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.