Submarine geological surveys in the area of the seikan tunnel have being carried out from 1946, by various methods, dredging of sea bottom, sonic logging, seismic survey, under sea drilling and the others. The geological structure of this area is shown at Fig. 1, 2, 3. Sea bottom contains so many faults at which are various fracturs and crushed rocks, that water may leak into the tunnel frequently during the excavation. F 50, one of theirs submarine faults, which is the nearest from the shore of Hokkaido have been excavated excavated at 1969 in the Yoshioka (Hokkaido site of Seikan tunnel) research tunnel. From conclusion of various surveys, we assumed F-50 would be 50m in width in research tunnel and practically it was 100m in width. But structures of F 50 were almost collect.
In 1962, Ministry of Agriculture and Forestry began three-year primary investigations on the remedies of groundwater troubles. They have run to twelve in number. The writers describe the outlines of the results on Yamagata basin and Mitoyo plain by way of example. (A) Yamagata Basin In the basin of Yamagata, thickly round the city of it, there were domestic artesian wells more than 1, 200 in former days. It is about fifteen years since deep wells were developed for irrigation, industry, and city water. In 1963, they had reached to 160 in number. Consequently, all the wells interfered each other, artesian wells were functionally disordered, and the water level dropped. For planning of artificial recharges, the writers attempted a pressure injecting test on Mamigasaki fan. The quantity of injection stayed at 44.1m3/day under a pressure of 0.2-0.8kg/cm2 because of high groundwater level. The writers had better try to shoot water into a trench at the apex of an alluvial fan. (B) Mitoyo Plain 1) Geology of Mitoyo plain is granite, Mitoyo group, the latest diluvial fan deposits (Allerod), and alluvium. Sinking diluvial fan deposits are placed in drowned valleys. Alluvium consists of three members, which are the uppermost, the upper, and th lower. The upper member has Jomon transgression clay. 2) Groundwater can be divided into two types-unconfined water in alluvium and in the terrace deposits and confined water in the submerged diluvial fan deposits and Mitoyo group. 3) Salt water has penetrated into the unconfined water and the confined water in the diluvial fan deposits. The confined water in Mitoyo group is only accompanied by a fall of the water level, as its strata have such a structure that salt penetration cannot occur easily. 4) The coefficient of injection was 50% in this case. Little is yet known as to the principle of such field measurements that injection rates are half as much as pumping rates. So the writers will suppose a mechanical simulation. 5) It is ineffective to inflict an excessive pressure on injection water.