To explain temperature differences at a given depth, several mechanisms can be
invoked. In volcanic areas, magma chambers heat the overlying crust and several
hundreds of °C may be expected at a few kms depth. Outside these regions, variations
in mantle heat flow can trigger differences in crustal temperatures, but they only
act over several tens to hundreds of kms. At a smaller scale, temperature
differences at shallow depths can only be explained by crustal heterogeneities
and/or fluid flows. When fluid motion is negligible, heat transfer processes and
thus subsurface temperatures are basically controlled by thermal properties of
rocks (mainly thermal conductivity and heat production rates). Actually, because of
the heterogeneous nature of the crust, one can say that heat refraction occurs
everywhere as soon as the working scale is comparable with typical lengths of
crustal bodies (layer thicknesses, granitoids sizes, etc).
Results from thermal modelling of heat refraction allow to quantify simple effects
due to crustal heterogenities, and thus may be compared with real data (heat flow
measurements and/or temperature profiles). However, some subtle additional
parameters, like the geometry of the heterogeneity, appear to play a significant
role in the interpretation of field data. In particular, an anomalous conductive
body with a small aspect ratio (width over depth) will not disturb the underlying
isotherms even if a high surface heat flow is measured (e.g. heat flow anomaly in
Manitoba). On the opposite, one may easily miss a large-scale temperature anomaly
when a large aspect ratio insulating body is considered, because surface heat flow
is only affected at the very edges of the heterogeneity (e.g. sedimentary basins,
ash-flow calderas). The case of one sedimentary basin in south France will be
discussed. Other heat refraction effects associated with contrasts in heat
production, or with depth-dependent thermal conductivity, will be presented.
Nevertheless, knowledge and measurements of appropriate thermal properties remain
unavoidable as soon as theoretical and modelling results are performed to locate
potential geothermal reservoirs.
|