Basement Structure by Airgun Reflection Survey in Osaka Bay, Southwest Japan YOShlnorl IWASAKI, Takao KAGAWA, Sumio SAWADA NOriko MATSUYAMA Geo-Research Institute, Osaka Soil Test Laboratory, 3-1-23, Nishihommachi, Nishi-ku, Osaka 550, Japan Kazuya OHSHIMA Hanshin Expressway Public Corporation, 16-1 Shin-minatomachi, Chuoh-ku, Kobe 650, Japan Takeshi IKAWA and Masazumi ONIsHI JAPEX Geoscience Institute Incorporation, 2-17-22, Akasaka, Minato-ku, Tokyo 107, Japan (Received August 18, 1993; Accepted October 25, 1993) Osaka Bay is surrounded by several mountains and considered as tectonically sinking block by fault movements in Quarternary period. Reflection sparker survey was applied in 1966 to study shallow geological structure in Osaka Bay. Since then, many reflection seismic investigations were conducted in Osaka Bay to study foundation problems in civil engineering and geotechnical purposes. These surveys have been limited in depth less than a few hundred meters. This papaer is the first to study deep basin structure of Osaka Bay area by airgun reflection seismic method. Because of the heavy marine traffic in the bay, the survey lines were limited as small as three lines to keep the safety on the sea. The first line is 40 km in length with NE-SW direction along long axis of Osaka Bay. The second line is 28 km in length with EW direction at the central part of the bay. The third line, parallel along the first line and 5 km east of the first line, was 10 km in length. The third line is planned to include the deepest boring (400 m) point in Osaka Bay and extend to cross the second line. The length of the streamer cable was shortened to 500 m and the number of the hydrophones was limited 20 groups with interval of 25 m. Two airguns with volume 300 inch3 were fired at every 25 m along the line under 7 m in depth from sea surface. The results obtained are as follows: 1) The base rock of Osaka Bay was found from 1, 200 m to 3, 000 m in depth, 2) The general trend of basin structure is monotonous dipping to NW direction, 3) Many clear reflection events from surface to base rock in the recorded section suggest the Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji 611, Japan
alternation of sand and clay formations, which are considered mainly young sediments based upon the estimated velocities from 2, 000 to 2, 500m/s, 4) An active fault is found in the west end of the EW line, the west block of base rock thrust over the east block with dip movement of 1,000m. Key words: Osaka Bay, Reflection survey, Sedimentary structure, Base rock depth, Fault. Fig. 1. Location map of the survey area in Osaka Bay. Each line indicats observation line with the location of end points. Triangle marks indicate radio navigation stations. Boring point is on the Line-C. Fig. 2. Airgun reflection survey with field dimensions in Osaka Bay.
Table 1. Survey parameters. Fig. 3. Flow diagram of data handling and processing.
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Fig o 5. Depth section along Line-C. At the points where numbers are labeled along the top of the panel, results of velocity analyses are shown in Tables 4. Arrows indicate the estimated base rock boundary.
Table 4. List of interval velocity along Line-C. The point numbers correspond to those indicated on the section in Fig. 5. Fig. 6. Fence diagram of Osaka Bay structure. Note the sinking movement towards North-West direction.
Fig. 7. Comparison between depth section along Line-C and 400 m deep boring. Strong reflection events correspond to the layer boundaries along the boring log.
NAKAGAWA, K., K. RYOKI, N. MUTO, S. NISHIMURA anc K. ITO, 1991, Gravity Anormaly Map and Inferrec Basement Structure in Osaka Plain, Central Kinki South-west Japan, J. Geoscience, Osaka City Univ.