CONCLUSION

A simple theory by Black and Brzstowski predicted that the response of zero-offset time migration to a point diffractor in a v(x) medium would be a distinctive cusp-shaped curve called a ``plume''. We have demonstrated that plumes exist by migration of synthetic data using various time-migration methods. We have also computed the shape of the plumes by two geometrical construction methods. These two geometrical methods compare well and explain the observed migration results. The simple first-order theory explains some of the features of the plume but lacks higher order terms which could account for asymmetry