Elastic impedance (Connolly, 1999) is a generalisation of acoustic impedance for a variable incidence angle. Elastic impedance provides a consistent and absolute framework to calibrate and invert non-zero offset seismic data. The elastic impedance approximation is derived from Schuey's linearisation of the Zoeppritz equations whereby AVO properties are approximately linear in sin^2(theta) where theta is the incidence angle in the pre-stack gathers.

The basic elastic impedance (EI) equation is:

Where:

Note that the factor K is assumed constant and is usually set to the average K over the zone of interest.

The elastic impedance equation thus defines an equivalent impedance as a function of incidence angle theta.