Figure 1. 4-D is multiple 3-D seismic surveys over the same fields. This example shows, from left, the results of a 1985 seismic survey acquired by Pennzoil, Mobil, and BP of Eugene Island Block 330, with blue showing the amplitude envelopes surrounding producing reservoirs; the same reservoirs imaged (in red) in a 3-D survey acquired by Texaco and Chevron in 1988, where improved technology lowered the noise and raised the images’ high frequency components; and a third survey of the same area (in green), shot by Shell and Exxon in 1992, showing the clear shrinkage in size of each reservoir and the noise-free definition of the bounds of each reservoir.