Comparison of VSP and sonic-log data in nonvertical wells in a heterogeneous structure

To compare the results of sonic-log measurements and of vertical seismic profiling (VSP), sonic-log velocities are used to estimate the corresponding traveltime in the geologic structure, which is then compared with the VSP traveltime. We show how to calculate the sonic-log traveltime in the geologic structure from the sonic-log velocities while taking into account the effects of the nonvertical propagation of seismic waves, resulting from the VSP-source offset and from heterogeneous velocity in the structure, together with the effects of the well trajectory deviating from strictly vertical. Errors caused by the commonly used assumption of vertical propagation may considerably exceed the difference of the measured VSP traveltimes from the sonic-log traveltimes.