Volume 38 (2016) Issue 3 Pages 71-84
The geological assessment of fractures, which usually play a role of paths of geothermal fluids and intrusive rocks, and consequently relate to heat sources and reservoirs, is essential to explore geothermal resources.
This paper aims to assess the characteristic of the intrusions such as depth, magnetization, age and so on in the geothermal prospective area. A method of its assessment in this paper is proposed using the integrated analysis of gravity and geomagnetic data sets. The two-layers model consisting of basement and overburden sediment rocks, where dikes and sheets intrude, is applied.
According to Poisson’s theorem, the gravity and geomagnetic data sets will be plotted linearly in the area where the contrast of density and magnetization is uniform. If the data are collected widely enough over the studied area, the linear gravity-geomagnetic correlations arising from the effects of the two-layers model are obtained, implying that their elimination makes a proper estimation of magnetic effects from such intrusive rocks as dikes and sheets which distribute locally.
Three methods, the histogram cross-plot, individual cross-plot and the gravitational-magnetic correlation efficient calculated by the Moving Window Poisson’s method, were applied. The regional linearity of the twolayers model and sporadic locality of the volcanic area are certified.
The result of the study in the central Hokkaido in Japan reveals that the NE-SW direction of strike-slip faults is present which is concordant with that of the stress field in the studied area. This may also support to the former study of the relationship between possible fracture distribution and compressive stress field in the geothermal prospective area. And the small uplifts and depressions along the faults can be analyzed by gravity and geomagnetic data respectively.