Nonlinear Semblance Processing For The Evaluation Of An Acoustic Altered Zone Around The Borehole

Abstract

This paper describes a simple method to evaluate an acoustic altered zone around the borehole using an array of sonic waveforms. The method is based on a semblance velocity analysis and a nonlinear travel time window is introduced. On the assumption that a velocity gradient of the altered zone may be linear, the nonlinear travel time window is calculated as a function of averaged velocity and a normalized velocity gradient. The method is applied to the synthetic acoustic data calculated by finite difference modeling. Case studies with several altered formation models confirmed that this method is a good estimator of the velocity gradients around the borehole.