Traveltime changes can be approximately converted to velocity changes with the following simple expression:

(1) |

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 *t*_{r} 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.

11/12/1997