During the drilling stage, VSP plays an important role in predicting the depth of target strata and the lithologic characteristics of the deep strata ahead of the drill bit. Therefore, using VSP data to predict ahead of the drill bit has become a popular topic. Highly precised acoustic impedance inversion is a key technique for ahead of drill bit prediction using VSP data. In this paper, we have proposed a method of predicting the acoustic impedance in the formation ahead of the drill bit using VSP data. First, the nonlinear iterative inversion is adopted to invert for acoustic impedance using the VSP corridor stack. Then, by modifying the damping factor in the iteration and using the preconditioned conjugate gradient method to solve the equations, the stability and convergence of the inversion results can be significantly enhanced. The results from theoretical models and real data demonstrate that the method is very useful and effective for ahead of drill bit prediction using VSP data.


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