Variations in fold coverage

It is difficult to acquire seismic data that is adequately distributed in azimuth and offset. Moreover, it is impossible to extract an adequately sampled 3D subset representing a single common-azimuth common-offset realization from the 5D prestack volume. An alternative solution is to consider a subset over a small range of azimuths and offsets for prestack processing. An example of such a subset from a 3D wide-azimuth data volume is shown in Figure 2. Traces whose source-receiver offset is between 7000 and 9000 ft and azimuth between -30^o and 30^o were extracted from a true 3D land survey acquired with Button-Patch geometry and we plotted the surface locations of their common mid-points. There is a fundamental problem with this subset. At a nominal bin size spacing, variations in fold between different bins may vary substantially. Figure 3 shows the fold distribution for this subset binned at nominal CMP spacing of 80 ft. The fold coverage varies from 0 to 5 with arithmetic average of about 1.5.

 

survey
Figure 1 Layout of the sources and receivers in the 3D land survey designed with a Button-Patch geometry; the dots and stars indicate receiver and source locations respectively.

 

irregular
Figure 2 CMP geometry of a subset from a wide azimuth 3D land survey. The corresponding traces have source-receiver azimuth between -30 and 30 dgrees and offset between 7000 and 9000 ft.

 

Fold
Figure 3 Fold diagram for the 3-D subset binned at nominal CMP spacing of 80 ft.