GNGTS 2016 - Atti del 35° Convegno Nazionale

GNGTS 2016 S essione 3.1 497 vary from 2000 m/s to 6000 m/s close to the surface and from 4000 m/s to 8000 m/s at the end of the model. In the centre of the model the ranges are linearly interpolated. As the misfit function, we used the L 2 norm between the observed and the predicted data at the end of the following steps of processing. At first the data are filtered in the same frequency band (from 0 to 10 Hz). Then we compute the envelope in the offset-time window enclosing only the direct and diving wave arrivals and finally carry out a trace by trace normalization. Results. In Fig. 2b is shown the final velocity model estimated after the 300 generation. Note on the top the topography. Below, the velocity increases in depth and range between 3000 m/s and more than 6000 m/s approximately. Also, the model highlight some features that are in agreement with the geological setting of the area (Scrocca et al. , 2003). The most evident is the important velocity contrast that delineates the structure that dip to the right part of the model. In Fig. 3 the observed data (in black) and the predicted data (in blue) are compared for the two expanding spread experiments and for a production shot. As can be noted, the matching between the envelope of the waveforms is satisfactory, increasing our confidence in the obtained result. Conclusions. In this work we have described the acoustic FWI experience made on a 2D seismic land data. A particular type of evolutionary algorithms (the genetic algorithms) is used in order to reduce the risk of getting trapped into local minima, and the two grid approach is adopted to reduce the computational costs. In order to obtain the observed data characterised by an improved signal-to-noise-ratio where the velocity information of the refracted and diving waves is preserved, we employ a processing sequence that include Fig. 3 – Comparison between observed (black) and predicted (blue) data for a) forward expanding spread experiment; b) a production shot located in the middle of the seismic line; c) backward expanding spread experiment.