Velocity Model Building
Success in depth imaging depends upon the accuracy and validity of the velocity model. PGS offers the model specification and updating methodology which is best suited to the geological setting and geophysical objectives of the depth imaging project. Model specification may be grid, layered, or solid. Initial model building is based on high density velocity analysis, in either a vertical sense (grid model) or horizon based (layered model). Layer attributes such as V(z) interval velocity and gradient are computed, and the model is rendered in depth via map migration of interpreted horizons. For VTI anisotropy, the Thomsen parameters delta, epsilon and eta are estimated. Delta is computed from the mistie between isotropic migration and well markers. A vertical velocity field and an eta volume are then created for input to anisotropic migration.
Velocity model updating is performed via tomographic inversion of depth delays in either grid or layered form. Traditionally in depth delay picking, a single value is picked based on a far or reference offset for a gather. Instead, our new tomographic update allows picking of delays which vary with offset, gather-by-gather.
These applications are resident in the holoSeisTM visualization system, so that the model building and delay picking for tomography is performed in the context of 3D visualization. This allows us to reap some significant benefits: First, there is the QC aspect, being able to visualize the model surfaces and attributes, together with the associated well control, as the model is being developed, and secondly, the rapid horizon growing capability within holoSeisTM means gather delay picking and horizon re-interpretation proceed quickly.
Model building along a fault zone (left), and Raytracing "on the fly" (right).
Top/base salt structures (left), and Well control during model building (right).
