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MINAMI Takuto
Graduate School of Science / Division of Planetology
Assistant Professor

Researcher basic information

■ Research Keyword
  • Geomagnetism
■ Research Areas
  • Natural sciences / Solid earth science
■ Committee History
  • Jan. 2025 - Present, Earth Planets and Space, Editors

Research activity information

■ Award
  • Feb. 2022 Earth, Planets and Space, EPS Excellent Reviewers 2021
    IRI

  • Jan. 2022 Earth, Planets and Space, EPS Highlighted Papers 2021, International Geomagnetic Reference Field: the thirteenth generation
    P. Alken, E. Thébault, C. D. Beggan, H. Amit, J. Aubert, J. Baerenzung, T. N. Bondar, W. J. Brown, A. Chambodut, A. Chulliat, G. A. Cox, C. C. Finlay, A. Fournier, N. Gillet, A. Grayver, S. Califf, M. D. Hammer, M. Holschneider, L. Huder, G. Hulot, T. Jager, C. Kloss, M. Korte, W. Kuang, A. Kuvshinov, B. Langlais, J.‐M. Léger, V. Lesur, P. W. Livermore, F. J. Lowes, S. Macmillan, W. Magnes, M. Mandea, S. Marsal, J. Matzka, M. C. Metman, T. Minami, A. Morschhauser, J. E. Mound, M. Nair, S. Nakano, N. Olsen, F. J, Pavón‐Carrasco, V. G. Petrov, G. Ropp, M. Rother, T. J. Sabaka, S. Sanchez, D. Saturnino, N. R. Schnepf, X. Shen, C. Stolle, A. Tangborn, L. Tøffner‐Clausen, H. Toh, J. M. Torta, J. Varner, F. Vervelidou, P. Vigneron, I. Wardinski, J. Wicht, A. Woods, Y. Yang, Z. Zeren, B. Zhou

  • Nov. 2021 SGEPSS, SGEPSS Paper Award, A candidate secular variation model for IGRF-13 based on MHD dynamo simulation and 4DEnVar data assimilation
    Takuto Minami, Shin’ya Nakano, Vincent Lesur, Futoshi Takahashi, Masaki Matsushima, Hisayoshi Shimizu, Ryosuke Nakashima, Hinami Taniguchi, Hiroaki Toh

  • Nov. 2020 SGEPSS, Obayashi Early Career Scientist Award, Studies on tsunami-induced electromagnetic field variation and time-varying electrical conductivity structure associated with volcanic activity, using numerical analyses based on finite element method
    Takuto Minami

  • May 2019 The Volcanological Society of Japan, Research Encouragement Award, 能動電磁探査ACTIVEを用いた阿蘇山マグマ噴火時の比抵抗構造時間変化の推定
    Takuto Minami

  • Jan. 2019 Earth, Planets and Space, EPS Highlighted Papers 2018, Temporal variation in the resistivity structure of the first Nakadake crater, Aso volcano, Japan, during the magmatic eruptions from November 2014 to May 2015, as inferred by the ACTIVE electromagnetic monitoring system
    Takuto Minami, Mitsuru Utsugi, Hisashi Utada, Tsuneomi Kagiyama, Hiroyuki Inoue
    共同研究・競争的資金等の業績IDIRI

  • Nov. 2013 SGEPSS, Student Presentation Award (Aurora Medal), Initial rise in the horizontal magnetic component induced by the tsunami passage
    Takuto Minami

■ Paper
  • Kazutaka Mannen, Ryosuke Doke, Akira Johmori, George Kikugawa, Takuto Minami, Tetsuya Takahashi, Mitsuru Utsugi, Koichiro Fujimoto
    Elsevier BV, Sep. 2025, Journal of Volcanology and Geothermal Research, 465, 108363 - 108363
    [Refereed]
    Scientific journal

  • Keiichi Ishizu, Yasuo Ogawa, Kuo Hsuan Tseng, Takahiro Kunitomo, Norihiro Kitaoka, Grant Caldwell, Takuto Minami, Sohei Serita, Hiroshi Ichihara, Edward Alan Bertrand, Wiebke Heise
    SUMMARY Although controlled-source electromagnetic (CSEM) methods have higher sensitivity to thin resistive bodies than the magnetotelluric (MT) method, their delineation by the inversion requires CSEM data with high signal-to-noise ratio (SNR). This study aims to enhance the SNR of CSEM data by increasing the number of stacks. To efficiently stack long-term data, we use an EM-accurately controlled, routinely operated signal system (ACROSS), which can transmit accurately controlled waveforms by synchronizing the transmitting waveforms with a 10 MHz Global Positioning System signal. We conducted a CSEM survey using the EM-ACROSS in the Kusatsu-Shirane Volcano to demonstrate that the SNR can be improved by extensive observation data and the CSEM inversion can delineate hydrothermal systems, including resistive bodies of vapour-rich reservoirs. Our EM-ACROSS simultaneously transmitted waveforms from two dipoles during a 192-h of the survey; five-component receivers located 4–6 km away from the transmitter captured EM-ACROSS signals ranging between 146 and 192 h. By stacking extensive observation data using a weighted method, the CSEM responses show minimal error levels, with standard errors <2 per cent for most frequencies. The SNR roughly followed the square root of the stacking times. 3-D inversion of the collected CSEM data delineated a relatively resistive body, interpreted as a vapour-dominated reservoir below a cap-rock layer, while the MT inversion failed to recover the same. This highlights the ability of an EM-ACROSS-based CSEM survey to delineate hydrothermal systems including vapour-dominated reservoirs, and provides a compelling rationale for establishing CSEM as a standard methodology in hydrothermal imaging. Furthermore, this study suggests that the enhanced imaging capabilities of CSEM data can be further improved when integrated with MT data.
    Oxford University Press (OUP), Dec. 2024, Geophysical Journal International, 240(2) (2), 1107 - 1121
    [Refereed]
    Scientific journal

  • Takuto Minami
    Electrically conductive seawater, moving in an ambient magnetic field, generates electromagnetic (EM) variations. Tsunamis are significant contributors to this phenomenon, inducing observable electric and magnetic fluctuations at seafloor and coastal observatories. While understanding of these occurrences in open oceans is robust, knowledge regarding their observation on islands remains limited. This article seeks, through the use of numerical experimentation, to enhance our understanding of tsunami-generated EM (TGEM) variations observed on islands. Utilizing simulations involving conical islands, we identify three key insights regarding EM intensity normalized by the height of incident tsunamis: (i) increased ocean depth surrounding the island amplifies tsunami EM signals, particularly for periods shorter than 20 min; (ii) magnetic field strength at the island is approximately comparable to that observed at the seafloor in the absence of the island when the island radius is smaller than 6 km; and (iii) electric field intensity at the island notably surpasses that observed at the seafloor, especially with smaller island radii ( 6 km). Additionally, we establish that employing the ratio of island radius to tsunami wavelength near the island coast facilitates the derivation of empirical functions for this phenomenon. This article is part of the theme issue ‘Magnetometric remote sensing of Earth and planetary oceans’.
    The Royal Society, Dec. 2024, Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 382(2286) (2286)
    [Refereed]
    Scientific journal

  • Toshitaka Baba, Zhiheng Lin, Takuto Minami, Hiroaki Toh
    Ocean-bottom pressure gauges are widely used for tsunami observations due to their established accuracy and stability. Recent advancements reveal that the magnetic field fluctuates when a large tsunami passes over the ocean, suggesting potential alternatives to pressure gauges in the form of ocean-bottom electromagnetometers (OBEMs). This article offers a comprehensive synthesis of recent findings concerning tsunami magnetic fields and their utility in tsunami source estimation. In addition, we scrutinize the effectiveness of tsunami observations employing OBEMs. Despite the promise of electromagnetometers, it is worth noting that the background noise inherent in electromagnetic observations tends to be approximately 10 times greater than that of pressure observations within the critical tsunami frequency bands. The Earth’s magnetic field sporadically disrupts tsunami magnetic fields, presenting a potential limitation to the utility of electromagnetometers in tsunami detection when compared with pressure gauges. Nevertheless, our investigation underscores the potential of electromagnetic observations in detecting tsunamis propagating over the ocean at magnitudes of a few centimetres. An invaluable advantage of electromagnetometers over pressure monitoring lies in their capability to observe tsunami velocity fields, suggesting a promising avenue for further research and development in tsunami observation technology. This article is part of the theme issue ‘Magnetometric remote sensing of Earth and planetary oceans’.
    The Royal Society, Dec. 2024, Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 382(2286) (2286), English
    [Refereed]
    Scientific journal

