2017 Volume 70 Pages 110-123
Kirishima area is one of the major active geothermal areas in Japan. In 1996, Ogiri geothermal power plant began operation and continuing operation without serious problems by Kyushu Electric Power Co., Ltd. and Nittetsu Mining Co., Ltd.
Japan Oil, Gas and Metals National Corporation (JOGMEC) conducts high-precision surveys using airborne gravity gradient and time domain electromagnetic survey by a helicopter. Airborne Gravity Gradient survey was conducted in 2013 and Airborne TEM was done in 2014 at Ogiri area. Also airborne magnetic survey data is available acquired by MMAJ at Origi area.
Analyses of the gravity gradiometer data and the airborne magnetic survey data at Ogiri area were carried out and the results were also compared with other geophysical data and geological data. Analyses applied for gravity and gravity gradient data were as follows. Surface rock density was estimated from gravity gradient data and this density was applied for the terrain correction. Three kind of average depths of stratum boundaries were estimated and frequencies for band-pass filters were determined by frequency analysis. On the survey line LKS13A-16, two-dimensional inversion was applied to gravity and gravity gradient data in two different range of wavelength: shorter wavelength indicating shallower structure and longer one indicating deeper structures. Two-dimensional inversion of three-layered model was applied for the shallow structure. The second layer with lower density reaches up to surface and thickness of the second layer is large around hot springs and steaming ground of Ginyu. The gravity basement is shallower at Ogiri area by two-dimensional two-layered analysis of deep structure. Analysis using a three-layered model was also applied for the airborne magnetic data. Second layer with low susceptibility reaches up to the surface at geothermal manifestations. A density cross section and a susceptibility cross section were compared with resistivity cross sections acquired by the airborne transient electromagnetic data (HeliTEM) and ground surface MT method. The cross sections show similar characteristics to each other. Resistivity of drilling core samples, closely connected to the degree of alteration, has a positive correlation with density and susceptibility. Low-density and low-susceptibility zone, obtained by interpreting data of airborne gravity gradiometry and airborne magnetic survey can be an indicator of alteration zones associated with geothermal activity.
