VELOCITY VARIATIONS

The equation (1) contains time delays which are computed using a velocity model. Usually this velocity model is rather simple (e.g., average velocity as a function of depth). The velocity structure in the medium can be much more complex and naturally the difference between the model and reality decreases the quality of the result. The errors in velocity model can be divided into two basic groups: (1) errors in average velocities, and (2) small scale velocity fluctuations which manifest themselves as small traveltime fluctuations.