Geology Reference
In-Depth Information
The bottom of the weathered-fractured layer is better constrained thanks
to the quality of the available observations. The number of fractures starts
to decrease between 14 and 16 metres (Fig. 9a) and is followed by an
absence of fractures below 20 metres while the quality ratio of observations
remains high down to 24 metres. This depth roughly corresponds to the top
of the fresh basement as identified by geological observations during the
drilling of the wells, and also to a sudden decrease in drilling rates at this
depth.
The comparison of the characteristics of the Maheshwaram aquifer,
resulting from a multiphase weathering process, with the ones of aquifers
where the landscape mainly results only from a single main weathering
phase (Dewandel et al., 2006), leads to a generalized 3-D geological and
hydrogeological conceptual model of granitic aquifers (Fig. 10).
The hydrodynamic properties of the fissured layer are thus controlled by
the distribution, the hydraulic conductivity, the anisotropy of hydraulic
conductivity and the connectivity of the fissures (Maréchal et al., 2004). It
thus constitutes an anisotropic medium. It is the most permeable layer of the
entire weathering profile and assumes most of the transmissive function of
the aquifer.
The global hydraulic conductivity of this layer appears to be a result of
the number of conductive fissures intersected by the considered well, and
not the result of a higher hydraulic conductivity of a single (or more)
conductive fissure (Maréchal et al., 2004). Indeed, the hydraulic conductivity
of the conductive fissure zones belonging to this layer is relatively similar
regardless of the case study location in the world; it ranges between 10
-6
and
10
-4
m/s. Thus, it appears that granitic rocks exposed to weathering are
affected by similar weathering processes that induce similar fissure hydraulic
conductivities.
As the hydraulic conductivity of this layer closely depends on the density
of the conductive fissures, the variations in hydraulic conductivity from one
well to another are explained by the variability in hydraulically conductive
fissure density itself and not by the variability of fissure hydraulic conductivity.
Moreover, the hydraulic conductivity of the fissures does not significantly
decrease with the depth (Fig. 9). The apparent decrease in hydraulic
conductivity toward the base of this layer, largely observed worldwide, reflects
only a downward decrease in density of the weathering-induced fissures
(Fig. 10).
The presence of two main fissure sets, a horizontal and a subvertical one,
recognized both from geological observations and from pumping test
interpretation (Maréchal et al., 2004), ensures good connectivity between the
fissures and induces an anisotropy of hydraulic conductivity (vertical
anisotropy ratio close to 10;
K
horiz
>>
K
vert
), which is in agreement with the
qualitative geological observations.
