GNGTS 2018 - 37° Convegno Nazionale

622 GNGTS 2018 S essione 3.2 Autorità Nazionale del Bacino del Fiume Adige,1998, note illustrative del progetto CARG - Carta Geologica d’Italia alla scala 1:50.000. Foglio 043, Mezzolombardo. Bassetti M., Borsato A., 2007, Evoluzione geomorfologia della Bassa Valle dell’Adige dall’ultimo massimo glaciale: sintesi delle conoscenze e riferimenti ad aree limitrofe, Studi Trentini di Scienze Naturali. Acta Geologica, 82, pp. 29-40. Binley A., 2015, Tools and techniques: Electrical methods. In G. Schubert, editor, Treatise on Geophysics (Second Edition), chapter 11.08, pp. 233 – 259, Elsevier, Oxford, second edition, doi :http://dx.doi.org/10.1016/B978- 0-444-53802-4.00192-5. Busato L., Boaga J., Peruzzo L., Himi M., Cola S., Bersan S., Cassiani G., 2016, Combined geophysical surveys for the characterization of a reconstructed river embankment. Engineering Geology 211 (2016), pp. 74–84. Donini F., 2017, Tomografie di resistività elettrica a supporto della modellazione idrologica: applicazioni su un argine dell’Adige. Tesi di Laurea Magistrale in Geologia e Geologia Tecnica, Padua@thesis archive. APPLICATION OF ACOUSTIC GLOBAL FWI ON A SHALLOW SEISMIC REFLECTION DATA SET ACQUIRED IN LUNI (TUSCANY) J. Brunod, S. Pierini, A. Tognarelli, E. Stucchi Department of Earth Sciences, University of Pisa, Italy Introduction. The archaeological site of Luni in Tuscany (Italy) is located on an alluvial plain where geological surveys carried out in the field recognized different units such as alluvial fans of the Magra river, sedimentary swamps and sand bars (Bini et al. , 2006), all indicating a flat layer stratification of the shallow subsurface. The archaeological context of the site makes Luni a very interesting location for the application of various geophysical techniques, such as GPR, electrical tomography and magnetic surveys. Furthermore, the good seismic response observed in previous seismic reflection and refraction tests, makes also Luni attractive for a seismic reflection survey aimed at investigating the near-surface layers for a more detailed geological study. All these motivations lead us to the acquisition of a reflection seismic data set that can also be used for the application of different processing and inversion algorithms especially tailored to the near-surface investigations. In this work, we describe the processing and the results of a preliminary 2D acoustic Full Waveform Inversion (FWI) of a seismic reflection data set acquired in Luni. The processing is carried out to obtain the best image in time and also to estimate a preliminary interval velocity field (by means of the Dix equation) that is later used as input to the pre-stack depth migration. Successively, a global 2D acoustic FWI is applied to produce a more accurate velocity field in depth. We make use of a parallel implementation of a genetic algorithm (GA) optimization (Sajeva et al. , 2016), where the data misfit is evaluated as the L2 distance between the envelope of the predicted and observed data on the first arrivals to reduce the non-linearity of the misfit function. Pre-stack depth migrated common image gathers (CIGs) computed with the obtained velocity field show a good horizontal alignment of the observed reflections. Seismic data set. In this experience, the equipment used consists of 48 vertical component geophones with characteristic frequency of 10 Hz connected to two seismometers by a cable with 48 channels. Receivers spacing is 2 m and shot distance 1 m. The spread configuration is mainly off-end, but towards the end of the line, to spare time, the cable is held fixed and the shot position gradually enters in the spread. This acquisition configuration allows to have a maximum offset of around 100 m and a mean fold coverage of 50, even if not uniformly distributed. The choice of the source is dictated by the requirement to have a high number of shots to grant the possibility of source array forming. Therefore a 10 kg sledgehammer is used even if the depth of penetration is lower with respect to a seismic gun. With this kind of source