Tomographic estimation of seismic velocity on an interactive workstation

Abstract

Tomographic estimation of seismic velocity from directly transmitted arrivals in offset VSP or crosshole data requires the following processing operations: (a) statics estimation and corrections; (b) stacking to enhance arrivals of interest and to remove coherent noise; (c) deconvolution and time picking; (d) iteration between tomographic inversion of picked source-receiver delay times and ray tracing. Velocity tomography with reflection data may also require migration to estimate the position of reflectors for picking two-way travel times. A workstation based software system has been developed which permits interactive tomographic velocity estimation from shallow crosshole and offset VSP data. The flexibility gained by performing velocity tomography interactively has many advantages, such as easy comparisons of intermediate results and the ability to update filtering sequences to accommodate the results of interactive analyses of signal and noise characteristics. We illustrate this for shallow crosshole velocity tomography with examples of (a) interactive tau-p filtering for arrival enhancement and coherent noise removal; (b) the dependence of tomogram resolution on raypath coverage; (c) interactive smoothing to compensate for resolution artifacts related to incomplete tomographic raypath coverage. The use of interactive smoothing to cater for different types of geological prior information in tomography is discussed.