Shallow subsurface thermal regimes in major plains in Japan with reference to recent surface warming

Abstract

It is generally recognized that the temperature profile in a borehole is affected by both groundwater flow and past climatic change. Solid earth scientists seek to remove such effects from a temperature profile for the corrections of terrestrial heat flow values. It has also been well recognized by hydrogeologists that the distribution of subsurface temperature is affected by groundwater flow and that those temperature data can be used to evaluate the direction and velocity of groundwater flow. There are studies dealing with the effect of surface temperature change on the subsurface thermal regime in the solid geophysical literature, and there are studies on the effect of groundwater flow on subsurface temperatures in the hydrogeological literature. However, there are few studies including both effects. This paper intends to provide some evidence of interactions between the two competing effects that we can see at depths about 30–300 m in Japanese basin or plains. We have compiled shallow subsurface temperature–depth profiles at depths about 30–300 m in Japanese basins and plains. Number of thermal data set is 15 areas from Hokkaido to Kyushu at present, and we can classify thermal data sets into four categories depending on the thermal regime. This paper, moreover, describes details of the subsurface thermal regimes in the Yamagata Basin and the Nobi Plain. Thermal regime in the Yamagata Basin was calculated by two-dimensional analytical solution and estimated the effect of regional groundwater flow. Result of three-dimensional heat transport simulation in the Nobi Plain shows that there are two effects of regional groundwater flow and surface temperature warming owing to urbanization on subsurface thermal regime.