The Experts below are selected from a list of 2040 Experts worldwide ranked by ideXlab platform
Yasuhiro Fujimitsu - One of the best experts on this subject based on the ideXlab platform.
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analysis and interpretation of Magnetotelluric data in characterization of geothermal resource in eburru geothermal field kenya
Geothermics, 2019Co-Authors: Justus Maithya, Yasuhiro FujimitsuAbstract:Abstract The Magnetotelluric Method (MT) is an essential geophysical Method for the exploration of geothermal systems. In this study, the MT Method was used to assess the extent of the geothermal resource in Eburru geothermal field, Kenya, with the aim of delineating the electrical conductivity structure of the area. Dimensionality analyses demonstrated that the MT data could be interpreted using two-dimensional approaches, but some localized 3-D effects were detected. A 2-D MT inversion was performed to generate resistivity models of Eburru geothermal field. Given its ability to recover complex resistivity models for the ground, three dimensional (3-D) MT inversion was also carried out, and a joint interpretation made from the 2-D and 3-D models. Both inversion approaches gave similar results and revealed a low resistivity layer (
Makoto Uyeshima - One of the best experts on this subject based on the ideXlab platform.
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magma reservoir beneath azumayama volcano ne japan as inferred from a three dimensional electrical resistivity model explored by means of Magnetotelluric Method
Earth Planets and Space, 2021Co-Authors: Masahiro Ichiki, Toshiki Kaida, Takashi Nakayama, Satoshi Miura, Mare Yamamoto, Yuichi Morita, Makoto UyeshimaAbstract:An electrical resistivity model beneath Azumayama Volcano, NE Japan, is explored using Magnetotelluric Method to probe the magma/hydrothermal fluid distribution. Azumayama is one of the most concerning active volcanoes capable of producing a potential eruption triggered by the 2011 Tohoku-Oki Earthquake. The three-dimensional resistivity model reveals a conductive magma reservoir (< 3 Ωm) at depths of 3–15 km below sea level (bsl). The 67% and 90% confidence intervals of resistivity are 0.2–5 Ωm and 0.02–70 Ωm, respectively, for the magma reservoir. We assumed dacitic melt + rock at a shallow depth of 4 km bsl and andesitic melt + rock at a greater depth of 9 km bsl. The confidence interval of resistivity cannot be explained by using dacitic melt + rock condition at a depth of 4 km bsl. This suggests that very conductive hydrothermal fluids coexist with dacitic melt and rock in the shallow part of the magma reservoir. For the depth of 9 km bsl, the 67% confidence interval of resistivity is interpreted as water-saturated (8.0 weight %) andesitic melt–mafic rock complex with melt volume fractions greater than 4 volume %, while the shear wave velocity requires the fluid and/or melt volume fraction of 6–7 volume % at that depth. Considering the fluid and/or melt volume fraction of 6–7 volume %, the conductive hydrous phase is likewise required to explain the wide range of the 67% confidence interval of resistivity. The Mogi inflation source determined from geodetic data lies on the resistive side near the top boundary of the conductive magma reservoir at a depth of 2.7 or 3.7 km bsl. Assuming that the resistivity of the inflation source region is above the upper bound of the confidence interval of resistivity for the conductive magma reservoir and that the source region is composed of hydrothermal fluid + rock, the resistivity of the source region is explained by a hydrothermal fluid volume fraction below 5 volume %, which is the percolation threshold porosity in an effusive eruption. This indicates that the percolation threshold characterizes the inflation source region.
