Velocity analysis is synonymous with velocity model building (VMB) because the goal is to produce a velocity model for the subsurface. VMB is among the most common practices in seismology for two reasons. First, for any study area, its seismic velocity model is one of the main measurable results from geoscience. Second, a velocity model is a precondition for seismic migration and other seismic imaging methods to map subsurface reflectors and scatters using reflected or scattered waves. Since seismic velocity is inferred from traveltimes of seismic waves, the resolution of each velocity model is limited by the frequency bandwidth and spatial coverage of seismic data. This chapter starts with definitions and measurements of different types of seismic velocities. The observations reveal the trends of seismic velocities as functions of pressure, temperature, and other physical parameters. The dominance of the 1D or V(z) velocity variation at large scale in the Earth leads to the classic refraction velocity analysis based on seismic ray tracing in a layer-cake velocity model.

The three common seismic velocity analysis methods based on NMO semblance, seismic migration, and seismic tomography are discussed in three sections. Assuming gentle to no changes in the reflector dip and lateral velocity variation, semblance velocity analysis provides stable estimates of the stacking velocity. The stacking velocity at each depth, marked by the corresponding two-way traveltime, is the average velocity of all layers above the depth. By taking advantage of the dependency of depth migration on velocity variations, migration velocity analysis enables the velocity model to be refined using horizons defined by data to accommodate lateral velocity variations. Nowadays most migration velocity analyses are conducted on common image gathers (CIGs). To constrain the velocity variation using all data together, tomographic velocity analysis derives or refines the velocity model through an inversion approach to update the velocity perturbations iteratively. Some practical issues in tomographic VMB are discussed in the final section, mostly on inversion artifacts and deformable layer tomography.