Geology Reference
In-Depth Information
Table 4
Properties and Representative Elementary Volume of Typical Geologic
Environments.
Porous marine
environment
Porous
continental
environment
Fractured
environment
Karst
environment
Homogeneity
10
-1
m
Homogeneous
Homogeneous
Homogeneous
Homogeneous
Homogeneity
10
2
m
Homogeneous
Very
heterogenous
Homogeneous
to slightly
heterogeneous
Very
heterogenous
Isotropy
Isotropic
Very anisotropic
Isotropic to
anisotropic
Very
anisotropic
Size of REV (m)
10
-2
to 10
1
10
1
to 10
3
10
1
to 10
2
10
2
to 10
4
Expected
productivity
Poor to irregularly
medium
Medium to
irregularly high
Poor to regularly
medium
Poor to
irregularly very
high
4.1 Ancient igneous and metamorphic units
Aquifers in igneous units (ancient masses and volcanic fl ows) often present a
complex geometry (shape of batholiths, fl ows following preexising valleys).
Intrusive units were exhumed at different time periods. This is the case for
the Variscan (Hercynian) units (Vosges, Massif Central, Armorican Massif,
Corsica, Maures), but also for the crystalline masses of the Alps and the
Pyrenees. In all of these units, made up of “hard” fractured rocks, fractured
aquifers predominate, as the rocks have, since the Paleozoic, undergone fi rst
the Variscan, then the Pyrenean or the Alpine orogenies. Their weathered
mantles are porous aquifers likely to store water restored to the fractures
of deeper, unweathered rock.
4.2 Sedimentary basins
(Paris Basin: Figure 16, Aquitanian
Basin, Alsacian Plain, Limagne)
In these basins, clay horizons alternate with porous aquifer horizons,
resulting in multilayered structures. The current layout of the basins, with
a center at low elevation and outcrops of permeable rock at higher elevation
around their periphery, results in centripetal drainage from the outcrops
towards natural outfl ow points or exploitation zones at structurally low
points. The result of these elevation differences between the peripheral
recharge zone and the outfl ow zone in the center is the frequent occurrence of
artesianism. In such a basin, under natural conditions, tectonic disturbances
affecting the intermediary aquicludes tend to allow deeper waters to fl ow
up into shallower aquifers.
Search WWH ::
Custom Search