PRE-STACK DEPTH MIGRATION USING TWO DIFFERENT TOMOGRAPHIC TECHNIQUES:

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, Trieste2 Dipartimento di Matematica e Geoscienze – DMG, Università di Trieste

IntroductionThe MS29 line, acquired in the 1971 by OGS and CNR’s vessel Marsili, is a longitudinal transect of the South Adriatic Sea, crossing the Otranto Channel.

Acquisition parameters:• Marine Multi-Channel Seismic survey• Source Flexotir (explosive)• 2400 m streamer length• 24 channels• 200 m shot point interval• Fold 600 %

The MS29 line intersect the Apulia carbonate platform and two basins: the Ionian/South Adritic Basin and the South Apulia one.

The carbonate platform margins are still debated. In figure is shown the most recent margins interpretation near the Otranto Channel.

Why do we use a tomographic approach? 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:

Coral reef

Steep dips

Carbonate platform surrounded by pelagic sediments

Irregular water-bottom topography

Processing in time domain

After SRMEBefore SRME Multiple subtracted

Fold-regularization (shot and receiver interpolation) is a fundamental procedure to adeguate modern processing steps (Surface Related Multiple Elimination) to our low fold coverage vintage data.

2D Grid Tomography

Starting-point for the 2D grid tomography: structural attributes (continuity and dip) and interval velocities in depth.

Structural attributes calculated on the CMP gathers. Note the well-fitting on the geological structures!

Unsmootehd RMS velocity by semblance picking every 40 CMP.

Afterwards Dix convertion to obtain the interval velocities.

a) Interval velocity in depth obtained after the Dix conversion of the RMS velocity.

b) Velocity field updated by 2D grid tomography.

Result of the 2D Grid Tomography

Horizon-Based Tomography

Starting point:Interpreted layers

Grid Tomography velocity

Coherency Inversion(ray tracing)

Velocity-depth model building via ray-shooting. The ray shooting tries to fit, layer-by-layer, the best velocity-depth model based on the Snell reflection’s law.

CIG before and after correction.

The Common Image Gathers are useful as a control on the quality of the RMO (residual move out) correction.

Before (unflattened CIG) After (flattened CIG)

Final velocity-depth model overlying the MS29 section.

c) Initial velocity-depth model obtained by coherence inversion.

d) Velocity-depth model updated by horizon-based tomography.

Result of the horizon-based tomography

Final resultsFrom the left to the right it is appreciable how the velocity field has been implemented on the real geology.

a) initial velocity field obtained by semblance picking.

b) velocity model updated by 2D grid tomography.

c) final velocity-depth model updated by horizon-based tomography.

Conclusion

MERLO WELL (projected) SSDS ?

Merlo well

THANK YOU FOR YOUR ATTENTION!