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Journal Articles

Three-dimensional magnetotelluric inversion using a heterogeneous smoothness-constrained least-squares method

Negi, Takeyuki*; Mizunaga, Hideki*; Asamori, Koichi; Umeda, Koji

Exploration Geophysics, 44(3), p.145 - 155, 2013/09

 Times Cited Count:1 Percentile:5.61(Geochemistry & Geophysics)

This paper presents a fast algorithm for electromagnetic inversion. The algorithm is distinct for the level of accuracy it attains when bypassing the sensitivity matrix update. A common sensitivity matrix for homogeneous half-space is used in all of the iterations. Instead of a fixed sensitivity matrix, the Laplace filter coefficients at each element are updated based on the spatial differences of resistivity calculated by a reconstructed structure derived from the former iteration. This substitution is expected not only to reduce the computation time required for large-scale inversions, such as those for three-dimensional surveys, but also to allow reconstruction for sharp boundaries in resistivity structures. This algorithm was applied to three-dimensional magnetotelluric inversion in order to confirm the effectiveness. This paper reports the results of several synthetic examples using different resistivity models and conditions.

Journal Articles

Magnetotelluric imaging of crustal magma storage beneath the Mesozoic crystalline mountains in a nonvolcanic region, northeast Japan

Umeda, Koji; Asamori, Koichi; Negi, Takeyuki*; Ogawa, Yasuo*

Geochemistry, Geophysics, Geosystems (Internet), 7(8), p.Q08005_1 - Q08005_8, 2006/08

 Times Cited Count:35 Percentile:62.78(Geochemistry & Geophysics)

Mesozoic crystalline mountains (Iide Mountains) in a non-volcanic region of the Northeast Japan arc were imaged by wide-band magnetotelluric soundings. A two-dimensional model shows that an anomalous conductive body is clearly visible beneath the Iide Mountains. The conductor widens with increasing depth, and extends from the near-surface down to the base of the crust and perhaps into the upper mantle. Considering several independent indications of the possibility of crustal magma storage, such as a high-temperature regime, hot spring gases with high $$^{3}$$He/$$^{4}$$He ratios, thinning of the brittle seismogenic layer and anomalous low seismic velocity, it is reasonable to suppose that the conductor reflects partial melt and/or high-temperature aqueous fluids in the crust, related to renewed magmatism in the present-day subduction system.

JAEA Reports

The Survey of the Omagari fault using electric survey

Matsuo, Koichi*; Kishimoto, Munemaru*; Negi, Takeyuki*; Teshima, Minoru*

JNC TJ5440 2005-001, 51 Pages, 2005/08

JNC-TJ5440-2005-001.pdf:15.08MB

The present document is to report the results of geophysical survey by electric syrvey around a site proposed for the Horonobe Underground Research Project at Horonobe-cho, Hokkaidou by JNC. Electrical survey using dipole-diple array was carried out at the area of Hokushin in Horonobe-cho near presumued Omagari-fault. Two dimensional resistivity inversion analysis was carried out and the resistivity structure from the surface to 250m below the sea level became clear.

JAEA Reports

None

Matsuo, Koichi*; Yokoi, Koichi*; Negi, Takeyuki*; Kasagi, Toshio*; Takahashi, Takeharu*; Teshima, Minoru*

JNC TJ5410 2005-001, 142 Pages, 2005/03

JNC-TJ5410-2005-001.pdf:18.35MB

The present document is to report the result of resistivity monitoring by using MT(Magnetotelluric) method near the site proposed for the Horonobe Underground Research Program at Horonobe-cho, Hokkaido by the Japan Nuclear Cycle Development Institute. The stationary MT observation system, installed near the HDB-1 borehole on November 2002, was moved to a site at the Hokusei-en, 4km west of the first site. This system is monitoring for the depth of underground resistivity. 0bservation data at the Hokusei-en from February 1st 2004 to January 31st 2005 was added in the investigation in 2004 fiscal year. But, data cannot be acquired from July 8th to November 11th of 2004 due to the disconnection trouble of the optical fiber cable for data transfer.The results were as follows

JAEA Reports

Resistivity Monitoring for the depth of underground Strycture by using MT Method

Yokoi, Koichi*; Takahashi, Takeharu*; Negi, Takeyuki*

JNC TJ5410 2004-001, 205 Pages, 2004/03

JNC-TJ5410-2004-001.pdf:42.0MB

The present document is to report the results of resistivity monitoring by using MT(Magnetotelluric) method around the site proposed for the Horonobe Underground Research Program at Horonobe-cho, Hokkaido by the Japan Nuclear Cycle Development Institute. One stationary MT observation system was installed near the HDB-1 drilling hole at the north of Horonobe-cho on November 2002.

