GNGTS 2018 - 37° Convegno Nazionale

GNGTS 2018 S essione 3.1 561 PRE-STACK DEPTH MIGRATION USING TWO DIFFERENT TOMOGRAPHIC TECHNIQUES: THE OTRANTO CHANNEL CASE STUDY N. Bertone 1,2 , G. Brancatelli 1 , R. Geletti 1 , A. Del Ben 2 1 Istituto Nazionale di Oceanografia e di Geofisica Sperimentale - OGS, Trieste, Italy 2 Dipartimento di Matematica e Geoscienze - DMG, Università di Trieste, Italy Introduction. Depth migration applied to vintage data is a challenging task but represents an important resource in order to reconstruct the real geometric setting of the geological horizons in a complicated geological framework. In this study, we present the methods and the results of the Pre-Stack Depth Migration (PSDM) using and combining two different tomographic techniques on the vintage seismic line MS29 acquired in 1971 in the Otranto Channel (Southern Adriatic Sea) (Fig. 1). This Multi-Channel Seismic (MCS) profile was acquired in the framework of the MS international project for exploration of the Mediterranean and Black Seas during the ‘70s - ‘80s by the Instituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS) of Trieste with the CNR’s vessel “Marsili”. The acquisition parameters of the line are: 2400 m streamer length, 24 channels, 200 m shot point interval, source Flexotir, fold 600%. We have re-processed the MS29 line applying a modern processing sequence to vintage data and performing the PSDM by a combined approach of two different kind of tomographic inversions: 2D Grid Tomography and Horizon-Based Tomography. We adopted this solution in order to better solve the velocity field and to find the best fitting velocity-depth model for the depth migration. Earth imaging and modeling is imperative when we need to depth migrate a section with strong lateral variations, like steep dips reflectors (carbonate platform surrounded by pelagic sediments), coral reefs, irregular water-bottom topography and imbricated structures. Therefore, we decided to use a grid tomography to update the starting velocity field in a semi-automatic way. Then we used this new velocity field to build a velocity-depth model through a coherency inversion of the interpreted formations velocities. Finally, we updated the velocity-depth model by the horizon- based tomography. Tomography uses the residual move out error showed by the reflectors on the Common Image Gathers (CIG) to find a new velocity-depth model which reduces as much as possible the errors and flattens the reflections. The final PSDM section has been interpreted, showing a higher resolution and a better reconstruction of the geological setting compared to the pre-stack time migrated section. Geological setting . The sedimentary succession could be directly linked to its geological evolution. During the Permo-Triassic age the Adria plate underwent a first rifting phase that lead to the formation of a thick carbonate platform (Vai, 1994; Finetti and Del Ben, 2005; Stampfli, 2005). From Upper Liassic to Lower Cretaceous the plate acted as an independent block separated from Africa. During this time interval the plate was affected by an extensional phase that originated horst and graben structures, determining an alternation of shallow water and pelagic conditions (Mattavelli et al. , 1991). The passive margin of the Adria Plate persisted until the Cretaceous, when the subduction brought to the enclosure of the Tethyan Ocean. The margins became active where overthrusted by the orogenetic chains of the Southern Alps (Picha, 2002), successively by the Dinarides and then by the Apennine Chain, becoming the foreland of all of them. The Mesozoic platform crops out only in the Apulian, Istria and Dalmatian regions. The Salento peninsula represents a relatively undeformed antiformal structure, NNW-SSE oriented, uplifted during the Pleistocene – Holocene time (Ricchetti et al. , 1988; Cosentino and Gliozzi, 1992). According to Mattavelli et al. (1991) the pelagic sequence of the Ionian/South Adriatic basin is composed by marls and limestones of the Liassic to the Paleogene, covered by a clastic succession discharged by growing orogens, followed by a regular alternation of sands and shales of the Bisciaro Formation of the Lower Miocene and by marly silty and clay turbidites of the Serravallian-Tortonian (Schieler Formation). This sequence is overlain by the Upper Messinian evaporites (Gessoso-Solfifera Formation), that is a guide horizon due to its strong