ユニブーム地層探査機による網走湖の湖底堆積構造

抄録

In June of 1976, the sedimentary structures of bottom deposits were surveyed by the seismic profiler “UNIBOOM” in Lake Abashiri. The profilings were a first trial in Japanese lakes.
Lake Abashiri is situated in the Eastern part of Hokkaido Island, 7.2km upstream from the estuary of River Abashiri which discharges to the Ohotsk Sea. It is a maritime coastal lake with mean altitude of 0.35m above sea level, and the morphometric features are as follows : area, 32.8km², maximum depth 16.1m : mean depth, 6.1m : length, 12km : maximum breadth, 4km : and shore development, 2.18.
Continuous seismic reflection profilings were made along five lines traversing the lake as shown by Fig. 1. As a result of the seismic profilings (Fig. 3), two peculiar evidences were observed in the features of strata. One was the disturbances in the strata which appeared at a water depth of about 10m, off both the shores on the each of the lines except for E-E' line, which is shallower than 10m. Another was the presence of strong reflection strata in the level of 12m below the water surface under the trench bottom. The trench (the so called fairway) is carved on the lake floor from the outlet to the point of 300m up D-D' line at the depth of 12.5m, on D-D' line the maximum water depth of trench is 12m, the floor depression is 1.2m in depth, 40m in width, while on E-E' line, the respective measurements are 8.8m, 2.6m and 100m. The trench cannot be carved by the present tidal actions because the water depth is too large to erode the lake floor.
If it is inferred from the above mentioned facts that the lake levels were lower about 10m than present lake levels at the ancient time, then the positions of disturbed strata may be situated at the ancient shore line of the lake reduced and the strong reflection strata under the trench of E-E' line may be corresponded to the past river bed.