GNGTS 2015 - Atti del 34° Convegno Nazionale

104 GNGTS 2015 S essione 1.2 deposited on the shelf (Fais et al. , 1996) and the sea bottom currents assumed a main role. The Upper Pliocene/ Quaternary sediments are generally scarce or absent, while in the western deep basin they are thicker because of the high sedimentation rate related to the rivers crossing the Gulf of Lions (Geletti et al. , 2014). According to Cornée et al. (2008) a Pliocene basaltic flow was emplaced on an erosional surface transecting both Messinian and Early Pliocene deposits. Plio-Quaternary Magmatism in West Sardinia. The Sardo-Provençal basin originated by the north-west subduction of the Tethys lithosphere. This caused the Oligo-Miocene calc-alkaline magmatism and the rotation of the Corso-Sardinian microplate, which reached its final stage about 18 Ma ago (Lustrino et al. , 2000). A new volcanic phase took place during the Pliocene - Pleistocene Tyrrhenian opening: in the Sardinian onshore it is divided in two different periods: from 5.3 to 5.0 Myr and from 1 to 0.9 Myr (Lustrino et al. , 2000). Large volcanic structures, plateau and basaltic lava flows from individual small volcanoes were produced, with tholeiitic to transitional to alkaline chemistry. Seismic processing: multiple attenuation method. The multiples problem. Multiples are signals generated by seismic waves repeatedly reflected between two seismic subsurface discontinuities. This phenomenon occurs when the seismic waves cross geological bodies that are characterized by large variations of the acoustic impedance: therefore, their paths are limited between interfaces with high reflection coefficients. The best example of multiples generation is the water column bounded by a top discontinuity with the air and the base with the sediments. The seismic waves crossing the water layer remain partially “trapped” and the repeated reflections are evidenced in seismic profiles as “ghost” reflectors. They will be not corresponding to real discontinuities present at pertinent depths. The occurrence of this phenomenon can obscure the presence of the primary reflectors, that are the purpose of a seismic acquisition. Fortunately, the recognition of a multiple reflection shows consistently distinctive characteristics: they are regularly localized at a depth with a not random position and this makes easy their recognition. Furthermore, multiple signals often cross the primary reflectors, and this represents an unrealistic geological condition that suggests the multiple analysis. Finally, the multiple waves travel in shallower/slower sequences relatively to their depth: this allows their identification in velocity spectra. Multiple attenuation process applied to the seismic data set. Since the studied area is sited on the west-Sardinian continental shelf and upper slope, the seismic profiles are affected by a shallow sea bottom. This implies the presence of strong multiple effects on the upper sedimentary sequence, that represent a major noise for the interpretation of data. Therefore we decided to reprocess the profiles to improve the possibility to recognize the primary signals. In the f-k domain (frequency - wavenumber) the energy of primary and multiple reflections can be separated in two different quadrants (Yilmaz, 2001). To achieve this result we apply a correction of NMO using a velocity intermediate between those of primary and multiple reflectors: the primary signals will be over-corrected, while the multiples will be under-correct. Consequently, the multiples can be suppressed by resetting the dial with values greater than zero. After the removal of multiples, we have to go back to the x-t domain through the inverse Fourier transform and to remove the incorrect NMO (NMO Removal) used for the primary reflections. At this point a standard process can be applied with all the steps normally used for the seismic processing: Deconvolution, Velocity analysis, correction of NMO and Migration (Fig. 2). Seismic interpretation. Our interpretation of seismic data focused on the Plio-Quaternary sequence. The base of PQ represents a very important and seismically clear horizon, which is represented by the MSC event. In the upper continental slopes and in the onshore Mediterranean regions the MSC is often highlighted by an erosional truncation caused by the drastic drop of the sea level: