Anisotropic Parameter Estimation
Our methodology for determining anisotropic parameters is as follows:
- Create the initial check shot-calibrated sedimentary velocity field
- Compute axis of symmetry angles for the TTI model & the Vnormal velocity model.
- Compute anisotropy parameters ε and δ at the well-locations using focusing analysis (FAN). FAN computes ε and δ by minimizing the difference between the calculated and true travel-times over a user-specified range of ε and δ values.
- Interpolate the ε and δ parameters away from the wells to create the initial anisotropy models.
Isotropic vs. Anisotropic
- In isotropic media, seismic waves propagate at the same velocity, regardless of their direction.
- In anisotropic media, seismic wave velocity varies with direction.
- The most typical anisotropic media to consider are transversely isotropic (TI), where the velocity normal to the bedding is lower than along it, and which may be vertical (VTI) or tilted (TTI).
- Transverse isotropy is usually seen in situations with neatly ordered sedimentary layers.
- HTI anisotropy compensation is achieved via a curve fitting approach in which delta_t time perturbations along an event after nominal (i.e., isotropic) moveout correction are fit to an elliptical velocity function. The underlying theory is based on the work of Tsvankin (1997, Geophysics), who showed that best-fit NMO velocities in an HTI medium exhibit an elliptical variation as a function of source-receiver azimuth. The curve fitting process yields estimates for Vfast, Vslow, and the axis of symmetry of the ellipse at all velocity control points. Once determined, these three parameters are used in conjunction with the observed source-receiver azimuth to perform either an azimuth-dependent NMO correction or a postmigration residual azimuthal moveout correction.
- Note that orthorhombic anisotropy can be handled empirically (although not theoretically) by cascaded application of the above VTI n and HTI corrections. Note also that TGS does not handle TTI anisotropic effects in time processing.