Initialising ...
Initialising ...
Initialising ...
Initialising ...
Initialising ...
Initialising ...
Initialising ...
Ogawa, Hiroki; Hama, Yuki*; Asamori, Koichi; Ueda, Takumi*
Butsuri Tansa, 75, p.38 - 55, 2022/00
In the magnetotelluric (MT) method, so as to identify the subsurface resistivity structure, the apparent resistivity and phase profiles are calculated by transforming time-series data into spectral data. The continuous wavelet transform (CWT) is well known as a new method of time-frequency analysis instead of the short-time Fourier transform. The CWT is superior in processing non-stationary wideband signals like the MT signal by adjusting the size of the wavelet according to the value of frequency. However, the calculation settings of the CWT, such as the type of basis function and the wavelet parameter, are often determined empirically because of the arbitrariness of the shape of the wavelet. Although there might be differences between the calculated MT responses and the true responses due to improper settings of the CWT, there are no detailed studies considering the effect of numerical errors derived from spectral transforms on MT data. In this study, focusing on the frequency band between 0.001 Hz and 1 Hz, we examined the optimum calculation settings of the CWT in processing MT data in terms of suppressing the numerical errors caused by the spectral transform of time-series data. We also show the validity of the proposed calculation settings by applying the CWT to MT survey data of different types. Superiority of the CWT with proposed settings is suggested especially when the signal-to-noise ratio of observed data is low. Consequently, the proposed calculation settings were confirmed to strike a balance between the resolutions of the time and frequency domains well and will therefore be effective in obtaining reliable MT responses.
Ishimaru, Tsuneari; Umeda, Koji*; Yasue, Kenichi; Kokubu, Yoko; Niwa, Masakazu; Asamori, Koichi; Watanabe, Takahiro; Yokoyama, Tatsunori; Fujita, Natsuko; Shimizu, Mayuko; et al.
JAEA-Research 2016-023, 91 Pages, 2017/02
This annual report documents the progress of research and development (R&D) in the 1st fiscal year during the JAEA 3rd Mid- and Long-term Plan (fiscal years 2015-2021) to provide the scientific base for assessing geosphere stability for long-term isolation of the high-level radioactive waste. The planned framework is structured into the following categories: (1) Development and systematization of investigation techniques, (2) Development of models for long-term estimation and effective assessment, (3) Development of dating techniques. In this paper, the current status of R&D activities with previous scientific and technological progress is summarized.
Ishimaru, Tsuneari; Yasue, Kenichi; Kokubu, Yoko; Niwa, Masakazu; Asamori, Koichi; Watanabe, Takahiro; Yokoyama, Tatsunori; Fujita, Natsuko; Shimizu, Mayuko; Hama, Yuki
JAEA-Review 2016-016, 44 Pages, 2016/08
This report is a plan of research and development (R&D) on geosphere stability for long-term isolation of high-level radioactive waste (HLW) in JAEA, in fiscal year 2016. The objectives and contents in fiscal year 2016 are described in detail based on the outline of 7 years plan (fiscal years 2015-2021). Background of this research is clarified with the necessity and the significance for site investigation and safety assessment, and the past progress in this report. In addition, the plan framework is structured into the following categories: (1) Development and systematization of investigation techniques, (2) Development of models for long-term estimation and effective assessment, (3) Development of dating techniques.
Umeda, Koji; Yasue, Kenichi; Kokubu, Yoko; Niwa, Masakazu; Asamori, Koichi; Fujita, Natsuko; Shimizu, Mayuko; Matsubara, Akihiro; Tamura, Hajimu; Yokoyama, Tatsunori; et al.
JAEA-Review 2015-019, 42 Pages, 2015/09
This report is a plan of research and development (R&D) on geosphere stability for long-term isolation of high-level radioactive waste (HLW) in JAEA, in fiscal year 2015. The objectives and contents in fiscal year 2015 are described in detail based on the outline of 7 years plan (fiscal years 2015-2021). Background of this research is clarified with the necessity and the significance for site investigation and safety assessment, and the past progress in this report. In addition, the plan framework is structured into the following categories: (1) Development and systematization of investigation techniques, (2) Development of models for long-term estimation and effective assessment, (3) Development of dating techniques.
