Heat Flow and Deep Underground Temperature in the Bergen Region

Abstract

Anomalously high concentrations in radioactive elements have been recently discovered in the Løvstakken Granite, southwest of downtown Bergen. Because it potentially implies radon hazard, this latter discovery has raised serious health concerns for the local populations. In addition, the presence of high levels of natural radioactivity for the Løvstakken Granite suggests unusually high underground temperatures in the Bergen area that might represent an economic interest in terms of geothermal potential. The main goal of this MSc thesis is to estimate heat flow values and deep underground temperatures in the Bergen area.

We collected 243 rock samples and measured their petrophysical properties. The densities of the samples were measured at the Geological Survey of Norway (NGU) in order to calibrate 2D crustal models. In particular, a dense database of gravity measurements was used for modeling of WNW-ESE crustal profiles across the major lithological units of the Bergen Arcs, including the Løvstakken Granite. The optimal crustal model served as input for 2D thermal modeling. Heat flow and geotherms were calculated and plotted using temperature values from the thermal models. Furthermore, input heat generation values in the thermal model were based on 401 radiometric field measurements made in the course of this thesis.

The main outcomes of the thesis are the following. The Løvstakken Granite showed an average heat generation value of 8.03 μW/m3 (i.e. highest ever recorded in Norway), while granitic gneisses of other areas along the profile gave an average value of 4.41 μW/m3, which is still above normal levels for Norwegian basement rocks. Other lithologies of the area present normal levels in heat generation.

Modeled surface heat flow values are from 62 to 105 mW/m2. This wide variation is strongly controlled by the type and geometry of the lithological units underlying the Earth’s surface.

In brief, the present results suggest that geothermal mining might be economically sustainable in some specific places of the Bergen area. However, the only way to validate or reject the results of this thesis remains to drill a pilot hole and measure temperature gradients and thermal properties.