CASE 1: SUBSALT IMAGING
CASE 1: 3D-LAND DATA, AREA 130 KM2
OBJECTIVE: SUB-SALT IMAGING AND BETTER RESOLUTION OF SALTDOME
INPUT DATA: PROCESSED CDP-GATHERS

PROCESSING SEQUENCE:

REFORMATTING FROM SEG-Y TO CRS INTERNAL FORMAT
EXTRACTION OF WAVEFIELD ATTRIBUTES
CRS-STACK
CRS-TOMOGRAPHY
POSTSTACK DEPTH MIGRATION (FIRST ITERATION)
UPDATING OF SALT INTERPRETATION USING GRAVITY MODELING
POSTSTACK DEPTH MIGRATION (FINAL)

Data example Case 1: Dipping laters and salt dome flanks

Strong dips are present in the land data of Figure 2, which displays a salt dome in the center of the sections.

The most remarkable feature of the CRS stack is the strong reflections from the salt dome flanks that do not appear in the DMO/NMO stack. The CRS method, which explicitly incorporates the reflector dip in the stacking planes, strongly enhances the dipping reflections of the DMO/NMO stack, and reveals other dipping structures, that have not been visible before.

Fig. 1-1: Processed Gathers. Input for CRS-Processing

Fig. 1-2: NMO/DMO snack (left) in comparison to CRS snack (right).
Please note enhancement of dipping events and signal-to-noise ratio

POSTSTACK DEPTH MIGRATION OF CRS-STACK

Comparison between reference PreSDM and CRS-PostSDM

Fig 1-3: Comparison between PreSDM (left) velocities from PreSDM and PostSDM of CRS, velocities from CRS-Tomography .

Fig. 1-4: Comparison between PreSDM velocities (left) and velocities from CRS-Tomography.

Conclusions

In numerous processing projects, the macro model independent CRS stack has proved to be a valuable supplement to the conventional NMO/DMO stack. The CRS reflection time surfaces are not constrained to a single CMP gather, and the fold is strongly increased, leading to a superior signal-to-noise ratio and a better reflector continuity.

Dipping reflectors are strongly enhanced, and faults are resolved much more distinctly, compared to the NMO/DMO processing. Though the reflection continuity is generally increased, the reflector discontinuities are not smoothed, or even removed by the CRS technique.

The CRS technique can be an alternative in areas where prestack depth migration fails to enhance the subsurface resolution. Poststack depth migration can be applied to transform the CRS stack into a high resolution depth section. As a further advantage, this depth processing scheme is less costly than prestack depth processing.

CASE 2: 3D LAND DATA OF GOM
OBJECTIVE: BETTER RESOLUTION OF SHALLOW REFLECTORS IN TERTIARY AND DEEP SEDIMENTS IN MIOCENE
INPUT DATA: SHOT'S GATHERS

PROCESSING SEQUENCE:

CONVENTIONAL PROCESSING UPTO DECONVOLUTION
EXTRACTION OF WAVEFIELD ATTRIBUTES
CRS-STACK
FINITE DIFFERENCE TIME MIGRATION

ABSTRACT

Variations in near surface geology or topography often cause problems in seismic data processing in many parts of the world. Abrupt changes of the velocity or depth of the low velocity layer are closely related to those problems as well as high velocity layers close to the surface. All this results in variations of seismic traveltimes and, thus, needs to be corrected by static shifts to the prestack traces before going on in the seismic data processing. We have applied conventional refraction statics to the given dataset plus a residual statics method based on the Common Reflection Surface (CRS) approach to solve for remaining errors within the static solution. After coming up with optimum statics we further enhanced imaging by applying the CRS stacking method instead of conventional NMO/DMO stacking.

CASE 3: 3D LAND DATA

OBJECTIVE: BETTER RESOLUTION OF SEISMIC EVENT IN SHALLOW REFLECTORS IN TERTIARY BY AMPLITUDE PRESERVED CRS-TIME PROCESSING FOR AVO.
TARGET ZONE: BETWEEN 50 MS AND 500 MS
INPUT DATA: SHOT'S GATHERS

PROCESSING SEQUENCE:

CONVENTIONAL PROCESSING UPTO DECONVOLUTION

• Calculation of static corrections
• Amplitude balance
• CRS-based residuals static

EXTRACTION OF WAVEFIELD ATTRIBUTES
OUTPUT OF CRS-GATHERS
CRS-STACK
FINITE DIFFERENCE TIME MIGRATION

COMPARISON IMAGES:

   Fig. 3-1

   Fig. 3-2

   Fig. 3-3

Fig. 3-1, 3-2, 3-3: (left) Result of the original PreSTM (right) Result of the CRS-Time Migration. The CRS-result shows a better resolution in the target zone TWT 0-600 ms.

Fig. 3-4: Left result of the conventional NMO/DMO Processing. Right result of the preserved amplitude CRS-Processing.

Fig. 3-5: CRS-Gathers to Imput for AVO-Analysis. The nominal survey coverage was 30 fold. The CRS-stacking operator elevates the number of traces in each CRS-Gathers.

Fig. 3-6: Stacking Velocity from CRS velocity search.

   Fig. 3-7

   Fig. 3-8

Fig. 3-7, 3-8: Comparison between the original PreSTM result (left) and CRS-Time migration (right).

Conclusion

The imaging advantages of the CRS method have already been exploited and discussed in detail in several publications. CRS imaging generally improves the signal-to-noise-ratio. It renders better reflector continuity, and enhances dipping structures. This paper points at another advantage of CRS processing. A case study shows that CRS supergathers may be used for an improved AVO analysis.

The AVO improvements by CRS are based on CRS supergathers, and on the CRS moveout correction. The moveout correction aligns reflection of any dip across the whole supergather, and the large portion of data in the supergather provides a stabilized AVO result.