Abstract:Offset is the most important parameter in offset VSP survey. The reasons are as follows. ?Offset variation results in the variation of incident angle of seismic wave, and different observation systems cause different variations of incident angles at the same reflection interface. Improper offset makes it possible that the reflection waves which come from the sas'ne reflector but are received at different observation stations have very different strengths, even opposite polarities. ?Strong reflector is an important factor resulting in refraction. However, other reflectors which may result in refraction must be considered in designing offset. ?NMO correction stretching is an important factor we should consider in designing offset. Then the formula for calculating NMO correction stretching coefficient is derived, and the basic criterion for offset choice is defined. It is illustrated how we remove big-angle incident seismic data and excessive NMO correction stretching.