**Robust Methods of Multistep Velocity Analysis**

**KATZ, SIMEON, University of Southern California, Los Angeles, CA**

The stability of * velocity* analysis results obtained in the presence of strong noise may be improved if several adjacent CMP gathers are processed, thus increasing the number of the input records for

*analysis, and if a better*

**velocity***analysis operator is utilized. The two classes of the*

**velocity***analysis techniques discussed here are based on the joint processing of a group of adjacent CMP gathers and are characterized by high stability of the output results.*

**velocity**The algorithms of the first class are less sensitive to perturbations in dips of the reflectors, so that approximate values of the dips may be used. The algorithms of the second group are dip sensitive so that depth dependent dips should be taken into account.

In each of these classes different * velocity* operators may be used to generate an output

*analysis section. I consider four types of the*

**velocity***analysis operators: (1) modified semblance, (2) penalty function defined as a monotonic function of the signal-to-noise ratio estimate, (3) linear stack*

**velocity***function, and (4) linear stack modulated by the penalty function.*

**velocity**The results of the experiments with these * velocity* analysis techniques indicate that in the case of semihorizontal interfaces the best results are obtained via use of the operator (4). If the dip of the reflector is unknown and may be large, the

*operators (2) and (1) may give better results.*

**velocity**

AAPG Search and Discovery Article #91009©1991 AAPG-SEPM-SEG-SPWLA Pacific Section Annual Meeting, Bakersfield, California, March 6-8, 1991 (2009)