64th EAGE Conference & Exhibition

A-34 FRESNEL-APERTURE PSDM Abstract 1 We introduce the Fresnel aperture which is the direct time-domain equivalent at the receivers’ surface of the subsurface Fresnel zone. Based on this concept we propose a new and efficient method for optimal aperture selection and Kirchhoff prestack depth migration. The problem with complex media is that multiple Fresnel apertures apparently exist for a given image point with just one of these corresponding to a possible real geological structure. Hence the contribution from these ‘false’ Fresnel apertures cause a noise-corrupted image of the subsurface. We propose a method by analogy with the velocity analysis where

A-36 RAY-BASED ANTI-ALIASING FOR DEPTH MIGRATION S.NGUYEN 1 R. BAINA 2 M. NOBLE 1 and P. THIERRY 1 Abstract 1 Operator/Imaging aliasing introduced in Kirchhoff migration is often tackled by trace tapering aperture truncation or a time and offset-variant filtering. The latter being the most suitable. However most implementations and published results using this technique are derived for Kirchhoff time migration and assume a constant velocity media. In this paper we introduce an anti-aliasing filter for ray-based pre-stack depth migration and valid for heterogeneous velocity models. We illustrate the benefits of such a scheme on a numerical and a real

A-38 HOW TO CHOOSE A SUBSET OF FREQUENCIES FOR FREQUENCY-DOMAIN FINITE-DIFFERENCE MIGRATION R.-E. PLESSIX and W.A. MULDER Abstract 1 Frequency-domain finite-difference migration can be accelerated by an order of magnitude if a subset of the available data frequencies is used. To avoid unwanted artefacts the choice of this subset should be guided by two principles: sufficient localisation of reflectors and the avoidance of wrap-around. The first is related to the spatial resolution and Heisenberg’s principle. The second is needed to avoid repetitive patterns and is related to the Nyquist sampling theorem. Here we study both by a simple analytical and

1 A-41 IMPROVING NEAR-SALT-FLANK IMAGING WITH SHOT-PROFILE WAVEFIELD- EXTRAPOLATION MIGRATION IN THE GULF OF MEXICO UWE ALBERTIN DAVE WATTS WENFONG CHANG S. JERRY KAPOOR Abstract Prestack Kirchhoff migration is widely used for imaging in complex areas and has been particularly effective in salt and subsalt imaging in the Gulf of Mexico. Despite this success there are areas where Kirchoff migration has difficulty imaging subsalt events. These difficulties are observed near salt flanks and under rugose salt bodies where migration artifacts often obscure primary sediment events. Among the causes for these artifacts are Kirchhoff’s use of a single traveltime arrival approximate

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A-45 PRESTACK PHASE-SHIFT MIGRATION OF SEPARATE OFFSETS IN LATERALLY INHOMOGENEOUS MEDIA TARIQ ALKHALIFAH Summary 1 Using the stationary-phase method prestack phase-shift migration is implemented one offset at a time. This separate-offset implementation allows for a Fourier (reasonably fast) wave-equation type migration on data with irregular offset sampling. However the separate-offset phase-shift migration like its zero-offset counterpart handles only vertically inhomogeneous media. Using the combination of the split-step and the phase-shift-plusinterpolation (PSPI) approaches the separate-offset phase-shift migration is extended to handle laterally inhomogeneous media. Like any PSPI-based method the cost of the new migration relies heavily on the complexity of the

B-02 P-WAVE AND C-WAVE VELOCITY ANALYSIS FOR DEEP WATER OBC DATA WEIZHONG WANG and LONG DON PHAM Summary 1 Deep-water OBC data are collected with sources and receivers at different depths. Seismic velocity estimation algorithms commonly used in processing assume that sources and receiver are at the same datum. To satisfy this requirement vertical shifts are applied to the data or straight ray projections are performed to bring shots and receivers to the same datum. If the sampling of the acquisition geometry is adequate then wave-equation datuming can also be applied. In this paper we present formulae for Pwave and

B-04 CALIBRATION OF DEPTH IMAGED SEISMIC DATA USING AN INTERVAL VELOCITY SCALING METHOD Summary 1 A. A. CURTIS D. D. STOUGHTON D. R. HILL J. F. RATCLIFFE and T. HARTNETT BHP Billiton Petroleum 1360 Post Oak Boulevard Houston TX 77056 USA As a consequence of anisotropy and other imaging effects horizon markers extracted from 3D seismic amplitude data frequently do not tie the equivalent formation intersections determined from well data. Such mis-ties between the seismic and well-log based formation markers can be quite significant in many Gulf of Mexico fields and are likely to be present in any 3D depth

B-06 THE KEY PRACTICAL ASPECTS OF 3D TOMOGRAPHY - DATA PICKING AND MODEL REPRESENTATION Abstract 1 There are a wide variety of approaches to building velocity models with prestack depth migration. The most common pitfalls we see in them are extracting reliable yet detailed information about velocity errors from migrated data and then the inability to produce geologically reasonable velocity models from these error estimates. We extract detailed error estimates along horizons from finely sampled migrated data edit them statistically and QC the edits in map view. Tomography converts these error estimates into new velocity models or velocity model updates

B-08 3D STEREOTOMOGRAPHIC INVERSION ON REAL DATA SET Abstract 1 EMANUEL CHALARD 1 3 PASCAL PODVIN 1 SOAZIG LE BEGAT 1 PHILIPPE BERTHET 2 BERNARD DAVID 3 Stereotomography is a new tomographic method introduced in 2D by Billette and Lambaré (1998). It uses the apparent slopes of locally coherent seismic events picked in the prestack data cube to constrain the velocity model. In previous papers (Chalard et al. 2000a 2000b) we introduced the extension of stereotomography to 3D. In particular we proved that in the case of multi-streamer marine data acquisition the picking of seismic events can be performed independently