2013 Honorary LecturerSponsored by Shell

South & East Asia

D. P. Sinha

EESH Energy Enterprises, EnerGeo India Ltd.

Earth velocity estimation–Bridge the gap of interdependency between geology and geophysics

 
Abstract

DP Sinha

3D depth migration has created many opportunities for exploration and production. At the same time, it has brought many challenges. An essential prerequisite for accurate imaging in depth is, of course, an accurate estimation of the velocity Earth model. Depth imaging essentially consists of two processes – traveltime computation and downward extrapolation of the observed wavefield through a suitable algorithm. For computing traveltimes, we need to start with an accurate depth-velocity interval model, which in turn, requires accurate reflection geometries in depth with layer interval velocities. During the process, one can check the various geologic hypotheses which match the observed data (i.e., whether the horizons and velocities used in the depth model make geologic sense). For this purpose, we combine the prestack velocity analysis and model building tools to obtain the accurate initial velocity model.

Either layered or non-layered velocity models or a combination of two can yield the best possible Earth velocity model which may bridge the gap between geology and geophysics. In many geologically complex areas, especially in hilly terrain, we do not get velocity semblance that is sufficiently adequate to obtain a good velocity Earth model for the area. In such areas, estimation of the Earth velocity model has a certain amount of uncertainty regarding layer velocity and layer geometries. This can be quantified with interpretation efforts and prior geologic information.