Three-dimensional (3-D) thermal investigation below high Alpine topography

The characteristics of severe topography in active mountain belts represent a special challenge for the evaluation of subsurface temperatures. These conditions require in particular a proper treatment of possible thermally relevant mechanisms. In the present analysis temperature data from depths of up to 1.5 kin are investigated which have been collected at the intermediate "point-of-attack" in the framework of the new Alpine transverse (NEAT) project in central Switzerland for the construction of a 57 km long base tunnel. Specialty designed temperature measurements were used in a 800 in deep shaft and along a 1200 in long access adit. Additional thermal information was provided by temperature logs from two nearby exploration boreholes and from laboratory measurements of various samples. For a detailed investigation of the temperature data a transient finite element (FE) model has been used which accounts for fluid and mass advection (uplift) as well as for climatic changes. The uplift and exhumation scenario assumed the surface to be in steady-state conditions. Special emphasis was given to structural effects like topography and anisotropy. The 3-D numerical model extends over an area of similar to 20 km x 20 km and includes Alpine high topographic relief with altitudes between 1500 and 3000 in a.s.1. Without modifying petrophysical parameters determined from laboratory measurements, all reliable temperature data could be nearly perfectly fitted by adjusting the two principal thermal boundary conditions at the surface and at the bottom. This study reveals that hydraulic influence is generally negligible at depths below similar to 500 m which is in contrast to results from lower-dimensional methods such as 1-D Peclet analyses. Vertical heat flow variations are rather due to topographic than to hydraulic impact. Sensitivity studies highlight the importance of uplift in the central Swiss Alps and of local ground surface temperature (GST) distribution which both can influence the temperature field even at greater depths. (C) 2001 Elsevier Science B.V. All rights reserved.