  • Takeshi Suzuki, Ryokei Yoshimura, Ken'ichi Yamazaki, Takuto Minami, Kazuki Sawayama, Naoto Oshiman
    Elsevier BV, Dec. 2023, Journal of Applied Geophysics, 105271 - 105271, English
    [Refereed]
    Scientific journal

  • Hiiro Yokoi, Toshitaka Baba, Zhiheng Lin, Takuto Minami, Masato Kamiya, Akino Naitoh, Hiroaki Toh
    American Geophysical Union (AGU), Jun. 2023, Journal of Geophysical Research: Solid Earth, 128(6) (6)
    [Refereed]
    Scientific journal

  • Toward application of electromagnetic observation to inference of tsunami sources and tsunami forecasts
    Takuto Minami
    Jul. 2022, The Earth Monthly, Japanese
    [Invited]
    Scientific journal

  • N. R. Schnepf, T. Minami, H. Toh, M. C. Nair
    Abstract On 15 January 2022, at around 04:00 UTC, the submarine volcano Hunga Tonga‐Hunga Ha'apai explosively erupted. We examine data from 10 Pacific Ocean geomagnetic observatories and process the data using both high pass filters and cross‐wavelet analyses to enable evaluating the time‐frequency characteristics of the magnetic signals across the Pacific region. At the Western Samoa observatory (API), magnetic signals of 3–8 min period, and visible in both vertical and horizontal fields, arrived at ∼04:44 UTC. The observatories at Chichijima Island (CBI) and Easter Island (IPM) both had local magnetic signatures concurrent with the eruption's water wave arrival and period ranges from, respectively, 13–93 and 5–100+ min. At CBI and IPM, the magnetic signal may be due to both the eruption's tsunami water wave and atmospheric/ionospheric sources. Our results suggest that the magnetic signatures from the eruption are identifiable and may be further separated in future studies.
    American Geophysical Union (AGU), May 2022, Geophysical Research Letters, 49(10) (10)
    [Refereed]
    Scientific journal

  • Ishibashi Sakura, Utsugi Mitsuru, Minami Takuto, Yoshikawa Shin, Inoue Hiroyuki
    The Volcanological Society of Japan, 2022, PROGRAMME AND ABSTRACTS THE VOLCANOLOGICAL SOCIETY OF JAPAN, 2022, 9 - 9, Japanese

  • Patrick Alken, E. Thébault, C. D. Beggan, H. Amit, J. Aubert, J. Baerenzung, T. N. Bondar, W. J. Brown, S. Califf, A. Chambodut, A. Chulliat, G. A. Cox, C. C. Finlay, A. Fournier, N. Gillet, A. Grayver, M. D. Hammer, M. Holschneider, L. Huder, G. Hulot, T. Jager, C. Kloss, M. Korte, W. Kuang, A. Kuvshinov, B. Langlais, J.-M. Léger, V. Lesur, P. W. Livermore, F. J. Lowes, S. Macmillan, W. Magnes, M. Mandea, S. Marsal, J. Matzka, M. C. Metman, T. Minami, A. Morschhauser, J. E. Mound, M. Nair, S. Nakano, N. Olsen, F. J. Pavón-Carrasco, V. G. Petrov, G. Ropp, M. Rother, T. J. Sabaka, S. Sanchez, D. Saturnino, N. R. Schnepf, X. Shen
    AbstractIn December 2019, the International Association of Geomagnetism and Aeronomy (IAGA) Division V Working Group (V-MOD) adopted the thirteenth generation of the International Geomagnetic Reference Field (IGRF). This IGRF updates the previous generation with a definitive main field model for epoch 2015.0, a main field model for epoch 2020.0, and a predictive linear secular variation for 2020.0 to 2025.0. This letter provides the equations defining the IGRF, the spherical harmonic coefficients for this thirteenth generation model, maps of magnetic declination, inclination and total field intensity for the epoch 2020.0, and maps of their predicted rate of change for the 2020.0 to 2025.0 time period.
    Springer Science and Business Media LLC, Dec. 2021, Earth, Planets and Space, 73(1) (1), English
    [Refereed]
    Scientific journal

  • Zhiheng Lin, Hiroaki Toh, Takuto Minami
    Abstract The motion of conductive seawater by tsunamis can generate magnetic fields in the presence of the background geomagnetic main field. Previous studies found that, using the tsunami‐generated seafloor magnetic field, it is possible to predict the propagation direction and wave height prior to the actual arrivals of tsunamis. This study correlates the tsunami magnetic field and the tsunami sea level change using observed data and three‐dimensional simulations of the 2009 Samoa and 2010 Chile tsunamis. Our direct comparison of the tsunami observed magnetic field and tsunami sea level change illustrate that the vertical tsunami magnetic component, , arrived earlier than the sea level change. The “initial rise” signal in the observed horizontal tsunami magnetic component, , which was arrived even earlier than also is found by combing the observation with the three‐dimensional simulations. We further examine the precision of conversion of the tsunami magnetic field to the sea level change and find that the magnetic field derived tsunami sea levels are as precise as those obtained from differential pressure gauge data. However, our simulation shows that existing tsunami source models are incompatible with our tsunami magnetic data. Therefore, it is necessary to include magnetic field derived tsunami sea level changes to improve those source models.
    American Geophysical Union (AGU), Oct. 2021, Journal of Geophysical Research: Solid Earth, 126(11) (11)
    [Refereed]
    Scientific journal

  • Takuto Minami, Neesha R. Schnepf, Hiroaki Toh
    AbstractA seafloor geomagnetic observatory in the northwest Pacific has provided very long vector geomagnetic time-series. It was found that the time-series include significant magnetic signals generated by a few giant tsunami events including the 2011 Tohoku Tsunami. Here we report that the tsunami-generated magnetic fields consist of the weak but first arriving field, and the strong but second arriving field—similar to the P- and S-waves in seismology. The latter field is a result of coupling between horizontal particle motions of the conductive seawater and the vertical component of the background geomagnetic main field, which have been studied well so far. On the other hand, the former field stems from coupling between vertical particle motions and the horizontal component of the geomagnetic main field parallel to tsunami propagation direction. The former field has been paid less attention because horizontal particle motions are dominant in the Earth’s oceans. It, however, was shown that not only the latter but also the former field is significant especially around the magnetic equator where the vertical component of the background magnetic field vanishes. This implies that global tsunami early warning using tsunami-generated magnetic fields is possible even in the absence of the background vertical geomagnetic component.
    Lead, Springer Science and Business Media LLC, Jan. 2021, Scientific Reports, 11(1) (1), English
    [Refereed]
    Scientific journal

  • Ishibashi Sakura, Utsugi Mitsuru, Minami Takuto, Inoue Hiroyuki
    The Volcanological Society of Japan, 2021, PROGRAMME AND ABSTRACTS THE VOLCANOLOGICAL SOCIETY OF JAPAN, 2021, 23 - 23, Japanese

  • Minami Takuto, Gresse Marceau, Anai Chisato, Utsugi Mitsuru
    The Volcanological Society of Japan, 2021, PROGRAMME AND ABSTRACTS THE VOLCANOLOGICAL SOCIETY OF JAPAN, 2021, 30 - 30, Japanese

