Heat Flow Density in Estonia - Assessment of Palaeoclimatic and Hydrogeological Effects

摘要

New measurements of heat flow density have been made in six boreholes in northern and western Estonia. The mean heat flow values range from 20 to 40 mW m~(-2). All holes display a vertical variation in apparent heat flow densities from 15 to 52 mW m~(-2). Since most of the holes are shallow and therefore sensitive to surficial disturbances, the effects of palaeoclimatic ground temperature changes and heat transfer by groundwater flow were studied with numerical models. Forward modelling of palaeoclimatic effects was made with a homogeneous half-space model in which the assumed climate history covered glaciations during the past million years, the Holocene, and the recent climatic warming. The influence of the sea cover after deglaciation was also included. The palaeoclimatic models suggested that the measured vertical variation might be partly attributed to the palaeoclimatic effects, but when the corrections were applied to the measured data they did not entirely eliminate the vertical variation in heat flow density. This is probably due to thermal conductivity structures that deviate from the assumed half-space conditions and the palaeoclimatic ground temperature history used in the models. Fluid and heat transfer in the subsurface were simulated with finite difference techniques. The simulation results of a 2-dimensional model indicate that the thermal effect of regional flow systems is less than 5 mW m~(-2) in most of Estonia. Larger perturbations may occur in the southeast of the country, where the hydraulic gradient is higher.