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PRE-STACK DEPTH MIGRATION USING TWO DIFFERENT TOMOGRAPHICTECHNIQUES: THE OTRANTO CHANNEL CASE STUDY N. Bertone1,2, G. Brancatelli1, R. Geletti1, A. Del Ben2

1 Istituto Nazionale di Oceanografia e di Geofisica Sperimentale - OGS, Trieste, Italy2 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 Triestewith the CNR’s vessel “Marsili”. The acquisition parameters of the line are: 2400 m streamerlength, 24 channels, 200 m shot point interval, source Flexotir, fold 600%. We have re-processedthe MS29 line applying a modern processing sequence to vintage data and performing the PSDMby a combined approach of two different kind of tomographic inversions: 2D Grid Tomographyand Horizon-Based Tomography. We adopted this solution in order to better solve the velocityfield and to find the best fitting velocity-depth model for the depth migration. Earth imaging andmodeling 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 agrid tomography to update the starting velocity field in a semi-automatic way. Then we usedthis new velocity field to build a velocity-depth model through a coherency inversion of theinterpreted 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 onthe Common Image Gathers (CIG) to find a new velocity-depth model which reduces as muchas 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 tothe 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 Adriaticbasin 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

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reflection. Finally, the Plio-Quaternary deposits, characterized by clay and marl sediments, both on platform and in the basin, are identified as Santerno Formation.

Methods. The MS29 line has been reprocessed with the Echos and GeoDepth Paradigm software. We can divide the processing into three major steps: processing applied to pre-stack data and the pre-stack time migration, PSDM with 2D grid tomography and finally PSDM with horizon-based tomography.

Processing in time domain. The main processing sequence steps adopted till the pre-stack time migrated section are: trace editing and geometry assignment, despiking, preliminary velocity analysis, deghosting, fold regularization (shots and receivers interpolation), SRME (Surface Related Multiple Elimination), amplitude recovery, predictive deconvolution and pre-stack time migration with iterative velocity analysis on the migrated gathers. After stack processing included an FX-deconvolution, a time variant filtering and a trace equalization. In particular the fold regularization is a procedure to adequate modern processing steps (SRME, pre-stack time migration) to our low fold coverage vintage data. Therefore, to mitigate the effects of data sampling aliasing, we increased fold coverage of the MS29 line before the SRME step (Verschuur, 1992) and the pre-stack migration (Yilmaz, 2001).

2D grid tomography. 2D grid tomography does not require an initial velocity-depth model, so it can be a useful starting point when it is difficult to make a preliminary interpretation due to

Fig. 1 - Location map of the studied area (by Del Ben et al., 2011, modified).

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complex structures. Therefore, we used it to improve our starting velocity field. Then we used the new velocity field to depth migrate the MS29 line and as input to the velocity-depth model building for the horizon-based tomography approach.

The iterative working flow for the grid tomography approach can be summarized into four steps:

1. Build of the initial velocity model. We converted the RMS velocity field (used for the pre-stack time migration) into interval velocities through Dix equations. Then we laterally and vertically smoothed the velocity model, starting from the water bottom layer, to avoid artifacts due to lateral variations. We performed a first PSDM with these velocities.

2. Calculate structural attributes (continuity and dip) from the depth migrated section and of the travel-time error through an automatic picking. These attributes act as source for the autopicking process and as ray shooting for the tomography. Therefore, the travel-time error (residual move-out, RMO) is calculated by an automatic picking on the migrated gathers and it is then used by the tomography to update the velocity model

3. Use the grid tomography to update the velocity model. Tomography tends to minimize the errors between modeled and real situation by small changes in the velocity parameter.

4. Depth migration with the updated velocities and quality control on the common image gathers (CIG). We repeated these steps until the quality control on the CIG showed a satisfactory flatness of the reflections.

The first step is performed only once, and the flow is stopped when a satisfactory result is reached.

Horizon-based tomography. This method requires a set of picked time horizons, an initial trial interval velocity field and the CMP time gathers in order to build the initial velocity-depth model through a layer-by-layer coherence inversion.

