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.
Geothermal energy laws in other countries may be great reference when Japan considers enhancing its own legal framework to promote geothermal energy development. This paper follows the previous paper of “Geothermal Energy Laws in the World” which was placed in the Journal of Geothermal Research Society of Japan, vol. 34, No.3 (2012). The previous paper surveyed geothermal energy laws of nine major geothermal countries including the U.S., the Philippines, New Zealand, Iceland and Japan. This paper is a supplemental paper to provide information of geothermal energy law of other countries such as Indonesia, Mexico, Italy, Turkey, Costa Rica, Peru and Ethiopia. The surveys show; (i) Each country has its own geothermal energy law to promote geothermal energy development. (ii) Hot Spring Law, that was enacted to regulate small wells for thermal baths, is also used to regulate large-scale wells for geothermal power plants in Japan. There are, however, many inconvenience arise from adapting Hot Spring Law to geothermal energy development. (iii) Japan needs to enact its own “Geothermal Energy Law,” of which objective is to control and to promote geothermal energy development, with reference of other countryʼs geothermal laws.