Anomalous hot temperatures at shallow depths can be due to several distinct causes,
such as the presence of a shallow magmatic reservoir, or a thick insulating
sedimentary cover, or a high heat-producing plutonic body, or a maintained
circulation of hot water within shallow fractures, etc. A combination of some (or
all) of these causes may explain famous geothermal sites. For example, the
Soultz-sous-Forêts geothermal anomaly (France) can be explained by both regional and
local factors. Locally, measurements of thermal properties have shown that
conductivity contrasts alone cannot explain the high temperature gradients, and that
the underlying granite exhibit high values for heat production rates (up to 6microW/m3).
At the Bouillante geothermal site (Guadeloupe, French West Indies), interbedded lavas
and tuffs in the 300 first meters probably result in a bulk thermally insulating
layer, overlying the active hydrothermal system. In both cases, as well in others,
vertical temperature profiles can be separated in three parts, typical of those
describing convective systems: a high temperature gradient in the upper part, then a
regularly decreasing temperature gradient yielding, at depth, a thick zone of nearly
constant temperatures.
These typical temperature profiles can be easily reproduced by simple one-dimensional
theoretical models, with sometimes unknown parameters. Transient cooling of a
magmatic body, as well as upwelling of hydrothermal fluids can explain similar
curvatures in temperature profiles. Such models however prevent any prediction for
geometry of the underlying geothermal system. On the opposite, the use of
neighbouring temperature profiles – when available – may help in the reservoir
characterization, provided that thermal properties of representative local
lithologies are measured. Examples of such neighbouring profiles at Soultz and
Bouillante will be discussed.

BRGM
3, avenue Claude-Guillemin
45060 Orléans cedex 02
France