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

Abstract

S U M M A R Y 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.

The thermal regime of the crystalline continental crust: Implications from the KTB

“…Depending on the resolution and on the size of the area considered, the topography around the KTB either can be simplified into a 2-D structure or must be treated in three dimensions. In order to quantify the potential effect of heat advection by topographydriven forced convection, coupled groundwater flow and heat transport were simulated in a regional 2-D and a local threedimensional (3-D) model using finite differences (FD) and finite elements (FE), respectively [Jobmann and Clauser, 1994;Kohl and Rybach, 1996]. The basic equations for these simulations are summarized in Appendix B. groundwater, whereas the borehole in the sedimets shows upward flow [Jobmann and Clauser, 1994].…”

“…(2) Regional processes can only be detected with appropriately distributed boreholes. This is emphasized in the Heat Advection by Forced Convection section by our discussion of simulations of advection due to groundwater flow, driven by a 2-D or 3-D topography [Jobmann and Clauser, 1994;Kohl and Rybach, 1996] , 1982] rather follows lithology. In a tectonically stable crust, chemical differentiation processes will lead to an exponential decrease of heat production with depth.…”

Determination of thermal conductivity for deep boreholes

“…The detailed knowledge of thermal conductivity components through the application of several determination methods for the KTB drill holes allows variations of heat flow with depth to be explained without invoking convective heat transport by water movement (see also Jobmann and Clauser [1994]). Figure 13 is integrated over large horizontal distances from the borehole (100-500 m).…”

“…For deep holes, heat flow often varies significantly with depth [see Sass et al, 1992b;Clauser and Huenges, 1993;Kukkonen and Clauser, 1994;Jobmann and Clauser, 1994]; diverse lithologies and complicated structures are often encountered in deep (3-5 km) and ultradeep (5-12 km) holes. Within measurement uncertainties it is usually impossible to document any changes in heat flow over a few hundred meters, and in these cases a single method is usually chosen to characterize thermal conductivity.…”

Heat transport analysis and three‐dimensional thermo‐hydraulic simulation in the Ishikari basin, Hokkaido, Japan

“…In general, high-porosity materials of the area express low thermal conductivity and diffusivity. An enhanced porosity, and hence higher water content, in the formation both decreases the thermal conductivity of the rock and increases its specific thermal capacity, which both lead to a reduced thermal diffusivity (Jobmann and Clauser, 1994). The anisotropy effect of the parameters measured in this study is not known, as measurements were made exclusively in the vertical direction.…”