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;

smartRefract

smartRefract is an open source seismic refraction software that implements the GRM method (Palmer 1980). The software allows you to select the first arrivals, assign the dromochrones to the layers and view the section.
The software is complete but it is to be considered a working prototype. It is compatible with SEG2, SU and some SEGY formats.

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)

smartTomo

 

SmartTomo is an efficient tool for seismic travel time tomography. Easily and in a very short time, it allows building an image of the underground velocity distribution. The output is the product of the iterative optimization of the starting model. Additionally, the user can define the initial model to fit the knowledge of the geology of the survey area.
SmartTomo uses an optimized implementation of the method by Moser, T. J. (” Shortest path calculation of seismic rays.” Geophysics 56.1 (1991): 59-67)) and optimizes the results through a specialized Simultaneous Iterative Reconstruction Technique. To guarantee efficiency, it exploits the computational power of modern multicore CPUs, while minimizing the required amount of memory.

Key points

  • Both manual and automatic first brake picking. Possibility to pick first breaks automatically or by hand.
  • The computation of tomographic at leasts needs the check by the user of automatics pickings. The computation of the tomographic profile can be almost completely automated. The user can supervise the computation and possibly provide specialized settings.
  • More seismic arrays can be combined to compose an unique seismic profile.
  • The initial model of velocity can be user-defined.
  • The initial model does not influence the result significantly.
  • The project size is limited only by available memory.
  • Users can set limit to the velocity range and smooth the result.
  • The result can be plotted using predefined or user-defined color palettes.
  • Both plot and data tables can be exported in different formats (PNG, PDF, ODT);
  • The tomographic profile can be exported in XZ-Velocity file compatible with Golden Software Surfer