Transmission Amplitude Variation with Offset (TAVO)

Abstract

Amplitude variation with offset (AVO) has been applied over the years in reservoirbgeophysics for various applications including fluid detection and lithology typing among others. Existing AVO methods utilize the reflected waves produced when a compressional body wave is incident on an elastic interface. This study presents new approximations for the transmission PP and PS coefficients from Aki and Richards (2002) equations. The new approximations were compared to the original Aki and Richards’ approximations using three different published simple reservoir models. The transmission P-wave was approximated by a linear function while the transmission converted S-wave was approximated with a polynomial function. Results show that there is a similar transmission P-wave amplitude versus offset response for the gas sand models which is different from the oil sand model’s response. All the three models showed similar trends for the transmission converted S-wave response, which is a general decrease of amplitude with offset. The TPS response was inverted for three elastic parameter contrasts: the average S-wave/P-wave velocity, the density difference/average density and the S-wave velocity difference/average S-wave velocity across the interface. Inversion resulted in a fairly good estimation for two parameters only (the average S-wave/P-wave velocity and the S-wave velocity difference/average S-wave velocity) up to intermediate angle of incidence but failed to recover one of the parameters (density difference/average density) with reasonable accuracy probably due to high non-linearity of the starting TPS equation and high dependence of this parameter on higher order terms in the TPS approximation. The presented approximations are ideal for use with VSP geometry with receivers below the target horizon.