Abstract
Seismic reflection survey and boring were performed in Hikami Drainage Basin in Hyogo Prefecture.
Surveyed area is located near Iso where the lowest watershed in the main devide of the Honshu Island is located.
Seismic profiles of the reflection survey show that unconsolidated sediments is 80 to 90 meters thick. The cored boring was drilled to depth of 100 meters on the profile line of the survey. This boring reached green rock of Tanba group at the depth of 96.6 meters (TP-5.92m).
Cored samples contain a volcanic ash layer in depths ranging from 4.10 meters to 4.30 meters. As a result of Tephra analysis, it became clear that this volcanic ash was Aira-Tn (AT) tuff having ages of 22, 000 year. Therefore Holocene sediments are less than 4.1 meters in thick in this boring point. It is considered that almost unconsolidated sediments are Pleistocene.
Sedimennts in depth from 0.6 meters to 6.5 meters contains rich silt and clays. Almost sediments under depth 6.5 meters are gravel and sand. The value of coefficient of permeability of these gravel and sand is 10-2-10-3cm/s which was obtained by the permeability test.
This stratum of gravel is aquifer which provide much ground water in this area.