  • Takuto Minami, Shin’ya Nakano, Vincent Lesur, Futoshi Takahashi, Masaki Matsushima, Hisayoshi Shimizu, Ryosuke Nakashima, Hinami Taniguchi, Hiroaki Toh
    Springer Science and Business Media LLC, Sep. 2020, Earth, Planets and Space, 72(1) (1)
    [Refereed]
    Scientific journal
    Link to Kobe Univ. Repository (Kernel)

  • Temporal variation in the resistivity structure of the first Nakadake crater, Aso volcano, Japan, during the magmatic eruptions from November 2014 to May 2015, as inferred by the ACTIVE electromagnetic monitoring system
    Minami Takuto, Utsugi Mitsuru, Utada Hisashi, Kagiyama Tsuneomi, Inoue Hiroyuki
    阿蘇山マグマ噴火に伴う地下熱水系時間変化の可視化に成功. 京都大学プレスリリース. 2018-08-23.During the last magmatic eruption period of Aso volcano (November 2014 to May 2015), a controlled-source electromagnetic volcano monitoring experiment (ACTIVE) was conducted. Here, we interpret the temporal variations in the electromagnetic responses. The ACTIVE system installed at the first Nakadake crater, the only active crater of Aso, consisted of a transmitter located northwest of the crater and four (before the eruptions) or three (after the eruptions) vertical induction coil receiver stations. The ACTIVE system succeeded in detecting temporal variations in the resistivity structure during the latest magmatic eruption period. The response amplitude started to increase in November 2014, peaked in February 2015, and decreased slightly in August 2015. An unstructured tetrahedral finite element three-dimensional inversion that accounted for topographic effects was used to interpret temporal variations in the ACTIVE response. The 3-D inversion results revealed that temporal variations in the ACTIVE response are attributed mainly to (1) a broad increase in resistivity at elevations from 750 to 850 m, not only directly beneath the crater bottom but also outside the crater, and (2) a thin layer of decrease in resistivity at the elevation of ~ 1000 m on the western side of the crater. The increase in resistivity can be ascribed to a decrease in the amount of conductive groundwater in the upper part of an aquifer located below the elevation of 800 m, while the decrease in resistivity implies that enhanced fluid temperature and pressure changed the subsurface hydrothermal system and formed a temporal fluid reservoir at the shallow level during the magmatic eruption period.
    Springer Nature America, Inc, Dec. 2018, Earth, Planets and Space, 70(1) (1), English
    [Refereed]
    Scientific journal

  • Takuto Minami, Hiroaki Toh, Hiroshi Ichihara, Issei Kawashima
    Dec. 2017, JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH, 122(12) (12), 9559 - 9579, English
    [Refereed]
    Scientific journal

  • Robert H. Tyler, Tim P. Boyer, Takuto Minami, Melissa M. Zweng, James R. Reagan
    The electrical conductivity of the ocean is a fundamental parameter in the electrodynamics of the Earth System. This parameter is involved in a number of applications ranging from the calibration of in situ ocean flow meters, through extensions of traditional induction studies, and into quite new opportunities involving the remote sensing of ocean flow and properties from space-borne magnetometers such as carried aboard the three satellites of the Swarm mission launched in 2013. Here, the first ocean conductivity data set calculated directly from observed temperature and salinity measurements is provided. These data describe the globally gridded, three-dimensional mean conductivity as well as seasonal variations, and the statistics of spatial and seasonal variations are shown. This "climatology" data set of ocean conductivity is offered as a standard reference similar to the ocean temperature and salinity climatologies that have long been available.
    SPRINGER HEIDELBERG, Nov. 2017, EARTH PLANETS AND SPACE, 69, English
    [Refereed]
    Scientific journal

  • Takuto Minami
    Sep. 2017, SURVEYS IN GEOPHYSICS, 38(5) (5), 1097 - 1132, English
    [Refereed]
    Scientific journal

  • Takuto Minami, Hiroaki Toh, Robert H. Tyler
    Apr. 2015, GEOPHYSICAL RESEARCH LETTERS, 42(7) (7), 2171 - 2178, English
    [Refereed]
    Scientific journal

  • Takuto Minami, Hiroaki Toh
    Sep. 2013, GEOPHYSICAL RESEARCH LETTERS, 40(17) (17), 4560 - 4564, English
    [Refereed]
    Scientific journal

  • Hiroaki Toh, Takuto Minami
    Feb. 2012, Earthquake Research and Analysis - Seismology, Seismotectonic and Earthquake Geology, (9) (9), 183 - 198, English
    [Refereed]
    In book

  • Takuto Minami, Hiroaki Toh
    2012, EARTH PLANETS AND SPACE, 64(5) (5), E9 - E12, English
    [Refereed]
    Scientific journal

■ MISC
  • Takuto Minami
    Lead, Mar. 2021, On Physical Sciences, English
    [Invited]
    Introduction commerce magazine

  • Preliminary results of ocean bottom pressure observation around Kikai submarine caldera, SW Japan
    ASAI Yuko, SEAMA Nobukazu, HANYU Tomoko, MATSUNO Tetsuo, HIROSE Hitoshi, MINAMI Takuto, SUGIOKA Hiroko
    2021, 日本地球惑星科学連合大会予稿集(Web), 2021

  • Minami Takuto, Mitsuru Utsugi, Utada Hisashi, Kagiyama Tsuneomi, Inoue Hiroyuki
    The Volcanological Society of Japan, 2018, PROGRAMME AND ABSTRACTS THE VOLCANOLOGICAL SOCIETY OF JAPAN, 2018(0) (0), 102 - 102, Japanese

  • Minami Takuto, Author Second
    The Volcanological Society of Japan, 2017, PROGRAMME AND ABSTRACTS THE VOLCANOLOGICAL SOCIETY OF JAPAN, 2017(0) (0), 52 - 52, Japanese

  • Detection of micro-tsunamis by using Vector Tsunameter
    HAMANO Yozo, SUGIOKA Hiroko, TADA Noriko, TOH Hiroaki, MINAMI Takuto
    2014, 地球電磁気・地球惑星圏学会総会及び講演会予稿集(CD-ROM), 136th

  • ベクトル津波計リアルタイム観測のための水中音響通信試験とウェーブグライダーの実海域長期運用
    浜野洋三, 杉岡裕子, 多田訓子, 伊藤亜紀, 藤浩明, 南拓人, 川嶋一生
    2014, ブルーアース要旨集, 2014

  • Observation of seafloor ElectroMagnetic(EM) signals by using Vector TsunaMeter
    HAMANO Yozo, SUGIOKA Hiroko, TADA Noriko, TOH Hiroaki, MINAMI Takuto, KAWASHIMA Issei
    2013, 地球電磁気・地球惑星圏学会総会及び講演会予稿集(CD-ROM), 134th

  • ベクトル津波計の開発・製作と海底観測
    浜野洋三, 杉岡裕子, 多田訓子, 伊藤亜妃, 藤浩明, 南拓人, 川嶋一生, 塩原肇
    2013, ブルーアース要旨集, 2013

  • Seafloor Electromagnetic Station with Differential Pressure Gauge
    TOH Hiroaki, MINAMI Takuto, SUGIOKA Hiroko, HAMANO Yozo
    2012, 地球電磁気・地球惑星圏学会総会及び講演会予稿集(CD-ROM), 132nd

  • 北西太平洋海盆で観測された東北地方太平洋沖地震津波による海洋ダイナモ効果
    南拓人, 藤浩明, 浜野洋三, 杉岡裕子, 多田訓子, 舘畑秀衛, 穂積裕太
    2011, 日本地震学会秋季大会講演予稿集, 2011

