The objective of the seismic survey method is a reliable measurement of the travel time between a signal source and a receiver. The results of a reflection survey are displayed on a time section, and geophysicists map structure in units of reflection travel time. A geologist, on the other hand, is more interested in the depth of the horizons, and the thickness of the section components measured in units of meters or feet.
The key which links the time domain to the depth domain is velocity, which in itself can be a significant diagnostic tool for the determination of lithology.
Geologists and geophysicists are often guilty of underestimating the influence that velocity exerts in all phases of the seismic process. In processing, the use of improper velocity can completely negate the benefits to be obtained by stacking or migrating data, for example.
Velocity variation causes the conventional time-display section always to have some degree of vertical distortion and if the velocity function changes laterally the geologic dip can be quite different from time dip. A relatively small change in velocity can either manufacture or wipe out a significant anomaly on a time section. Migration, which is very dependent upon precise velocity determination, can greatly enhance the definition of structural features when it is properly applied, but it can also be very damaging to the data if the velocity function is not correct.
The potential danger of velocity distortion of time structure maps has always existed, but prior to the advent