A seismic multi-approach method for characterizing contaminated sites

A geophysical campaign to characterize the subsurface of a contaminated site down to a depth of several tens of meters was carried out under the HYGEIA-CEE project. On this site, seismic techniques were combined to image the geological structures; i.e. seismic reflection, P-wave tomography and spectral analysis of surface waves. Because these techniques consider different wave components in the processing, they can be expected to provide complementary information concerning the site lithology. The special feature of this experiment is the fact that the same seismic acquisition device, consisting of a mobile central unit, a drop-weight seismic source, and a sensor line of gimbal mounted geophones, was used for each of the techniques. Two perpendicular seismic lines were set up in the field for testing two geophone spacings. Three processing procedures, one each for the seismic reflection, P-wave tomography and spectral analysis of surface waves, were developed for producing seismic images from the P-wave reflectivity, the first P-wave arrivals and the dispersion of Rayleigh waves, respectively. The images show good complementarity in terms of investigation depth. The results are also in good agreement with available borehole data: the sandy layers seem to be related to low velocities, since the high velocities are better explained by the presence of clayey and gravelly intervals. The contribution and the limits of this seismic multi-approach method is discussed.