Date of Award
Master of Science (MS)
W.D. Gosnold Jr.
Low heat flow has been previously reported in Minnesota (406 mW m-2) and the Superior Province of the Canadian Shield (42 ± 8 mW m-2). Low heat flow in Minnesota is due to heat flow measurements made in the mafic rocks of the Mid-Continent Rift System (MCRS) and attributed to the lack of correction for post-glacial climate fluctuations. Radioactivity is a critical heat flow component, particularly in Minnesota where all published heat flow sites are located within the MCRS, creating a bias in the data. The rift is a massive mafic intrusive complex that extends from Moho to the surface, and has characteristically low radioactive heat production. Therefore, there is a sampling problem where heat flow has not been measured in the granitic terranes in Minnesota.
Post-glacial climatic fluctuations affect temperature gradients in the upper two kilometers of the crust and are not consistently accounted for in previously published heat flow values. Thermal gradient measurements in Minnesota require re-analysis in order to account for the effects of recent climate change and by post-glacial warming affecting the calculations of heat flow by 25-40%. New heat flow measurements, and the sampling of over two-hundred rock samples, were obtained from cores and outcrops, for 238U, 232Th, and 40K concentrations and thermal conductivity in Minnesota.
Several lines of evidence show that steady state heat flow in the region has been underestimated by measurements in shallow boreholes, and geothermal potential in the region is not too low for geothermal development. This study finds the average heat flow in Minnesota to be 44.1±6.6 mW m-2 after corrections for post-glacial warming
Newly acquired data from this study on heat flow, thermal conductivity, and radiogenic heat production allows for a re-assessment of the EGS resources at depths of 4 to 10 kilometers for Minnesota. This reassessment determines that EGS resources are available at 6 km than previously at 8.5 kilometers. Total Enhanced Geothermal System (EGS) resources for Minnesota are 98,754 EJ or 18,409 of MWe at a 2% recovery factor. This amount of resources is three times larger than previous estimates by Tester et al. (2006).
Klenner, Robert Car Lloyd, "Terrestrial heat flow and an assessment of enhanced geothermal system resources in Minnesota" (2012). Theses and Dissertations. 163.