Characterizing Aquifers - Hydrogeology: Objectives, Methods, Applications - page 150

7.4.1 Locating accessible drainages
A karst aquifer and its compartments and annex-to-drain systems are
drained by passageways converging towards a major drain feeding the
emergence. The resource is generally accessible by drilling over the entirety
of the saturated zone, as the distribution of discontinuities, paired with
the polyphasage of karst systems, generally allows the interception of
productive zones, with the condition that drilling should be expected to
extend well below the piezometric surface. Encountering blocks of totally
impermeable limestone extending over 100 m is common. The exploitation
of a karst aquifer is optimal when it is possible to draw water directly from
a drainage passageway. In addition, conduits, perched above the karst
aquifers that they contribute to sustaining, can be interesting resources
close to the surface in zones with sharp relief.
The detection of submerged or vadose drains from the surface, and
their precise mapping in order to place a borehole or a gallery, is therefore
an interesting objective, which unfortunately remains problematic, even
when the cavities have already been explored by speleologists. Another
diffi culty is ensuring a perfectly vertical borehole.
a) Topographic surveying
Using classic speleologic techniques, compass, level, and laser range-
fi nder, a topographic survey is done in successive steps, working through
the galleries of a cave, and correction by triangulation is impossible. Even
though errors generally balance out, it is rarely possible to exceed a precision
of 1%, rarely suffi cient when attempting to reach, by drilling, a target a
few meters across at a depth of 300 to 400 m. When, in 1957, EDF wished
to capture the underground river in the Pierre-St-Martin chasm (Pyrénées
Atlantiques) by digging a tunnel into the side of a mountain, in order to
produce electricity for the valley of Ste. Engrace, the error in the position
of the Verna chamber (200 m in diameter), was of 400 m. The work of a
geometer with a theodolite and declinometer is necessary, and a precision
of 1% is then possible, but at the price of diffi culties in the execution.
b) Radiolocation
The best results were obtained by placing an electromagnetic emitter
below ground, and detecting the emissions at the surface with a receptor.
The emitting coil is placed horizontally, so as to create a vertical magnetic
fi eld. It can be installed in an infl atable fl oat on the surface of a lake. The