  • Electromagnetic Observations in Sea and Land to Image Resistivity Structure beneath the Back-Arc Region of Southwest Japan
    大志万 直人, 藤 浩明, 笠谷 貴史, 塩崎 一郎, 下泉 政志, 吉村 令慧, 新貝 雅文, 藤井 郁子, 山口 覚, 中尾 節郎, 村上 英記, 山崎 明, 矢部 征, 高橋 幸裕, 森山 多加志, 増子 徳道, 清水 淳平, 源 泰拓, 大和田 毅, 豊留 修一, 岩瀬 由紀, 上嶋 誠, 西山 陽子, 新村 華奈子, 栫 裕亮, 平野 裕紀, 杉山 大輔, 鳥越 孝一郎, 伊藤 洋輔, 玉井 信太朗, 望戸 裕司, 高木 悠, 南 拓人, 丸谷 良博, 久田 大樹, 姫野 康一郎, 安松 潤二, 岳田 和也, 山本 優二, 臼井 嘉哉, 寺本 万里子, 西村 和浩, 三浦 勉
    In the San-in region, Japan (the Tottori and surrounding region in the northern part of Chugoku district, southwestern Japan), epicenters of seismic activities are distributing within a line belt with a width of about 4-9 km along the coastal line of the Sea of Japan. In order to investigate deep resistivity structure beneath the seismic belt and clarify the relationship between subducting Philippine Sea plate and the deeper resistivity structure beneath the San-in region, we have carried out not only seafloor EM measurements using OBEM and OBE but also longer period MT surveys on land since 2006 along two profiles, SW100 and SW200 passing through almost N-S lines of longitudes, 134.3E and 133.4E, respectively. We report the outline of the measurements in the region of sea and land beneath the San-in region, and preliminary results of the data analysis in this manuscript.
    京都大学防災研究所, 01 Jun. 2010, 京都大学防災研究所年報. B = Disaster Prevention Research Institute Annuals. B, 53(0) (0), 181 - 190, Japanese

  • Estimation of the bathymetric effect in the back-arc region of SW Japan using non-uniform thin sheet approximation
    南 拓人, 藤 浩明, 笠谷 貴史
    CA研究グループ, 2010, Conductivity anomaly研究会論文集, 87 - 94, Japanese

  • Long-period electromagnetic responses of the conducting Earth in the back-arc region of Southwest Japan
    南 拓人, 藤 浩明, 笠谷 貴史
    CA研究グループ, 2009, Conductivity anomaly研究会論文集, 56 - 63, Japanese

■ Lectures, oral presentations, etc.
  • 火星地下の比抵抗構造推定におけるHSG法の実現可能性について
    星野咲華, 南拓人, 原田裕己, 寺田直樹, 佐藤雅彦
    2024年度CA研究会「地球物理学・地球電磁気学におけるインバージョン理論の実践と発展」, Mar. 2025, Japanese
    Poster presentation

  • 2011年東北津波に起因する電場変動を用いた神津島地下比抵抗構造推定の試み
    武林哲志, 南拓人
    2024年度CA研究会「地球物理学・地球電磁気学におけるインバージョン理論の実践と発展」, Mar. 2025, Japanese
    Poster presentation

  • ⻤界カルデラ⽕⼭海域での地下⽐抵抗構造
    小畑拓実, 松野哲男, 南拓人, 臼井嘉哉, 市原寛, 巽好幸, 杉岡裕子, 大塚宏徳, 島伸和
    2024年度CA研究会「地球物理学・地球電磁気学におけるインバージョン理論の実践と発展」, Mar. 2025, Japanese
    Oral presentation

  • CSAMTデータを用いた箱根大涌谷西部の三次元比抵抗構造の推定
    南拓人, 萬年一剛, 宇津木充, 城森明
    2024年度CA研究会「地球物理学・地球電磁気学におけるインバージョン理論の実践と発展」, Mar. 2025, Japanese
    Oral presentation

  • CSAMT データを用いた大涌谷噴気帯西部の三次 元比抵抗構造推定
    南 拓人, 萬年一剛, 城森 明, 宇津木 充
    火山学会2024年度秋季大会, Oct. 2024, Japanese
    Poster presentation

  • Estimation of 3-D resistivit y structure under the Kikai submarine caldera volcano
    Obata, T., Araki, M., Hirose, T., Matsuno, T., Minami, T., Usui, Y., Otsuka, H., Ichihara, H., Tatsumi, Y., Sugioka, H., Seama, N.
    Electromagnetic Induction Workshop 2024, Sep. 2024, English
    Oral presentation

  • Attempt to estimate the underground resistivity structure of Kozushima using the electric field variation caused by the 2011 Tohoku tsunami
    Takebayashi, T., Minami, T.
    Electromagnetic Ineduction Workshop 2024, Sep. 2024, English
    Poster presentation

  • Controlled-source electromagnetic survey for investigating Kusatsu-Shirane volcano, Japan
    Ishizu , K., Ogawa, Y., Kitaoka, N., Tseng , K. H., Serita , S., Minami , T., Ichihara, H., Kunitomo, T., Caldwell, G., Bertrand , E.A., Heise, W.
    Electromagnetic Ineduction Workshop 2024, Sep. 2024, English
    Poster presentation

  • Controlled source electroma g netic monitorin Crater Lake , Waimangu, New Zealand
    Caldwell , T.G., O Kitaoka, N. , Kirkb, g awa , Y., Ishizu , K., y , A., Minami , T.
    Electromagnetic Ineduction Workshop 2024, Sep. 2024, English
    Poster presentation

  • Temporal Variation in Controlled Source Electromagnetic Response at Inferno Crater Lake, New Zealand
    Norihiro Kitaoka, Yasuo Ogawa, T. Grant Caldwell, Alison Kirkby, Keiichi Ishizu, Takuto Minami
    Electromagnetic Ineduction Workshop 2024, Sep. 2024, English
    Poster presentation

  • Attempts to detect tsunami-induced electromagnetic fields using machine learning methods: Towards tsunami early warnings
    Chiaki Mita, Takuto Minami, Hiroko Sugioka, Hiroaki Toh
    Electromagnetic Ineduction Workshop 2024, Sep. 2024, English
    Oral presentation

  • Quantifying the DC effect of temporal lake changes at Inferno Crater, Waimangu Geothermal System, New Zealand
    Kirkby, A., Caldwell, G., Ogawa, Y., Ishizu, K., Kitaoka, N., Minami, T.
    Electromagnetic Ineduction Workshop 2024, Sep. 2024, English
    Poster presentation

  • Time-domain simulation of magnetic fields due to tsunami-driven electric currents in the ocean and ionosphere
    Takuto Minami
    Electromagnetic Ineduction Workshop 2024, Sep. 2024, English
    Poster presentation

  • 箱根大涌谷噴気地帯西部の地下比抵抗構造
    萬年 一剛, 城森 明, 道家 涼介, 宇津木 充, 藤本 光一郎, 南 拓人
    JpGU Meeting 2024, May 2024, Japanese
    Oral presentation

  • 火星における地表磁場観測による比抵抗構造探査の可能性
    南 拓人, 原田 裕己, 佐藤 雅彦
    JpGU Meeting 2024, May 2024, Japanese
    Poster presentation

  • 3-D resistivity structure under the Kikai submarine caldera volcano
    小畑 拓実, 荒木 将允, 廣瀬 時, 松野 哲男, 南 拓人, 臼井 嘉哉, 大塚 宏徳, 市原 寛, 巽 好幸, 杉岡 裕子, 島伸和
    JpGU Meeting 2024, May 2024, Japanese
    Oral presentation

  • Benchmark study toward DGRF2020 for IGRF14 - Reproducing DGRF2015 using magnetic data from ground observatories and Swarm satellites
    南 拓人, 松島 政貴, 近藤 優子, 清水 久芳, 藤 浩明
    JpGU Meeting 2024, May 2024, Japanese
    Oral presentation

  • 機械学習による津波誘導磁場の検出の試み
    美田 千璃, 南 拓人, 藤 浩明, 杉岡 裕子
    JpGU Meeting 2024, May 2024, Japanese
    Poster presentation

  • Evaluation of the effect of lake level and temperature on CSEM transfer functions at Inferno Crater Lake, New Zealand
    北岡 紀広, 小川 康雄, Caldwell Gran, 石須 慶一, 南 拓人, Kirkby Alison
    JpGU Meeting 2024, May 2024, English
    Poster presentation

  • 2011年東北津波に起因する電場変動を用いた神津島地下比抵抗の検討
    武林 哲志, 南 拓人
    JpGU Meeting 2024, May 2024, English
    Poster presentation

