Estimation of thermal conductivities of overburden in northern Germany

In the course of a groundwater investigation program in east Schleswig-Holstein, groundwater temperature measurements were performed in groundwater observation wells. First, the heat flux density of the investigation area was determined. The heat flow ranged from 20 to 77 mW m(-2). Near the boundaries of the area, subsurface salt structures cause an increase in heat flow to the earth's surface. In the centre of this region the heat flow is due to convective processes, and is somewhat low. Here the groundwater recharge as well as the influx from top to bottom affects the temperature field. By means of heat flux densities and the measured temperature gradients, the thermal conductivity was estimated. For clays in the study area the thermal conductivity averaged 1.4 W m(-1) K-1 ranging between 1.1 and 1.7 W m(-1) K-1. For the coarse grained sand the conductivity was 3.0 W m(-1) K-1 and for the fine sands it was 2.8 W m(-1) K-1. Due to its heterogeneity, the thermal conductivity of the till ranged between 1.6 and 2.8 W m(-1) K-1, depending on the mineralogical composition, the bulk density and the water content of the till. The average thermal conductivity was 2.1 W m(-1) K-1.