Model 4: Sort, Velocity Analysis
From the previous chapter, we have a 2-D line of seismic data. In this chapter, we:
sort the shot gathers to common midpoint (CMP) gathers and
perform velocity analysis on selected CMP gathers.
Because our synthetic data are noise-free, we can proceed quickly from shot gathers to migration. If we wanted to, we could use suaddnoise to add noise to the data; this is often useful. You should consider adding some noise to the data set, then processing it using the examples we provide in this chapter and the next.
Let's discuss “common-depth-point” (CDP) and “common midpoint” (CMP) (Sheriff, 2002). We try to avoid the term CDP because there is no common (same) point at the reflector if the reflector dips. On the other hand, common-midpoints almost always exist because they are defined by the geometric midpoint between sources and receivers. Because SU does not have a cmp key, we reluctantly use the cdp key.
Script sort2cmp.sh does two jobs:
We use program suchw (Change Header Word) to create header (key) cdp and assign values to it. Using the equation below, values of cdp (key1) are computed from gx (key2) and sx (key3).
Scalars a, b, c, and d need to be determined by sketching the geometry. Our geometry dictates a = 1525, b =1, c = 1, and d = 50. Therefore,
We use program susort to sort the traces based on primary sort key cdp and secondary sort key offset. A secondary sort is the order within the