PLUMES - RESPONSE OF TIME MIGRATION TO LATERAL VELOCITY VARIATION

We analyze how time migration mispositions events in the presence of l ateral velocity variation by examining the impulse response of depth m odeling followed by time migration. By examining this impulse response , we lay the groundwork for the development of a remedial migration op erator that links time and depth migration. A simple theory by Black a nd Brzostowski predicted that the response of zero-offset time migrati on to a point diffractor in a upsilon(x, z) medium would be a distinct ive, cusp-shaped curve called a plume. We have constructed these plume s by migrating synthetic data using several time-migration methods. We have also computed the shape of the plumes by two geometrical constru ction methods. These two geometrical methods compare well and explain the observed migration results. The plume response is strongly influen ced by migration velocity. We have studied this dependency by migratin g synthetic data with different velocities. The observed velocity depe ndence is confirmed by geometrical construction. A simple first-order theory qualitatively explains the behavior of zero-offset time migrati on, but a more complete understanding of migration velocity dependence in a upsilon(x, z) medium requires a higher order finite-offset theor y.