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.
Ninety-one carbonate samples in the Sumikawa geothermal wells (37 core samples from well S-2 and cutting samples from other 9 wells) were analyzed for their carbon and oxygen isotopic compositions to discuss the origin of fluids in equilibrium with them. Chemical compositions as well as CO2 content of rocks from well SC-1 were also analyzed in details. The δ13C values of carbonates studied are -7.8 to -0.6‰ similar to those reported in the world wide geothermal systems. The calculated δ13C values of HCO3- in fluids or CO2 gas in equilibrium with the carbonates are -12 to -3 ‰ and -9 to -2 ‰, respectively, and show no good relationship between them and geological features such as rock type, distribution of clay minerals and so on. The δ18O (SMOW) values of carbonates in the northern wells (low temperature zone at Sumikawa) vary from -4.2 to +12.8‰, whereas those in the southern wells (high temperature zone) are lower than +5‰. The calculated δ18O (SMOW) values of fluids in equilibrium with the carbonates above 200 mASL are -14 to -10 ‰ similar to those of the local meteoric waters. The CO2 contents in rocks from well SC-1 abruptly increase up to 10 wt% with increase in depth to 700 mASL, where all of Ca in rocks is fixed as carbonates (calcite), and gradually decrease up to 200 mASL. These features indicate that meteoric water penetrated from surface down to cap rock at 200 mASL without oxygen isotopic shift. In contrast, δ18O (SMOW) values of fluids in equilibrium with carbonates below 200 mASL show an oxygen isotopic shift up to 0‰. From these results, most of carbonates above 200 mASL are thought to be deposited from meteoric water with high water / rock ratios, whereas carbonates below 200 mASL were formed from meteoric water which suffered oxygen isotopic exchange by water-rock interaction with low water / rock ratios.
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