Fast ship versus slow ship

For marine seismic data, the spacing between shots $\Delta s$ is a function of the speed of the ship and the time interval between shots. Naturally we like $\Delta s$ small (which means more shots) but that means either the boat slows down, or one shot follows the next so soon that it covers up late arriving echos. The geophone spacing $\Delta g$ is fixed when the marine streamer is designed. Modern streamers are designed for more powerful computers and they usually have smaller $\Delta g$.Much marine seismic data is recorded with $\Delta s = \Delta g$and much is recorded with $\Delta s = \Delta g/2$.There are unexpected differences in what happens in the processing. Figure 5 shows $\Delta s = \Delta g$,and Figure 6 shows $\Delta s = \Delta g/2$.

 

geqs Figure 5 $\Delta g = \Delta s$. The zero-offset section lies under the zeros. Observe the common midpoint gathers. Notice that even numbered receivers have a different geometry than odd numbers. Thus there are two kinds of CMP gathers with different values of the lead-in x0 = x0

When $\Delta s = \Delta g$ there are some irritating complications that we do not have for $\Delta s = \Delta g/2$.When $\Delta s = \Delta g$, even-numbered traces have a different midpoint than odd-numbered traces. For a common-midpoint analysis, the evens and odds require different processing. The words ``lead-in'' describe the distance (x₀ = x0) from the ship to the nearest trace. When $\Delta s = \Delta g$ the lead-in of a CMP gather depends on whether it is made from the even or the odd traces. In practice the lead-in is about $3\Delta s$.Theoretically we would prefer no lead in, but it is noisy near the ship, the tension on the cable pulls it out of the water near the ship, and the practical gains of a smaller lead-in are evidently not convincing.

 

geq2s Figure 6 $\Delta g = 2\Delta s$. This is like Figure 5 with odd valued receivers omitted. Notice that each common-midpoint gather has the same geometry.