| Co-Investigator(Kenkyū-buntansha) |
MATSUMOTO Satoshi Akita University, Faculty of Engineering and Resource Sciences, Lecturer, 工学資源学部, 講師 (40221593)
FUKUDOME Takaaki Akita University, Faculty of Engineering and Resource Sciences, Associate Profes, 工学資源学部, 助教授 (50125738)
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| Budget Amount *help |
¥2,900,000 (Direct Cost: ¥2,900,000)
Fiscal Year 1997: ¥2,900,000 (Direct Cost: ¥2,900,000)
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| Research Abstract |
The purpose of this research is to improve the S-wave reflection seismic prospecting and to extend its availability to active structures. The fundamental policy of the research is to extend merits of S-wave in cooperation with other methods.
Along the coast of the Sea of Japan in Akita prefecture, there are two major active faults, the Kitayuri thrust faults in the south and the Noshiro thrust faults in the north. They are assumed to meet in Hachiro-gata Reclaimed Land, where active faults have not yet been found. Across the assumed fault line in this area, the reflection seismic prospectings ofS- and P-waves were carried out with some auxiliary experiments. Their main points and the results are described in the following.
(1) In usual shallow grounds, S-wave velocities are very lower than P-wave ones, and reflect facies variation. Hence, S-waves are characterized by shorter wavelengths and effective reflection. Such properties of S-waves are adequate to the shallow high resolution prosp
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ecting. Its field work efficiency, however, is lower, because receiver spaces should be shorter. Moreover, the depths to be explored are shallower, because powerful SH-wave sources can not be obtained easily. Accordingly, it is important to cooperate the SH-wave method with the P-wave method, which can be applicable to deeper exploration. This fact was confirmed through this research.
(2) Clear understanding of Velocity characteristics in surface layrs is essential to perform effective static correction in both methods of SH- and P-waves. In this field, important problems to be dealt with were very soft mud layr (deposited on the original lake bottom) for S-waves, and thick low velocity layr (probably caused by partial gas saturation) for P-waves. To solve these problems detailed experiments about surface layrs were conducted.
(3) Reflected SV-waves included in the P-wave records were also analyzed. In this case, clear understanding of wave conversion characteristics in the surface layr was very important. Experiments were conducted for it. The CMP stack section of SV-wave fairy well coincides with that of SH-wave.
(4) In depth of about 500m, a fault (F_1) with throw of 500m at least was estimated by investigating the CMP stack sections of P-wave with help of gravity data. In the CMP stack section of SH-wave over F_1, a vertical displacement (F_2) of 1 to 2m is found at horizons of about 40 and 50m in depth. Faults connecting F_1 and F_2 can not be found in the present accuracy. Therefore, F_2 can not be recognized evidently as fault. Less
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