Rapid Updating and Visualization of 3D Velocity Models
Authors: Schultz Phil , Saunders Mike 
Published: October 2008

Abstract
Seismic velocity model building for 3D depth imaging or depth conversion is to a large extent an interpretive process, As with all interpretive processes rapid visualization is key, Such visualization requires examination of the model in multiple domains, such as rms velocity interval velocity. linear gradient coefficient, and time and depth, At the same time, initial velocity models are almost always in need of refinement. Experiences have taught that informed interpretive input accessibility of the model, rapid visualization and rapid response are at least as important to building an effective velocity model in a timely manner as adherence to any specific updating scheme.

Introduction
Velocity model building is a relatively new concept in integrated interpretation and processing, Traditionally, velocities have been described for stacking and poststack time migration in such a way that formal modeling was not required. For these purposes, it was (and still is) necessary only to describe the velocity as a processing parameter which varies over space and time, In such a scheme the geologic environment over which the seismic data have been collected provides little guidance to the quantitative selection of parametric values of velocity, For stacking purposes, the distribution of the velocity parameter can be envisioned as a field of numbers: the velocity field, For certain purposes such as depth migration and depth conversion the accuracy of the velocity estimates are required to be quite high, The velocity field evolves to a velocity model when interpretive geological and well log input is necessary to achieve the required accuracy, This paper will describe the aspects and the flow of successful 3D velocity model building for depth imaging and depth conversion, focusing on the role of interpretive interaction, The process is illustrated with an example.

The Initial Velocity Model
Normally a velocity model is built from structural information merged with regional and point velocity observations, Velocity interfaces often correspond to mappable surfaces, This is true to varying degrees depending on the local geology, For example. if seismic velocities arc compaction driven. the depth of burial of a layer is normally the most important controlling factor in establishing local velocity nlues. such as in the Gulf of Mexico. On the other hand. in some areas the rock lithology drives the local velocity more than compaction or age. such as for intrusive bodies like thrust wedges or tabular salt lenses. In either case the structural component of the velocity model is initially defined and later refined to impose overall structure on the velocity model. Familiarity with the local geology is required to identify the type of structural constraints to impose, Point velocity observations originate from either surface or downhole measurements, Surface measurements involve the interpretation of moveout of event coherence versus offset. While these observations are made at a point they represent an average value in the subsurface and a correction step (such as those described in Ref. I) is often required. On the other hand point velocity observations made at depth in the well represent true point values but often measure a different component of velocity than do seismic estimates. Regional velocity information usually is incorporated m the form of known fixed gradients of velocity in specific layers or in fixed values of velocity based on rock type.