Thermal data from recently drilled boreholes near Bergen, Stavanger and Moss, southern Norway

Abstract:

We have done an attempt to reveal the major features of the thermal pattern in the Fyllingsdalen, the Ullrigg and the Årvollskogen boreholes, which are located near Bergen, Stavanger and Moss, respectively. Based on 2D gravity and magnetic modelling, the 2D structural models have been constructed for the Bergen, Stavanger and Moss areas. All available shallow and deep structural data have been used to constrain these models. The constructed 2D models were used as a structural base during the 2D thermal modelling.

The results of the 2D thermal modelling indicate that a significant decrease of the Earth's surface temperatures during the two last glaciations still affects the subsurface thermal field of the study areas in terms of the reduced temperatures within the uppermost crystalline crust. Besides, the advective cooling due to groundwater flow is an additional factor for the reduction of temperatures within the Bergen and Stavanger areas where the normal annual precipitation is one of the highest in Europe, reaching roughly 4000 mm/year. On the other hand, the influence of the groundwater flow on subsurface temperatures is not clearly recognizable in the Moss area.

The modelled temperatures are higher in the Fyllingsdalen and Årvollskogen boreholes compared to the Ullrigg borehole. This difference is in agreement with the low measured thermal gradient in the Ullrigg borehole which is less than 13.0 °C/km compared to 16.5 °C/km in the case of the Fyllingsdalen borehole and 19.3 °C/km in the Årvollskogen borehole. The differentiation in radiogenic heat production of the crystalline crust is one of the main reasons for the higher measured and modelled temperatures within the Bergen and Moss areas. This resulted in a higher heat flux in the Fyllingsdalen and the Årvollskogen boreholes in comparison with the Ullrigg borehole.