microtremor array, seismic survey, gravity survey and borehole data which contain information about S-wave and P-wave velocity, density and layer thickness. We developed a joint inversion program as a tool to integrate these geophysical and geological data and build a 3D velocity structure model. To analyze different kinds of survey data simultaneously, we introduced a new frame work of inversion model which employs global and local models in parallel, and formulated a non-linear least squares joint inversion. In addition, P-wave velocity and density are handled as functions of S-wave velocity to reduce unknown parameters. Since it is important to set an appropriate weighting factor for each survey data in the joint inversion, we firstly examined a simple normalizing method using L2 norm of a row vector of Jacobian matrix. Further, to equalize the difference of noise levels among different survey data, we applied ABIC minimizing method using Simulated Annealing technique (SA) and estimated an optimum combination of weighting factors. As the result of simulation, we confirmed that these two normalization methods are effective to choose appropriate weighting factors. Next, we compared accuracy of six solutions inverted from different combinations of survey data. The simulation results showed that model accuracy is increased by the joint inversion using multi types of survey data. Further, resolution matrix analyses for these results showed that increases of data information provided by different types of survey data improve uniqueness of solutions. Finally, to discuss convergence of a solution from the perspective of RMS residuals, we mapped distributions of RMS residuals on a 2D curved surface that is extracted from a multi-dimensional parameter space using a newly developed method. With those distribution maps, we understood visually improvements of accuracy and uniqueness of a solution and initial model dependency. The joint inversion technique is a powerful and effective tool to build a 3D velocity model. Further development of this method will be expected.

The thermal resistance of soil around underground power transmission lines is an important parameter affecting transmission capacity. Thermal resistance of soil is conventionally measured by inserting a heating element and a thermocouple into a borehole. Because this method is expensive, the thermal resistance is often assumed to be the standard value (80 ℃cm/W). However, this is a conservative estimate, which inadvertently leads to the condition of cables that have a large diameter than is required. Kubota et al. (2015) and Suzuki et al. (2015) suggested a method to estimate the thermal resistance from resistivity data using an empirical formula and artificial soil samples. However, it is important to use the measured resistivity and thermal resistance of actual soil samples surrounding the survey lines. In this study, we propose a new soil thermal resistance model composed of sand particles, clay particles, pore water, and air which was based on the unconsolidated sand model (Avseth et al., 2005). The theoretical thermal resistance derived by this model is comparable to the saturation–thermal resistance relationship measured by laboratory tests performed on artificial unconsolidated soil samples. Additionally, we propose a method to estimate the thermal resistance profile by combining the S-wave velocity–porosity (Avseth et al., 2005) and resistivity–porosity relationships (Katsube and Hume, 1983). This method was applied to geophysical survey data obtained using the surface wave and the electrical methods in the field where underground power transmission lines are planned. Thermal resistance profile obtained using integrated geophysical data matches the values measured directly in the borehole drilled along the survey line. In conclusion, this method is suitable for use in designing power transmission cables.

The geometry/angle of the deeper extension of active fault is one of the fundamental information to evaluate the magnitude of inland earthquake associated with active faulting. The information is mainly revealed by geophysical exploration that is executed across the active fault. Although the geophysical explorations were scarcely applied for strike-slip fault in Japan, we discuss the applicability of several geophysical explorations, seismic reflection and refraction survey, CSAMT (controlled source audio-magnetotelluric) survey and gravity survey for strike-slip fault in this study. The study area is located in northwest area in Kinki region in Japan, and Go-mura fault zone and Yamada fault zone are distributed in a granite area (Miyazu granite). In the seismic reflection profile, discontinuity and inflection of reflector shows the deeper geometry of active fault in 200-300 m below the surface of the ground. In some profile, discordance between subsurface structure and slip direction of the active fault were shown. Hence, the comparison between seismic reflection profile and other geophysical explorations is important in the interpretation of subsurface geometry of the active fault. Although, the seismic refraction image cannot define the strict angle of the active fault in principle, the image well indicates the low-velocity layer crushed by the strike-slip faulting. In the CSAMT survey, low-specific resistance area extended in depth clearly shows the geometry and angle of deeper part of active fault. In some image, pseudo-specific resistance can be seen in some CSAMT image, comparison to other information of geophysical survey is necessary for interpretation. Because the vertical displacement is not dominant in strike-slip faulting, the gravity survey across the strike-slip fault is generally insufficient method to detect the fault. Dense spacing of gravity stations and precise measurement allow sensing the Bouguer gravity anomaly associated with strike-slip faulting.