  • UAV搭載電磁探査による熱水噴火発生場の比抵抗構造監視に向けた挑戦
    萬年一剛, 城森 明, 道家 涼介, 宇津木 充, 藤本 光一郎, 南 拓人
    JpGU Meeting 2024, May 2024, English
    Oral presentation

  • Attempts to produce candidate models for the IGRF-14 (1)
    Masaki Matsushima, Takuto Minami, Shin’ya Nakano, Hiroaki Toh
    第154回地球電磁気地球惑星圏学会・総会, Sep. 2023
    Oral presentation

  • 機械学習を用いた津波誘導磁場検出手法の開発 と実観測データによる検証
    美田 千璃, 南 拓人, 杉岡 裕子, 藤 浩明
    第154回地球電磁気地球惑星圏学会・総会, Sep. 2023
    Poster presentation

  • 海底圧力データと海底磁場 データのジョイントインバージョン による 2007 年千島列島地震の津波 波源推定
    柴原 澪, 南 拓人, 藤 浩明
    第154回地球電磁気地球惑星圏学会・総会, Sep. 2023
    Oral presentation

  • 鬼界カルデラ火山海底下の 三次元比抵抗構造解析
    小畑 拓実, 荒木 将允, 廣瀬 時, 松野 哲男, 南 拓人, 大塚 宏徳, 巽 好幸, 杉岡 裕子, 市原 寛, 島伸和
    第154回地球電磁気地球惑星圏学会・総会, Sep. 2023
    Oral presentation

  • ニュージーランド Inferno Crater LakeにおけるEM-ACROSS 連続観測
    北岡 紀広, 小川 康雄, T. Grant Caldwell, 石須 慶一, 南 拓人, Alison Kirkby
    第154回地球電磁気地球惑星圏学会・総会, Sep. 2023
    Oral presentation

  • Development of 3-D joint inversion code for MT and CSEM data sets for both land and ocean survey situations
    Takuto Minami
    第154回地球電磁気地球惑星圏学会・総会, Sep. 2023
    Oral presentation

  • Finite Fault Model of the 2009 Samoa Earthquake Estimated from Simultaneous Inversion of Ocean-bottom Pressure and Electromagnetic Tsunami Records
    Toshitaka BABA, Hiiro YOKOI, Zhiheng LIN, Takuto MINAMI, Hiroaki TOH
    AOGS 2023, Aug. 2023, English
    Oral presentation

  • Magnetic signatures due to motional induction in the ocean after the 2022 Tonga eruption
    Takuto Minami, Seiki Asari, Rei Shibahara, Neesha Schnepf, Nair Manoj, Hiroaki Toh
    The 28th IUGG General Assembly, Jul. 2023
    [Invited]
    Invited oral presentation

  • Forward modeling of the EM-ACROSS at Inferno Crater Lake, New Zealand
    Norihiro Kitaoka, Yasuo Ogawa, Keiichi Ishizu, Takuto Minami, T. Grant Caldwell, Alison Kirkby
    JpGU Meeting 2023, May 2023
    Oral presentation

  • Tsunami source of the 2007 Kuril earthquake inferred by joint inversion of seafloor pressure and seafloor magnetic data
    Rei Shibahara, Takuto Minami, Toshitaka Baba, Hiroaki TOH
    JpGU Meeting 2023, May 2023
    Poster presentation

  • 3-D joint inversion of the AMT and ACTIVE datasets obtained in Aso volcano
    Takuto Minami, Mitsuru Utsugi, Wataru Kanda
    JpGU Meeting 2023, May 2023
    Poster presentation

  • Imaging 3D resistivity structure under the seafloor of Kikai caldera volcano
    小畑 拓実, 荒木 将允, 廣瀬 時, 松野 哲男, 南 拓人, 大塚 宏徳, 巽 好幸, 杉岡 裕子, 羽生 毅, 田中 聡, 市原 寛, 島 伸和
    JpGU Meeting 2023, May 2023
    Oral presentation

  • Attempt to detect tsunami-generated magnetic variation using machine learning
    美田 千璃, 南 拓人, 杉岡 裕子
    JpGU Meeting 2023, May 2023
    Poster presentation

  • Numerical simulation of tsunami and magnetic signals at Chichijima Island, Japan, after the eruption of Hunga volcano, Tonga, on January 15, 2022
    Takuto Minami, Rei Shibahara, Seiki Asari, Hiroaki TOH
    JpGU Meeting 2023, May 2023
    Oral presentation

  • Hydrothermal system of Aso volcano, Central Kyushu, Japan, inferred by electromagnetic soundings
    T. Minami, M. Gresse, M. Utsugi
    The IAVCEI Scientific Assembly 2023, Jan. 2023, English
    Oral presentation

  • Development of 3D resistivity structure analysis method considering the effects of seafloor topography
    Takumi Obata, Masamitsu Araki, Tetsuo Matsuno, Takuto Minami, Nobukazu Seama
    SGEPSS 2022 Fall Meeting, Nov. 2022, Japanese
    Oral presentation

  • 3D forward calculation of tidally Induced magnetic field in the Lau Basin
    Tokuma Nakaya, Takuto Minami, Nobukazu Seama
    SGEPSS 2022 Fall Meeting, Nov. 2022, Japanese
    Oral presentation

  • Effects of the difference in sensitivity between ACTIVE and MT on the joint inversion in volcanic regions
    Takuto Minami
    SGEPSS 2022 Fall Meeting, Nov. 2022, Japanese
    Oral presentation

  • Estimation of subsurface resistivity distribution after the Aso 2021 phreatomagmatic eruption
    Sakura Ishibashi, Mitsuru Utsugi, Takuto Minami, Shin Yoshikawa, Hiroyuki Inoue
    2022年度日本火山学会, Oct. 2022, Japanese
    Oral presentation

  • The dependence of the tsunami electromagnetic signals observed at islands on the subsurface resistivity
    Rei Shibahara, Takuto Minami
    Electromagnetic Induction Workshop 2022, Sep. 2022, English
    Poster presentation

  • Hydrothermal model of Aso volcano, Central Kyushu, Japan, inferred from AMT and ACTIVE datasets
    T. Minami, M. Gresse, M. Utsugi
    Electromagnetic Induction Workshop 2022, Sep. 2022, English
    Oral presentation

  • EM-ACROSS System: Installation at the Kusatsu-Shirane Volcano, Japan
    S. Serita, Y. Ogawa, K. Ishizu, K.H. Tseng, T. Kunitomo, T. Minami, H. Ichihara, T. G. Caldwell, W. Heise, E. A. Bertrand
    Electromagnetic Induction Workshop 2022, Sep. 2022, English
    Poster presentation

  • Magnetic Signatures of the January 15 2022 Hunga Tonga-Hunga Ha`apai Eruption
    N. R. Schnepf, T. Minami, H. Toh, M. C. Nair
    AOGS 2022, Aug. 2022, English
    Oral presentation

  • Development of joint inversion code to obtain resistivity structure of Nakadake, Aso volcano, consistent between AMT and ACTIVE data sets
    Takuto Minami, Mitsuru Utsugi
    JpGU Meeting 2022, May 2022
    Oral presentation

  • The dependence of the tsunami-generated electromagnetic field on the subsurface resistivity
    Rei Shibahara, Takuto Minam
    JpGU Meeting 2022, May 2022, Japanese
    Oral presentation

  • Development of 3D forward calculation code of tidally-induced magnetic variation
    Tokuma Nakaya, Takuto Minami
    JpGU Meeting 2022, May 2022, Japanese
    Oral presentation

  • Tsunami-related magnetic signals observed at Chichijima Island, Japan, after the eruption of Hunga volcano, Tonga, on January 15, 2022
    Takuto Minami, Rei Shibahara, Seiki Asari
    JpGU Meeting 2022, May 2022
    Oral presentation

  • 地中電磁誘導の数値シミュレーションの現状と将来展望について
    南拓人
    地磁気誘導電流(GIC)研究会, Mar. 2022, Japanese
    [Invited]
    Invited oral presentation

