| Abstract |
Poisson’s theorem describes the relation existing between gravitational and geo-magnetic potentials. The integrated analysis of surface gravity and aero-magnetic datasets is adequate to calculate subsurface magnetization-to-density ratio by converting aero-magnetic data to those of pseudo-gravity data and repeatedly applying the theorem within a small, moving data window. The results show the regional variation of the ratio, and lead to delineate not only appeared to surface but buried volcanic conduits and intrusive rocks, and to estimate an approximate age of volcanic activities. These delineation trends are considered as being along possible deep strike-slip faults, which are normally oriented along the NW-SE. The analysis is extended to the application for the gravity gradient and magnetic datasets, which are acquired by helicopter at low flight height and with extremely dense measurement density in the mountainous areas where several geothermal prospects are located, in the south-eastern part of Akita prefecture, Japan. The magnetic data are converted to those of reduction to pole. The grid interval can be 25 m. The correlations between the two datasets are extremely well. The results of analysis show the detailed variations of mainly near-surface (up to about 500m from surface) magnetization-to-density ratio. These help to assess not only the precise near-surface distribution of volcanic conduits and intrusive rocks, but the possible extension of near-surface lava distributions. the delineation of possible strike-slip faults more in details and the local stress distributions under the regional E-W compressional field. The geological information obtained by the analysis helps to interpret the distributions of heat sources and releasing type fractures, and consequently contributes to evaluate geothermal potentials more accurately. |