GNGTS 2015 - Atti del 34° Convegno Nazionale

122 GNGTS 2015 S essione 3.3 Unfortunately, this situation does not usually happen. When only OBS data are available, missing surface traces can be obtained by transforming OBS data in surface data with a datuming process: a quite expensive preprocessing step (both in terms of computational cost and I/O overburden) is required to apply standard data-driven demultiple techniques. Any additional I/O is a manifest drawback, especially when dealing with the dimensionality of modern 3D acquisitions. Conversely, a fully model-driven approach has been proposed by Pica et al. (2006), extending to OBS data their approach based on wavefield modeling. While data-driven methodology requires only a description of the water layer (implicitly needed in the datuming step), the latter requires a good-quality reflectivity model (i.e., migrated data volume, that cannot be available in the early processing steps). Another OBS data demultiple algorithm has been proposed by Jin et al. (2012) by extending the MWD technique to OBS geometries (requiring the knowledge of the water layer only). This technique can predict only source-side and receiver-side water layer reverberation (travelpaths a, c, d, e, f in Fig. 1). Even if the MWD approach cannot predict all multiple paths, it must be noted that water-layer related multiples (also defined as source-side or receiver-side water- layer reverberations) prevail over the other interfering wavefields, at least in the shallow water environment. Furthermore, its computational cost is smaller than those of the approaches previously discussed and it proves to be an alternative method to perform up-down separation without the need of different wavefields recorded by hydrophones and geophones (the quality of the separation relies on a subsequent adaptive subtraction processing step). Unfortunately, none of the proposed techniques can deal with all the different multiple travelpaths (source-side and receiver-side) in a single processing step, without requiring a full reflectivity volume or an intermediate 3D surface acquisition. We propose a hybrid approach that blends OBS data and computed travelpaths in water layer and that is able to estimate all the possible source-side and receiver-side surface related multiples shown in Fig. 1. Single-pass OBS surface related multiple estimation. We introduce a (partially) data- driven method for the estimation of multiple reflections for OBS data that neither requires integration of surface data nor reflectivity models, but only the knowledge of the bathymetry (information that, if missing, can be anyway extracted from the geometry of OBS data). Unlike the techniques based on a separate datuming step, this method has also the advantage to perform Fig. 1 – OBS SRME multiple generation schemes: a) first order downgoing multiple (receiver ghost); b) first order upgoing multiple; c) second order downgoing multiple; d) water layer multiple; e) first order upgoing multiple (source side reveberation); f) second order downgoing multiple (source side reveberation).