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.
Figure 11.5-3 Survey geometries of two time-lapse 3-D seismic data as in Figure 11.5-4: (a) the grid orientations, and (b) grid sizes. See text for details. (; figure courtesy Rickett, Stanford Exploration Project, and 4th Wave Imaging; data courtesy Chevron.)
Figure 11.5-4 Time-lapse 3-D seismic data: (a) an inline section and (b) a time slice from the time image volume associated with the 1979 survey; (c) an inline section and (d) a time slice from the time image volume associated with the 1991 survey. The interior of the broken yellow line defines the overlapping area between the two surveys. See text for details. (; figure courtesy Rickett, Stanford Exploration Project, and 4th Wave Imaging; data courtesy Chevron.)
Figure 11.5-5 Amplitude spectra of the two time-lapse 3-D seismic data as in Figure 11.5-4 (a) before and (b) after cross-equalization; (c) a difference section following cross-equalization. (); figure courtesy Rickett, Stanford Exploration Project, and 4th Wave Imaging; data courtesy Chevron.)
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.