Apparent horizontal displacements in time-lapse seismic images

In addition to vertical time shifts commonly observed in time-lapse seismic images, horizontal displacements are apparent as well. These apparent horizontal displacements may be small relatively to seismic wavelengths, perhaps only 5 m at depths of 5 km, but they consistently suggest an outward lateral expansion of images away from a compacting reservoir. It is well known that apparent vertical displacements are caused mostly by a decrease in seismic wave velocities above compacting reservoirs. Those same velocity changes contribute to horizontal displacements. This contribution can be computed from the velocity changes that, in turn, can be estimated from measured vertical displacements. Horizontal displacements computed in this way are similar to those measured, and this similarity suggests that horizontal as well as vertical displacements may be largely due to velocity changes.