地熱探査におけるMT法の測点配置と3次元逆解析による探査分解能の関係

抄録

Resistivity structure of a conventional geothermal reservoir is usually characterized by a combination of low-resistivity cap rock and relatively higher-resistivity hydrothermal reservoir underneath. Depth of the boundary between these two zones roughly ranges from 500 m to 1,500 m. Several recent magnetotelluric (MT) surveys revealed the existence of deep low-resistivity anomaly beneath the geothermal reservoir at a depth of several kilometers. These deep anomalies are interpreted as an upflow zone from a deeper part to the geothermal reservoir or a high-temperature supercritical geothermal system. Appropriate setting of MT survey stations is a crucial factor for accurately delineating the resistivity structure of these geothermal features by three-dimensional (3D) inversion. In this study, utilizing an accurate 3D inversion code that uses the finite-element method (FEM) for the forward modeling, we conducted 3D inversion of MT data obtained from a geothermal field as well as several sets of synthetic MT data. We then examined the effects of the station interval and the size of the survey area (survey aperture) on the resolution of both shallow and deep low-resistivity anomalies. Our experiments confirmed: 1) that the station interval is suggested to be less than about 700 m to delineate the resistivity boundary between the cap rock and the hydrothermal reservoir and a smaller interval (e.g., 500 m) can increase the accuracy, and 2) that the size of the MT survey area should be wider than twice of the estimated depth of the deep anomaly in order to accurately delineate such deep resistivity structure.