Heat advection versus conduction at the KTB: possible reasons for vertical variations in heat-flow density

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SUMMARY 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 PCclet 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 PCclet numbers of -0.37f0.13 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 k, = 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.