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 is 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 travel time picks from the zero-offset VSP (ZVSP) data. However the accuracy of these vertical velocities fade away quickly as the source 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 approach type of ray tracing using horizontally layered model may not be sufficient to calculate the first arrival travel times accurately enough from a source (S) to receiver (R). We introduce here 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 searches and finds travel time ellipses from S to R and determines the boundaries of the layers where the interval velocities change. The algorithm calculates S to R travel time ellipses, searches for the best matching ellipse to the travel time pick from the data and finds the layer interface between the S to R pair. Using the multiplicity of the S-R pairs pseudo-ellipse images of the interfaces are created. The method is 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 are used in the evaluations.