日本地熱学会誌

Tracer test is one of the most useful tools to evaluate geothermal reservoirs, since tracer response provides valuable information about geothermal reservoirs and underground fluid flow behavior. In this study, tracer response analysis is examined on the basis of the finite difference method. Using a model of geothermal reservoirs with a large planar dominant flow path, TVD schemes are compared with the first and the third order upwind schemes. Examination of time integration methods and grid spacing is made. The selection of grid points for the evaluation of tracer concentration at outlet on tracer response curves is also discussed. As the result, we found that the finite difference method using TVD shceme with φ=-1 and the first order explicit Euler method as the time integration method is an effective and accurate one when the grid spacing and the grid points for the evaluation of tracer concentration at outlet are set adequately.

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Thermal gradients were obtained from temperature logging data of 687 drillholes in Hokkaido. Most of the data are from the drillholes for hot spring explorations: A mesh data set of thermal gradientswas created from all of the data of the drillholes through a spatial averaging interpolation algorithm.The map from the mesh data represents a regional geothermal structures of Hokkaido. The high density data coverage of the map also delineates local thermal anomalies of volcanic and geother-mal areas. The first-order thermal structures were calculated based on the following assumptions: (1) 10°C at the level; (2) a one-dimensional and steady state model; (3) a constant thermal conductivity of 2.5 Wm^-1K^-1 for the upper crust; (4) a constant heat generation of 1.5μWm^-3 for the upper crust; (5) existence of partial melting zones. The temperatures at 3 km depth below sea level, depths of isotherm of 200°C, and four thermal corss sections across volcanic and geothermal areas were mapped . The results present local thermal structures of geothermal areas as well as regional structures of the whole Hokkaido area. They are concordant with local thermal structures mapped by reliable temperaturedata of deep drill holes of geothermal areas. The consistency suggests that the temperature data from the drillholes for hot spring explorations give a reasonable geothermal gradient map.

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