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network Magnetotelluric Method and its first results in central and eastern hokkaido ne japan
Geophysical Journal International, 2001Co-Authors: Makoto Uyeshima, Hisashi Utada, Yasunori NishidaAbstract:SUMMARY A new field observation technique based on the Magnetotelluric (MT) Method has been developed to determine deep and large-scale 3-D electrical conductivity distributions in the Earth. The Method is named ‘Network-MT’, and employs a commercial telephone network, to measure voltage differences with long dipole lengths ranging from 10 to several tens of kilometres. This observation configuration enables us to obtain the telluric field distribution with nearly continuous coverage over a target region. Response functions are estimated between the respective voltage differences and the horizontal magnetic fields at a reference point. Owing to the long electrode spacing, the observed responses are relatively free from the effects of small-scale near-surface heterogeneity with a scalelength shorter than the typical electrode spacing. Therefore, physically meaningful direct comparison between the observations and model responses is feasible even if the fine-scale features of near-surface heterogeneity are ignored. This extensively reduces the difficulty, especially in 3-D MT interpretation. The first Network-MT experiment was performed in central and eastern Hokkaido, NE Japan, in 1989. It took about five months to complete all of the measurements, and used 209 dipoles to cover the target area of 200(EW)r200(NS) km 2 . The long electrode spacing enabled us to obtain the voltage differences with a high signal-to-noise ratio. For 175 dipoles, the squared multiple coherency between the voltage difference and the horizontal magnetic field at Memambetsu Geomagnetic Observatory was determined to be more than 0.9 in the period from 10 2 to 10 4 s. 193 MT impedances were computed in tensor form by linear combination of the response functions. The estimated impedances generally possessed smooth period dependence throughout the period range. No drastic spatial change was observed in the characteristics of the tensors for neighbouring sites, and some regional trend could be detected in the spatial distribution. Thus, we confirmed the merit of the Network-MT Method, that its responses are little affected by small-scale near-surface structures. The regional feature of the response implied a significant influence of the coast effect, and was well correlated with the regional geological setting in Hokkaido. Conventional Groom‐Bailey tensor decomposition analysis revealed that the target region is not regionally one- or two-dimensional. Therefore, we developed a 3-D forward modelling scheme specially designed for the Network-MT experiment, and tried to reproduce the Network-MT responses directly. In the 3-D model, a realistic land‐sea distribution was considered. The resistivity of sea water was fixed to be 0.25 V m and, as a first trial of 3-D modelling, the resistivity of the land was assumed to be uniform and its value was determined to be 200 V m by a simple one-parameter inversion. Overall agreements between the observations and the best-fit model responses indicated the importance of the 3-D coast effect in the target region. However, there remained significant discrepancies, especially in the phase of the responses, which provide a clue to determining a regional deep 3-D structure.
Justus Maithya - One of the best experts on this subject based on the ideXlab platform.
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analysis and interpretation of Magnetotelluric data in characterization of geothermal resource in eburru geothermal field kenya
Geothermics, 2019Co-Authors: Justus Maithya, Yasuhiro FujimitsuAbstract:Abstract The Magnetotelluric Method (MT) is an essential geophysical Method for the exploration of geothermal systems. In this study, the MT Method was used to assess the extent of the geothermal resource in Eburru geothermal field, Kenya, with the aim of delineating the electrical conductivity structure of the area. Dimensionality analyses demonstrated that the MT data could be interpreted using two-dimensional approaches, but some localized 3-D effects were detected. A 2-D MT inversion was performed to generate resistivity models of Eburru geothermal field. Given its ability to recover complex resistivity models for the ground, three dimensional (3-D) MT inversion was also carried out, and a joint interpretation made from the 2-D and 3-D models. Both inversion approaches gave similar results and revealed a low resistivity layer (
Yasunori Nishida - One of the best experts on this subject based on the ideXlab platform.
