Imaging of subsurface structure beneath Obama area, Japan using gravity data

Obama area, Kyushu Island, has one of the most promising geothermal fields in Japan and therefore the study of the basement structure contributes to the understanding of the geothermal reservoir features. Two gravity survey of the Obama area were conducted in 2003 and 2004 included 203 stations using Scintrex CG3-3M gravimeters. The deduced Bouguer anomaly is positive, ranges from 10.2 mGals in northern part of Obama and 13.8 mGals in south and west-center parts of Obama. The relief of the basement is confirmed by Cordell and Henderson [1968] which is an inversion three-dimensional solution of gravity anomaly. The basement appears uplifted in the southern, northeastern and western parts of the Obama area. The local structure is mainly composed by horsts and grabens related to the general structure: Beppu-Shimabara graben.