Active seismic techniques study the subsoil by analyzing the propagation of artificially produced seismic waves on the ground. Therefore, these techniques involve the generation (seismic source), transmission and detection of seismic signals (seismic sensors).

Seismic data acquisition scheme and the main types of waves according to their propagation characteristics.

The propagation of energy is carried out elastically fulfilling the equation of seismic waves. The solution of this equation allows to establish the existence of different types of waves: internal waves and surface waves. Internal waves can be compressional (P waves) or shear (S waves) and are characterized by a movement of the particles in the same direction of the propagation of the waves (P waves) or perpendicular to it (S waves). Surface waves are generated by the presence of discontinuity in the media and travel parallel to the surface of discontinuity. The penetration of these waves is a function of the wavelength. This fact causes surface waves to be dispersive: different frequencies have different propagation speeds. Depending on the part of the wave field studied, active seismic techniques can be classified into the following:

Example of the record of seismic waves produced by a seismic source and detected by a group of 48 sensors. Three sectors are marked with the arrivals of the three types of waves (refracted, reflected and surface) differentiated by the arrival pattern. Each of these types is the basis of one of the seismic techniques applied to the ICGC.

Refraction seismic

This technique uses the refraction of seismic waves (P or S) along the contacts between subsurface layers to obtain a seismic velocity model. The data used to obtain this model are the arrival times of these waves as a function of the receiver-seismic source distance. Currently, the most widely used treatment technique is refraction seismic tomography, which consists of inverting this data to obtain a subsurface velocity model. The resulting velocity model can be related to the lithology, depth and/or mechanical state of the subsurface materials (degree of compaction, rippability, etc.).

P-wave velocity model obtained using seismic refraction tomography, with its interpretation.

Reflection seismic

The reflection seismic method involves the use of seismic waves that travel from the seismic source on the surface to the subsurface discontinuities and return, after being reflected, to the receivers located on the surface (reflected waves). The isolation of these reflected signals from other types of waves and noise and their processing allows to obtain an image of the main discontinuities of the subsurface (seismic section).

Seismic reflection section where the variations in amplitude of the waves reflected at depth are observed. The continuity of the observed reflections allows the different subsurface structures to be delineated.

Surface wave seismic

Surface wave analysis allows the S-wave propagation velocity profile to be obtained with depth. This technique consists of measuring the dispersive characteristics of surface waves (phase velocity as a function of frequency) and inverting them to estimate the soil properties (Vs).

Example of a seismic record with surface waves and their dispersion curve in a frequency-velocity diagram. S-wave velocity model with depth.

Applications

• Degrees of alteration, rippability of formations (geotechnics and civil engineering).
• Study of aquifers (hydrogeology, environment).
• Location and study of fractures and faults (civil engineering, geotechnics and geological risks).
• Stability of slopes (geological risks).
• Location of cavities and landslides (geological risks).
• Obtaining Vs (geotechnics, seismology).
• Characterization of geological structures (geology).