JAEA Reports

The Survey of the Omagari fault using Audio frequency Magnetotelluric method in Horonobe research project for the Subsurface Environment

Matsuo, Koichi*; Negi, Takeyuki*; Yokoi, Koichi*; Takahashi, Takeharu*; Teshima, Minoru*

JNC TJ5410 2004-002, 188 Pages, 2004/02

JNC-TJ5410-2004-002.pdf:109.99MB

The present document is to report the results of geophysical survey by electromagnetic method around a site proposed for the Horonobe Underground Research Program at Horonobe-cho, Hokkaido by the Japan Nuclear Cycle Development Institute. Remote-reference AMT(Audio-frequency magnetotelluric) method was carried out at 59 stations over 4 km2 in the area of Hokushin in Horonobe-cho. Two dimensional and three dimensional resistivity inversion analysis was carried out and the resistivity structure from the surface to 1,000m below sea level became clear. The resistivity structure was studied by comparing geological structure presumed from existing geological information. Resistivity contour lines extends NNW-SSE trend in plan of resistivity that is coincident with geological structure. The resistivity distribution was good agreement with existing electrical resistivity log data. At 500m below sea level, the resistivity of South-West part from presumed Omagari-fault was ranges from 1 to 3 ohm-m, and that of North-East part is ranges from 3 to 7 ohm-m.

Oral presentation

Magnetotelluric imaging of crustal magma storage beneath Mesozoic crystalline mountains in a non-volcanic region, Northeast Japan

Umeda, Koji; Asamori, Koichi; Negi, Takeyuki*; Ogawa, Yasuo*

no journal, , 

Mesozoic crystalline mountains (Iide Mountains) in a non-volcanic region of the Northeast Japan arc were imaged by wide-band magnetotelluric soundings. A two-dimensional model showed that an anomalous conductive body ($$<$$ 10 ohm-m) is clearly visible beneath the Iide Mountains. The conductor widens with increasing depth, and extends from the near-surface down to the base of the crust and perhaps into the upper mantle. Considering several signals imaging the presence of crustal magma storage, such as high-temperature regime, hot spring gases with high $$^{3}$$He/$$^{4}$$He ratio, thinning of the brittle seismogenic layer and anomalies of low seismic velocity, it is reasonable to suppose that the conductor reflects partial melt and/or high-temperature aqueous fluids in the crust, related to renewed magmatism in the present-day subduction system.

Oral presentation

Resistivity and seismic velocity structure beneath Iide mountains, Northeastern Japan

Asamori, Koichi; Umeda, Koji; Negi, Takeyuki*; Ogawa, Yasuo*

no journal, , 

no abstracts in English

Oral presentation

Three dimensional magnetotelluric inversion using heterogenious smoothness-constraint least-squares method

Negi, Takeyuki*; Mizunaga, Hideki*; Asamori, Koichi; Umeda, Koji

no journal, , 

Electromagnetic surveys have been widely used for several decades in geothermal, mining and oil exploration or disaster prevention. More recently, three-dimensional surveys have been increasingly carried out. However, they require the large-scale inversion consuming significant computation time. Traditional inversion algorithms based on the linear sensitivity matrix consume time to update the sensitivity matrix in the several iterations. In the presentation, we show a rapid calculation algorithm for the electromagnetic inversion procedure that allows to bypass updating of the sensitivity matrix. In the procedure, a fixed sensitivity matrix for a homogeneous half-space is used in all iterations. Instead of the fixing sensitivity matrix, the Laplace filter coefficients in the each element are updated based on the resistivity smoothness calculated by reconstructed structure. This substitution is not only expected to reduce the computing time required for the large-scale inversion such as for the three dimensional surveys, but also expected to allow the reconstruction for sharp boundary structure. We applied the algorithm for the three dimensional magnetotelluric inversion algorithms to confirm the effectiveness. In the presentation, we show the calculation results using several synthetic data examples with different structures and conditions.

