Shallow Subsurface Thermal Regime in Japan

New Concept of Subsurface Thermal Regime

Abstract

Subsurface thermal regime is affected not only by thermal conduction but also by advection owing to groundwater flow. The effect of thermal advection is especially large in shallow sedimentary layer with active groundwater flow. In geophysical field of study, thermal gradients have been evaluated by means of linear interpolation of surface and bottom hole temperatures to estimate terrestrial heat flow values. Since this method is based on the removal of the advection effect due to groundwater flow in shallow layer, relationship between groundwater flow and subsurface thermal regime should be investigated for further discussion. In this paper, subsurface temperature profiles are investigated from hydrogeological points of view. Introducing a database of subsurface temperature in basins and / or plains in Japan, field evidences of subsurface thermal effects that can be found at depths about 30 m to 300 m will be shown. Main factor of subsurface thermal regime is regional groundwater flow system. The seasonal subsurface temperature fluctuation can reach at deeper layer in the alluvial fun than another area, because alluvial fun deposits have high permeability and thermal regime is affected by infiltration from paddy field. In urbanized areas, temperature inversion due to surface warming can be recognized, and distribution of inversion is closely connected with groundwater flow system.