1. A method comprising: obtaining a sparsely sampled monitor data set for a subsurface region; and processing the obtained sparsely sampled monitor data set with external information to generate a more accurate 3D representation of the subsurface target area than could be obtained from the sparsely sampled monitor data set alone, wherein the external information comprises a plurality of alternative models of the subsurface region that each represent the subsurface region as the subsurface region is predicted to exist at an additional point in time when the sparsely sampled monitor data set is acquired, said alternative models being generated from an earlier, more fully sampled base survey using varying assumptions of how the subsurface region will change over time.