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In the study of mineral resources, ore deposit models are often created to show the distribution of valuable minerals. In ore deposit models of hydrothermal deposits and porphyry copper deposits, conceptual diagrams of hydrothermal systems and hydrothermal alteration are presented from the viewpoint of geology and geochemistry, expressing knowledge on the evolution of alteration and formation of ore deposits by the reaction between hydrothermal fluids from deep underground and rocks and groundwater. Recently, resistivity structures inferred from MT surveys have been referenced to understand magma-hydrothermal system models, and depth information is added to qualitative models. We are conducting research aimed at upgrading electrical and electromagnetic prospecting methods applied to mineral resource surveys. First, we conducted a demonstration test in a hydrothermal deposit in the Nansatsu area of Kagoshima Prefecture to evaluate the applicability of the SQUITEM method and the AMT method to the mineral resource exploration. As a result, both methods were able to obtain resistivity structures that could be used to interpret the geological structure of the hydrothermal deposit. Then, we examined measurement and analysis techniques for both methods and identified their advantages and disadvantages.

Phreatic eruptions are one of the eruption styles that are difficult to forecast and occasionally cause significant damage. Understanding the mechanisms of phreatic eruption is essential for minimizing damage. Mt. Motoshirane, one of the pyroclastic cones of the Kusatsu-Shirane Volcano located in central Japan, experienced a phreatic eruption in 2018 that resulted in casualties. The eruption was abrupt with no precursory phenomena. Mt. Motoshirane has been dormant for the last 1500 years, so it has been poorly monitored and studied. Studies conducted after the eruption suggested that the migration of hydrothermal fluids triggered the eruption, but those studies lacked information on shallow subsurface structure. In this paper, we report on the three-dimensional resistivity structure around the craters of the 2018 eruption, which was estimated from the audio-frequency magnetotelluric data obtained in 2020 and 2022. The inferred resistivity structure basically has a two-layer structure composed of a high-resistivity layer corresponding to the Quaternary lavas near the surface and low resistivities corresponding to the altered Neogene lavas below. Shallow moderately high resistivity regions in the low-resistivity layer are considered fluid reservoirs that have become infiltrated as a result of the eruption. Regions around the crack associated with the eruption showed relatively high resistivity, implying that the low resistivity zone is decoupled beneath the eruption site. We will discuss how the phreatic eruption occurred based on these interpretations in the presentation., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

Additional file 1: Fig. S1 Sounding curves of the apparent resistivity and phase for four components (Zxx, Zxy, Zyx, and Zyy), and the real (upper panel) and imaginary (lower panel) parts of tipper vector, respectively, at three sites (402, 403, and 405) around Naka-dake crater. Black dots with error bars and open red circles indicate observed and synthetic data, respectively. Blue crosses indicate responses calculated from the model of Hata et al. 2018. Fig. S2 Sounding curves of the apparent resistivity and phase for four components (Zxx, Zxy, Zyx, and Zyy), and the real (upper panel) and imaginary (lower panel) parts of tipper vector, respectively, at three sites (407, 408, and 410) around Naka-dake crater. Black dots with error bars and open red circles are observed and synthetic data, respectively. Blue crosses are responses calculated from the model of Hata et al. 2018. Fig. S3 Results of sensitivity test model 1. The uppermost panel shows a NNE–SSW-oriented cross-section passing through Naka-dake crater for the test model. The three lower panels show a comparison of the sounding curves for observed data (black dots), final model (red circles), and test model (blue triangles) at sites 022, 409, and 410. Fig. S4 Results of sensitivity test model 2. The uppermost panel shows a NNE–SSW-oriented cross-section passing through Naka-dake crater for the test model. The three lower panels show a comparison of the sounding curves for observed data (black dots), final model (red circles), and test model (blue triangles) at sites 022, 409, and 410. Fig. S5 Results of sensitivity test model 3. The uppermost panel shows a NNE–SSW-oriented cross-section passing through Naka-dake crater for the test model. The three lower panels show a comparison of the sounding curves for observed data (black dots), final model (red circles), and test model (blue triangles) at sites 022, 409, and 410. Fig. S6 Results of sensitivity test 4. The upper panels show a horizontal section for the test model at 4.25–5.00 km BSL and the locations of the sites (028, 032, and 035) selected for this test (left figure), and a E–W-oriented cross-section passing through Naka-dake crater for the test model (right figure). The three lower panels show a comparison of the sounding curves for observed data (black dots), final model (red circles), and test model (blue triangles) at sites 028, 032 and 035.

