Fresnel zone binning analysis and quality of seismic data processing

Paul Boughton

Hejie Wang, Usman Raja, Jingdong Liu and Ottar Sandvin evaluate the Fresnel Zone Binning infill criteria used in a 3D survey acquired for Statoil at Nordkapp in the Barentsea.
For a typical 3D seismic survey, infill shooting may account for as much as 25 per cent or more of the total cost of prime line seismic acquisition. The Fresnel Zone Binning (FZB) infill criterion is an approach designed to minimize infill costs further based on FZB calculated coverage in combination with steerable streamers.

Two decimated datasets were generated in addition to the full dataset, to simulate the  more severe lack of coverage in the acquisition. The three data scenarios were all processed independently, including data regularization, and finally 3D time migrated.
The processing has demonstrated that the FZB infill criteria have proven sufficient.

Even for the decimated data set with a drop in coverage of -3dB to -4dB, high quality output from the 3D pre-stack time migration was obtained.
Most marine 3D seismic surveys are acquired with infill to ensure adequate subsurface seismic reflection coverage in areas where the surveys may have been affected by adverse currents, sub-optimal streamer feather angle matching or source/streamer separations.
By understanding the importance and the size of the Fresnel Zone, requirements for coverage as a function of offset, angle of incidence, frequency content and depth can be evaluated accurately.
These criteria can be used to set steering tolerances for streamers during acquisition, to further minimize the amount of infill required in a marine streamer survey. This approach is called Fresnel Zone Binning (FZB) and has been described by Dave Monk, see Monk 2010 SEG, Denver 2011, Annual Meeting and see also Fig. 1.
In the figure FZB dB fold is measured in decibels (dB) down from the nominal fold contributions; with colour scale included.
Nordkapp Data Scenarios
A 3D seismic survey was acquired for Statoil at Nordkapp in the Barent Sea in 2010. Additional decimation of the acquired data was done by removing some of the data from a set of streamers and some offset ranges along a number of streamers for a number of sail lines.

In addition to the full data set S1, the data decimation simulated two additional data sets S2 and S3. The S2 data were decimated down to -2 to -3 dB and the S3 data further decimated down to -3 to -4 dB. The three independent data sets were processed independently and used for comparison and data quality analysis.
FZB infill analysis
The FZB calculations are performed for all shot and receiver combinations along all the sail lines.
A subsurface model of the survey area is defined and used in the FZB calculations, consisting of target horizons, an overburden macro velocity model and  estimated seismic bandwidth. The output from FZB is a set of output coverage colour plots for each target horizon and for a selected set of offset ranges. An example of the Far Offset FZB coverage for the S3 data scenario is shown in Fig. 2.
Note that the lowest dB levels for the FZB coverage (-3dB to -4dB) are located at the light coloured areas in the figure. The question is whether the FZB dB threshold used (S3) is conservative enough to generate high quality imaging of the subsurface
Data processing and interpolation

The three data sets were all subject to the same processing flow prior to the final 3D Kirchhoff time migration, including the regularisation step.
The Fresnel zones or data holes may be large for deeper targets and larger offsets and it is therefore essential that prior to pre-stack migration, the offset data are accurately interpolated with advanced algorithms capable of handling multiple and conflicting dips. High quality processing algorithms for wave field reconstruction and regularization are today available in the seismic processing industry.
Fig. 3 below displays a time slice of the S3 data at 1300 ms of one offset data plane (2800m).  Data holes and amplitude stripes are clearly visible in the data. All offset planes are interpolated and the data finally prepared for 3D Kirchhoff time migration.
Data analysis
Data quality comparison and difference analysis has been done on a set of time slices, in- and cross-line sections for the three alternative data sets.
The migrated time slices at 1300 ms for the S3 data sets are displayed in Fig. 4.
The corresponding difference plots between S3 minus S1 is displayed in Fig. 5. Note that the amplitude differences between the full data set S1 and the decimated data sets S3 are very small across the entire section.
A migrated inline section is displayed in Fig. 6 for the S3 data.
The main horizon seen in the inline section in Fig. 6 was interpreted and amplitude analysis and data comparison done along the interpreted horizon. In Fig. 7 the RMS amplitude variation for all 3 data scenarios have been plotted together.

The FZB processing tests performed on the Nordkapp data have demonstrated that the infill requirements based on Fresnel Zone criteria have proved to be appropriate in order to obtain high quality output images after 3D pre-stack time migration. The most severe data decimation (S3) has simulated coverage which is -3 to -4 dB down from a theoretical coverage of one. Although with relative large holes in the Nominal Fold for the far offsets, the lack of coverage can be accommodated if FZB sampling criteria are used and high fidelity regularization/interpolation is performed.

Enter at

Hejie Wang, Usman Raja, Jingdong Liu and Ottar Sandvin are with with Fugro Seismic Imaging, Oslo, Norway.
Acknowledgements: We would like to thank Statoil and its partner GdF Suez E&P Norge, for access to seismic data and authorization to show the obtained results, and Fugro for permission to present this work.

References: Monk, D J, 2010, Reducing infill requirements using Fresnel zone binning and Steerable streamers. SEG Denver 2010 Annual Meeting; Monk, D J, 2010, Fresnel zone binning: Fresnel-zone shape with offset and velocity:Geophysics, 75, no.1, T9-T14; Monk, D J, 2010, Fresnel zone binning: Application to 3D seismic fold and coverage assessment, Leading Edge; Young P and Monk D J 2010, Alternative coverage analysis method reduces infill Shooting. World Oil Magazine, September 2010.

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