Construction of Evaluation System for Hydrothermal Resources Using Soil Gas Method

提出了一种利用土壤气中氡浓度的时空变化来判断地下浅层热液状态的方法。所得结果总结如下。（1）在日本西南部九州中部的阿索火山西侧的三个带小孔的温泉区，配备了用于氡测量的1 m深的观测孔。这些地区被认为位于南北走向的断层上。使用闪烁计数器方法，对20分钟内由^ Rn和^ Rn衰变产生的α粒子数<220><222>进行计数。利用氡数的计算理论对测量数据进行处理，得到了<222>半衰期大于^ Rn的^ Rn数<220>和氡气年龄。结果表明，这些量的时间变化与火山活动密切相关， ...更多信息 在阿索地区发生的地震。（2）利用SPOT全色影像的高空间分辨率子景，采用分段跟踪算法自动提取温泉区的线性特征。通过这些线性构造和数字高程模型数据的组合，具有热液流体通道作用的断层被估计为反向断层类型，倾向东80度。(3)A利用有限元法建立了热液流体三维流动分析的数值模拟方法。为了简化分析，将研究区划分为基底岩、第四纪火山岩和断层破碎带3种岩石类型，并根据岩石类型的分布模拟地质构造。利用该模型和模拟方法，计算了深部热液流体条件变化可能引起的浅部温度和压力的变化。计算结果表明，这些物理量在热液流体对流较大的场地变化敏感，因此氡浓度变化显著。在此基础上，提出了水热资源评价的三个步骤：结合卫星影像和数字高程模型，估算连续裂隙的三维分布;探测氡浓度时间变化特征明显的场地，并考虑氡浓度变化和断层分布，进行热液流数值模拟。少

The system to estimate the condition of hydrothermal fluids in the shallow subsurface has been proposed based on the temporal and spatial variations of radon concentration contained in soil gas. The results obtained are summarized as follows.(1)The observation holes of 1 m depth for the radon measurement were equipped at the three hot spring areas with fumaroles in the western side of the Aso volcano, central Kyushu, southwest Japan. These areas are considered to be located on a north-south striking fault. Using the scintillation counter method, the number of alpha particles attributable to the decay of ^<220> Rn and ^<222> Rn for 20 minutes were counted. These measurement data were processed by the calculation theory for the number of radon at the start of measurement, and the number of ^<222> Rn which has longer half-life than ^<220> Rn and the age of radon gas were obtained. It was found that the temporal variations of those quantities correspond with the activities of volcanic micr … More o-earthquakes in the Aso area.(2)The lineaments around the hot spring areas were automatically extracted using the Segment Tracing Algorithm and the subscene of the SPOT Panchromatic imagery with high spatial resolution. By the combination of these lineaments and the digital elevation model data, the fault which has a role of conduit for the hydrothermal fluids was estimated to be a reverse-fault type and dip 80ﾟ east.(3)A numerical simulation method for the three-dimensional flow analysis of hydrothermal fluids has been constructed using a finite element method. Three rock types, the basement rocks, the Quaternary volcanic roocks, and the fault fracture zone, were assigned to the study area for the simplification of analysis, and the geologic structure was modeled by the distribution of these rock types. Using this model and the simulation method, the changes of temperature and pressure in the shallow subsurface, which may be caused by the change of hydrothermal fluids condition in a deep part, were calculated. This calculation clarified that these physical quantities change sensitively at the site with large convection of hydrothermal fluids, and consequently, the radon concentration varies remarkably. The site at which the temporal changes of radon concentration are related closely to the volcanic earthquakes, therefore, is considered to have high geothermal potential.Based on the above results, an evaluation of hydrothermal resources reguires the three steps : estimation of three-dimensional distribution of continuous fractures through the combination of satellite images and digital elevation model ; detection of a site having distinctive feature in temporal variation of radon concentration ; and execution of numerical simulation for hydrothermal flow, taking the change of radon concentration and the fault distribution into account. Less