ALBERT

Notes: Data from the 4 km deep KTB pilot hole (VB) show a strong vertical variation in heat-flow density (HFD) by as much as 50 per cent. This may be caused both by heat conduction, by advection, and by transient diffusion. At the moment it is not possible to quantify exactly the contribution of each of these. However, 2-D simulations help to define the parameter ranges and structural features required if these processes are to be thermally efficient. the main results are: (1) thermal conductivity contrasts combined with structural heterogeneities as seen in the drilled profile give rise to steady-state, lateral refraction of heat. 2-D simulations of heat conduction indicate that this effect alone is sufficiently strong to account for the observed variation of HFD with depth. (2) Vertical Péclet number analyses of T-logs in shallow boreholes and the KTB-VB indicate a NE-SW flow of meteoric water across the Franconian Line (FL). However, average Péclet numbers of -0.37 ± 0.13 in the potential recharge zone east of the FL are compatible with 2-D, steady-state simulations of heat and fluid flow only up to a distance of about 10 km east of the FL, and only if a crystalline permeability kc= 10−14 m2 is assumed. (3) A permeability this high, however, is not confirmed by a comparison of temperature and HFD from numerical simulations and data from the KTB boreholes, neither for a model focusing on shallow flow systems nor a deep structural model investigating potential contributions of convection in the entire upper crust. (4) Alternatively, a joint inversion of T-logs from the same shallow holes yields a ground-temperature history (GTH) that is in remarkably good agreement with long-term meteorological records. (5) It appears, therefore, as if the thermal regime at the KTB was generally dominated by conduction, with additional advective, topography-driven contributions mainly at shallow depths. the conductive regime, however, is a complicated one, characterized by lateral heat flow due to structural heterogeneity (and possibly anisotropy), and, at least at shallower depths, by transient diffusion of paleoclimatic temperature signals into the subsurface.

Notes: The drill hole SG-3, 12 261 m deep in the Pechenga-Zapolyarny area, Kola Peninsula, Russia, is currently the deepest drill hole in the world. Geothermal measurements in the hole reveal a considerable variation (30-68 mW m-2) with depth in the vertical component of heat-flow density (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 refraction 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, thermal conductivity and heat-production rate in the deep-drilling area, is the basis of all forward-model calculations. A conductive, steady-state simulation indicates that heat production and refraction create a variation of about 15 mW m-2 in the uppermost 15 km, but are insufficient to reproduce the measured HFD-depth curve in the uppermost 2-4 km. However, if topography-driven groundwater flow is considered in the model, the measured HFD variation is easily explained. The most sensitive parameters in fitting the model results to the observed HFD-depth curve are the permeability of the top 4 km (10-14-10-15 m2) and the (constant) HFD applied at the base of the model at 15 km depth (40-50 mW m-2). The palaeoclimatic effect for the Kola structure was calculated with a conductive transient simulation. A simplified ground surface-temperature history (GTH) of the Kola area was simulated by varying the surface temperatures of the model during different intervals of the simulation. Our results indicate that the measured variation in the vertical HFD cannot be explained by the palaeoclimatic effect alone, because its amplitude decreases rapidly from about 16 mW m-2 near the surface to less than 2 mW m-2 at depths in excess of 1.5 km.