Extracting True Anisotropic Values Using Raytraced Two-parameter Velocity Analysis

We present a methodology that allows the simultaneous determination of velocity and true eta in a production setting. Rather than deal with non-hyperbolic gathers by higher-order moveout equations, we take a new approach of using a different velocity function for each offset (Hellman, et al., 2012). At each analysis location, interval velocities are inverted from the near-offset rms velocities and then traveltimes are determined by anisotropic ray tracing through the implied model determined by the velocity and eta picks. These raytraced traveltimes are inserted into the hyperbolic moveout equation and rearranged to form offset dependent velocities, which are used to move out the gathers. Etas and velocities are iterated until the updated gather is flat. A primary benefit of the method is that the eta values are completely decoupled from the non-hyperbolic effect of the layered overburden, so that the derived etas are geophysical, and not “effective”. The velocities and etas are not necessarily picked on the same grid, allowing the processor to control the velocities in a much finer grid than is necessary for the anisotropy. The final set of picks can then be re-gridded to be on the same grid for further processing steps.