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 40km in length with NE-SW direction along long axis of Osaka Bay. The second line is 28km in length with EW direction at the central part of the bay. The third line, parallel along the first line and 5km east of the first line, was 10km in length. The third line is planned to include the deepest boring (400m) point in Osaka Bay and extend to cross the second line.
The length of the streamer cable was shortened to 500m and the number of the hydrophones was limited 20 groups with interval of 25m. Two airguns with volume 300inch3 were fired at every 25m along the line under 7m in depth from sea surface.
The results obtained are as follows:
1) The base rock of Osaka Bay was found from 1, 200m to 3, 000m 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 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
