Heat transport analysis and three-dimensional thermo-hydraulic simulation in the Ishikari basin, Hokkaido, Japan

Temperature profiles of 22 observation boreholes in the Ishikari basin were investigated in order to extract information on the ground thermal processes and the associated water flow disturbances. Data analyses methods were based on temperature profile typology, thermal gradient–temperature plots, and a thermo‐hydraulic model. The distributed data sets of measured temperature profiles and thermal conductivity show lateral heat flow variations from 34 to 162 mW m−2. These thermal conditions revealed the co‐existence of a highland geothermal field, a lowland central subnormal heat flow area, and a strong heat flow anomaly near the northeast Ishikari River. To explain this specific configuration of the thermal field, possible effects of petro‐physical properties, topography and groundwater movements were examined with a three‐dimensional thermo‐hydraulic simulation model. Laboratory measurements of thermal conductivity, porosity, density, and permeability from borehole core samples were used to constrain the model parameters. The main results suggest that: the lateral increase of geothermal gradient and heat flow density with elevation, i.e. in the southwest direction, are essentially due to hydro‐ and thermo‐physical differences between the hydrogeologically active Quaternary sedimentary system and the underlying Tertiary volcano–sedimentary formation; the low heat area in the central lowland may be explained by the effect of its local groundwater flow recharge, and then the positive heat anomaly near the Ishikari River would result from a basin‐wide heat flow redistribution by water flow. In general, the thermal regime in the Ishikari basin appears dominated by conduction processes in the southwest area and strongly influenced by advection phenomena in the northeast. Copyright © 2002 John Wiley & Sons, Ltd.