GNGTS 2014 - Atti del 33° Convegno Nazionale

GNGTS 2014 S essione 3.1 57 Rehault J. P., Boillot G. and Mauffret A.; 1984: The Western Mediterranean Basin Geological Evolution . Marine Geology, 55, 447-477. Ryan W.B.F., Hsü K.J., Cita M.B., Dumitrica P., Lort J., Maync W., Nesteroff W.D., Pautot, G. Stardner, H. Wezel, F.C.; 1973: Boundary of Sardinia slope with Balearic Abyssal Plain - Sites 133 and 134 . In: Ryan et al. (Ed.): Init. Rep. DSDP, 13, 465–514. Sage F., Von Gronefeld G., Déverchère J., Gaullier V., Maillard A. and Gorini C.; 2005: Seismic Evidence for Mes- sinian Detrital Deposits at the Western Sardinia Margin, Northwestern Mediterranean . Marine and Petroleum Geology, 22, 757-773. Yilmaz O.; 2001: Seismic Data Analysis: Processing, Inversion and Interpretation of Seismic Data. Stephen M. Doherty – SEG. SEPARATION AND IMAGING OF WATER-LAYER MULTIPLES FOR VSP SURVEYS S. Fiorentino, M. Codazzi, P. Mazzucchelli Aresys, Milano, Italy Introduction. Vertical Seismic Profile (VSP) surveys allow to obtain information about the subsurface structure thanks to (multi-component) geophones in the borehole, that record the transmitted and reflected energy originating from a seismic source at the surface. Such surveys can provide data with higher resolution compared to surface seismic acquisitions, although their coverage is limited around the well area. In this paper the focus will be on the walkaway VSP (WVSP) offshore surveys in which the vessel, equipped with an air-gun, moves progressively along a line (or in an area for 3D surveys) at the sea surface and an array of receivers is held at fixed depths in the well. WVSP surveys, compared to Zero-offset VSP acquisition geometries, provide a continuous illumination of a wider area close to the well. WVSP surveys are usually performed to improve the knowledge of the subsurface, and in particular to refine the estimates of AVO (Amplitude Versus Offset) and anisotropic parameters. However, it must be noted that also WVSP surveys have a limited fold of coverage if compared to surface seismic (thousands of traces vs. millions of traces): thus, the stacking power of imaging algorithms is of little help in attenuating the coherent noise that contaminates recorded data. A meticulous pre-processing phase is mandatory prior to seismic imaging and any quantitative analyses: either techniques typical of VSP borehole data standard processing and approaches borrowed from surface data processing should be applied to WVSP surveys. Among all the spurious wavefields that can contaminate reflection data, multiple arrivals can be the most energetic source of coherent noise that can hide information recorded at target level. While surface related multiple arrivals are ubiquitous in marine data recordings and their effect must be always addressed, energetic internal multiple reflections (which can be tougher to be separated) are related to the presence of sharp variations in the subsurface (i.e., salt-dome flanks), thus their effect can be neglected in areas with limited geological complexity. Multiple reflections in walkaway VSP data. The conventional multiple removal technique for the (Zero-Offset) VSP data is the “up-by-down” deconvolution: after up- and down-going wavefield separation, a predictive deconvolution is applied to the up-going wavefield using an operator built with the down-going wavefield. Although this approach gives good results in case of zero/near-offset VSP when the subsurface model can be considered 1D (i.e., horizontally layered model), more sophisticated techniques are needed for multiple elimination inWalkaway VSP data, because of the extended acquisition geometry. Similarly, (surface-related) multiple elimination plays a fundamental role in marine data processing and nowadays data-driven convolutional methods, as SRME (Verschuur and Berkhout, 1992), have become the de-facto standard procedure for multiple removal for