Page 38 - Geothermal Energy Systems Exploration, Development, and Utilization

14 1 Reservoir Definition
in compiling available thermal data that correspond to the boundary conditions,
as well as petrophysical parameters controlling heat transfer. Surface temperature
is well known, but may however show significant spatial variations as detailed
below. The few direct measurements of surface heat flow in Europe are also shown
together with temperature gradients and thermal conductivity values. At mantle
depths, indirect evidence for temperature variations in Europe has been evidenced.

1.2.1
Far-field Conditions

In order to constrain thermal regime of the shallow crust, one need to constrain
far-field thermal boundary conditions, say at the surface and at the base of the
lithosphere. Even if spatial distribution of heat producing elements within the crust
is of major importance, it is necessary to estimate the amount of heat supplied at
the base of the lithosphere (which is also the heat supplied at the base of the crust)
andthatlost atthe surface.
At the surface, the ground surface temperature has been measured for centuries
and can be considered as constant over length scales of several hundreds of
kilometers (Hansen and Lebedeff, 1987). In Europe, ground surface temperature
◦
◦
increases from north to south France (∼1000 km) by about 5 C, and by ∼10 C
from Denmark to south Italy (Figure 1.8) separated by a distance of 2000 km
(Haenel et al., 1980). If thermal regime of the crust is to be studied, such large-scale
variations can thus be neglected. Apart from the effect of latitude, ground surface
temperature can be locally disturbed by surface heterogeneities such as topography
(Blackwell, Steele, and Brott, 1980) or the presence of lakes. These permanent
disturbances should be theoretically considered when subsurface temperatures
are studied, especially if representative length scale of surface features compare
with the studied depths (e.g., warm water outflows in tunnels of Switzerland,
Sonney and Vuataz, 2008). Transient changes in surface conditions such as those
induced by forest fires may also affect subsurface temperatures but only for a short
period. Long-period surface temperature changes such as climatic warming or
cooling periods affect underground temperatures as it can be deciphered through
measured temperature profiles (Guillou-Frottier, Mareschal, and Musset, 1998),
but associated thermal disturbances are damped with depth, and basically cancelled
at several hundreds of meters.
Contrary to the upper surface, there is no reason to consider the base of the
crust as an isotherm. Seismic tomography studies have indicated that this is indeed
not the case. Even if seismic velocities vary with temperature and composition,
Goes et al. (2000) suggested that the inferred variations at 100 km depth revealed
temperature differences (Figure 1.8). At shallower depths (Moho depth +20 km),
Figure 1.9 shows possible large-scale temperature differences as deduced from the
shear velocity model of Shapiro and Ritzwoller (2002). Local studies of seismic
tomography also suggested anomalous hot zones at the base of the European
crust, such as beneath the FMC and beneath the Eifel area in Germany (Granet,
Wilson, and Achauer, 1995; Ritter et al., 2001). These anomalously hot zones and