Seismic methods

 Seismic methods are based on the study of the propagation of elastic waves in the media. The seismic signal, which can be generated artificially (hammer, drop weights,...) or natural (seismic passive), is registered by sensors (geophones) distributed appropriately in the field.

 The main seismic methods are:

 

Dromochronic plot

  • seismic refraction, based on the study of direct and critically refracted waves, defines the layers present in the media with their seismic velocities as well as alteration, faults, fractures, fillings, landslide,... It is a widespread technique in civil engineering and geotechnics as it allows defining degrees of cavability and excavability.

  • seismic reflection, 

    based on the study of the reflected waves, allows defining objectives very similar to the refraction method but reaching much higher depths (several kilometres).

  • Surface seismic tomography, has field works very similar to the seismic refraction but with a higher density of trigger points. It allows obtaining, through an iterative process, inversion of seismic velocities on the basis of an initial model.

  • seismic passive ReMi is based on the spectral analysis of surface waves to define the distribution of S waves velocity (Vs) with depth. This technique is frequently used in media with strong seismic noise (i.e. urban, areas with traffic or works). Besides, it is used as a complement to the methods that define P wave velocity (Vp).

    seismic passive ReMi device

  • Down-hole and cross-hole tests are geophysical techniques in wells that study direct and refracted waves to obtain the distribution of P and S seismic velocities. These values, together with the density of the materials, define the elastic modules.

  • Seismic tomography in wells allows to define accurately the Vp distribution between two wells by a high density of point pairs shooting - receiver.

  • Parallel seismic is a seismic survey in wells that defines the depth of buried building elements (walls, screens, piles...) due to the sudden seismic speed change that occurs at its limit.