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We have developed a computer program to calculate geomagnetic field changes due to the piezomagnetic effect, which is caused by hydrothermal pressurization. The program uses the distribution of pore fluid pressure that is the result of a numerical simulation for hydrothermal fluid circulation after magma intrusion. First, we present a mathematical formulation of three-dimensional (3-D) effective stress equilibrium equations that contain the hydrostatic buoyancy effects of the pore fluid and the seepage force associated with fluid flow. Then, we develop a 3-D finite element program for solving the elastic effective stress field using the spatial distribution of body forces resulting from both gravity and seepage force, with appropriate boundary conditions. On the basis of the solved elastic effective stress field, the distributions of stress-induced magnetization were estimated from the linear piezomagnetism to calculate the piezomagnetic effect on the ground. We examined the results of numerical experiments on the effects of permeability and the influence of a caprock as important factors in changing the physical state within a volcanic edifice. From these evaluations, we found that both effects influence not only the amount of piezomagnetic anomalies, but also the duration time of the anomalous piezomagnetic field. Finally, our method was applied to actual volcanomagnetic field variations observed at the Merapi Volcano in Indonesia. As a result, our model reasonably explained the observed pattern of variations.

240Pu in marine particle samples by sector field high-resolution ICP-MS was developed. The method was applied for large and small particle samples (particle diameter: >70 μm and 1–70 μm, respectively, collected with a large volume in situ filtration and concentration system at different depths in the water column off Rokkaho, Japan, where the spent nuclear fuel reprocessing plant of Japan Nuclear Fuel Ltd. has started test operation since March 2006.

マグマ貫入後の熱水流動のシミュレーションを行い，熱水系の消長によって地表でのピエゾ磁場がどのように変化するかを数値的に評価した。シミュレーションで得られる間隙水圧の分布から火山体内部の磁化変化分布を見積もり，地表でのピエゾ磁気変化を計算するポストプロセッサを開発し，浸透率および不透水岩体であるキャップロックがピエゾ磁気変化にどの程度影響を与えるかをケーススタディ化することで調べた。その結果，浸透率が小さい方が，地磁気変化が大きく，地磁気変化が観測される時間が長いこと，またキャップロックが存在する場合は，存在しない場合と比べて地磁気変化が大きくなることがわかった。, Geomagnetic field variations observed at many volcanoes suggest temperature changes and/or stress changes inside those volcanoes. Such volcanomagnetic effects are largely controlled by a behavior of volcanic fluids or hydrothermal systems that transport heat and mass from the deep source. In this study, we have developed postprocessors to calculate piezomagnetic effects caused by hydrothermal pressurization. By using a newly developed postprocessor, we carried out numerical experiments on the effect of host-rock permeability and the influence of caprock, as a factor to change the physical state within the volcanic edifice. From these evaluations, it turns out that the both effects influence not only the amount of piezomagnetic changes but also the duration of the anomalous piezomagnetic field.

Thorium-230 concentrations were determined in the Sulu Sea, a small 5000m deep basin surrounded by the Philippine and Malaysian Islands, and the South China Sea. The mid-water of South China Sea flows into the Sulu Sea through the Mindoro Strait (sill depth 420 m). Thorium-230 concentrations in deep waters at two stations (PA-1 in the Sulu Sea and PA-11 in the South China Sea) are considerably lower than those in the open ocean (average concentrations below sill depth: 0.49 dpm/m3 (PA-1), 0.79 dpm/m3 (PA-11)). The low concentration and homogeneity of 230Th in deep water below sill depth are consistent with the trend of other semi-closed basins such as the Japan Sea. We also measured 232Th concentration in the Sulu Sea. From these data, terrigenous 230Th was calculated to be 19% of the measured 230Th in the basin. The most probable explanation for the 230Th deficit in the deep water of the Sulu Sea is an enhanced scavenging effect by the particles supplied from the seafloor. Using a simple box model, the amount of 230Th scavenged by particles supplied from the seafloor is calculated to be 0.052 dpm/m3/y. r 2006 Elsevier Ltd. All rights reserved.