Fig. 2 - a) Interval velocity in depth obtained after the Dix conversion of the RMS velocity. b) Velocity field updated by 2D grid tomography. c) Initial velocity-depth model obtained by coherence inversion. d) Velocity-depth model updated by horizon-based tomography.

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The iterative working flow for the horizon-based tomography approach can be summarized into these main steps:

1. Build initial velocity-depth model. Therefore, we interpreted the main horizons on the MS29 pre-stack time migrated section. We picked the water-bottom and 5 horizons (Water layer, Quaternary base, 2 strong reflectors of the Plio-Pleistocene age, Messinian unconformity, Carbonate platform), subdividing the section into 7 formations. It is convenient to pick significant horizons, that are those with major changes in velocity (principal reflectors).

2. Perform the demigration of the picked horizon using the RMS velocity field. As a matter of fact, coherency inversion (Landa et al., 1988) requires time horizons picked. This kind of inversion estimates the layer velocity using the ray tracing for a set of trial velocities. We used the velocity model obtained from the grid tomography as set of trial velocities.

3. Pick the highest semblance value velocity on the CMP locations along the picked horizon.4. Smooth this velocity to avoid unrealistic oscillations caused by noisy data. The

coherency inversion method honors the ray bending at layer boundaries, considering a non-hyperbolic CMP traveltime so, it is more realistic (Yilmaz, 2001) (Fig.2).

Finally, we have performed a first PSDM with the initial velocity-depth model. Thereafter, to check the consistency of our velocity-depth model, we analyzed the CIG by a RMO analysis, which represent the input (the measure of non-flatness of the CIG) to the tomography. After tomography we newly migrated our data. We performed this process as iterative way for three times to minimize the errors and to get best as possible velocity-depth model.

Conclusions. The MS29 seismic line investigates the Otranto Channel along its NW-SE axis. It intersects the Apulia carbonate platform and the two sedimentary basins: Ionian/South Adriatic Basin and South Apulia basin, respectively located at NW and at SE. To have a real geometry of the geological structures we proceeded to a PSDM, also using the well data present in the study area, achieving a good result allowing to better identify the following horizons: Quaternary base, Middle Pliocene reflector, top of the Messinian layer, top of the Apulia carbonate platform and base of pelagic sediment basins.

A preliminary result obtained by grid tomography showed a velocity-depth model with high frequencies lateral variations that affect the migrated section, producing unrealistic oscillations of the horizons. This effect is probably due to the noise present on the data, which affect the compute of the structural attributes and the successive tomographic inversion. Nevertheless, grid tomography flattens the CIG’s. On the other hand, the layer-by-layer coherence inversion lead to a starting velocity-depth model that produce a realistic depth migrated section. Finally, the model-base tomography improves the velocity field with a low RMO error on CIG’s. The

Fig. 3 - Part of the pre-stack depth migrated MS29 seismic line. In detail it is shown the carbonate platform with reef, the adjacent sedimentary basin and the topping sedimentary sequence. (WB=Water Bottom; QUAT=Quaternary Base; MID PLIO=Middle Pliocene; BS=Bright Spot MES=Messinian Layer; CARB=Carbonate Platform).

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final PSDM section (Fig. 3) shows reflectors with a good lateral continuity without artifacts or under/over-migration effects.

The seismic processing phase presented in this work was made within the master thesis of the first author. Now a day we are currently working on the interpretation of the MS29 profile. This study will be integrated by other geophysical data (high-resolution multichannel seismic lines, CHIRP profiles and Multi-Beam data acquired by OCSS15 OGS project in 2015, ViDEPI dataset: seismic lines and wells). The focus is to correlate deep regional structures, as platform margins, faults and possible bright spots to shallow local structures and morphologies, as fracture systems, gas seeps, mud volcanoes and carbonate mounds that are present in the area.References Cosentino, D., & Gliozzi, E. (1988). Considerazioni sulle velocità di sollevamento di depositi eutirreniani dell’Italia

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data. Deep Seismic Exploration of the Central Mediterranean and Italy, CROP PROJECT, 19, 447-470.Landa, E., Kosloff, D., Keydar, S., Koren, Z., & Reshef, M. (1988). Method for determination of velocity and depth

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