Ottenere sezioni stack da dati di sismica a rifrazione: un tentativo di migliorare la conoscenza sulla geometria del sottosuolo

Shallow refraction seismic survey is a very common and useful subsurface investigation method. However, traditional shallow seismic refraction processing methods show long-standing limitations due to the almost complete reliance on the first arrival signal. On the other hand, full wave form processing of data can by-pass inherent limitations and improve the imaging of the subsurface. In this sense, better results can be achieved through digital processing that has been successfully developed in reflection seismology. In particular, a time cross-section, similar to the well known reflection cross-section, can be generated through processing of reflected signals, using tools like CMP stacking that improves the signal-to-noise ratio. Other tools imply deconvolution and migration for improvement of, respectively, vertical and lateral resolution. Muting and dip filtering for deletion of coherent noise. This note presents a discussion on the feasibility of this kind of approach. Different time cross-sections from different processing tests were compared with a HR reflection seismic section used as calibration: particularly, triangular mute windows and the exclusion of the traces relative to the external shots enable the elimination of the strong ringing due to the first arrivals and this tends to be the main problem; furthermore, the external shots prove to be useful for the reconstruction of the deeper structures (with arrival times greater than 400 ms) but not as useful for the reconstruction of the superficial structures

VSP processed down-hole data within local seismic response assessment

Local effects evaluation may be solved using different methodological ways. VEL Project of Tuscany district approach, foresees the evaluation of the surface effects in a specified zone of a "forecasted" earthquake, through geological, geomorphologic, geotechnical, geophysical and numerical modelling. The knowledge of bedrock depth and geometry is a fundamental issue in such a multidisciplinary integrated approach. Down-hole tests carried out in not sufficiently deep borehole to reach bedrock, have been processed as VSP (Vertical Seismic Profiling) method, allowing the knowledge of bedrock depth. This has been obtained developing a processing aimed to the study of reflected signals that provided important information on discontinuities occurring below the borehole bottom. Results obtained by VSP processing from conventional SH waves down-hole data, have been calibrated by comparison with an high-resolution shear wave reflection line carried out in the same site. This comparison provided new cognitive elements and work prospects

Time cross-sections generated from shallow seismic refraction data: preliminary results

Shallow refraction seismic survey is a very common and useful subsurface investigation method. However, traditional shallow seismic refraction processing methods show long-standing limitations due to the almost complete reliance on the first arrival signal. On the other hand, full wave form processing of data can by-pass inherent limitations and improve the imaging of the subsurface. In this sense, better results can be achieved through digital processing that has been successfully developed in reflection seismology. In particular, a time cross-section, similar to the well known reflection cross-section, can be generated through tools like CMP stacking that improves the signal-to-noise ratio. Other tools imply deconvolution and migration for improvement of, respectively, vertical and lateral resolution. Muting and dip filtering for deletion of coherent noise. This note presents a discussion on the feasibility of this kind of approach. Time cross-sections obtained with these methods are shown and compared with a HR reflection section

Application of high resolution shear wave seismic methods to a geotechnical problem

Applied geophysical techniques in underground exploration can be useful in defining the subsoil geometry and the spatial relationships of the physico–mechanical properties of the materials. An example is presented to demonstrate how highresolution seismic techniques using shear waves can assist with a geotechnical issue. The survey has been carried out to elucidate the subsoil geometry and characterize lithotypes in terms of physicomechanical parameters, useful in the geotechnical study of an historic monument in which fractures developed following a downslope excavation