The Study of the Geometry of Aquifers - Hydrogeology: Objectives, Methods, Applications - page 108

to the recorder. The shock is created by a falling weight (for shallow
investigations) or by explosives (allowing a better vertical penetration).
Starting at the time of the shock, the time for the waves to propagate to each
geophone is measured, allowing the creation of a T-X graph of the arrival
times as a function of distance from the source (Figure 57, Figure 58). This
curve can then be used to calculate the apparent velocities, then the true
velocities and the thicknesses of various beds. Generally, two shock waves
are used, one direct, E and one inverse, E', in order to determine whether
the contacts between layers are horizontal or inclined, and in the latter case,
to determine their slope.
Figure 57 T-X graph for a seismic setup.
Figure 58 Interpretation of a seismic study along a road project in Upper Corsica.