Seismic Velocity Analysis Techniques
Seismic ray paths, as shown below, are rarely straight lines, but rather follow the laws of refraction, resulting in curved ray paths. The exact shape of the path is determined by the rock velocity distribution versus depth and the spread geometry. In the seismic record shown below, the apparent curvature of the seismic reflections is caused by these factors.

Sound velocity in the subsurface can be found directly from the seismic data by observing the difference in arrival time of reflectors versus distance from the source. The time difference is called normal moveout or NMO.


Raw 24 trace seismic record showing normal moveout

A large number of semi-automatic techniques have been developed to derive seismic velocity from seismic data independently of well log data, The illustration below show two types of velocity analysis display. Many others have been developed over the years. The principle of such techniques is based on computerized approximations to normal moveout analysis which used to be done by hand calculation. The normal moveout is often called delta-T, just as the sonic log interval transit time is called delta-T by log analysts (abbreviated DTC in this Handbook), so we will always use the term normal moveout (abbreviated NMO) to avoid confusion. My first job as a geophysical engineer in 1967 was to plot NMO versus depth, pick the slopes of the graph and enter the data into a TIAC computer to calculate velocity.

Velocity analysis display - velocity panels

Velocity analysis display - cross correlation panels

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