New heat flow data from three boreholes near Bergen, Stavanger and Moss, southern Norway

Abstract

New heat flow data have recently been obtained for three boreholes, Fyllingsdalen, Ullrigg and Årvollskogen, which are located in southern Norway near Bergen, Stavanger and Moss, respectively. The obtained topographically and palaeoclimatically corrected values of average heat flow density are 51mW/m2 within the Ullrigg borehole, 72mW/m2 within the Fyllingsdalen borehole and 80mW/m2 within the Årvollskogen borehole, in the depth interval of 120–400m. According to the preferred palaeoclimatic scenario, the highest tentative palaeoclimatic corrections vary from 21 to 26mW/m2 within the shallow parts of the investigated boreholes. Therefore, a significant decrease of the Earth's surface temperatures as a result of the continuous cooling during the two last glaciations in Weichselian and Saalian still affects the subsurface thermal field of the study areas in terms of the reduced heat flow density within the uppermost crystalline crust. Topographic corrections are characterised by rather minor values compared to the palaeoclimatic ones. Moreover, the groundwater flow can be a significant factor for the reduction of heat flow density in the Fyllingsdalen and Ullrigg boreholes, whereas hypothesised subsurface radioactive sources may have contributed to a higher heat flow density at Årvollskogen. The variation in heat production related to different lithologies appears to be one of the main reasons for the higher heat flow density in the Fyllingsdalen and Årvollskogen boreholes in comparison with the Ullrigg borehole.