  • ACTIVEとAMTのジョイントインバージョンによる阿蘇中岳の比抵抗構造推定を目指して
    南拓人, 宇津木充
    2021年度CA研究会, Jan. 2022
    Oral presentation

  • Distribution of subsurface resistivity before and after the Aso 2014 magmatic eruption
    Sakura Ishibashi, Mitsuru Utsugi, Takuto Minami, Hiroyuki Inoue
    SGEPSS 第 150回総会・講演会, Nov. 2021, Japanese
    Oral presentation

  • A hydrothermal model of Aso volcano inferred by resistivity structure
    Takuto Minami, Marceau Gresse, Chisato Anai, Mitsuru Utsugi
    2021年度日本火山学会秋季大会, Oct. 2021, Japanese
    Oral presentation

  • Distribution of subsurface resistivity before and after the Aso 2014 magmatic eruption
    Sakura Ishibashi, Mitsuru Utsugi, Takuto Minami, Hiroyuki Inoue
    2021年度日本火山学会, Oct. 2021, Japanese
    Oral presentation

  • Ionospheric ring current system accounting for ground-level magnetic observations during the 2011 Tohoku earthquake tsunami
    Takuto Minami, Masashi Kamogawa, Tsutomu Ogawa, Makoto Uyeshima, Toru Mogi, Hiroshi Ichihara
    Japan Geoscience Union Meeting 2021, Jun. 2021, English
    Oral presentation

  • Toward joint inversion of AMT and ACTIVE data sets for better understanding of the hydrothermal system of Aso volcano
    Takuto Minami, Mitsuru Utsugi, Wataru Kanda, Marceau Gresse
    Japan Geoscience Union Meeting 2021, Jun. 2021, Japanese
    Oral presentation

  • How about using the magnetic field for tsunami warning? Comparison of the tsunami magnetic field and sea level change
    Lin Zhiheng, Toh Hiroaki, Minami Takuto
    Japan Geoscience Union Meeting 2021, Jun. 2021, English
    Oral presentation

  • 1-D forward calculation of tidally-induced magnetic variation in the Lau Basin: Comparison of calculation and seafloor magnetic data and sensitivity analysis
    Tokuma Nakaya, Takuto Minami, Nobukazu Seama
    Japan Geoscience Union Meeting 2021, Jun. 2021, Japanese
    Poster presentation

  • 電磁アクロスデータの時系列解析
    芹田創平, 小川康雄, 曾國軒, 石須慶一, 國友孝洋, 南拓人, 市原寛, T. G. Caldwell
    2020年度CA 研究会, Jan. 2021, Japanese
    Oral presentation

  • Summary: A candidate secular variation model for IGRF-13 based on MHD dynamo simulation and 4DEnVar data assimilation
    Takuto Minami, Shin’ya Nakano, Vincent Lesur, Futoshi Takahashi, Masaki Matsushima, Hisayoshi Shimizu, Ryosuke Nakashima, Hinami Taniguchi, Hiroaki Toh
    2020年度CA 研究会, Jan. 2021, Japanese
    Oral presentation

  • Simulation of tsunami-generated electromagnetic fields for the 2009 Samoa and 2010 Chile earthquakes
    林 智恒, 藤 浩明, 南 拓人
    SGEPSS Fall Meeting 2020, Nov. 2020, English
    Oral presentation

  • A hydrothermal model of Aso volcano based on multiphase flow simulation and resistivity structures from ACTIVE and AMT survey data
    Takuto Minami, Marceau Gresse, Mitsuru Utsugi, Wataru Kanda
    SGEPSS Fall Meeting 2020, Nov. 2020, Japanese
    Oral presentation

  • Forecasts of geomagnetic secular variation using core surface flow models (2)
    Masaki Matsushima, Hisayoshi Shimizu, Futoshi Takahashi, Takuto Minami, Shin'ya Nakano, Ryosuke Nakashima, Hinami Taniguchi, Hiroaki TOH
    JpGU-AGU Joint Meeting 2020, Jul. 2020, English
    Poster presentation

  • Inferring geomagnetic secular variation using MHD/kinematic dynamo modeling with data assimilation
    Futoshi Takahashi, Shin ya Nakano, Takuto Minami, Hinami Taniguchi, Ryosuke Nakashima, Masaki Matsushima, Hisayoshi Shimizu, Hiroaki TOH
    JpGU-AGU joint Meeting 2020, Jul. 2020, English
    Poster presentation

  • Iterative ensemble variational methods and its application for the prediction of geomagnetic secular variation
    Shin'ya Nakano, Takuto Minami, Futoshi Takahashi, Masaki Matsushima, Hisayoshi Shimizu, Hiroaki TOH
    JpGU-AGU joint Meeting 2020, Jul. 2020, English
    Oral presentation

  • Contribution to IGRF-13 from Japan: A secular variation model using a numerical dynamo model and 4DEnVar data assimilation
    Takuto Minami, Shin’ya Nakano, Vincent Lesur, Futoshi Takahashi, Masaki Matsushima, Hisayoshi Shimiz, Ryosuke Nakashima, Hinami Taniguchi, Hiroaki Toh
    JpGU-AGU Joint Meeting 2020, Jul. 2020, English
    Oral presentation

  • Analysis of evolution of the hydrothermal system of Aso volcano from 2014 to 2015 using multiphase flow simulation and 3-D resistivity variation model
    Takuto Minami, Marceau Gresse, Toru Mogi, Mitsuru Utsugi
    JpGU-AGU Joint Meeting 2020, Jul. 2020, English
    Poster presentation

■ Affiliated Academic Society
  • IAGA Division VI

  • The Volcanological Society of Japan

  • Society of Geomagnetism and Earth, Planetary and Space Sciences

  • The Society of Exploration Geophysicists of Japan

■ Research Themes
  • 海底の潮汐起因磁場変動を用いた新しい比抵抗構造推定手法の確立
    南 拓人
    日本学術振興会, 科学研究費助成事業, 基盤研究(B), 神戸大学, 01 Apr. 2025 - 31 Mar. 2029

  • New wave sources estimated by tsunami-generated electromagnetic fields
    藤 浩明, 馬場 俊孝, 南 拓人
    Japan Society for the Promotion of Science, Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (B), Grant-in-Aid for Scientific Research (B), Kyoto University, Apr. 2022 - Mar. 2026

  • Monitoring and evaluation of volcanic activity using UAV magnetotelluric survey
    萬年 一剛, 道家 涼介, 宇津木 充, 藤本 光一郎, 南 拓人
    Japan Society for the Promotion of Science, Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (A), Grant-in-Aid for Scientific Research (A), Hot Springs Research Institute of Kanagawa Prefecture, Apr. 2021 - Mar. 2026
    調査地の箱根火山大涌谷噴気地帯では2008年頃から、地上設置型のCSAMT探査による地下の比抵抗分布解析が何回か実施され、火山活動の消長に応じて、比抵抗分布も変化していることが明らかになっている。本研究課題では、ドローン搭載型電磁探査により、こうした比抵抗分布変化を高時間分解能で明らかにし、噴火切迫性評価に結びつけることを目標としている。このため、ドローン搭載型電磁探査が、CSAMT探査と少なくとも同等の探査深度と分解能を有するよう改良を図ることが最終目標のひとつとなる。 一方、大涌谷における従来のCSAMT探査は南北に伸びた（＝南北走向）発振源からの電磁波を観測点で受信し、比抵抗構造の南北断面を取得してきた。しかし、大涌谷には東西にロープウェイが走っているため、南北方向にドローンを飛行させると、ロープウェイを横断する形になり安全面から好ましくない。そのため、ドローン電磁探査の運用を考えると、東西走行の発振源と断面取得に移行する必要がある。 そこで、本年度は（１）新たに東西走行の発振源を構築すること、（２）CSAMT探査で東西走行の発振源と南北走行の発振源で得られる比抵抗構造に相違がないか確認すること、（３）CSAMT探査とドローン探査で同等の探査深度と分解能が得られるかを確認すること、の３つの課題に取り組んだ。 この結果、（１）については、構築して探査が実施出来た。（２）に関しては大きな相違が見られない結果が得られたが今後精査する。（３）については、探査領域の比抵抗が低いことや、原因不明のノイズにより、ドローン搭載型電磁探査では測線の半分以上で探査深度がCSAMT探査に比べて劣る結果となった。

