SIMULATION OF HEAT-TRANSFER AT THE KOLA DEEP-HOLE SITE - IMPLICATIONSFOR ADVECTION, HEAT REFRACTION AND PALEOCLIMATIC EFFECTS

The drill hole SG-3, 12 261 m deep in the Pechenga-Zapolyarny area, Ko la Peninsula, Russia, is currently the deepest drill hole in the world . Geothermal measurements in the here reveal a considerable variation (30-68 m W m(-2)) with depth in the vertical component of heat-flow de nsity (HFD). We simulate heat and fluid flow in the bedrock structure of the Kola deep-hole site. Various potential sources for the observed HFD variation are discussed, with special emphasis on advective heat transfer, palaeoclimatic ground surface-temperature changes and refrac tion of heat flow due to thermal conductivity contrasts. A 2-D finite- difference (FD) porous-medium model of the Kola structure, constructed from all available data on lithology, hydrogeology, topography, therm al conductivity and heat-production rate in the deep-drilling area, is the basis of all forward-model calculations. A conductive, steady-sta te simulation indicates that heat production and refraction create a v ariation of about 15 m W m(-2) in the uppermost 15 km, but are insuffi cient to reproduce the measured HFD-depth curve in the uppermost 2-4 k m. However, if topography-driven groundwater how is considered in the model, the measured HFD variation is easily explained. The most sensit ive parameters in fitting the model results to the observed HFD-depth curve are the permeability of the top 4 km (10(-14)-10(-15) m(2)) and the (constant) HFD applied at the base of the model at 15 km depth (40 -50 m W m(-2)). The palaeoclimatic effect for the Kola structure was c alculated with a conductive transient simulation. A simplified ground surface-temperature history (GTH) of the Kola area was simulated by va rying the surface temperatures of the model during different intervals of the simulation. Our results indicate that the measured variation i n the vertical HFD cannot be explained by the palaeoclimatic effect al one, because its amplitude decreases rapidly from about 16 m W m(-2) n ear the surface to less than 2 m W m(-2) at depths in excess of 1.5 km .