Effects of Vertical Velocity Heterogeneity on Stacking Velocity and Depth Conversion
Qadrouh, Ayman N.
Master of Science Degree 2008
University of Adelaide
It is assumed a constant velocity is usually incorrect because velocity regularly increases with increasing depth. In oil and gas investigations, the subsurface layers of the earth are generally considered to be horizontal. Therefore, the suggestion raised was to study the effects of vertical velocity heterogeneity on stacking velocity and depth conversion with different spread lengths, i.e., a small spread with a maximum offset of 2000 m and a large spread with a maximum offset of 4000 m. This study focused on the variation between stacking velocity and average velocity. In addition, the traveltime equation of Taner and Koehler (1969) for two terms and three terms was examined in order to find out which one provided better results.
Understanding the variations between the different types of velocities was crucial to this approach, which was carried out using data from the Tirrawarra-29 well in the Cooper Basin, South Australia. Well log data are used to calculate different types of velocities such as average velocity, root-mean-square velocity (for both short offset and three terms) and stacking velocity.
The results for both the plots and the plots for small (2000 m) and large spreads (4000 m) proved that the variation between average velocity and stacking velocity increases with offset. Furthermore, using the traveltime equation for three terms on the residual moveout plots for small and large offsets provided better results than using only two terms.