Using the velocity models of Figures 15 and 16, I simulated shot gathers by finite difference acoustic modeling. The shot is located at the injector location, and the data are modeled with about 200 Hz maximum frequency content, to match the field data. Figure 17 shows a snapshot of the wavefield before and after steam injection. Note that the steam snapshot has a much larger upgoing reflection branch, and is slightly flattened due to time delay on the leading downgoing branch.
Figure 18 shows the shot gathers corresponding to the two point-source shots before and after steam injection. Note that the top of reservoir reflection at 210 ms has changed dramatically. It is much brighter and contains a polarity reversal. This polarity reversal occurs where the low-velocity steam zone transitions into the high-velocity pressure front. This polarity effect might explain the polarity reversal seen in the field data of Figures 6 and 7, and suggest that there is a pressure front in the field data. Finally, note the diffractions from the steam zone. These also match diffractions seen in the field data, and suggest the lateral extent of a steam zone.