  • International joint research on precise frequency comb electromagnetic monitoring technology for phreatic eruption prediction
    小川 康雄, 石須 慶一, 南 拓人
    Japan Society for the Promotion of Science, Grants-in-Aid for Scientific Research, Fund for the Promotion of Joint International Research (Fostering Joint International Research (B)), Tokyo Institute of Technology, 07 Oct. 2021 - 31 Mar. 2024
    日本及びニュージーランドは火山国で、火山防災研究への期待が大きい。2014年御嶽山噴火、2018年本白根山噴火、2019年ニュージーランド・ホワイト島噴火は、水蒸気噴火であり、数多くの犠牲者・被災者を出した。水蒸気噴火は、現状ではその前兆を捉えることが困難である。これは、火山体に広がる熱水やガスの分布する構造やその時間変化が、地震や測地の観測から捉えることが難しいことによる。一方、比抵抗は熱水やガスの分布に敏感な物理量であるため、人工電磁場を用いることによって、熱水やガスの分布の時間変動を検知することができる。申請者は、高精度に制御された人工的な周波数コム電磁信号源を用いて、火山体を精密にモニタリングするシステムを構築中である。ニュージーランド国ワイマング地熱地域のインフェルノ火口湖は短期間に湖面が10m上下することが知られており、同システムによるモニター観測に適している。そこで、ニュージーランドのGNS Scienceの研究者と共同して観測を行い、火口湖の熱水系の時間変動を捉える観測を実施する。また共同研究を通じて、精密周波数コム電磁モニタリングシステムに関して、観測方法や解析ソフトウエアの高度化をはかる。 今年度は、現地観測の準備として、現地の共同研究者と実際に現地の予察を行い、送信信号装置や観測点の配置の検討および現地での立入許可の申請を開始する予定であった。ところが、新型コロナウイルスの感染拡大のために日本からの渡航が困難でとなった。そこで、共同研究者の協力を仰いだ。観測ターゲットとなるの地元の土地所有者に実験内容を説明することにより、観測に関して基本的な協力を得ることに成功した。実際の人工電磁信号の観測に関しては、ニュージーランド国GNS Scienceと東京工業大学では同一の測定装置を有することから、時系列解析ソフトウエアに関する情報交換を行った。

  • Precise volcano monitoring by actively controlled electromagnetic signal using frequency comb
    Ogawa Yasuo
    Japan Society for the Promotion of Science, Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (B), Grant-in-Aid for Scientific Research (B), Tokyo Institute of Technology, Apr. 2020 - Mar. 2023, Coinvestigator
    An electromagnetic monitoring system using precisely controlled frequency comb signals was constructed and was demonstrated at Kusatsu-Shirane volcano. Two transmitter dipoles were installed at the southern foot of the volcano, and electromagnetic receivers were placed around the peak. The distances between the transmitters and receivers are approximately 5 km. The current dipoles were in two directions one in the east-west direction and the other in the north-south direction, transmitting sine waves consisting of frequency sets with logarithmic equal intervals in the frequency range of 0.02 Hz to 4.6 Hz. By synchronising the function generator with a 10 MHz GPS signal, the transmitted signal could be repeated with high accuracy over a long period of time. The stacking method using the inverse of the noise as weights was found to be effective in extracting weak repetitive signal components from the received signals, and the response functions could be easily obtained.

  • Ovservational study of hydrothermal system of Aso Volcano with respect to the phreatic eruption mechanism
    Minami Takuto
    Japan Society for the Promotion of Science, Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (B), Grant-in-Aid for Scientific Research (B), Kobe University, Apr. 2020 - Mar. 2023, Principal investigator
    As initially predicted, a phreatic eruption occurred at Aso volcano in October 2021 during the project period, and valuable geophysical observation data was obtained before and after the eruption. Particularly in electromagnetic surveys utilizing artificial electric currents, the three-dimensional analysis of the obtained data revealed that the electrical conductivity, a measure of how easily electrical currents flow in the medium, was low before the phreatic eruption in October 2021 and gradually increased after the eruption, in the region up to several hundred meters below the first crater of Nakadake, Aso volcano. This indicates that the amount of subsurface hot water is changing in conjunction with the phreatic eruption. This study provides observational evidence that the underground electrical conductivity structure changes over time during periods including phreatic eruptions.

  • Basic research on tsunami propagation by seafloor array data of electromagnetic fields
    藤 浩明, 南 拓人
    Japan Society for the Promotion of Science, Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (C), Grant-in-Aid for Scientific Research (C), Kyoto University, 01 Apr. 2019 - 31 Mar. 2022
    昨年度に引き続き今年度は，世界で唯一津波通過時に電磁場と海底圧力が同時に観測されたタヒチ周辺海域のSOC8観測点のデータ解析を行い，それと並行して同海域で観測された2009年のサモアおよび2010年のチリ地震津波の三次元時間領域数値シミュレーションを行なった。 データ解析では，磁場成分の津波の波高への変換に着目した研究を進めた。2009年サモア／2010年チリ地震津波共に，タヒチ周辺海域では遠地津波として観測され，使用した海底アレイの空間スケールでは二次元伝播とみなせることが分かった。そこで，昨年度発表した二次元線形分散波に対する解析解（Minami, Schnepf and Toh, 2021）を用い，観測された津波磁場成分を波高へ変換してSOC8における海底圧力データと照合した所，両者は高い精度で一致することが新たに明らかになった。これは，昨年度発見した磁場各成分と津波波高との位相関係と並び，津波に伴う電磁場を津波の早期警戒に応用する際には非常に有用な性質となる。これらの成果は，既に米国地球物理学連合（AGU）の国際誌上で公表し（Lin, Toh and Minami, 2021），2021年12月にAGUからPress Release，その翌日には京都大学からも記者発表を行なった。また成果の一部は，日本地球惑星科学連合オンライン大会でも研究協力者らと口頭発表済みである。 数値シミュレーションでは，昨年度明らかになった既存の波源モデルの限界を改善するため，タヒチ周辺海域における津波磁場データを最も良く説明する初期水位分布を求めることに成功した。 さらに，2022年1月に発生したトンガ海底火山噴火に伴う津波についても，研究分担者および米国NOAAの研究者と急遽共同研究を行ない，太平洋の海洋島における津波磁場を速報した。この成果は，現在AGUの学術誌で改訂中である。

  • Study on anisotropy of resistivity structure of Aso and Izu-Oshima volcanos
    Minami Takuto
    Japan Society for the Promotion of Science, Grants-in-Aid for Scientific Research Grant-in-Aid for Early-Career Scientists, Grant-in-Aid for Early-Career Scientists, The University of Tokyo, Apr. 2018 - Mar. 2021
    This project focused on the anisotropy of the resistivity structure of the magma routes of Aso volcano, Central Kyushu, Japan, and Izu-Oshima volcano, Izu-Bonin arc, Japan, using the controlled-source electromagnetic (EM) sounding method called ACTIVE and the traditional magnetotelluric EM sounding method. We tested continuous ACTIVE monitoring in Aso volcano as well as campaign observations of ACTIVE and MT in Izu-Oshima volcano. Although the test succeeded in obtaining daily data and transferring them to laboratory, we found that a large amount of gas emission in Aso volcano makes ACTIVE monitoring for long term difficult. We are still analyzing the EM data obtained by this project, which will be completed by the following project, Grant-In-Aid for Scientific Research (B) "Observational study on hydrothermal system of Aso volcano associated with phreatic eruptions".

