Analyzing the Aki-Richards (1980) equation for converted waves (PS), I find that it is possible to decouple the effect of density contrast from that of shear velocity contrast. The two terms are mixed when the P-wave incident angle is less than 30°, but they start to separate at a middle angle range (approximately 40°). The term related to shear-wave velocity reaches zero at an incident angle around 60°. However, the other term which is related to the density contrast does not reverse polarity until 90°. Furthermore, this density term reaches almost the maximum magnitude around 60°. Based on those characteristics, a new method called "S-Zero Stack" has been designed to capture the density contrast reliably at the interface. In this paper, the theoretical decoupling is introduced and the method of S-Zero Stack is developed. A synthetic example is presented. Finally, the new method has been applied in a real 4C/3D PS data and the result is calibrated with density log. S-Zero Stack captures reliably the subsurface density anomalies without going to inversion. It is simple but robust, even with noise data. Combined with the traditional P-wave AVO technique, S-Zero Stack of PS waves may help discriminate commercial gas from fizz.


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