GNGTS 2015 - Atti del 34° Convegno Nazionale

GNGTS 2015 S essione 3.3 121 Single-Pass Surface Related 3D Multiple Estimation for OBS Data with Data Driven + Bathymetry Approach M. Codazzi, P. Mazzucchelli Aresys, Milano, Italy Introduction. Ocean Bottom Seismic is a type of survey where recording nodes are laid out on sea-floor and seismic sources are energized at sea surface and it is an alternative acquisition method in marine environments when the presence of obstacles or a shallow water layer prevents the conventional seismic acquisition by towed streamers. Despite higher acquisition costs, OBS data compare favorably to conventional surface marine acquisitions because of their inherently full azimuth coverage (as OBS geometries usually show similarity to land acquisition geometries), providing better illumination of complex subsurface structures. Furthermore, OBS surveys record both pressure and particle motion through hydrophones and geophones respectively: the multicomponent nature of OBS data broadens the number of applicable processing algorithms. More specifically, the availability of different recordings of the wavefield allows to process OBS data for the separation of up-going and down-going wavefields. OBS data suffer from the contamination by surface-related multiple reflections exactly like conventional marine data, amplified by the shallow water environment where OBS acquisitions are employed: it must be noted that surface related-multiple reflection estimation is tightly joined to up-going and down-going wavefield separation. However, taking advantage of the multicomponent nature of OBS data is not straightforward, becauseof thedifferent technologies of hydrophones andgeophones that implya trickycalibration process. Moreover the sparse sampling of sea bottom nodes (compared to conventional surface marine acquisitions) prevents the direct application of multidimensional filtering algorithms, usually available for surface marine acquisition geometries, that cannot cope with aliased wavefields (at least in CSG -Common Shot Gather- subdomain). Surface related multiple suppression for OBS data. Surface related multiples can occur both at source side and at receiver side: examples of surface related multiple travelpaths are sketched in Fig. 1. More complex patterns can be generated while dealing with higher order multiples. Each travelpath have been visually separated into three different sub-paths: red and blue sub-paths represent recorded OBS data (source and receiver related, respectively), while the “missing” water-layer propagation is shown in green. A well-known and widely-used way to deal with multiple wavefields and multicomponent data is the so-called PZ summation: the multicomponent nature of OBS data is exploited in order to separate up-going and down-going wavefields, as multiples contaminating pressure and vertical particle velocity data have opposite polarity. However, PZ summation can correctly handle only receiver-side surface-related multiples (or equivalently, ghosts, corresponding to travelpaths a, c, d, f in Fig. 1). Thus, up-going and down-going wavefields after separation are still contaminated by source-side multiples, and further processing is needed. Moreover, an effective PZ summation requires the calibration of geophones and hydrophones, that is known to be generally difficult because of differences in frequency responses and signal to noise ratios, and their variability with respect to emerging angles. Different techniques based on the adaptation of the standard data-driven SRME approach (Berkhout et al. , 1997) have been proposed to address the estimation and suppression of surface related multiples while dealing with OBS acquisitions: in fact, multiple estimation techniques that are generally used in the processing of conventional streamer acquisitions cannot be applied to OBS data, as the assumption that both sources and receivers are located at the sea surface is not met by this type of acquisition geometries. However, in case that conventional streamer data are available in the same acquisition area, a full data-driven approach can still be implemented by combining them with the OBS data (Verschuur et al. , 1999).