  • 火山体浅部比抵抗の三次元イメージング手法の開発と阿蘇火山・伊豆大島への適用
    南 拓人
    Japan Society for the Promotion of Science, Grants-in-Aid for Scientific Research Grant-in-Aid for JSPS Fellows, Grant-in-Aid for JSPS Fellows, The University of Tokyo, 22 Apr. 2016 - 31 Mar. 2019
    研究実績の概要
    平成30年度は、前年度末にEarth, Planets and Space誌に投稿した論文の改訂作業を行い、論文は2018年8月に出版された。本論文は、阿蘇山において2014年11月に始まったマグマ噴火の際に、地下の比抵抗構造がどのように変化したかを能動電磁探査ACTIVEのデータを用いて明らかにしたものである。論文投稿時点ではACTIVEレスポンスの振幅データのみの使用であったが、論文の改訂作業の中で、位相データを新たに加えた三次元インバージョンを実施し、より信頼度の高い比抵抗構造の時間変化の推定に成功した。この時点において、現実的なデータセットに対するインバージョンの高速化が実現し、本研究で開発したコードが今後の火山モニタリングにおいて重要なツールになったと言える。本論文の成果は、2018年8 月にデンマークで開催された国際ワークショップにて、口頭発表を行った。またこの成果は、2018年地球電磁気・地球惑星圏学会において、約400件の投稿論文の中から3件のプレスリリース論文に選出された。さらに、2019年１月には本成果の重要性が認められ、本論文はEPS誌のEPS Highlighted Papers 2018に選出されている。 一方で、平成30年度に計画していた2017年8月のACTIVEデータセットの解析は、多くの観測点で振幅が振り切れていた問題があり、現在も解析を進めている段階である。また、伊豆大島のACTIVEデータに関しては、平成31年度に予定されているMT観測結果との同時解析に向けて準備を進めている段階であり、平成30年度中の成果物の出版には至らなかった。今後、阿蘇のACTIVE研究で培った技術を土台に、伊豆大島を含む多くの火山において、他の観測手段との併用を行いながら、ACTIVE観測の応用を進めていきたい。

  • 阿蘇山中岳噴火に伴う地下比抵抗構造時間変化の解明
    南 拓人
    Japan Society for the Promotion of Science, Grants-in-Aid for Scientific Research Grant-in-Aid for Research Activity start-up, Grant-in-Aid for Research Activity start-up, Kyoto University, 28 Aug. 2015 - 31 Mar. 2017
    平成27年度は、ACTIVEデータから阿蘇火山の浅部比抵抗構造を推定することを目的に、第一段階として三次元順計算コードの開発を行った。当初の予定どおり、フリーソフトGmshを用いて阿蘇火山の地形を表現する四面体メッシュを作成した。四面体メッシュでは、観測データと比較可能な精度の高い計算結果を得る為、ACTIVE観測の送信電極付近と、受信機付近のメッシュを特に細かく設定できる仕様とした。一方、順計算の計算手法について、当初の計画では、磁場の誘導方程式を鉛直磁場について解くことを計画していたが、複雑な地形の下では、磁場三成分について解くことが必要であることが明らかとなり、開発方針の変更を余儀なくされた。本研究では新たに、磁場三成分をベクトルポテンシャルによって表現し計算する有限要素(FEM)法を採用し、節点要素を用いる手法(node-based FEM法)と辺要素を用いる手法(edge-based FEM法)の二種類のコード開発を開始した。平成27年度が終了した現時点において、edge-based FEM法による三次元コードの開発が完了し、地形のない層状大地において、電流水平双極子の解析解(Ward and Hohmann, 1988)を、鉛直電流を含まないPMモードにおいて精度よく再現することに成功した。今後、node-based FEM法でも開発を完了させ、精度と計算コストの両面からedge-based FEM法と比較検討し、適した計算手法を採用して阿蘇火山の地下構造の推定を目指していく。平成27年度における本研究の意義は、諸外国に比べてやや立ち遅れている、人工電流を考慮できる三次元有限要素計算手法の開発において、大きな前進を遂げた点にある 。今後、この研究を引き続き進めていくことで、日本における火山電磁モニタリング計算の基礎を確立することが期待できる。

  • Tsunami early warning by realtime transfer of seafloor electromagnetic data
    Toh Hiroaki, IMAI Kazumasa, OHTSUKI Keiji, NAKAMICHI Akika, YAMAZAKI Akira, KAWASHIMA Issei, IWASHITA Kohdai
    Japan Society for the Promotion of Science, Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (B), Grant-in-Aid for Scientific Research (B), Kyoto University, 01 Apr. 2014 - 31 Mar. 2017
    The tsunami is a dreadful natural disaster that can give enormous damage to human society. In this research, we conducted a geomagnetic study that leads to their prevention / mitigation. We aimed to apply to the existing tsunami early warning system the fact that observable electromagnetic fields are generated by coupling of tsunami particle motions of the conductive seawater with the Earth's main magnetic field. As a result, the following two were proved probable by both actual observation and three-dimensional numerical simulation: (1) real-time transfer of seafloor electromagnetic data for tsunami early warning purpose, (2) improvement of the existing tsunami early warning system by adding information from the tsunami-generated electromagnetic fields.

  • 津波電磁場に関する観測的研究
    南 拓人
    Japan Society for the Promotion of Science, Grants-in-Aid for Scientific Research Grant-in-Aid for JSPS Fellows, Grant-in-Aid for JSPS Fellows, Kyoto University, 01 Apr. 2013 - 31 Mar. 2015
    報告者は、2014年6月から、NASA Goddard Space Flight Center の客員研究者として、アメリカで研究を開始した。前年度までに開発した二次元時間領域の津波電磁場シミュレーションコードを三次元に拡張し、平成26年度内に津波誘導磁場の鉛直成分の計算に成功した。今回開発した三次元シミュレーションでは、非構造四面体メッシュを採用した。非構造四面体メッシュは、メッシュ生成アルゴリズムが煩雑になるものの、格子状の数値メッシュに比べて海底地形を高い精度で表現することが可能となる。報告者は、フリーソフトのGmsh (http://geuz.org/gmsh/)を採用し、大気、海水、海底下の四面体メッシュの作成に成功した。報告者は、2014年8月にドイツ・ワイマールで開催されたElectro-Magnetic Induction Workshop に参加し、2011年東北地方太平洋沖地震津波によって誘導された磁場鉛直成分の計算結果を報告した。報告者の発表は、多くの聴衆の関心を集め、津波電磁場現象の津波早期予測への応用について活発な議論が行われた。現在の三次元津波電磁場シミュレーションコードでは、計算コストの問題から、津波の伝搬領域が500km×500km程度の領域に制限される。今後は、津波電磁場の広域計算を目指し、並列計算手法等を導入した計算コストの削減にとり組む予定である。軽量化されたシミュレーションコードを用いれば、今後、従来の津波波源インバージョンに津波電磁場データを組み合わせ、波高と電磁場データの両方を用いた、ジョイント波源インバージョンが可能となる。本研究では、津波の早期予測、及び、津波現象の解析への応用を目指し、引き続き三次元津波電磁場シミュレーションコードを改良していく。

  • Detecting tsunami dynamo effect using vector geomagnetic data on land
    TOH Hiroaki, KAWASHIMA Issei, MINAMI Takuto, YOSHIMURA Ryokei, FUJII Ikuko, OOGI Junpei, YAMAZAKI Akira, ABE Satoshi
    Japan Society for the Promotion of Science, Grants-in-Aid for Scientific Research Grant-in-Aid for Challenging Exploratory Research, Grant-in-Aid for Challenging Exploratory Research, Kyoto University, 01 Apr. 2013 - 31 Mar. 2015
    In order to improve the detection capability of the existing tsunami early warning system, a basic research on detection of the tsunami-generated magnetic field on land using vector geomagnetic difference between a coastal station and an inland reference site was conducted. As a result, an observation system that monitors real-time vector geomagnetic difference between Muroto and Umaji in Kochi Prefecture has been constructed. It is now possible to provide new data for tsunami early warning at the time of he Nankai/East Nankai megatquakes.

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