Heat transfer processes in the upper crust: influence of structure, fluid flow, and palaeoclimate
Numerical models constrained by geological and geophysical data form the basis of understanding the thermal regime of the Earth's crust. This dissertation focuses on modelling heat transport in the upper crust, studying the relative contributions of different processes to the specific heat flow distribution. Its vertical variation is a well known fact, caused by different processes such as changes in surface temperature, fluid flow, and heterogeneity. In particular, the first one can provide valuable information. Since the subsurface temperatures are directly related to past temperatures, their inversion into ground surface temperature histories are the only method available in palaeoclimatology to construct palaeotemperatures without using indirect proxy methods. Furthermore, a general better understanding of the processes affecting the thermal regime of the upper crust is needed for better downward continuation of thermal data, which is important for considerations about the thermal evolution of the lithosphere. A large geothermal data set from the Kola peninsula is processed and described in detail in order to prepare it for a numerical case study simulating heat transport processes in the Kola super-deep hole area. The data set includes 3400 measurements of thermal conductivity on 1375 samples from 21 boreholes with a depth up to 1.6 km and 36 temperature logs. The modelling involves 3-D forward simulation of both conductive and advective heat and mass transfer, and 1-dimensional inverse modelling for the palaeoclimatic ground temperature changes in the study area. Steady-state and transient 3-D models as well as the inverse modelling allow to estimate and quantify systematically the influence of fluid flow, spatial heterogeneity of thermal properties of rock, and palaeoclimate on the subsurface temperature field. Being aware that the information on permeability is sparse, the modelling results suggest that advection has a major influence on the vertical specific heat flow distribution. This is confirmed by inversion results which show higher temperatures during the last glacial maximum than in other areas, indicating an insulating effect of a persisting ice cover. However, forward modelling demonstrates that transient changes in surface temperature cannot be totally neglected, because their influence may reach more than half of the magnitude of the advective effects, depending on the assumed permeability and the particular climate model. The northern location of the study area required to implement latent heat effects by thawing and freezing of pore water in the numerical forward and inverse codes. So far, most geothermal investigations on past ground temperature histories in northern areas and during cold climatic episodes have not taken into account these effects. Depending on different parameters, such as the freezing period, surface temperature, and porosity, the influence on modelling results can be substantial. Since the modelling results show that latent heat effects can be neglected in the low porosity crystalline environment of the Kola area, the impact of freezing processes is shown for an example in the East European Platform. Whereas the inversions including freezing effects yield a postglacial warming of about 18 K, the neglect of latent heat effects would overestimate this result by some 6 K. This result is generalised by a study about the freezing and thawing processes in subsurface inverse modelling for a wide range of the above-named parameters. This allows to provide a more universal characterisation of the influence of latent heat effects on past temperature reconstructions by inversion. For possible corrections of existing ground surface temperature histories derived from borehole measurements, parametric relationships are developed which describe quantitatively the magnitude of these effects in terms of porosity, basal specific heat flow, present-day and past ground surface temperature history. Since a large number of synthetic model runs were were required, it was necessary to modify the applied Tikhonov inversion method. In this approach, a regularisation parameter has to be determined, representing a trade-off between data fit and model smoothness. This is achieved by the general cross validation method which makes the inversion for past temperatures faster, more automatic, and more objective. It is employed in a synthetic example, as well case studies from the Kola ultra-deep drilling site and another borehole from northeastern Poland. Although the convergence of the inversion iterations are rather different in these three cases, a satisfactory final result was obtained in each of them. Thus, this novel approach in the field of palaeotemperature inversions contributes to the current efforts to optimise the inversion methods for palaeotemperature reconstructions.
info:eu-repo/classification/ddc/550, Wärmestrom, Modellierung, Inversion <Mathematik>, Paläoklima, Petrophysik, Geowissenschaften, heat flow, temperature gradient, petrophysical properties, inversion, palaeoclimate
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