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Comparison of different strategies for velocity model building and imaging of PP and PS real data

CGG, Massy, France

Constantin Gerea, Gisèle Etienne and Alexandre Stopin

Institut Français du Pétrole

Mathias Alerini, Soazic Lebegat and Gilles Lambaré

Armines

Philippe Berthet, Serge Nebieridze and Jean-Luc Boelle

TotalFinaElf Exploration Production

Corresponding author: aherrenschmidt@cgg.com

The first 20% of the full text of this article appears below.

In 1999, a 2-D/4-C data set acquired with ocean-bottom cable on Mahog-any Field, Gulf of Mexico, was distributed at the SEG-EAGE Research Workshop to test the feasibility of using converted waves to image under a salt diapir. We show and discuss results obtained by several imaging methods—some in the time domain, others in the depth domain—applied to both PP and converted-wave fields (PS, also referred to as the C-wave).

After a review of the standard time-domain approach (DMO, common conversion point or CCP binning, NMO stack, and poststack time processing), we will consider more elaborate approaches. We start with methods that use observations in the unmigrated time domain, inversion of stacking velocities and inversion of picked prestack traveltime. We finish our review with methods that use observations in the migrated domain, migration velocity analysis using prestack time migration, and migration velocity analysis using prestack depth migration.

The multicomponent 2-D Mahog-any data set is oriented E-W and was acquired with a 1.5 km 4-C cable at a quasiconstant water depth of 118 m (Figure 1). Maximum offset is 11.5 km. Record length is 10 s with a 2-ms sampling rate. The line was shot vertically above the receivers, and the line direction chosen to minimize 3-D effects. Raw shot gathers show that PP waves were mainly recorded on the P (pressure) and Z (vertical) components. PS waves were essentially recorded on the X (in-line horizontal) component. Some time processing steps specific to OBC acquisition were undertaken—surface-consistent compensation of coupling between the receivers and the sea bottom, P-Z summation for the PP waves, and medium wavelength surface-consistent receiver statics for the PS waves to account for the extremely low S-wave velocity in the near-sea bottom. The polarity of positive offsets was reversed to obtain radial polarity . . . [Full Text of this Article]