|USING CRS GATHERS TO IMPROVE PRESTACK MIGRATION IMAGES IN THE GULF OF MEXICO|
In the Gulf of Mexico, the main challenges to seismic imaging and model building are provided by extensive salt geology. High-velocity salt bodies of arbitrary shape distort the seismic wave propagation, and produce strong scattering at the rugged salt surfaces. Seismic reflections from sedimentary structures in the vicinity of salt, or below the salt body are masked by strong salt reflections and diffractions, and obscured by irregular wave-paths.
The CRS method, or Common-Reflection-Surface method, is a time domain imaging method that was originally developed by Hubral et al. (1999), Mann et al. (1999), and Jaeger et al. (2001) within the concept of macro-model independent imaging (e.g. Gelchinsky 1988). CRS zero-offset stacking assumes local reflector elements with dip and curvature in the subsurface that give rise to the seismic reflections.
|CRS GATHERS FOR GOM CASE STUDY DATA|
The case study data comprised a 3D onshore seismic survey acquired in the 1990s on the southern coast of the Golf of Mexico (GOM) in Mexico. The depth model building in this project made use of CRS tomography, and was presented in detail in a previous paper by Pruessmann et al (2008).
|CRS GATHERS FOR DEPTH IMAGING|
In the CRS depth imaging workflow the CRS gathers are input to several runs of prestack depth migration (PreSDM) using the preliminary and final velocity models. The CRS-based increase of signal quality is first observed in the common-reflection-point (CRP) gather. A CRP gather calculated from CMP gathers, and the corresponding CRP gather from CRS gathers are compared in Figure 2. The CRS-based CRP gather shows the residual moveout more clearly, and facilitates moveout analysis in the gather and semblance displays. CRS depth processing thus stabilizes automatic moveout picking, requiring much less manual intervention. Based on this analysis, the depth model is updated from the initial CRS tomography model.
The Common-Reflection-Surface, or CRS methodology offers a powerful technique for conditioning and regularizing seismic prestack data prior to migration. Taking advantage of the detailed event description provided by the CRS attributes, a partial CRS stacking is performed in order to map the original seismic data to regular trace locations on a CMP/offset grid. The resulting regular CRS gathers exhibit a significant increase of the signal-to-noise ratio in addition to the regularisation.
We thank PEMEX for the permission to present their data.
Duveneck, 2004, Velocity model estimation with data-derived wavefront attributes. Geophysics, 69, pp 265-274.