MASW and Remi surveys for seismic characterization of soils.

The MASW and ReMi surveys are carried out with the DoReMi seismograph with 16 channels and 16 bits of SARA Electronic Instruments using 4.5 Hz geophones. The MASW investigations are tests based on the analysis of surface waves generated by the use of an active seismic source. On the contrary, in the presence of a lot of environmental noise, it is possible to use the ReMi technique, which is based on the analysis of environmental noise (passive).
The MASW tests allow to reconstruct the seismostratigraphy of the S waves in a similar way to a well log, so that unlike the refraction tests they are not affected by any reversal of speed. From the seismostratigraphy we obtain the parameter Vs30 necessary to seismically characterize the site.

The results of the survey are:

  • a 1D profile of the velocity of shear wave;
  • a 2D profile of the velocity of shear wave if more acquisition are performed;

Seismic tomography

Refractive seismic tomography uses refracted seismic waves to reconstruct the underground distribution of the speed of seismic waves. This technique improves the results that can be obtained with traditional refraction seismics and prevents problems of interpretation due to reversed velocity layers or reduced thicknesses as well.

Refractive seismic tomography is applicable to all those situations in which it is necessary to map the variations of speed (therefore of ” strength “) in the subsoil. It is not particularly suitable for less complex situations, where the site to be investigated simply consists of a uniform soil cover placed on a very fast bedrock. In these cases, traditional refraction better identifies the passage between the topsoil and the bedrock (refractor).

From an executive point of view, the test is carried out in the same way as the traditional refraction test. The only difference is the necessary number of energisations which, in the case of the tomographic test, must be higher to ensure greater coverage.

smartTomo tomographic profile
This profile shows on the left a ramp in the bedrock and on the right a shear zone

The inversion procedure consists of the generation of an initial simplified model (often this procedure is automatic) then the arrival times of the seismic waves in the model are calculated and the times are compared with the measurements actually recorded. Based on the errors, the model is updated and the procedure is repeated until a reliability threshold is reached.

This type of test has been successfully applied in the reconstruction of the geometry of landslide bodies, landfills and in the survey of stratigraphy of sites on which civil engineering works were being planned.

Our smartTomo software is available for the tomographic processing of seismic data.

References:

ARPAL SUPPORT FOR MANAGEMENT OF THE REGIONAL NETWORK FOR HILL-SLOPE MONITORING, G. Beccaris; F. Di Ceglia; S. Pittaluga; E. Scotti; EUREGEO 2012 (PDF)