In recent years, many diving surveys for acquiring submarine bathymetric, geophysical, biological and chemical data and offshore drillings have been conducted for researches on marine mineral resources. Therefore, practical methods to comprehensively analyze these survey data are expected in order for effective exploration and development of marine mineral resources. JAMSTEC has been developing useful tools for integrated interpretation of the data based on earth scientific aspects by comparing and referring to the acquired data and geological information. As a part of this program, the authors have started to compile geographical and geological information of the survey sites using video image data acquired with the manned submersible "Shinkai 6500", the remotely operated vehicle "Hyper Dolphin" and the Deep-Tow system at the Mid-Okinawa Trough. The compiled data are imported into GIS so as to easily search the existing geo-information around the site, to plan future surveys, and to discover new research themes. As an example of utilization of the GIS-based data, the authors created a geological information map that can be effectively utilized for analysis and interpretation of survey data, through grasping the positional relationship, continuity, distribution of seabed topographic/geologic features and hot water/spring water, and chimney. By integration with other drilling and geophysical data acquired by various research institutions, the database created in this study can provide detailed three-dimensional geological information around the target area, which can be effectively used for marine mineral exploration.

Ultra-high resolution 3D seismic survey (UHR3D) is a novel concept of three-dimensional seismic survey which is characterized by very short cables (< ~100 m), high frequency seismic sources (> ~500 Hz), high density shot and receiver intervals (< ~5 m), and a very small bin size (< ~5 m crossline). This compact system enables flexible and low-cost field operations. UHR3D could be a promising tool for "Seismic Trenching" to bring us detailed subsurface images which have been difficult to obtain by conventional seismic methods.

To demonstrate the feasibility, a UHR3D survey with 1km x 2 km coverage was planned and carried out at Yatsushiro Sea where the southwestern part of Hinagu Fault Zone is extended seawards. Delineation of the active fault system along Hinagu Fault Zone has become very important for earthquake hazard mitigation because the northernmost part of Hinagu Fault Zone was ruptured at the first event of 2016 Kumamoto Earthquakes (Mj 6.5 and Mj 7.3).

In this UHR3D survey, a boomer seismic source and four short streamer cables of Autonomous Cable System (ACS) were employed. ACS continuously records acoustic signals in flash memories, then cumbersome electric wires between the onboard recording system and the streamer cables are not necessary. This enables easy cable handling and reduction of operation times for the cable deployment and retrieval. The positionings of vessel, cables and receivers were conducted by ORCA navigation system. ORCA's real-time binning function was used for the optimization of in-fill line design to have uniform fold coverage.

Processing of the acquired field data were elaborately made to obtain a high-quality 3D data volume. Special attention was paid for the elimination of severe field noises, water column statics, tidal corrections, footprints suppression, etc. These noise reduction and signal enhancement were comprehensively implemented with a combination of various methods to achieve the most effective results.

The processed data has successfully revealed the detailed fault distributions as well as the precise sub-bottom structures. This data will shed new light on the detailed investigation of Hinagu Fault Zone in the near future.

Various geophysical explorations have been recently conducted in the hydrothermal field of the middle Okinawa trough for exploration of seafloor massive sulfide. The sub-seafloor distribution of physical properties such as electric conductivity, density can be obtained by geophysical explorations, however, the implications of geological structure based on the physical information are restricted. For this reason, currently, a number of seafloor drillings are required to decide the prospect of mineral deposit in detail and to evaluate the quality, while the high drilling costs and low recovery rate of rock cores prevent the application. Therefore, it is necessary to extract the geological information appropriately on substances from the various physical properties obtained by the geophysical exploration, and a new technological development is required. In this study, we obtained rock samples from several seafloor areas in the Okinawa trough where the germination of seafloor massive sulfide is predicted. We measured the various physical properties, mineral composition and element concentration at laboratory, and investigated their relationship. We constructed a rock physics model to estimate the electrical conductivity from the other physical properties such as porosity. As a result of applying our rock physics model to the measurement data, we succeeded in reproducing the electric conduction characteristics of samples showing the high electrical conductivity, which could not be explained by the previous rock models. Furthermore, a clear positive correlation was found between an estimated parameter of the physics model and the volumetric content of conductive sulfide minerals in rock samples. This result indicates that the electrical conductivity of rock samples obtained from the seafloor hydrothermal areas can be explained by new rock physics model proposed in this paper.