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network Magnetotelluric Method and its first results in central and eastern hokkaido ne japan
Geophysical Journal International, 2001Co-Authors: Makoto Uyeshima, Hisashi Utada, Yasunori NishidaAbstract:SUMMARY A new field observation technique based on the Magnetotelluric (MT) Method has been developed to determine deep and large-scale 3-D electrical conductivity distributions in the Earth. The Method is named ‘Network-MT’, and employs a commercial telephone network, to measure voltage differences with long dipole lengths ranging from 10 to several tens of kilometres. This observation configuration enables us to obtain the telluric field distribution with nearly continuous coverage over a target region. Response functions are estimated between the respective voltage differences and the horizontal magnetic fields at a reference point. Owing to the long electrode spacing, the observed responses are relatively free from the effects of small-scale near-surface heterogeneity with a scalelength shorter than the typical electrode spacing. Therefore, physically meaningful direct comparison between the observations and model responses is feasible even if the fine-scale features of near-surface heterogeneity are ignored. This extensively reduces the difficulty, especially in 3-D MT interpretation. The first Network-MT experiment was performed in central and eastern Hokkaido, NE Japan, in 1989. It took about five months to complete all of the measurements, and used 209 dipoles to cover the target area of 200(EW)r200(NS) km 2 . The long electrode spacing enabled us to obtain the voltage differences with a high signal-to-noise ratio. For 175 dipoles, the squared multiple coherency between the voltage difference and the horizontal magnetic field at Memambetsu Geomagnetic Observatory was determined to be more than 0.9 in the period from 10 2 to 10 4 s. 193 MT impedances were computed in tensor form by linear combination of the response functions. The estimated impedances generally possessed smooth period dependence throughout the period range. No drastic spatial change was observed in the characteristics of the tensors for neighbouring sites, and some regional trend could be detected in the spatial distribution. Thus, we confirmed the merit of the Network-MT Method, that its responses are little affected by small-scale near-surface structures. The regional feature of the response implied a significant influence of the coast effect, and was well correlated with the regional geological setting in Hokkaido. Conventional Groom‐Bailey tensor decomposition analysis revealed that the target region is not regionally one- or two-dimensional. Therefore, we developed a 3-D forward modelling scheme specially designed for the Network-MT experiment, and tried to reproduce the Network-MT responses directly. In the 3-D model, a realistic land‐sea distribution was considered. The resistivity of sea water was fixed to be 0.25 V m and, as a first trial of 3-D modelling, the resistivity of the land was assumed to be uniform and its value was determined to be 200 V m by a simple one-parameter inversion. Overall agreements between the observations and the best-fit model responses indicated the importance of the 3-D coast effect in the target region. However, there remained significant discrepancies, especially in the phase of the responses, which provide a clue to determining a regional deep 3-D structure.
Hongtao Liu - One of the best experts on this subject based on the ideXlab platform.
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explorations of gold and lead zinc deposits using a Magnetotelluric Method case studies in the tianshan xingmeng orogenic belt of northern china
Ore Geology Reviews, 2020Co-Authors: Qingdong Zeng, Tiebing Liu, Ping Shen, Hongtao LiuAbstract:Abstract The Tianshan-Xingmeng Orogenic Belt (TXOB) is located in China’s northern region, and is one of the country’s most important Au-Pb-Zn-Ag-Mo-Cu metallogenic belts. This metallogenic belt is known to have good minerogenetic conditions and exploration potential. However, the engineering detection depths of the majority of the metal deposits are generally less than 300 m. Therefore, high precision geophysical detection techniques are necessary to achieve the required accurate detection results. Among the commonly used techniques, the Stratagem EH4 system has been found to be the better choice for the deep prospecting of the metal deposits in the TXOB. Furthermore, a good understanding of the physical characteristics of the different rock formations in the region is essential for effective and efficient geophysical prospecting. This study revealed that the resistivity values of the ore and rock formation walls were different. In previous related research, the EH4 Conductivity Image System, which is a system combining MT and CSAMT, has been successfully used for ore explorations in China. The results of the geophysical prospecting processes indicated that there was good exploration potential of the aforementioned system in the TXOB. In the current study, the hidden gold ore bodies of disseminated (altered rock-type) gold deposits within the study area were defined by resistivity values of less than 150–1000 Ωm. The concealed gold orebodies in the quartz-type gold deposits were confirmed by resistivity values of 300 Ωm. The mineralization anomalies of the epigenetic lead-zinc deposits were defined by the smaller resistivity values, which were lower than the 100–1000 Ωm range.