Geology Reference
In-Depth Information
Figure 10.
Generalized 3-D geological and hydrogeological conceptual
model of granite aquifers controlled by single or multiphase weathering.
(a) Maheshwaram (southern India) multiphase weathering and (b) Classical
profile - single weathering - (idealized profile). Dfiss: density of conductive
fissures, Kfiss: hydraulic conductivity of fissures and Kglobal: equivalent hydraulic
conductivity of the layer (from Dewandel et al., 2006).
The effective porosity of the fissured layer is relatively low, about 10
-2
,
and is mainly (90%) ensured by the low permeability fissure zones affecting
the blocks (block permeability, including very low permeability fissure zones:
5 × 10
-8
m/s), while the conductive fissure network contributes only 10% to
the effective porosity (Maréchal et al., 2004).
The top of the fissured layer is characterized by a few metres-thick high
density fissured zone, named the 'transition zone' (density: 0.7-0.5 m
-1
)
(Figs 9 and 10). This zone, 2-metre-thick in the case of the young rejuvenated
weathering profile studied in Maheshwaram, can reach more than 12 metres
in the case of old thick weathering profiles (French Brittany, Wyns et al.,
2004). The hydraulic conductivity of the fissures belonging to this transition
zone is comparable to that of the deeper part of the fissured layer. This
transition zone is the more permeable part of the fissured zone, thus of the
entire weathering profile.