マグマ貫入後の熱水流動のシミュレーションを行い，熱水系の消長によって地表での地磁気がどのように変化するかを数値的に評価した。シミュレーションで得られる温度分布から火山体内部の磁化分布を見積もり，地表での地磁気変化を計算するポストプロセッサーを開発し，マグマの貫入の深さおよび浸透率が地磁気変化に与える影響を調べた。その結果，より浅くマグマが貫入する方が，地磁気変化が大きく，地磁気変化が観測される時間が短いこと，また，浸透率が小さいほうが，地磁気変化が大きく，地磁気変化が観測される時間が長いことがわかった。, We have developed a postprocessor to calculate the geomagnetic field variations caused by hydrothermal activities in the volcanic region. They use distributions of temperature inside the volcano, which are results of a numerical simulation for hydrothermal activity after magma intrusion. The most dominant mechanism of volcanomagnetic effects was considered here: thermomagnetic effect. We examined the influences of host rock permeability and depth of magma intrusion on thermomagnetic effect. Results showed that the shallower depth of intrusion causes larger total intensity anomaly and earlier decline of it and that larger anomalies appear at later times in cases of lower permeability host rocks.

空中磁気データから雲仙火山の広域的な磁化構造を推定した。雲仙火山地域の過去の空中磁気研究のうち、Nakatsuka(1994)だけが、深さ方向に対する磁化構造解析が行われている。Nakatsuka(1994)では、地殻一様磁化と仮定した底部深度解析により雲仙地溝の磁化構造が決められていたが、地殻一様磁化では、表現しきれない問題が残された。そこで本研究では、Nakatsuka(1994)の1991 年に収得された島原半島中部を覆う高高度(海抜高度2300m)な空中磁気データを再度利用して、掘削データから得られた深度方向の情報を取り入れ、かつ磁化強度マッピングにより求められた磁化強度値を採用して、2 次元フォワードモデリングから雲仙地溝の磁化構造を推定した。また高高度なデータからは断層のバウンダリを明瞭に示せなかったことから、2002 年の低高度なデータを用いて、磁気的構造境界の検出をすることで、断層の落ち込みに対する定性的な議論を行った。これらの結果、磁化構造の観点からも、雲仙地溝の構造は、重力、地質断面およびMT 探査の結果から指摘されているように、雲仙西部地域では北落ちのhalf-graben、また、雲仙東部地域では南落ちのhalf-graben となることを示した。, Aeromagnetic analyses have been conducted in and around Unzen Volcano, Kyushu, Japan, in order to reveal the subsurface structure of the Unzen graben. Finally, also from a viewpoint of magnetization structure as pointed out by the result of other geophysical data, it turns out that the Unzen graben has the feature of half-graben, which northern fault fallen in the western Unzen region and southern fault fallen in the eastern Unzen region, moreover, it clarified that magnetization lows (expand from east to west) corresponding to the hydrothermal alteration exists in the center part in the graben.

