EAGE/SEG Workshop - Depth Imaging of Reservoir Attributes

Seismic survey design is often a relatively qualitative procedure that relies on experience and economic constraints rather than quantitative predictions of the image quality produced by different acquisition geometries. Simple ray tracing procedures provide one technique for obtaining some quantitative input by measure binning along target interfaces.

3-D Prestack Depth Migration has greatly improved our ability to image reservoirs in complex areas, such as below salt and carbonates in the North Sea or beneath salt bodies in the Gulf of Mexico. But what effect does the overlying structures and abrupt velocity variations have on seismic attributes such as amplitude anomalies or amplitude variation with offset (AVO)?

Data acquired with the parallel geornetry can be described as a collection of common-offset gathers. Prestack migration of such data can be conveniently considered as the repeated application of the migration proces to all common-offset gathers.

The convergence of conventional reflection tomography is often uncertain when the starting velocity function is too far from the correct one. The time-domain reflection tomography that we present in this paper is more robust than conventional depthdomain reflection tomography.

The "flatness" of events in the Common Image Gathers (CIGs) used for AVO analysis, is directly related to the quality of the velocity model [4,6]. The method we present here for velocity estimation in 2D media is precisely adapted to further AVO analysis, as its aim is to flatten the depth migrated CIGs. For an automatic velocity estimation where no picking is introduced, the choice of the objective cost function which measures the "flatness" of the coherency panels, is crucial. We explore some proporties of the Differential Semblance function, introduced by [7], to prove its efficiency in 2D velocity estimation. Synthetic tests are presented here, and show that at least in these cases, total optimization algorithms may be applied.

Common image panels are commonly used for migration based velocity analysis and for migration based AVO analysis. The advantages of common offset image gather with respect to common shot image gathers are now well established [4] but also their poor quality in case of complex models [3]. The reasons of this failure may appear when the problem is analyzed in the frame of the asymptotic linearized inverse theory [7, 6]. We propose here an analysis of the relevance of common image gather analysis in case of velocity models with triplicated ray fields. We show that individual common shot or common offset may provide inconsistent images of the subsurface, and we propose a strategy for common anale imaging avoiding the former artefacts. Comparisons of common shot, common offset and common angle image gathers are presented for 2D complex Marmousi dataset.

The proposed reflector migration is a fast and accurate prestack depth migration designed for interval velocity and depth interface determination and for AVO analysis. The output of the migration consists of CRP gathers in windows centered at the reflecting horizons (CRP migrated panels) and the corresponding CRP ray paths for the given output offsets. Related approaches for 2D data were described in a patent by Johnson (1990), Landa and Sorin (1993) and Kosloffet et. al. (1995). This work is designed for 3D complex geological structures with a special treatment for obtaining true amplitude reflection events.

AVO migration/inversion has been developed for estimating AVO attributes for the case of moderately complex structures. There are several sources of uncertainty in this procedure, the largest of which is usually the estimation of a suitable depth migration velocity field.

Irregular and sparse sampling of the wavefield in 3-D surveys generates spatial aliasing. Wavefield interpolation yields densely sampled data either operating in a suitable domain where sampling is more regular and can then be densified (e.g., in common shot domain) or by exploiting the predictability of the wavefield as parameterized by one (or more) plane events. In this paper we present an analysis of the sensitivity to velocity of the 3-D shot continuation operator (3-D SCO) for interpolation of 3-D acquisition geometries and we compare the 3-D SCO interpolation with prediction-error-filter (PEF) approach.

Three basic processing methodologies for amplitude-versus-offset (AVO) / amplitude-versus- angle (AVA) studies are discussed and compared: time processing, time imaging and depth imaging.