The emerging imaging technique, muography, takes advantage of highly penetrating and ubiquitous cosmic-ray muons, and has been applied to volcano eruption prediction, social infrastructure monitoring, and cultural heritage investigations in the world. Muons arrive from the upper hemisphere in every direction and thus, the topographically prominent objects can be targeted without the necessity of burying the detector. However, researchers have started to develop underground muography to apply the technique to investigations of buried ancient cities, mineral explorations, and underground water monitoring. This review describes the present status of the observational studies in which underground muography has been used. The outlook for the future deep underground muography will be briefly discussed.

MUOGRAPHY, a novel technique of non-destructive monitoring for gigantic structure, is expected to apply to underground exploration. In this article, the possibility to incorporate machine learning into this interesting and challenging technical topic is studied. After looking at overview of both of physics in muography and deep learning briefly, to realize precise tomography and make observation time shorter as possible, the promising methods such as super resolution, GAN, and LSTM are reviewed. As a practical activity, the abandoned mine exploration using muography is reported.

Subsurface density investigation using cosmic ray muons (muography) has been evolving and diversifying its application due to continuous improvement of measurement systems. Investigation of its applicability in oil and gas exploration can promote wider discussion to take forward its application to different promising areas. To increase the reliability and applicability of density information, it is important to identify issues in existing exploration schemes in terms of estimating subsurface density distribution and to investigate possible contributions of muography. This review paper first clarifies the superiority of density information, and then reviews the technical progress of gravity and seismic surveys which have been playing an important role for estimating subsurface density distribution, and finally, based on the reviews, proposes several suggestions to encourage the application of muography to oil and gas exploration.

Muography, a density tomography technique using cosmic-ray muons, is widely used to visualize the internal structure of large objects, such as pyramids or volcanos. However, because large muon detectors are used in existing muography technology, the development of a smaller logging tool is required for muography in boreholes to visualize underground structures. We developed a muon density logging tool that can be installed in a borehole around 10 cm in diameter. In this logging tool, plastic scintillators were scaled down to achieve a high angular resolution for the incoming muon direction and were rotated mechanically to give a wider view of incoming muon particles. The verification of the logging tool function was conducted in a horizontal borehole located beneath a small mountain at a depth of 50 m. The density distribution of the mountain obtained via muography with the logging tool was roughly consistent with the density values measured by using cores in a borehole drilled adjacent to the mountain. We also discussed the application of muography in a deep borehole using the logging tool and the tool’s suitability for muography in oil and gas resource development, CO₂ geological storage, methane hydrate resource development, underground hydrocarbon energy storage, compressed air energy storage, and geothermal power.

Muography is an emerging visualization technique for inspection of large-sized objects with the measurement of the absorption rate of cosmic-ray muons. Present paper introduces the first prototype of a Multi-Wire-Proportional-Camber (MWPC)-based borehole detector. The designed tracking system is based on the so-called Close Cathode Chamber (CCC) concept, which provides easily handling and robust detectors. The 18-cm-length detector is covering a sensitive area of 20 cm × 32 cm and an angular acceptance up to 60 deg with close to full tracking efficiency (99 %), reasonable position resolution of 1.8 mm and angular resolution of 10 mrad. The detector has been tested inside a shallow shaft and an underground iron pillar with concrete basement has successfully been imaged with the resolution of 15 cm within 15 days, which indicates the future industrial usage of MWPC detectors and encourages the application oriented development of this technology for borehole-based muography.

Recently, cosmic-ray muon radiography, called muography, has been widely developing by many researchers. The muon has high energy enough to penetrate large scaled objects such as volcanos and pyramids. There is no existing method to investigate such objects except muography, and the muography technique will reveal volcano eruption dynamics for disaster defense or provide new archaeologic knowledge. We have focused on muography of small or middle scaled infrastructure buildings, such as incinerators, bridges and dams. In the present paper, structure and characteristics of our muography detector are described. In addition, an example of results of infrastructure muography is shown, and its feasibility is discussed.