Geology Reference
In-Depth Information
MAPPING THE LAYERS CONSTITUTING
THE HARD ROCK AQUIFERS
The formation of the fissured layer is, therefore, closely related to the
development of the alterites, and hence to the weathering front. An important
consequence of this process is that, at the catchment scale, these weathered
layers are parallel to the paleo weathering surfaces (paleo landscape)
contemporaneous with weathering. At the local scale (from about 10 to
about 100 m), it can be influenced by lithological or structural variations, the
latter being well-known to basement hydrogeologists and geophysicists who
use them to site water wells: the presence of fractures enables water to
circulate or diffuse, causing a local deepening of the base of the alterites.
These ancient surfaces have been affected by erosion processes posterior
to the main weathering phase. Therefore, the saprolite and the fissured layer
make up stratiform beds whose geometry at the regional scale can be linked
to the more or less good preservation of paleoweathering surfaces. On the
basis of the knowledge of this genetic principle, it is quite easy to map, at
the watershed scale (Lachassagne et al., 2001; Dewandel et al, 2006):
the altitude of:
o the limit between the saprolite (laminated layer) and the fissured layer,
o the base of the fissured layer,
and thus to respectively compute the residual thickness of:
o the saprolite,
o the fissured horizon.
In most cases, the geometry of the base of the saprolite is determined at
first, through the combined use of well data, geophysical data if any, and
specific field surveys. Since the surface of the base of the saprolite is, in
many places, cut by the present-day ground surface, notably near valleys, it
is relatively simple, in the field, to determine the position of the saprolite/
weathered-fissured layer interface.
On the basis of observations of the thickness of the fissured layer
(observations on outcrops, statistical treatment of existing well data), the
altitude of its base can also be directly computed from one of the alterite
base. The altitudes of (i) the saprolite/fissured layer interface and (ii) fissured
layer/fresh rock interface can therefore be subtracted from that of the present
ground surface, as inferred from Digital Elevation Model (DEM) data, in
order to compute the residual thickness of respectively the saprolite and the
fissured layer. In regions where the paleosurface(s) have partly been preserved
from erosion, slope analysis from DEM data can also be used for the
elaboration of such maps.
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