The cartoon in Figure 1 shows schematically how an upcoming wave-train, , will reflect from the surface and act as a source at receiver r1 for the subsurface reflection recorded at r2. An intuitive understanding of this experiment and the manufacture of pseudo shot gathers can be developed by progressing through the panels of Figure 1 from the earth model in panel (a) through the raw data recording in panel (b) to the correlated records in panel (c) where the fast-axis is now in units of correlation lag.
The correlation step has two important functions when applied to raw passive data. First, it collapses to an impulse the long source functions of the ambient sound energy recorded at each station. Second, it develops hyperbolic move-out of these impulses as a single input trace is correlated with data from stations at increasing offset. Figure 2 shows a representative gather from a synthetic passive dataset over a two-layer earth-model.
Figure 2 Representative pseudo shot gather from data cube generated by cross-correlating all traces from a modeled passive seismic dataset. The raw data trace that has been correlated with all the others serves as the source location for the gather. Notice the ``virtual multiple'' at 0.11 seconds. This arises due to the partial correlation of the two reflectors between themselves. The greater the velocity contrast, the less visible this event. Its zero-offset time is equal to the difference in time between the two events.
After the pseudo shot gathers are constructed through correlation, a conventional five-dimensional data volume is ready for any common processing flow such as sorting and migration. However, I show that it is possible to migrate the raw data directly with a shot-profile migration algorithm that has been modified to use an areal source rather than a conventional impulse or wavelet. I present comparisons of both methods performed on modeled data and comment on some of their characteristics and costs.