The coda waves are very sensitive to small changes in the propagation medium. When the medium changes over time, the most important terms to quantify these changes are ‘Cross correlation’ and ‘Velocity change’. We can extract information about changes in a medium by monitoring the velocity of wave propagation. If the propagation path before and after the velocity change remains fixed, the relative time lag is a direct measure of the relative velocity change.
Contributor: Xin Wang (ESR1)