The source generates a mechanical disturbance which propagates in the ground, is re-
ected, refracted or diracted, and returns to the surface. When the disturbance propagates
in a
uid such as water a temporary variation of pressure is created. Elastic
deformation results in movements of the surface and at some point of the surface the
acceleration, the velocity or the displacement of a point can be measured. In any case,
whether a movement or a variation of pressure is observed, we have to represent it by
some other physical quantity which can be easily stored and manipulated. Considering
the development of electronic technology, a representation by an electrical voltage is evidently
a good solution. The rst eld component of a seismic data acquisition system is
the detector group. The detectors convert the seismic disturbance into a voltage of which
the variations represent faithfully the variations of the mechanical disturbance detected,
a voltage which is the analog of the seismic disturbance.