Hydrothermal fluid migration and supercritical phase transitions around the brittle-ductile transition are key to understanding volcanic activity and geothermal energy, but it remains challenging due to limited subsurface imaging. Here we conducted active-source seismic surveys at Kuju volcano, Japan, applying extended common-reflection-surface analysis to image magma-related and hydrothermal structures. We further estimated earthquake hypocenters, focal mechanisms, and seismic velocity. A continuous horizontal reflector could indicate a low-permeability seal confining supercritical fluids, while a low-amplitude reflection zone reveals fractures disrupting the seal. This permeable window, located at the magma margin, coincides with earthquake clusters of various focal mechanisms, suggesting upward fluid migration. A plume-shaped zone with low P-wave and S-wave velocity ratio (Vp/Vs) above the permeable window indicates thermally-altered formations and gas release from phase transitions. These results provide 3D visualization of the interplay among trapped supercritical fluid, its migration through permeable window, and related seismicity near the brittle-ductile transition.

We have developed a portable active seismic source (PASS) to monitor CO2 storage reservoirs at a depth of approximately 1 km. Despite its small size, stacking the signals generated by the PASS improves the signal-to-noise ratio of the seismometer data far from the source. The smaller size and lower cost of the PASS enables its permanent deployment in many locations to continuously monitor CO2 storage reservoirs. To achieve continuous monitoring, distributed acoustic sensing (DAS) is also a vital technology. Based on DAS, we can continuously record the signal from the PASS in an extensive area, including within boreholes and offshore fields. Here we report application of the PASS for the borehole DAS system. We confirmed the PASS signal propagation to a depth of similar to 1 km when we used a PASS with 630N at 50 Hz close to the wellhead and recorded the signal by the borehole fiber optic cable. The ability of the system to propagate the PASS signal to a depth of similar to 1 km enables continuous monitoring of most CO2 storage reservoirs with high temporal resolution. Furthermore, deploying multiple PASS systems could improve the spatial resolution of monitoring results. (c) 2023 Society of Chemical Industry and John Wiley & Sons, Ltd.

To delineate detailed geothermal reservoir structures, we have progressed a R&D project for geothermal exploration since 2013. Gravity and electromagnetic surveys are commonly used for geothermal exploration in Japan. However, the resolution of these surveys is much broader than the size of faults or fracture zones we see from well data. Thus, there is considerable uncertainty to locate the reservoirs by these surveys. To resolve this problem, we focused attention on the seismic method which tends to have higher spatial resolution. As the first step of this project, we reprocessed a few legacy records with present data processing techniques and confirmed improvements in its data quality. Then, we acquired a 3D seismic test data in Yamagawa geothermal field. Analyses with a variety of advanced methods including 3D visualization of discontinuous structures indicating fractures, and integrated analyses with well and other geophysical data, demonstrated that the seismic survey provided detail subsurface models which help us to build a geothermal model. Meanwhile, Yamagawa field had a significant advantage that its flat geography made it easy to deploy survey instruments, which is very rare among Japanese geothermal fields. Therefore, we conducted the second verification seismic survey in Onikobe geothermal field in 2017 to confirm its efficacy even in mountainous areas where most geothermal resources are located in Japan. In mountainous areas, the survey design is crucial because a limited number of survey lines are available. We visited and investigated the survey area several times before the survey to find as many available survey lines as possible. Four types of sources and two types of recorders were used according to surface conditions in the data acquisition. These efforts allowed us to obtain a 3D seismic volume in the deep part which included potential target zones in this field. In the data processing, static correction to compensate precipitous geography was important. Analyzed results showed consistency with existing data and added new insights in the reservoir structures.