Umeda, Koji; Yasue, Kenichi; Kokubu, Yoko; Niwa, Masakazu; Asamori, Koichi; Fujita, Natsuko; Shimizu, Mayuko; Shimada, Akiomi; Matsubara, Akihiro; Tamura, Hajimu; et al.
JAEA-Review 2015-012, 43 Pages, 2015/08
The concept of geological disposal of high-level radioactive waste (HLW) in Japan is based on a multibarrier system which combines a stable geological environment with an engineered barrier system. Potential geological host formations and their surroundings are chosen, in particular, for their long-term stability, taking into account the fact that Japan is located in tectonically active zone. This report is to outline 7 years plan (fiscal years 2015-2021) of research and development (R&D) for geosphere stability for long-term isolation of the HLW in JAEA. Background of this research is clarified with the necessity and the significance, and the past progress in this report. The objectives, outline, contents and schedule during the next 7 years are described in detail. In addition, the plan framework is structured into the following categories: (1) Development and Systematization of investigation techniques, (2) Development of models for long-term estimation and effective assessment, (3) Development of dating techniques.
Umeda, Koji; Asamori, Koichi; Makuuchi, Ayumu; Kobori, Kazuo; Hama, Yuki*
Journal of Geophysical Research; Solid Earth, 120(4), p.2279 - 2291, 2015/04
Times Cited Count:3 Percentile:12.07(Geochemistry & Geophysics)Following the Mw 9.0 Tohoku-Oki earthquake, an unusual, shallow normal-faulting swarm sequence occurred near the Pacific coast in the southeast Tohoku district. The observed He/He ratios are significantly lower than the atmospheric value, indicating mantle helium contributed less than 10%. The plausible source of fluids can be attributed to waters released owing to sediment porosity collapse, and smectite-illite and opal-quartz reactions in the subducting sediments, rather than dehydration reactions of the subducting altered basalts and/or hydrated mantle. The aqueous fluids driven off the subducting slab migrate into the fore-arc crust, because of the pressure gradient between lithostatic pore pressure along the plate interface and hydrostatic pore pressure in the overriding crust. The swarm earthquake sequence would have been triggered by stress change associated with the Tohoku-Oki earthquake, enhanced by fluid flow along inherited weakened zones in the crust.
Hama, Yuki*; Makuuchi, Ayumu; Negi, Tateyuki*; 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.
Umeda, Koji; Asamori, Koichi; Kobori, Kazuo; Hama, Yuki
no journal, ,
Earthquake swarms, often interpreted to result from fluids invading the brittle seismogenic zone, have seismicity patterns that are significantly different from an aftershock sequence. Following the Mw 9.0 Tohoku-Oki earthquake, an unusual, shallow normal-faulting swarm sequence occurred near the Pacific coast in the southeastern Tohoku district. An integrated approach combining geophysical and geochemical methods was utilized to establish the presence of aqueous fluids around the seismic source region and their derivation.
Negi, Tateyuki*; 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.
Asamori, Koichi; Hama, Yuki; Umeda, Koji
no journal, ,
no abstracts in English
Asamori, Koichi; Hama, Yuki; Umeda, Koji*; Tanaka, Hidemi*
no journal, ,
no abstracts in English
Ogawa, Hiroki; Asamori, Koichi; Hama, Yuki*
no journal, ,
no abstracts in English
Ogawa, Hiroki; Asamori, Koichi; Hama, Yuki*
no journal, ,
It is generally difficult to improve the quality of magnetotelluric (MT) data if noise is correlated between the electromagnetic fields. Independent component analysis (ICA) is a statistical method for transforming the observed data into signals that are mutually as independent as possible. This paper proposes the way to process MT data accompanied with intentional coherent noise in the time-frequency domain by utilizing continuous wavelet analysis and frequency-domain ICA. At each frequency, two procedures are conducted that are composed of lowering the effect of noise through ICA applied to the wavelet coefficients of horizontal electromagnetic components and selecting these reconstructed coefficients in terms of S/N ratio. As a result of the remote reference processing using the calculated spectra, the apparent resistivity curve is clearly improved, while the way to handle the separated signals leaves much to be desired in the ICA.
Ogawa, Hiroki; Asamori, Koichi; Hama, Yuki*
no journal, ,
no abstracts in English