Surface Wave Analysis using an innovative acquisition and processing scheme

The thesis is carried out in the framework of a cooperative project between Total E&P and Politecnico di Torino. The project is aimed at implementing new seismic processing methods for estimating velocity models from data acquired with irregulalrly spaced receiver and source carpets. A dataset was acquired for research purpose at a mining site in the province of Aurignac, south France, characterized by stiff materials. We extracted Rayleigh dispersion curves (DCs) by means of an innovative multichannel method, practically our processing methodology consists of a moving square-window that slides over the acquisition area for the selection of the receivers and a moving circumference, having the same center of the square-window, for the selection of the shots. For each position of the moving window, we obtain a local DC, as a consequence, the dataset to be inverted is composed of a unit of DCs that are associated with a series of spatial coordinates, represented by the center of each square-window. To consider the topographic variation in the analyzed area and to build a 3D VS model, we invert the single fundamental mode of the acquired DCs using the Laterally Constrained Inversion (LCI) algorithm, which consists in a deterministic inversion where every 1D model is related to its neighbors with a mutual constraint to offer a single pseudo-3D model. Then, to take advantages of all the benefits of including the higher modes in the inversion, increase the investigation depth, increase the resolution, and so on, we invert the higher modes using a Monte Carlo inversion, method based on a misfit function consisting od the Haskell-Thomson determinant. This allows higher modes to be considered without the need to associate experimental data points to a specific mode. The retrieved velocities obtained from the two different approaches are in good agreement, with an average dofference of 16%. We conclude that the data acquired with the novel approaches envisaged in the METIS project merged with an expeditious processing approach is functional to build shear wave velocity models in an area characterized by stiff materials.