| Abstract |
The undisturbed temperature profile measured before a thermal response test (TRT) and the recovery temperature profile measured after a test can be used as a valuable complement to the results from a thermal response test. The temperature profiles are also useful to detect water bearing fractures and groundwater flow (Liebel et al. 2012 and Holmberg et al. 2018). Water passing through fractures in boreholes can be observed as local temperature changes in the recovery temperature profiles. Data from laboratory measurements from several rock types in the larger Oslo area (Ramstad et al. 2014) and several unaffected TRTs gives a good knowledge of the expected thermal conductivity of the bedrock. Groundwater flow, however, may affect the results from a TRT leading to a considerably higher effective thermal conductivity as compared to the expected value for the rock based on its mineral composition and layering. In the present paper, temperature profiles measured before and after 60 TRTs in Norway are systemized, analyzed and presented. The temperature profiles are combined with site geology and topography in an attempt to quantify the presence and explain the flow of groundwater through fractures in the bedrock. The data shows that there is a relation between high water yield of the boreholes and groundwater flow. In about 50 % of the cases the groundwater is stagnant and is not detectable in a TRT or in the temperature profile. The prerequisites for groundwater flow are, a hydraulic gradient e.g. as induced by topography and minimum a moderate water yield of the borehole, thus a water bearing fracture system. The groundwater flow can either be perpendicular to the well and vertically between two fracture zones with different hydraulic gradient. In cases where differences in the hydraulic gradient causes a vertical groundwater flow, it is expected that the drilling of several energy wells equalizes the hydraulic gradient in the area and reduces the groundwater flow. This method and knowledge from ground source investigations can also be considered as a useful supplement to understand other hydrogeological applications, e.g. drinking water supply, underground infrastructures, tunnels etc. in urban areas. |