Converted-wave (P-S) prestack migration and migration velocity analysis

Shaowu Wang, John C. Bancroft, Don C. Lawton, and Darren H. Foltinek

ABSTRACT

Based on the consideration of prestack migration by equivalent offsets and common scatter point (CSP) gathers, a new approach for converted-wave prestack migration and velocity analysis is proposed. The theories for converted-wave prestack migration and migration velocity analysis are described. The equivalent offset is depth- and velocity-dependent. Velocity error has an effect on the equivalent offset, especially at the early time, but with the increasing time or depth, this effect becomes negligible. By resorting the data into the common conversion scatter point (CCSP) gathers with equivalent offset, this new method is very simple and fast.

In CCSP gathers, the relationship between the two-way travel time and equivalent offset is proven to be hyperbolic making the conventional P-P velocity analysis tool suitable to P-S velocity analysis. This provides a simple but powerful way to perform converted-wave migration velocity analysis and simplify the P-S processing.

The creation of a CCSP gather and its velocity analysis are fairly insensitive to the velocity error. Moreover, the velocity analysis is convergent when the initial velocity estimation is inaccurate, and the convergence rate is rapid.

The feasibility of the new algorithm was demonstrated by physical modeling data, numerical modeling and field data examples. The numerical modeling example proves the ability of dealing with a depth-variant velocity model, and also the ability of imaging dipping reflector using converted waves. The real data example shows the possible improvement of signal-to-noise ratio, as well as the improvement of velocity analysis and the images of the seismic events, while not reducing the bandwidth.

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