Combined effect of topography and hydrogeology on subsurface temperature — implications for aquifer permeability and heat flow. A study from the Bohemian Cretaceous basin

Abstract

The subsurface temperature field of the northern Bohemian Basin (Cretaceous) was studied by using a group of 8 boreholes (depth 75–220 m) drilled in a hydrogeologically active area (2 horizontally layered aquifers separated by an aquiclude) with a pronounced relief (400 m within 2 km from the borehole sites). Reliable temperature logs from closely spaced holes allowed evaluation of both vertical and horizontal temperature trends. The vertical temperature gradient varied from 20 to 60 mK m −1 depending on lithology. In spite of relief the effect of topography was not identified: temperature at one altitude did not vary between the individual boreholes — the isotherms were nearly horizontal. The temperature field was simulated numerically along a 2-D profile to assess the influence of horizontal water flow in the aquifers on temperature. The permeability of aquifers was varied in a broad range. The switch to the convective thermal regime occurred after an increase of permeability from 2 × 10 −12 m 2 to 2 × 10 −11 m 2. The water flow velocity was then sufficient to reduce lateral temperature variations induced by topography so that the model and measured temperatures were in agreement. The temperature field was sensitive especially to permeability changes in the upper aquifer which allowed to constrain its permeability to more than 5–10 × 10 −12 m 2. The vertical temperature gradient in the aquiclude and the lower aquifer was not substantially affected by water flow in any of the models and the gradients determined from the measured temperatures could be therefore used for the heat flow density determination. The estimate (90–100 mW m −2) corresponds well to the increased heat flow in other parts of the Bohemian Cretaceous Basin.