Technical Paper: Sub-salt Velocity Prediction With a Look-Ahead AVO Walkaway

Society: SEG
Paper Number: 00035
Presentation Date: 2004


Drilling through salt in deep water is expensive and risky. Abnormally high pore pressure just underneath salt is a common problem, and one of the essential inputs to pore pressure prediction is seismic velocity. Estimating velocity beneath salt using even the most sophisticated surface seismic techniques is difficult due to poor and irregular illumination. In this paper we explore the possibility of using a walkaway VSP survey to predict seismic velocities just below the base salt interface. The idea is to acquire a walkaway survey with an array of three component receivers clamped in the salt just above the base salt interface. As with walkaways acquired for AVO calibration, the similarity of the down going path for direct and reflected arrivals is exploited together with true amplitude processing to produce a multiple-free, zero phase seismic AVO response just beneath the receivers. We pick the base salt P-p and P-s reflection amplitudes versus offset, transform to incidence angle using measured polarizations and invert for sub-salt elastic parameters. Salt properties are assumed constant and the base of salt interface is assumed to be locally planar. The AVA inversion is Bayesian and Monte Carlo, providing posterior uncertainty estimates in estimated parameters. Given realistic noise we find that a soft constraint on density is required as it is poorly constrained by the data. Using this look-ahead AVO walkaway approach the uncertainty in Vp should be significantly reduced from pre-drill bounds. A good estimate of Vshear is also provided, opening up new opportunities for sub-salt pore pressure work.

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