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CRS Technique For Advanced Prestack Merging And Regularisation of Vintage 3D Seismic Data
Gierse, Guido (TEEC) | Otto, Dennis (TEEC) | Berhorst, Arnim (TEEC) | Trappe, Henning (TEEC) | Pruessmann, Juergen (TEEC)
Summary The merging of seismic data of different origin is a common task in the reprocessing of vintage 3D seismic data. In contrast to poststack merging, the prestack merging is rewarded by the much broader possibilities of prestack migration and analysis techniques but requires a larger effort to adjust different acquisition and bin geometries, and to interpolate missing data in the binning grid of the merged dataset. In this case study, a new strategy is proposed using Common-Reflection-Surface (CRS) partial stacking for both, the merging and the regularisation of the prestack data from two 3D marine surveys. The acquisition already provided some irregularities in the CMP and offset coverage of both surveys, which are increased by the adjustment of the binning grids requiring a smaller grid cell in one of the datasets. In a addition, the overlap zone of the two surveys exhibits a general decrease of the coverage. The data which are missing in the regular CMP/offset grid of the merged dataset are recovered by partial CRS stacking of original traces in the CMP/offset vicinity of a missing regular trace. This data mapping benefits from the detailed event description in the CRS attributes derived in the CRS zero-offset stacking workflow. It combines a dipconsistent interpolation of the prestack data with a significant increase of the signal-to-noise ratio as part of the partial CRS stacking. Introduction Modern acquisition equipment, and increasing processing capacities based on high-performance IT technology have stimulated a steady growth of project sizes in 3D seismic surveying. This trend to larger units has also influenced the reprocessing of old 3D seismic data, that often had been acquired in much smaller patches. The merging of several small or medium 3D seismic surveys of different vintages has thus become a common task in seismic exploration projects. Since contemporary processing sequences have replaced former poststack imaging by prestack migration techniques the merge is generally performed in prestack domain. Before merging, different acquisition footprints, signal characteristics, amplitude levels, and static shifts are commonly adjusted separately in the individual 3D surveys. The prestack merging then aims at a maximum homogeneity of the resulting dataset, not only comprising the similarity of the seismic events in traces from different sources, but also the structure of the dataset. With a consistent regularisation throughout the dataset prestack migration is expected to minimize migration noise, and produce the best results. This case study concentrates on the aspect of adapting the dataset structure in a 3D seismic merge project, and proposes a new workflow based on CRS regularisation. CRS interpolation strategy The CRS method, or Common-Reflection-Surface method, 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. The corresponding CRS stacking parameters, the so-called CRS-attributes, accordingly comprise the wavefield dip together with wavefront curvatures observed at the surface.
- Geophysics > Seismic Surveying > Surface Seismic Acquisition (1.00)
- Geophysics > Seismic Surveying > Seismic Processing > Seismic Migration (1.00)
Geometry-preserving CRS Shot Gathers For Enhanced Resolution In Prestack Depth Migration
Trappe, Henning (TEEC) | Gierse, Guido (TEEC) | Pruessmann, Juergen (TEEC) | Eisenberg-Klein, Gerald (TEEC) | Zehnder, Melanie (TEEC)
Summary Noise suppression and signal enhancement prior to prestack depth migration (PreSDM) may significantly increase the resolution of the depth image, and the effectiveness of the PreSDM workflow. The Common-Reflection-Surface (CRS) technique was previously used for this enhancement of seismic prestack data, providing so-called CRS gathers with regularized CMP and offset coverage, and with a strong noise suppression. These CRS gathers considerably improved the depth image in Kirchhoff PreSDM but were not suited for shot-based PreSDM algorithms. This case study now presents a straightforward way to produce geometry-preserving CRS gathers that similarly increase the signal-to-noise ratio. In a first implementation, CRS prestack data interpolation is performed at the existing trace locations providing a straightforward and automatic preservation of the original shot geometry. Application to 3D seismic land data demonstrates the improved signal-tonoise ratio and resolution both in the geometry-preserving CRS shot gathers, and in the corresponding QC stack. As in the Kirchhoff migration of regularized CRS gathers, such enhancements are expected for Reverse Time Migration of CRS shot gathers as well. Introduction Prestack depth migration (PreSDM) techniques have evolved as a standard procedure in seismic exploration projects in order to obtain a maximum resolution of the subsurface. New algorithms have been developed to provide depth images with increased accuracy, mostly coupled with increased costs of large computation times, and sometimes extensive disk space consumption. In all cases, however, the full benefit of advanced migration algorithms may be missed if a low signal-to-noise ratio is present in the prestack input data. Hence, a careful data preparation and noise suppression is a necessity before entering this costly depth imaging step. The Common-Reflection-Surface (CRS) technique is known as a powerful tool for noise reduction that has previously been used for producing both, enhanced poststack and prestack data. In prestack data, this noise reduction has been combined with data regularisation establishing a uniform data coverage in a given 2D or 3D seismic dataset. CRS applications to regularize the trace distribution in common-offset data have been demonstrated by Hoecht et al. (2009), and by Gierse et al. (2009). This type of prestack data preparation has been performed by Eisenberg-Klein et al. (2008) for strongly reducing the noise level in PreSDM, using a Kirchhoff depth migration algorithm. The migration noise is suppressed by the CRS regularisation of the CDP and offset coverage, whereas random noise is suppressed by partial CRS stacking. However, the regularisation of CDP and offset coverage usually does not preserve the shot geometry. Traces pertaining to an individual shot may receive different shot coordinates corresponding to the offset regularisation. Such data may well be used in Kirchhoff common-offset PreSDM, but not in some of the most advanced PreSDM algorithms. Many PreSDM implementations of one-way wave equation algorithms, and Reverse Time Migration require shot data as input. This case study presents a new strategy of CRS-based prestack data enhancement that enhances the signal-tonoise ratio of the data but preserves the shot geometry in so-called CRS shot gathers.