Additional file 1: Fig. S1. Plot of data misfit versus model roughness. Various models calculated with several values of hyper-parameters (α2 and β2) are shown. This plot was used to determine α2 under the L-curve criterion. Fig. S2. Comparisons of the observed data with the results of the final 3-D model. Comparisons of the apparent resistivity and the phase of the (a) Zxx component, (b) Zxy component, (c) Zyx component, (d) Zyy component, and (e) tipper component. Red dots with error bars indicate observed data, and green lines indicate model results. Fig. S3. Histograms of (a) sensitivity density and (b) sensitivity. Histograms for meshes within the core region and within the whole calculation domain are shown by red and green bars, respectively. The core region was defined as a cubic region ranging from −1000 to 1000 m in the N–S and E–W directions, and from − 2000 to 0 m in the vertical direction, which covers most of the AMT survey area. The vertical dashed line in each histogram indicates the lower limit of the relatively sensitive mesh (10−6.4 for sensitivity density and 100.5 for sensitivity), which was determined so as to contain most of the mesh of the core region.

An audio-frequency magnetotelluric (AMT) survey was conducted at the foot of Sakurajima volcano in November 2007. This survey was carried out within the framework of the 7th National Project for Prediction of Volcano Eruptions. The main objective was to clarify the shallow layers of Sakurajima volcano for better understanding of the volcanic activity. We measured electromagnetic fields at frequencies from 1 to 10400 Hz at 27 locations along only three lines set on the northern, western, and southeastern flanks of the volcano. The three profiles enclosed the summit area where access is prohibited because of frequent explosive vulcanian eruptions. The data quality was generally considered to be good at frequencies higher than 2 Hz. Two-dimensional and three-dimensional (3-D) modeling approaches were applied to the data. As a result of 3-D modeling, we obtained the following features in the resistivity model. The surface layer showed a high resistivity from several hundreds to thousands of ohm-meters and corresponded to the lava. This surface layer covered a highly conductive layer with a resistivity of less than tens of ohm-meters, interpreted as containing seawater or groundwater. However, the basement structure was not clearly detected, except for in an area of northern Sakurajima. The first lava layer was distributed thickly in northern and western parts of Sakurajima, and the boundary between the first and second layers was typically located at around sea level. The conductive second layer was found at deeper levels beneath areas such as the Haruta-yama lava dome and the Nabe-yama pumice cone. This depression of the low resistivity layer is probably caused by the fracture zone associated with previous eruptions, which is formed above the conductive layer. In the proximity of Hikino-hira lava dome and the Taisho crater, the second conductive layer was found at a depth shallower than sea level, suggesting that a hydrothermal system has developed underneath this area. In contrast, elevation of the second layer was not seen on the side of Taisho crater located nearest to the edge of Nabe-yama, implying that a heat source is not present beneath the eastern foot of Sakurajima.

