Large-offset approximation to seismic reflection traveltimes

Conventional approximations of reflection traveltimes assume a small offset -to-depth ratio, and their accuracy decreases with increasing offset-to-dep th ratio. Hence, they are not suitable for velocity analysis and stacking o f long-offset reflection seismic data. Assuming that the offset is large, r ather than small, we present a new traveltime approximation which is exact at infinite offset and has a decreasing accuracy with decreasing offset-to- depth ratio. This approximation has the form of a series containing powers of the offset from 1 to -infinity. It is particularly accurate in the prese nce of a thin high-velocity layer above the reflector, i.e. in a situation where the accuracy of the Taner and Koehler series is poor. This new series can be used to gain insight into the velocity information contained in ref lection traveltimes at large offsets, and possibly to improve velocity anal ysis and stacking of long-offset reflection seismic data.