Seismic methods are a type of geophysical method, and constitute tests carried out to determine the geotechnical characteristics of a terrain, as part of the recognition techniques of a geotechnical survey.

Seismic waves that pass through terrain can be:

• - Longitudinal or compression.

• - Transverse or shear.

• - Superficial.

The propagation speed of seismic waves in the terrain depends on its deformability characteristics. In the hypothesis of assuming elastic behavior for the terrain, the speed of the longitudinal and transverse waves is a function of the elastic modulus and the Poisson's coefficient (both dynamic), so these parameters can be obtained with both expressions.

The longitudinal waves (P waves) are faster than the transverse waves (S waves), which makes the detection of the latter difficult in the field. Therefore, in general the elastic modulus is obtained from the longitudinal velocity, establishing hypotheses regarding the value of the Poisson coefficient. The dynamic modulus has a greater value than the static one, since it is obtained for small tension increases such as those produced by seismic waves. The ratio of dynamic to static modulus is normally considered 4, but the range can be between 1 and 20.

The seismic refraction method is based both on the fact of the difference in speed of the seismic wave in different terrains, and on the fact that waves when crossing the border between two types of different terrain suffer refraction, (as happens with light waves), changing their direction at an angle whose value depends on the relationship between the wave speeds of each terrain.

The wave generating impulse can be an impact or a small explosive charge that is generally placed at a point on the surface. Using geophones (which are detectors of small vibrations in the ground), arranged at different distances from the point of impact, the moment at which the first wave that reaches a given geophone arrives is measured. In this way the transmission speed is obtained.

When the impulse is produced, waves are emitted in all directions. A particular wave travels a path across the surface of the ground in the direction of the geophone (direct wave). Other waves descend at various angles with respect to the horizontal. Upon encountering a lower stratum with seismic velocities, the wave is refracted in the contact plane between both terrains.