Inversion for velocities through phase shift downward continuation of walkaway vertical seismic profiling wavefield

  • Lian Hou

Student thesis: Master's Thesis

Abstract

A new walkaway VSP processing technique to build a velocity model is proposed in this study. It uses a combination of two seismic processes. First the common receiver gathers are downward continued to their respective receiver depths. This will allow sorting the data into Common Offset Gathers (COGs). Second a velocity analysis is performed on each common offset gather leading to a velocity model of the subsurface. Two new approaches of VSP NMO corrections are proposed that differ substantially from those of surface seismic. This new walkaway VSP velocity model building procedure is tested using synthetic data based on a velocity model derived from the subsurface around well ZK327-01D in Zakum oil field in Abu Dhabi. The results were satisfactory and the method was able to extract a good velocity model and a good stacked seismic section of the deep subsurface around the well location at the reservoir level. Another aspect of the present study is to look at Walkaway VSP subsurface illumination. It is found to be different from that of surface seismic and highly dependent on the overburden and depth of the target. The illuminated area in the subsurface is much reduced compared to the maximum source offset. However the illumination density (fold) is very high with a maximum very close to the well location. It is also found that the location of the receivers in the borehole with respect to the zone of interest plays a major role on the subsurface illumination. Indeed the more receivers deployed above a given reflector the better the imaging quality of the final stacked section and the better the illumination farther away from the well. These observations affect the way to design walkaway surveys to achieve optimum imaging of the region of interest.
Date of Award2015
Original languageAmerican English
SupervisorYoucef Bouzidi (Supervisor)

Keywords

  • Earth sciences
  • Petroleum geosciences
  • Petroleum Geology
  • 0583:Petroleum Geology

Cite this

'