Course description

Velocity is one of the most important parameter that can be derived from the seismic data. Velocity values are indicative for layer identification and thus convey rock properties. Moreover, velocities relate the seismic measurements that are in (two-way) traveltime to the end product of seismic data processing, i.e. the depth picture of the subsurface that can be obtained via migration or seismic imaging. New developments in acquisition geometries enable the determination of velocities with greater accuracy and also require to take into account anisotropy. The relationships between elastic constants and velocities are explained; this includes the phenomenon of anisotropy. In the end it is the wave equation that describes all wave propagation phenomena. Different types of velocities play a role during the processing sequence; stacking and migration being the most important ones. Migration or seismic imaging is the typical end product of conventional seismic data processing. The process of migration, whereby a proper image in time or depth of the subsurface is obtained, is directly related with the velocity model that both serves as input for the migration process as well as is the result of such a migration. Therefore migration and velocity model building are intimately related processes. There are several methods to implement the process of migration, and there are also several methods to build a velocity model. This course gives an overview of all aspects of velocity that one encounters during seismic data processing, of the relevant migration processes and velocity model building methods, i.e. depth conversion based on seismic data. In addition, VSP data acquisition and processing will be discussed, together with the application of well data, if available, to enhance depth conversion using the process of kriging. 


Geologists, geophysicists, petrophysicists, reservoir engineers, and drilling engineers, who need to understand how the various types of velocity information, necessary for imaging and depth conversion, can be derived from seismic data and, if available, well data and VSP, and who need to understand how subsurface images are generated.


Course content

•    introduction
•    Velocity: definition and comparison of the many types of velocity 
o    average, 
o    interval, 
o    RMS, 
o    stacking, 
o    migration, 
o    P-wave
o    S-wave
•    Velocity Inputs: 
o    accuracy and regional extent of each, 
o    check shots, 
o    VSPs, 
o    sonic logs, 
o    time/depth functions, 
o    well picks and pseudo velocities, 
o    seismic velocities, 
o    horizons for structural control
•    Synthetic Seismograms: 
o    creation, 
o    upscaling, and tie to seismic data. 
o    Advanced synthetics including synthetic gather creation, Zoeppritz equations, AVA, and AVO 
•    Matching Synthetics to Seismic: calibrating the seismic data to the well data 
•    Seismic Velocities
•    Semblance analysis, 
•    velocity picking, 
•    multiples, 
•    how seismic velocities differ from well velocities
•    Migration and Migration Velocities: introduction to pre- and post-stack algorithms, tomography.
•    Iterative velocity analysis
•    Velocity Model Building: 
o    workflows to integrate stacking velocities, 
o    time/depth curves, 
o    well picks associated with seismic horizons (pseudo-velocities), 
o    structure from horizons
•    Time-to-Depth Conversions: 
o    vertical stretch, 
o    inverse ray tracing, 
o    migration, and uncertainty
•    Introduction to Advanced Topics
o    Anisotropy, 
o    pore-pressure prediction, 
o    geostatistics, 
o    forward modeling

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