Polarity changes and vanishing reflectors

The time-lapse seismic images show some dramatic changes in amplitudes along baseline reflectors. Strong reflectivity changes are visible in the monitor 2 survey, compared to the baseline survey, along the reflector at the base of the P formation (200 ms). In the monitor 2 images, at the west (left) side of the injector, the baseline reflection has almost completely vanished out to the survey edge. This feature is labeled ``Z'' in the stacked and migrated images of Figures 6 and 7, and indicates that an impedance match has occurred across the P/K formation boundary, perhaps by differential heating from above and below, or pressure changes in the P formation alone. On the other side of injector, towards the east (right) the monitor 2 sections show what appears to be a polarity reversal along the P/K formation boundary at 200 ms, labeled ``Pr''. This feature is also clearly evident in both the stacked and migrated and difference sections, and suggests that the fluid-flow activity in the lower portion of the P reservoir is different to the east from that to the west.