Oral presentation

Three dimensional magnetotelluric imaging of the source area of the 2000 Western Tottori Earthquake using heterogeneous smoothness-constraint least-squares method

Negi, Takeyuki*; Mizunaga, Hideki*; Asamori, Koichi; Umeda, Koji

no journal, , 

no abstracts in English

Oral presentation

A Data processing technique of removing DC railway noise from time domain magnetotelluric data

Makuuchi, Ayumu; Asamori, Koichi; Negi, Takeyuki*

no journal, , 

Even though applying the far remote reference magnetotelluric (MT) method, we need long recording period to obtain usable data from the contaminated data by strong and coherent noise in DC railway area. In the recorded MT time series sampled continuously at 15Hz, the noises from DC railway are recorded as square waves in electric time series and as pulses in magnetic time series measured by induction coil. In this study, we attempt to identify and delete the short time windows which are contaminated by the DC railway noises in magnetic time series automatically, and then we attempt to interpolate these windows by plausible data which are calculated as a response of reference magnetic time series. MT sounding curves obtained by above processing are improved over curves obtained from raw data.

Oral presentation

A Data processing technique of removing DC railway noise from time domain magnetotelluric data, 2

Makuuchi, Ayumu; Asamori, Koichi; Negi, Takeyuki*

no journal, , 

Even though applying the far remote reference magnetotelluric (MT) method, we need long recording period to obtain usable data from the contaminated data by strong and coherent noise in DC railway area. In this study, we consider the electric time series model including a trend component, natural magnetic signal response, correlated noise components, and white noise, then attempt to separate to each component with a Kalman filter algorithm. The method was applied to the magnetotelluric data observed near the DC railway and seems to work well in the time domain.

Oral presentation

The Influence of the noise included in far remote reference site on the magnetotelluric survey, and the noise reduction method, 2; A Validation study using survey data

Hama, Yuki*; Makuuchi, Ayumu; Negi, Takeyuki*; Asamori, Koichi

no journal, , 

The remote reference technique is widely used in the magnetotelluric method to decrease local noise by acquiring data simultaneously at a remote site. This technique can reduce the local noise component and extract the magnetotelluric signal by stacking the cross power spectrum between the remote data and field data. Remote reference is especially efficient when applying ideal remote data with no noise component. Yet it is usually difficult in practical surveys. The best way to compensate in such a difficulty is to select multiple remote sites. With multiple remote sites we can expect the same effect obtainable from an ideal remote site without noise. In this paper we applied multiple remote reference method using robust weighted stacking to noisy survey data, in order to redact the noise component included in the remote site which causes the escalation of data scattering.

Oral presentation

Three-dimensional magnetotelluric inversion regarding distortion tensors as unknown parameters

Negi, Takeyuki*; Makuuchi, Ayumu*; Hama, Yuki; Asamori, Koichi

no journal, , 

In magnetotelluric (MT) soundings, Galvanic distortions, which might disturb the reliability of the resistivity model estimated by inversion analyses, are often recognized. In order to solve this problem, we developed a new three-dimensional MT inversion method that can estimate both resistivity model and distortion tensors without prior information. Using synthetic examples of data under distorted and noisy conditions, we demonstrated that the method can estimate both resistivity model and distortion tensors, and that the method could be expected to enhance the reliability of the resistivity model estimated by the MT inversion.

Oral presentation

Seismic velocity and resistivity structure beneath Iide mountains, northeastern Japan

Asamori, Koichi; Umeda, Koji; Negi, Takeyuki*

no journal, , 

no abstracts in English

Oral presentation

Three-dimensional resistivity distribution by magnetotelluric survey to evaluate the distribution of fossil seawater

Sato, Naomi; Hayano, Akira; Teshima, Minoru*; Negi, Takeyuki*; Yamane, Terumasa*; Kawasaki, Shinji*

no journal, , 

This paper reports the preliminary results of the magnetotelluric (MT) survey aiming to evaluate the three-dimensional distribution of fossil seawater in the Horonobe area. The MT survey was conducted in the area of 3 km square adjacent to the Horonobe Underground Research Laboratory at 60 points. Our results showed that the distribution of the resistivity varied in accordance with the geological structure. This survey was funded by the Ministry of Environment, Trade and Industry.

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