InVA: Dual-parameter Anisotropic Velocity Analysis

Build superior anisotropic velocity fields

Conventional methods for anisotropic velocity analysis are often based solely on the Alkalifah & Tsvankin technique for deriving effective velocity measurements: Vrms and effective eta. However, this approach can yield unrealistic velocity fields as changes in Vrms and effective eta are introduced to compensate for curved-ray effects.

The solution is to use imaging algorithms that apply full ray tracing, compensating for both turning-wave (curved-ray) effects and VTI. This requires a velocity analysis technique that can generate interval parameters for both vertical velocity and eta.

WesternGeco has developed a workflow that derives true vertical interval velocity and interval eta. This is achieved by ensuring that full ray tracing (including curved-ray effects) is used during both the velocity analysis stage in InVA and subsequent imaging steps.

There are several benefits in following this approach:

• Improved imaging using Anisotropic Turning-wave Kirchhoff Prestack Time Migration.
• A major component in creating 'depth-ready' velocity fields, which can be used to reduce the number of iterations in a depth imaging workflow.
• More reliable vertical velocity functions for identifying overpressure zones as part of a Pore Pressure Prediction (PPP) workflow.

InVA provides the ability to perform either automatic or manual dual-parameter velocity analysis in a single interactive environment. Both Vnmo and the anisotropic parameter effective eta are analyzed simultaneously, enabling the geophysicist to build an anisotropic field that complements the conventional stacking velocity equivalent.

The resultant Vnmo and eta fields can subsequently be gridded, smoothed, or transformed to generate the interval parameters of vertical velocity and eta, while correctly accounting for curved-ray effects.