Production sustainability of geothermal resources requires accurate and high definition conceptual model focusing on temperature and permeability. Probabilistic approach facilitates investigation and analysis of complex subsurface structures regarding geothermal reservoirs, cap rocks, recharge areas and faults associated with hydrothermal system. In this paper, we present a case study of 3D conceptual modelling at the Onikobe geothermal field in northeast Japan. There has been multimodal dataset consisted of surface seismic, magnetotelluric (MT), airborne gravity gradiometry (AGG) and gravity surveys in addition to borehole data. These geophysical data provide subsurface structures regarding rock properties to interpret hydrothermal system. Especially, surface seismic data have valuable information on not only velocity structure, but also fault and fracture images represented by geometric attributes that are automatically extracted such as Thinned Fault Likelihood (Hale, 2013), dip azimuth, and curvature attributes (Roberts, 2001). Thus, the geometric attributes are significant to understand spatial distributions of fault-controlled geothermal reservoirs. These seismic attributes and rock properties can be quantitatively integrated by means of multi-attribute analysis (MAA). We have implemented a probabilistic MAA that with the artificial neural network (ANN) algorithm developed by Hampson et al. (2001) for a cost-effective solution of the non-linear problem in geothermal modelling. In this workflow, the probabilistic approach makes it possible to provide a detailed geothermal model while taking advantage of higher spatial resolution of 3D surface seismic data. Another probabilistic approach, Bayesian classification, is a robust tool for permeability assessment in the conceptual modelling. We performed Bayesian classification based on non-parametric probability density functions (PDFs) using resistivity, P-wave impedance and Thinned Fault Likelihood attribute. The comprehensive interpretation from the detailed litho-facies model is consistent with the observed distributions of temperature, permeability and hydrothermal fluids beneath the Onikobe caldera. It reveals that the probabilistic conceptual modelling yields significant benefits for geothermal exploration and exploitation.

Summary Exploration and development of geothermal fields requires quantitative interpretation of different geophysical data. In this paper, we introduce the novel approach to three-dimensional inversion and interpretation of multimodal geophysical data, which incorporates the known geological/geophysical constraints. In a general case, geophysical inverse problem is ill-posed, i.e., it is non-unique and unstable; however, appropriate a priori information can help reducing the nonuniqueness and increasing the stability of this ill-posed problem. The developed approach uses the principles of inversion “guided” by known information. In the framework of this approach, the 3D inversion itself is data driven, but a priori geological/geophysical model is used as the initial and reference model during the iterative inversion process. We have applied the developed method to the integrated interpretation of magnetotelluric, gravity, and magnetic data in Yamagawa geothermal field of Japan, using the constraints based on seismic and well-logging data acquired in the same area. The results have demonstrated that the developed method produces reliable 3D models of different physical properties, which provide a solid basis for geological interpretation of the complex subsurface structures in the survey area, critical for geothermal exploration.

In this paper, we have developed a novel approach to three-dimensional inversion and interpretation of multimodal geophysical data, which incorporates known geological/geophysical constraints. In a general case, the geophysical inverse problem is ill posed, i.e., it is non-unique and unstable. However, appropriate a priori information can help reduce the non-uniqueness and increase the stability of the ill-posed problem. The developed approach uses the principles of inversion “guided” by known information. In the framework of this approach, the 3D inversion itself is data driven, but a priori geological/geophysical model is used as the initial and reference model during the iterative inversion process. We have applied the developed method to the integrated interpretation of magnetotelluric, gravity, and magnetic acquired in Yamagawa geothermal field of Japan, using constraints based on seismic and well-log data acquired in the same area. The results have demonstrated that the developed method produces reliable 3D models of different physical properties, which provides a solid basis for geological interpretation of the complex subsurface structures in the survey area, critical for geothermal exploration. Presentation Date: Wednesday, October 17, 2018 Start Time: 8:30:00 AM Location: 213A (Anaheim Convention Center) Presentation Type: Oral

阿多カルデラ内部に位置する鹿児島県山川地熱地域において，地熱貯留層の位置や形状の高精度把握技術の開発を目的とする弾性波探査実証試験が実施された。本実証試験の目標は，山川地域の地熱貯留層構造の詳細検討を通じて，反射法・屈折法統合弾性波探査の断裂系評価法としての有効性と，電磁探査や重力・磁気探査と組み合わせる統合物理探査の地質評価法としての可能性を示すことであった。本試験データの3次元反射法記録とそのジオメトリカルアトリビュート解析からは断裂系の連結構造が可視化されたほか，屈折トモグラフィ解析とフルウェーブインバージョン（FWI）解析からは複雑な弾性波速度構造が明らかとなった。さらに，弾性波探査をガイドにした既存のMT / 重力 / 磁気データの3次元逆解析により，比抵抗・密度異常・磁化ベクトルのモデルを推定し，弾性波探査や坑井の情報との対比や地球統計学的な検討を通じて，岩相および温度構造のモデルを推定した。そして，これら一連の作業手順は3次元地質モデル構築ワークフローとして総括した。また，本試験データを用いたデシメーションテストでは，多くの国内地熱地域が位置する山岳部における弾性波探査の適用性を評価した。ここでは，準3次元的なデータ取得配置であっても，5D内挿法や共通反射面（CRS）重合法などのデータ補間技術を適用することにより，3次元構造把握が可能であることが示された。

