Geology Reference
In-Depth Information
the statistical analysis of several hundreds existing well yields from Korea
(Cho et al., 2003) reveals no evidence to suggest a relationship between well
yield and tectonic fracturing (Fig. 7).
Conversely, a marked relationship is observed (Cho et al., 2003) between
the well's specific yield (yield [m
3
/day] divided by well depth [m] below the
base of the saprolite) and its depth below the base of the saprolite (the first
water bearing zone always appears below this horizon), shown as a "classical"
sharp decrease with depth (Fig. 8). The most interesting point is that this
relationship is well expressed for wells drilled in zones where a weathered
cover (saprolite and/or fissured layer) is preserved (Fig. 8), but is not consistent
where the weathering profile has been totally eroded. It thus appears that the
weathered cover, and thus its lower part, the fissured layer, plays a determining
role in terms of the transmissive properties of the hard rocks and not only
in term of their water-retention capacity. The weathered-fissured horizon,
which develops within the uppermost tens of metres of the "fresh" basement
substratum, is clearly due to the weathering processes. Thus, well yields are
statistically much higher in regions where a weathered cover (alterites +
fissured layer or fissured layer only) is preserved than in places where these
horizons have been eroded.
Despite providing different absolute values of specific yield, similar results
are obtained for granite and metamorphic rocks.
The underlying fresh basement substratum is permeable only very locally
due to tectonic fracturing. Despite local significant yields from certain
fractures, the overall specific yield is lower than that measured in the
weathered-fissured horizon (Fig. 8).
The
hydrodynamic properties
(permeability, storativity, anisotropy, fissures
density, double porosity, scale effects) of the
fissured layer
of a granite
aquifer have been characterized in details in southern India, within a
multiphase weathering profile (
see
previous subsection) using hydraulic tests
at different investigation scales. Slug tests, injection tests, flowmeter tests,
and pumping tests are interpreted using various analytical solutions (Table 1)
specific for fractured media (Maréchal et al., 2004 and Dewandel et al.,
2006). A first comprehensive hydrodynamic model of the fissured layer has
then been proposed.
The application of flowmeter profiles during injection tests determines
the vertical distribution of the most conductive fissure zones (CFZ) and their
hydraulic conductivities (Fig. 9c). The geometric mean of available data (19
flowmeter tests) is
K
CFZ
= 8.8 × 10
-5
m/s (according to the geometry of the
well and the observed drawdowns, the sensitivity of the flowmeter limits the
identification of fissure zones to those with a hydraulic conductivity higher
than 1 × 10
-5
m/s; hydraulic transmissivity
T
> 5 × 10
-6
m
2
/s). The rather
narrow range of hydraulic conductivities (which do not exceed 2.10
-3
m/s)
suggests a similar genesis for all of them, namely the weathering processes.
