Transmission amplitude variation with offset (tavo)

Amplitude Variation with Offset (AVO) has been applied successfully in reservoir geophysics for various applications including fluid detection and lithology typing. Existing AVO methods utilize seismic waves reflected off elastic interfaces. Amplitudes of transmitted waves, usually recorded in vertical seismic profiling (VSP) surveys, also show angle dependence and can be used for AVO-like analysis. In this study, we derive new approximations of the exact Zoeppritz transmission PP (TPP) and PS (TPS) coefficients that are convenient for conventional AVO analysis. Testing on a published reservoir model showed that the new approximations deviated from their corresponding exact coefficients by less than 7% at an incidence angle equal to 90% of the critical angle. Furthermore, a quantitative approach is described to invert the new TPP and TPS approximations for the P-wave velocity contrast (Δα/α), density contrast (Δρ/ρ), S-wave velocity contrast (Δβ/β) and S-wave/P-wave velocity ratio (β/α) across a subsurface interface. Testing this approach using the same reservoir model resulted in error amounts of 0%, 0%, 6% and 6.5% for Δα/α, Δρ/ρ, Δβ/β and β/α, respectively. These small errors show that the new approximations and inversion approach are relatively accurate.