Top to bottom interactive velocity analysis using travel-time ellipses and first break times from walkaway vertical seismic profiling data: a synthetic study

An interactive velocity analysis method is introduced in this paper and its efficacy was tested on various walkaway vertical seismic profile (WVSP) modeled data sets. In WVSP surveys, interval velocity profiles are determined fairly accurately along the VSP borehole using first break (FB) travel time picks from the zero-offset VSP (ZVSP) data. However, the accuracy of these vertical velocities fades away quickly as the source (S) distance (offset) from the VSP borehole increases due to possible horizontal changes in the geology or dips of the horizons. If the layer boundaries are not known, a simple type of ray tracing approach using a horizontally layered model may not be sufficient to accurately calculate the first arrival travel times from a S to a receiver (R). Introduced herein is an interactive velocity determination method to overcome the problem of finding layer interfaces in the events of unknown layer dips. Starting from the interval velocities of the ZVSP data, the algorithm successfully searched for and found travel time ellipses from the S to R and determined the boundaries of the layers where the interval velocities changed. The algorithm calculated the S to R travel time ellipses, searched for the best matching ellipse to the travel time pick from the data, and found the layer interface between the S and R (S-R) pair. Using the multiplicity of the S-R pairs, pseudo-ellipse images of the interfaces were created. The method was tested on several synthetic modeled data sets where it successfully expanded the interval velocity structures laterally from the borehole. Both direct as well as refracted wave arrivals were used in the evaluations.