GNGTS 2017 - 36° Convegno Nazionale

544 GNGTS 2017 S essione 3.1 remove the effects of the near-surface velocity variations, and sometimes the near-surface coherent noise such as the Rayleigh waves. A more standard static correction solution was also tested: since the imaging and migration is performed at the topography and not at a CMP- consistent floating datum, the statics are computed between the actual position of source and receivers, an intermediate datum and a final smoothed topography or depth-defined, surface- consistent floating datum. The small scale near-surface velocity variations and the topographic irregularities are actually a key challenge of the CROP-03 dataset. And the conventional reflection-based residual static corrections struggle to compensate them unless an initial solution is successful. A tomography based and refraction-residual static solution is proven to be particularly effective in compensating for the near-surface effects. The effectiveness of the perturbation corrections is evident from the comparisons between the images created using data with and without the short period perturbations. Then the noise attenuation is applied using an iterative strategy, starting from mild and short FX filters and rank reduction filters and avoiding large multichannel dip filters. This is essential to preserve low-amplitude primary energy, including reflections and diffractions, needed to image the targets. To ensure a correct imaging of the complex structures, an accurate, horizon consistent pre- stack migration velocity picking is performed. Conclusions. The results are very encouraging, as we managed to obtain a considerable improvement of the images: the coherency and continuity of events, with locally large dips and complex structures. The improvement of the images reveals shallow structures that were not seen in previous processing, which were targeting mainly deep crustal features. References Giustiniani, M., Tinivella, U. and Nicolich, R.; 2015: Reflection seismic sections across the Geothermal Province of Tuscany from reporcessed CROP profiles. Geothermics, 53 , 498-507. Experience of FWI on marine seismic data using a robust optimization procedure B. Galuzzi 1 , A. Tognarelli 2 , E. Stucchi 1 1 Department of Earth Sciences, University of Milan, Italy 2 Department of Earth Sciences, University of Pisa, Italy Introduction. Full Waveform Inversion (FWI) represents an important tool to obtain high resolution model of subsurface from active seismic data (Tarantola, 1986; Virieux, et al. 2009). In the last years many aspects about FWI have been studied concerning the implementation of efficient modelling algorithm (Chaljub et al , 2007; Moczo et al., 2007) and the formulation of inversion procedures (Fichtner, 2010). However, its application to real data requires specific operations on the seismograms to obtain an observed data that can be reproduced by a modelling algorithm. Besides, the use of an iterative gradient method requires the estimation of a starting model that must be as close as possible to the valley of the global minimum of the misfit function (Beydoun et al., 1988). In this work, we present an application of acoustic FWI on a marine seismic data. Specific processing operations are applied on both predicted and observed data to increase the robustness of the inversion procedure against the cycle skipping phenomenon, thus improving the reliability of the final model estimation. The predicted data are obtained by solving the 2D acoustic wave equation, whereas in the local optimization procedure the steepest descend algorithm is employed, using the L 1 norm difference between the predicted and observed data