雲仙科学掘削の一観測項目として、長崎県防災ヘリの協力をえて、2002 年9 月18 日に雲仙岳を中心とする低高度な高密度空中磁気探査を実施した。近年の噴火は、1990 年11 月17 日に地獄跡火口と九十九島火口における水蒸気噴火に始まり、翌年の 2 月の屏風岩火口からの火山灰噴出の後、5 月に地獄跡火口に溶岩ドームが出現した。噴出した溶岩の多くは、ローブ状に東部斜面に張り付くとともに内性的成長を続けドーム状の平成新山を形成した。山頂近くで磁場の連続および繰り返し観測がなされているが、地上観測では測点が限られ、面的に磁化構造を把握しきれない。また雲仙火山地域における過去の空中磁気探査では、Nakatsuka(1994)、Mogi et al(1995)、 Honkura et al(1995)などが挙げられるが、飛行高度、Target 領域および位置精度の問題が存在し、雲仙火山とその周辺における詳細な磁化構造は把握しきれていないのが現状である。また、空中磁気探査界において飛行航跡をスパイラル飛行としたのは、世界初であることも特記しておきたい。本研究の目的は、1.雲仙火山とその周辺の詳細な磁気異常分布から地下の磁化構造を推定し、火山活動と関連する地質構造や熱構造等を明らかにすること、2．雲仙の火山活動に関連する、雲仙地溝の詳細な2 次元磁化構造を明らかにすることである。調査飛行は、2 高度面(対地高度約320m および180m)同じ領域で行われ、両高度面ともに普賢岳を中心とする概ね10km 四方の範囲のスパイラル測線である。本調査では急峻な地形に沿っての飛行の為に、普賢岳平成新山地域では1600-1700m と高く、飛行高度に約1200m 程度の差があると同時に、スパイラル測線のため、データの密度に偏りもある。そこで、観測高度を滑らかな曲面にグリッド化させ、且つ磁気異常のノイズを取り、長周期分の波長を抽出する目的で、観測高度下に地形補正済み磁気異常を説明するequivalent anomaly(Makino et al, 1993)をinversion により求め、観測高度面に並行する鉛直500m 上の基準面に引き直した。この磁気異常値を入力値として、2 高度面を同時に、ジョイント・インバージョンを行った。なおデータの個数は2 高度分の6498 個、未知数は入力データ範囲の1.8km を覆う6400 個とし、データと未知数のグリッド間隔は各々150m で、20 回の反復計算より求めた。今回得られた磁化強度マッピングから、以下の点を明らかにした。1. 雲仙火山における溶岩の磁化強度は各々の噴火に対し、値がかなり異なる.2. 平成新山は低磁化強度を示し、溶岩が完全に冷却されていないことが示唆される。3. 崩壊壁や温泉と今研究で得られた低磁化強度域は一般的に良く一致している。, On September 18, 2002, we conducted a high-resolution, low-altitude helicopter-borneaeromagnetic survey over Unzen Volcano. Therefore, we conducted a magnetization intensitymapping on the volcano, on the assumption that the magnetic anomalies are caused by the terrainmagnetized in the same direction as the present Earth's magnetic field and the magnetizationintensity varies only laterally. This map shows good agreement with the geologic features, especially the hydrothermal alteration zone and the collapsed pyroclastic deposits. In addition, even in the area covered by lavas, the magnetization intensities show various values correspondingto each eruption event. Local magnetization lows on Heisei-Shinzan suggest that theHeisei lava produced by the 1991-1995 eruption has not yet been cooled enough.

2002年9月18日に雲仙科学掘削の一観測項目として, 雲仙岳を中心とする空中磁気測量を実施した。雲仙火山では, 1990年11月に約200年ぶりに噴火活動が始まり, 1991年5月には地獄跡火口より溶岩ドームが出現した。噴出した溶岩の多くは, ローブ状に東部斜面に張り付くとともに内性的成長を続けドーム状の平成新山を形成した。山頂近くで磁場の連続や繰り返し観測がなされているが, 測点が限られていて十分でない。そこで本研究の目的は, 空中磁気探査により, 広域かつ精密な磁気構造を把握すること, 噴出した溶岩の冷却過程を調べることにある。ここでは今回得られた空中磁気データを用いて, 磁気異常分布を示し, 得られた磁気異常を考察する。, We conducted helicopter-borne magnetic surveys over Unzen Volcano, southwestern Japan on September18, 2002 in order to investigate the subsurface structure of Unzen graben and the cooling process of thelava dome formed in the latest eruption spanning between 1991 and 1995.Unzen volcano started a phreatic eruption in November, 1990 at summit craters after a quiescenceof 198 years. After intensive ash ejection in February 1991, a lave dome appeared at the eastern neighborof the previous peak of Mt. Fugen in May, 1991.The lava dome, named Heiseishinzan, gradually grew on theeastern flank of Mt. Fugen and yielded frequent pyroclastic flows until the surface activity ended in1995.The survey consists of three flights of 1 hour for each. The first flight covers an area over Futsu, Chijiwa, and Kanahama faults, the major normal faults that form Unzen graben system. The second and thethird flights cover the summit area of Unzen volcano with spiral trajectories at altitudes of 1000 and500 ft, respectively. The spacing between the survey lines is about 500 m. Geomagnetic total field wasrecorded by an optical pumping magnetometer (GEOMETRICS) and an Overhauser proton magnetometer (GEM)installed in and over the sensor bird, which is suspended with a wire of 20 m long under the airframe.The sampling intervals of these magnetometers are 0.1 sec and 0.5 sec, respectively. While real timenavigation was achieved by a portable GPS receiver with a PC monitor, precise positioning data of thesensor bird was obtained by the differential GPS technique with a time resolution of 1 sec.Diurnal magnetic variations of extra-terrestrial origin were removed by subtracting the total fielddata recorded at a temporal station nearby. As a next step, we estimated the effect of surface topographyusing a statistical correlation method (Grauch, 1987) in order to model a deeper magnetic structure. Inthe present study, we propose a plausible model of magnetic structure beneath Unzen volcano.Main features of the magnetic map are positive anomalies on the summit area of the volcano(Heiseishinzan lava dome and Mt. Fugen) and a negative anomaly on Unzen hot-spring area.

