Refining 3-D Velocity Models For Depth Migration Using Tomography: Application to Rapid Permafrost Variations In Alaska¿s North Slope Transition Zone Province

The presence of permafrost near the surface creates special problems for accurate imaging of seismic data. Permafrost thickness can exceed 2000 ft (600 m) onshore and completely disappear offshore, and the high contrast between permafrost velocity and unfrozen sediment velocity produces severe distortions in reflections observed in seismic data. Images from time migration have false structures and positioning errors that compromise their quality and increase uncertainty. In this type of environment 3-D depth migration should be strongly considered. In this study we use tomography to construct the velocity model of the near-surface. Tomography is an ideal tool for this estimation problem where refraction information is limited or completely absent due to the decrease of velocity with depth, and where lack of short offsets, irregular offset sampling and multiple azimuths compromise velocity resolution near the surface. The image obtained with velocities from tomography attenuates false structure introduced by the permafrost. This image is comparable to an alternative image obtained with a velocity field built using well and seismic information.