Traveltime changes can be approximately converted to velocity changes with the following simple expression:
where is the change in velocity in the reservoir, v is the background rms velocity before steam injection, is the time delay measured between two time-lapse surveys, as plotted in Figure 28, and tr is the two-way interval traveltime through the reservoir using the background velocity.
For the Duri reservoir, I assumed an rms velocity value of 1.75 km/s, and a reservoir interval transit time of 50 ms. I used these valued with (1) and the curves of Figure 28 to compute changes in velocity in the reservoir for each time-lapse survey. The velocity changes are computed as an average value vertically within the reservoir.