| Abstract |
Detailed gravity anomaly map over wide areas is thought to be useful to understand and to analyze underground structure in geothermal fields. Gravity measurements in and around the Japanese Islands were started from a century ago. In 1899, Dr.H. Nagaoka connected the absolute gravity in Potsdam with Tokyo by using Sterneck pendulum. Until 1915, more than 100 gravity stations were settled over the Japanese islands. In early period of gravImetrical studies, the measurement of gravity itself was one of important phys ica 1 s tud.ies. The first gravity measurement in oceanic area was carried out off Sanriku district, around the Japan trench axis,' in 1932 by Dr.M. Matsuyama. A submarine and a Vening Meinesz type pendulum were employed for the measurements. After the second world war, gravity measurements by using spring type gravimeters increased rapidly. In 1954, gravity measurements at the second order leveling stations along national roads were performed by Dr.C. Tsuboi and his colleague by using Wordon gravimeter and number of measurement points was 4,500. In 1969, Geographical Survey Institute (G.S.1.), completed the precise gravity measurements one round allover Japan by using North American gravimeters and the G. S. 1. type pendulums. Measured data were increased in about 10,000 points. Their gravi'ty anomaly maps showed already complicated pattern of the anomaly reflecting complicated geological structures of the Japanese Islands. However, gravity stations were still restricted only along national roads and then wide vacant spaces were remained inside of them. Furthermore, many of national roads are running along faults or geological boundaries, it is obvious that many of gravity stations were situated in geologically singular locations. Geological structures in Japan are so complicated as those extensions are ordinarily limited within tens of kilometer. sparse gravity stations may not give real information on subsurface structures. |