Processing of 4-D seismic data
Just as in the case of the medical example given above, processing, inversion, and interpretation of 4-D seismic data are influenced by the evolving technologies in 3-D seismic exploration. The different vintages of 3-D seismic data that are used in a 4-D seismic project are often recorded with different vessels, source and cable geometries, and source and receiver types and arrays. In fact, some 4-D seismic projects may involve, say, two time-lapse 3-D surveys — one conducted using the conventional streamer cable and the other conducted using the ocean-bottom 4-C technique (next section). The 3-D surveys most likely would be conducted using different recording directions and bin sizes. Figure 11.5-3 shows the base maps for two time-lapse 3-D surveys with different recording directions and bin sizes . The data associated with the 1979 survey and the 1991 survey were recorded with a 34-degree difference in grid orientation. Also, the bin size for the 1979-survey was 80 × 27.5 m, whereas the bin size for the 1991 survey was 12.5 × 12.5 m. Additionally, these 3-D seismic data sets most likely would be processed differently — not only the processing sequences would be different but also the processing parameters. Figure 11.5-4  shows a section and a time slice from each of the two time-lapse 3-D surveys which are referred to in Figure 11.5-3. The 1991 survey data have produced a more accurate image of the salt flank.
Hence, the time-lapse 3-D data sets used in a 4-D seismic project need to be cross-equalized prior to the interpretation of the results. Cross-equalization involves the following steps .
- Align the grids of the time-lapse data to a common grid orientation. In many cases of 4-D seismic projects, grid alignment and subsequent steps in cross-equalization are applied to poststack data. As such, grid alignment may be achieved by remigrating the poststack data to a specified common grid orientation. If you have access to prestack data, one way to align the grids of the time-lapse data is by crossline migration (3-D prestack time migration), the output of which would be common-azimuth gathers.
- Apply spectral balancing to the time-lapse 3-D data to account for the differences in the spectral bandwidth and shape. Figure 11.5-5 shows the amplitude spectra computed from the 1979 and 1991 survey data shown in Figure 11.5-4. Note the significant differences in the shape and bandwidth of the spectra before cross-equalization. These differences have been minimized by cross-equalization.
- Derive amplitude gain curves from the time-lapse 3-D data based on trace envelopes, and apply the gain curves for amplitude balancing.
- Estimate static shifts between the time-lapse data traces and apply them to eliminate vertical time diffferences.
- Examine and determine differences in event positioning in the migrated data volumes associated with the time-lapse 3-D surveys. Eliminate the differences in event positioning by residual migration.
- Rickett and Lumley, 1998, Rickett, J. and Lumley, D. E., 1998, A cross-equalization processing flow for off-the-shelf 4-D seismic data: 68th Ann. Internat. Mtg., Soc. Expl. Geophys., Expanded Abstracts, 16–19.
- MacLeod et al., 1999, MacLeod, M. K., Hanson, R. Bell, R. C., and McHugo, S., 1999, The Alba Field ocean bottom cable seismic survey: Impact on development: The Leading Edge, 1306–1312.