Extended elastic impedance (Whitcombe et al, 2002) redefines elastic impedance to allow an equivalent impedance and associated reflectivity to be calculated for any arbitrary angle, including imaginary angles not necessarily recorded in the gathers. The projection allows rock physics moduli to be expressed in terms of impedance volumes or the generation of impedance volumes which are optimal for prediction of lithology, porosity or fluid properties.

For example, an impedance log corresponding to shear modulus is equivalent to an extended elastic impedance (EEI) volume calculated at an angle of sin^2(theta) = -1.25. Clearly, negative angles are not physically recordable but the imaginary Schuey amplitude response for this angle can be computed using recorded angle gathers.