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own parameters (resistance to the drill bit, fl uid pressure), which affect its
value (drilling parameters).
Once the boreholes have been created, the cross-sections of the
destructive boreholes can be optimized and calibrated to the cored boreholes
by using drill logs. These consist of measurements along the entire depth
of the borehole, taken with the help of specifi c probes, of the terrain's
characteristic parameters. The principal methods are as follow: gamma
ray probes (measuring natural radioactivity), electric logs (measuring
resistivity), and sonic logs (measuring the propagation of longitudinal
waves).
4 WELLS, DITCHES, AND EXPLORATORY GALLERIES
Wells and ditches can be easily put in place in sites accessible to construction
machinery. They lead to relatively destructive investigations, limited to a
depth of a few meters, and sometimes requiring the creation of an access
route.
This method is generally limited to verifying the thickness of an
unconsolidated cover (weathering mantle, scree cover) and to the excavation
to the bedrock (in order to identify its nature, its age, even its strike and
dip, in particularly covered regions).
The digging of an exploratory gallery is extremely onerous, and remains
an option available only in large-scale civil engineering projects (dams,
tunnels). It allows the observation of the actual geologic conditions in
the heart of the rock mass, and it obviously provides information of great
importance.
For certain construction sites, subhorizontal core or destructive drilling
can provide comparable results, as long as their length does not exceed a
hundred meters.
5 GEOMETRIC SYNTHESIS
During the fi nal survey phase, the geometry of the aquifer(s) being studied
is synthesized and reconstructed through maps and cross-sections. Figure
59 illustrates such a representation, concerning the region around Peille
(Alpes-Maritimes), located at the heart of the subalpine range of Nice. They
consist of sedimentary units running N-S to NNW-SSE, with a structure
consisting of recumbent folds in which certain anticlina axes are broken and
overlapping. This fi gure presents the geometry of two distinct aquifers, one
Jurassic karst reservoir and one Turonian fractured reservoir, separated by
a few hundreds of meters of impermeable Cenomanian marl.
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