独立行政法人石油天然ガス・金属資源機構（JOGMEC）が地熱発電技術研究開発事業として実施する地熱貯留層探査技術開発では，地熱開発での掘削成功率やフィールドの長期経済性の向上において求められる地熱貯留層構造の3次元的詳細把握の課題に対し，電磁探査，重力探査，磁気探査，及び弾性波探査の統合物理探査技術の開発を行ってきた。平成27年度には阿多カルデラ内部に位置する鹿児島県山川地熱地域において，第1回実証試験として3次元弾性波探査実証試験が実施され，平成28年度には既存のMTデータ，重力データ，磁気データを用いた3次元逆解析が実施された。物理探査データの3次元逆問題は，一般的にill-posedであることが知られている。一方で，複雑な地質構造を解釈するためには，信頼性の高い3次元モデルを得る必要がある。地下の地質構造を反映したモデルを初期モデルとして与えた3次元逆解析手法がこの問題を解決する手助けとなることが期待される。本研究では，山川地熱地域における弾性波探査データや坑井データ等の既知情報を統合した先験的モデルを初期モデルとし，MTデータ，重力データ，磁気データの3次元逆解析を実施した。本稿では，先験的モデルの構築において，弾性波探査データからの情報を加味することの利点を探るとともに，RMSミスフィットや坑井データと比較することで弾性波探査データからの寄与の度合いによる3次元逆解析結果の違いを検証する。

Over the past 30 years, towed streamer has evolved into a mature and highly effective technology for 3D subsurface imaging. Time-lapse 3D seismic monitoring at Sleipner in the North Sea has clearly demonstrated the practical usefulness in monitoring CO2 injected into the deep saline aquifers. Although sometimes it is unsafe to towed streamers when acquiring data in areas of shallow water or surface infrastructures associated with production platforms and fishing gears. That is why seabed seismic worth considering in some situations. In addition, the quality of time-lapse 3D seismic acquisition depends strongly upon the positioning of seismic sources and receivers. Improving the resolution of seismic data has been selected as the most important factor in increasing the value of seismic data. A permanent OBC (Ocean Bottom Cable) system offers a number of potential benefits for obtaining increased resolution by permanently deployed seismic sensors on the seabed or under the seabed. Compared to the towed streamer the permanent OBC also offers the opportunity to record shear waves, not only the P-waves. We are developing a permanent OBC system to meet requirements of Japanese offshore CO2 sequestration in coastal areas.

鹿児島県山川地熱地域において，地熱貯留層探査技術開発の一環である弾性波探査実証試験として取得された3次元弾性波探査データを対象として，不連続箇所を強調・抽出することができるcoherenceアトリビュートを評価した。得られたcoherenceアトリビュートは，dip/dip azimuth，ならびにcurvatureアトリビュートから推察される地質構造や，既往文献（Okada and Yamada, 2002）に報告されていた坑壁比抵抗イメージによって検出された天然フラクチャーの分布と整合的であることが確認された。総じて，ジオメトリカルアトリビュート解析によって信頼性の高い断裂系分布の推定ができており，地熱地域における断裂系把握への有効性が示されたものと考えられる。