13th European Winter Conference on Plasma Spectrochemistry

The activities of anthropogenic and natural occurring radionuclides in marine environment have been monitored near the nuclear facilities. These data has been used to construct a dynamic model in the ocean. However, some radionuclides have been used as a tool to elucidate particle dynamics on time scales ranging from day to year in the ocean. Because the concentrations of radionuclides and particles are very low in the ocean, it is difficult to concentrate and fractionate the particulate matters with the filtration systems in seawater. The large volume in situ filtration and concentration system (LV-FiCS) was developed to collect various forms of trace radionuclides and particles in seawater. The LV-FiCS has been operated during several cruises in the coastal waters off Japan, and four to eleven m3 of seawaters were filtered through different kinds of filters and then pass through the adsorbents to concentrate radionuclides simultaneously. We report the vertical profiles of plutonium and thorium with the size-fractionated method and those of cesium and technetium. These results suggested the behavior of these nuclides in the ocean.

Goldschmidt2008 Conference

Thorium is one of the least soluble elements in seawater: its dissolved species is considered as Th(OH)n(4-n)+ [Turner et al., 1981]. Th-232 (t1/2 = 1.4 * 1010 years) in the oceans is almost entirely contributed by continental weathering, whereas 230Th (t1/2 = 75,200 years) is produced in situ within the water column through the decay of a parent nuclide 234U in the seawater. Vertical distributions of 230Th have been well described by reversible scavenging model in the Pacific Ocean [Nozaki et al., 1981; Bacon and Anderson, 1982; Nozaki, 1983; Nozaki and Nakanishi, 1985; Nozaki et al., 1987]. Scavenging-mixing models have been adopted in regions where horizontal advection is the dominant factor controlling 230Th distribution, such as in the Weddell Sea [Rutgers van der Loeff and Berger, 1993], the Atlantic Ocean [Vogler et al., 1998; Moran et al., 1997; 2001; 2002] and the Andaman Sea [Okubo et al., 2004]. Previous studies on thorium isotopes in seawater have elucidated scavenging processes in water columns in various oceans. Nevertheless, thorium isotope studies have not been performed sufficiently in mid-latitude of the Pacific Ocean. This study clarifies vertical distributions of thorium isotopes, especially 230Th, to study scavenging processes of thorium and trace metals in this region. We investigated the vertical distribution of thorium isotopes in mid-latitudes of the Pacific Ocean especially 230Th as a test case of scavenging of metals, and discuss the control factor of the distribution of 230Th. In comparison with previous studies in the Pacific Ocean [Nozaki et al., 1981; Nozaki and Nakanishi, 1985; Nozaki et al., 1987], the higher latitude stations show lower 230Th concentrations. This tendency corresponds to the primary productivity in surface oceans. The prominent feature is the depletion of 230Th concentrations compared with that estimated by reversible scavenging model calculations in deep water in BO-3 (30o N, 160oW, Depth: 5778 m) and BO-5 (19oN, 175o W, Depth: 5480 m). For these deficiencies, two possibilities were considered 1) lateral transport of re-suspended particles 2) enhanced scavenging effect in deep water., American Geophysical Union 2007 Fall Meeting

Indian Ocean Planning Workshop

005年6月および2006年6月の六ヶ所村沖の調査航海では、超大容量現場型濾過装置を用いて、5000Lの海水を各層において濾過し、粒子態のプルトニウム同位体の鉛直分布を測定した。プルトニウムの比放射能は深度とともに増加し、生物生産の活発な表層では、試料中のプランクトン破砕物によって、プルトニウムが希釈されていた。, International Symposium Environmental Modeling and Radioecology

International Ocean Research Conference

We examined the vertical distributions of 239+240Pu activity and 240Pu/239Pu atom ratio in particles collected by large volume water in-situ pump in the western North Pacific Ocean (off Rokkasho, Japan), where the spent nuclear fuel reprocessing plant of Japan Nuclear Fuel Ltd. has started test operation since March 2006. This is the first information of vertical diatribution of plutonium activity and plutonium atom ratio in small aprticle (1-70 micrometer) and large particle (.70 micrometer).