The basic concept of velocity model building is to use the travel time of the acoustic waves to image the subsurface. Unlike the amplitudes, which are linked to acoustic properties (density and Lame parameters), the travel time and ray geometry between source and receiver are function of the velocity (and anisotropy) field(s). This is a consequence of the ray geometry obeying Fermat’s principle of least time.
A number of migration algorithms, like the Kirchhoff migration, compute travel times based on the velocity field. These computed travel times are used in a second processing step for the migration of the recorded seismic amplitudes into an image of the subsurface, in depth. This process is called Pre-Stack Depth Migration (PSDM). This technique is today one of the best imaging tools available for complex underburden with strong lateral variations. Its weakness resides in a convergence problem with respect to the input velocity model: The process quality is linked to the velocity model being relatively close to the actual velocity field.
The Pre-Stack Depth migration/Velocity Model Building system consists in the following three workflows:
Build a velocity model
Apply Kirchoff migrations
Run tomography
The velocity model building phase is performed in OpendTect, while the Kirchoff migrations and tomography are run from Geokinetics's Ethos Job Deck Builder.
The velocity model building requires migrated Common Image Gathers(CIG), as input. CIG are pre-stack seismic gathers migrated to depth, thus NMO corrected. The velocity field used for this migration will also be a necessary input for semblance calculation and thus RMO picking, and is linked to OpendTect CIG data store. The corresponding post-stack volume might also be appended to the pair CIG/Velocity model to create a link between the post stack horizons and the pre-stack events and to perform horizon-based velocity analysis.
Picking velocities is possible since the velocity model determines the travel times and ray geometry. A correct travel time and ray geometry will allow the image gather (migrated seismic in depth) to be stacked constructively. The constructiveness of the gathers is measured via a semblance function which outputs a semblance value for each possible RMS velocity. The picking of the high semblances enables, therefore, the interpreter to retrieve the correct velocity function. The process is made available on a semi-automated base in two modes: (1) Vertical velocity update and (2) horizon-based velocity update. The vertical velocity update presents the semblance panel for a single trace. The picked RMO’s generate a new velocity function with depth, which can be applied on-the-fly on the corresponding common image gather (CIG). Therefore, the flatness of the CIG can be very quickly appreciated during the picking phase. The maximum of semblance, therefore the change in the velocity function, is tracked between the picked positions and the next position. Therefore, the picking is called semi-automated. The picking may also be done along a post-stack horizon instead of on a trace-by-trace basis. A standard workflow would be to track horizons on a migrated volume, pick the RMO’s along each horizon performing the horizon-based velocity update, and QC them in the vertical velocity update window.