Abstract Surface based microseismic monitoring during hydraulic fracture stimulations is an emerging technology in the last decade. The method observes acoustic emission (AE) or microseismic events using a surface receiver array and estimates source locations and origin times of the events by analyzing a time series of migration images from observation data. However, the accuracy and monitoring cost of the technology highly depends on the array-geometry. The migration images can be blurred if a too coarse array is used. Monitoring cost can be unnecessarily high if a too dense array is used. Some sort of adequate array evaluation methods are required for designing a cost effective survey plan. We introduce an array evaluation method based on the point-spread function (PSF). The PSF describes the response of an imaging system to a point source. Our proposed method evaluates an array in the following way: First, a point source of which excitation time and source location are known is defined. Second, synthetic seismograms using the array are computed. Third, the synthetic traces are migrated and a time series of source images are obtained. Finally, the images are examined so that the abilities of the array are understood. We demonstrate the method by diplaying attributes calculated from the dataset which represent the precision and accuracy of the source location determination. Introduction The use of microseismic monitoring during the hydraulic fracture stimulations is growing worldwide as the development of tight oil/gas shows phenomenal growth. The technology is based on seismic source location methods, which observes microseismic events using a receiver array network and estimates source locations and origin times of the events by analyzing the characterestics of microseismic events. The source location methods can be divideed into two categories: downhole based and surface based (Duncan and Eisner, 2010). Downhole based method deploys the sensors in the well and configures an array network near the stimulated area. Most of the source location methods applied to the downhole data are based on the Geiser's scheme (Geiser, 1912), which minimizes the arrival-time residuals in an iterative maner. Thanks to a quiet environment in the borehole and short travel paths from the microseismic sources to the receivers, detectability of the method is higher than surface based method. However, because of the limited array configuration, the area that can cover with an array network in a well is also limited around the well. To increase the accuracy, many monitoring wells are necessary to retain an observable area. This may cause the increase of the monitoring cost. Even though higher noisy environments than downhole based method, surface based microseismic monitoring has been developed as an emerging technology in the last decade. This method plants or buries the sensors to configure a large 2D array network on the surface or near the surface. Migration-type methods are typically used for the surface based monitoring because the signal to noise (S/N) ratio can be improved by stacking of the events (Gajewski et al., 2005). To overcome the higher noise level of surface environments, a temporary array network consisting of 1,000+ geophones, or a permanent sensor array network consisting of several buried sensors per square mile, has been used in these days.

As the deep water drilling technology continues to evolve, sub-salt basins has become one of the most prospective targets for hydrocarbon reserves. Even though sub-salt is a difficult target for seismic imaging, various advanced technologies have been developed in the lost decade and difficulties encountered in the new prospect have been reduced. New seismic acquisition technologies such as broadband seismic, full-azimuth and ultra-long offset acquisition mitigate the problems from the presence of salt. Sub-salt imaging techniques has also evolved with the aid of the advanced new acquisition technology, high oil price, and low cost high performance computing (HPC). Anisotropic RTM has became a usual technique in the Gulf of Mexico, and FWI grows in use and popularity nowadays.

Although sub-basalt exploration has been achieved great attention, the progress is very limited due to the difficulty of imaging beneath the basalt. Most of seismic energy are usually scattered and attenuated by thick basalts. Also Basalts usually generate a lot of multiples. As a result, quality of seismic data are usually not enough for interpretation. But recently, seismic techniques are advanced rapidly and some of those new techniques are expected to be effective in sub-basalt imaging. In this article, we review issues relating to sub-basalt imaging and some of the new recently advanced techniques, especially de-ghosting techniques in data acquisition are reviewed because low-frequency is a key factor in sub-basalt imaging.

In the CCS monitoring, time-lapse 3D seismic survey is necessary and microseismic monitoring is also required to assure the safe CCS operations. In order to fulfill these requirements efficiently, we are developing a permanent Ocean Bottom Cable system (OBC) in shallow marine environments. The system can figure out the extent of CO2 injected area and assure that CO2 is not escaped above seal rocks. The use of the system is advantageous from the long term view of the cost. In this paper, we, firstly, present a simulation study based on the data from the Nagaoka CCS test site. 4D response was evaluated using crossplot ofNRMS and PRED indices so that we could estimate the discrimination degree between 4D responses caused by CO2 injection and other 4D noises. Basic test experiments of a permanent OBC system have been conducted in Sites A and B in Japan and repeated seismic surveys using airgun source were shot for evaluating the 4D noise response. Tidal correction is found not to be simple as the wavelet shape itself is modified in the shallow water condition. P-SV reflections are recorded with three-component geophones installed in the OBC which will contribute to the CCS reservoir characterization and monitoring. A microseismic event with the magnitude of 0.6 occurred at the depth of 103 kilometers just below Site A was detected. As the CO2 geological sequestration will be conducted within several thousand meters, detectable limit is estimated down to magnitude minus two in an ideal condition.