地下水流動系は水平・垂直方向に広い範囲にわたるが, この解明は地下水資源の量と質に関する保全に不可欠である. これには深部地質構造の推定を可能とする電磁探査法の適用が有効である. 本研究では, この推定精度の向上を目的とし, 電磁波の周波数帯が異なるMT 法と AMT 法を組み合わせ, 比抵抗分布の 3 次元モデリング法を検討した. これを地下水資源が豊富な熊本平野を研究対象に適用したところ, 比抵抗分布より浅部と深部の地下水流動系は類似していることが明らかになった. また, 対象領域は第 1・2 帯水層を含む深度0～500m 間の浅層地下水流動系, 深度500～1500m 間で先阿蘇火山岩類を主とする浅層水理地質基盤, 深度2000～3500m 間で御船層群を主とする深層帯水層, およびそれ以深で肥後変成岩を主とする深層水理地質基盤から構成されることが推定できた., Clarification of groundwater system over wide horizontal and depth ranges is indispensable to the management and maintenance of the amount and quality of groundwater resource. Electromagnetic surveys are suitable to this clarification because of their capability of detecting deep geologic structures. This study proposes a spatial modeling of resistivity by a joint inversion of MT and AMT surveys data and a 3D interpolation method Kumamoto plain covering 35 km ×32 km, which is well-known as being rich in groundwater resource, is selected as a case study. Resultant 3D resistivity model elucidates that the shallow and deep groundwater flows are similar. In addition, a hydrogeologic model of the study is constructed as the shallow groundwater system including the first and second aquifers (0 to 500 m depth range), the shallow hydrogeologic basement rocks consisting chiefly of pre-Aso volcanic rocks (500 to 1500 m depth range), the deep groundwater system containing a large aquifer composed of the Cretaceous sandstone, Mifune formation (2000 to 3500 m depth range), and the Higo metamorphic rocks forming the deep hydrogeologic basement at the bottom.

In the 2000 eruption of Usu volcano, it is widely believed that magmatic intrusion has reached the shallow subsurface to produce significant ground deformation. We performed a magnetotelluric survey over this area aiming for prospecting the intruded magma. The resistivity section exhibits a High-Low-High three-layered structure. The surface layer with a resistivity around 100 Ωm is likely to correspond with the lower Pleistocene andesite. The second layer is very conductive (0.1 to 1 Ωm), thinning and shallowing in the middle. It is difficult to explain such a low resistivity by anything but the Neogene units containing conductive clay minerals with high salinity ground water. Below it the resistivity gradually increases. The top middle part of the third layer shows the resistivities around 10 Ωm. It looks that this part is pushing the second layer upwards. We investigated whether this block is explainable by intruded magma or not from a viewpoint of the resistivity values. We at first calculated the resistivities of rhyolitic melt by using Gaillard (2004) 's model. The melt resistivities are in a range from 1 to 10 Ωm for 800 to 600 ℃. Secondly, we examined the partial melt by using the MBL model of Schilling et al. (1997). Given that the resistivities of melt and solid are 1 and 10,000 Ωm, respectively, the melt fraction corresponding to the bulk resistivity of 10 Ωm should be 15 vol%. Higher melt resistivity requires a larger melt fraction. The discussion above predicts that the bulk resistivity of the intruded magma will increase by some orders of magnitude in the course of cooling and solidification and that this process can be monitored by MT repeat measurements.

Clarification of groundwater system over wide horizontal and depth ranges is indispensable to the management and maintenance of the amount and quality of groundwater resource. Electromagnetic surveys are suitable to this clarification because of their capability of detecting deep geologic structures. This study proposes a spatial modeling of resistivity by a joint inversion of MT and AMT surveys data and a 3D interpolation method Kumamoto plain covering 35 km ×32 km, which is well-known as being rich in groundwater resource, is selected as a case study. Resultant 3D resistivity model elucidates that the shallow and deep groundwater flows are similar. In addition, a hydrogeologic model of the study is constructed as the shallow groundwater system including the first and second aquifers (0 to 500 m depth range), the shallow hydrogeologic basement rocks consisting chiefly of pre-Aso volcanic rocks (500 to 1500 m depth range), the deep groundwater system containing a large aquifer composed of the Cretaceous sandstone, Mifune formation (2000 to 3500 m depth range), and the Higo metamorphic rocks forming the deep hydrogeologic basement at the bottom.