Geology Reference
In-Depth Information
current circulating through a coil on the ground. The intensity of the signal
is a function of the amount of water present below ground. Good results
have been obtained with respect to fi nding karst drains close to the surface
(see chap. B3-7.4.2).
Such lightweight methods generally allow shallow investigations
(<50 m).
2.1.4 Gravimetric methods
These consist of searching for anomalies due to density variations below
the surface. Microgravimetric methods are well-adapted to searching for
shallow or more or less fi lled-in or fl ooded cavities, veins or fractured zones,
or even altered karst zones or zones of alluvium.
2.1.5 Seismic methods
a) Seismic refraction method
This method is based on the propagation of elastic waves emanating from
a purposeful shock. The travel velocity of the seismic waves is measured
between the source and regularly spaced receivers (geophones). This
velocity increases with the density of the rock it is travelling through and is
highly variable, from 200 m·s -1 for surface formations to several thousands
of m·s -1 for more compact terrain.
Figure 56 Principle of seismic refraction.
Seismic refraction only considers longitudinal refracted waves, and its
practice requires an increase in velocity with depth. The equipment consists
of a recording station, aligned geophones, and a cable linking the geophones
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