AB FOCAL VELOCITY ANALYSIS

摘要

A conventional semblance velocity analysis is an analysis in which it is assumed that there is no amplitude-variation-with-offset (AVO) on a hyperbolic event in an input CMP gather. As a result, events with a strong AVO anomaly, especially the ones with polarity reversal, will not produce a sharp semblance attribute. Such an AVO anomaly can be seen from the top of some class 1 and class 2 sands. To overcome this disadvantage, data modeling with amplitude variation expressed by an intercept and gradient AB semblance was introduced. On the other hand, an AB semblance has a resolution approximately twice as low as that of the conventional semblance. In this study, a new method called the AB focal velocity analysis (AB-FVA) was introduced as a tool for high resolution velocity analysis in the presence of class 2 AVO. In this method, a velocity analysis is run in the focal domain to achieve a higher resolution. The AB-FVA was introduced by combining the AB semblance and the focal velocity analysis (FVA). The AB-FVA uses the same explicit formula for the generalized semblance attribute as used in the AB semblance. Two synthetic CMP datasets were used to demonstrate an improvement in the velocity analysis by AB-FVA in comparison with the AB semblance and the other two methods (the FVA and the semblance). The assessment of these four methods over synthetic datasets proved that the AB-FVA has the least smearing along time and velocity directions compared to other methods. Also, two real datasets were used to check the quality of the velocity analysis by the AB-FVA compared to the other three methods. The first dataset was a deep marine CMP gather containing strong long period multiples. The second dataset was a shallow part of a CMP gather showing class 2 AVO. By comparison of the results obtained from synthetic and real datasets it was concluded that AB-FVA was a promising method for velocity analysis.