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signal is detected at the surface by a vertical receptor coil, for which the
signal cancels out when its surface is oriented towards the magnetic axis.
Different surface measures therefore intersect above the zone holding the
emitter. This method allowed the precise calculation of the lake at the end
of the Qattine Azar chasm in Lebanon, below approximately 300 m of
rock, the goal being to increase the water resources for the east of Beyrouth
(Courbon, 2002).
Several catchments using similar methods have been emplaced, among
which can be cited that of the Trou-de-Garde underground river by a
borehole 120 m below the Féclaz plateau in Savoy, and that of the Trou-
qui-Souffl e in Vercors, which supplies the towns of Méaudre and Autran,
thanks to a borehole 300 m deep drilled in 1989.
c) Magnetic methods
When the gallery is entirely submerged and accessible only to divers, it is
possible to deposit a magnetic bar and to detect it from the surface with a
magnetometer. This method enabled the precise mapping of the drain for
the Lez spring (see chap. C4-4.2).
7.4.2 Locating unknown drains
Different geophysical methods were defi ned in chapter B2-2. Although
results are generally disappointing, certain methods have sometimes
confi rmed, from the surface, the presence of cavities, either empty or fi lled
with water or sediment:
￿ electric surveying,
￿ microgravimetry,
￿ MRS (magnetic resonance sounding).
This last method is based on exciting the protons in the hydrogen atoms
of the water molecule with an electromagnetic fi eld, and measuring their
resonance signals once they are no longer being stimulated. The signal is
proportional to the amount of water below ground. It can reveal important
volumes of water when they are close to the surface. Tests in the causse
de l'Hortus (Hérault) detected the karst conduit, 40 m below the surface,
supplying the Lamalou spring (Vouillamoz et al. , 2003).
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