A B S T R A C TBy applying seismic inversion, we can derive rock impedance from seismic data. Since it is an interval property, impedance is valuable for reservoir characterization. Furthermore, the decomposition of the impedance into two fundamental properties, i.e. velocity and density, provides a link to the currently available rock-physics applications to derive quantitative reservoir properties. However, the decomposition is a challenging task due to the strong influence of noise, especially for seismic data with a maximum offset angle of less than 30 • . We present a method of impedance decomposition using three elastic impedance data derived from the seismic inversion of angle stacks, where the far-stack angle is 23.5 • . We discuss the effect of noise on the analysis as being the most significant cause of making the decomposition difficult. As the result, the offset-consistent component of noise mostly affects the determination of density but not the velocities (P-and S-wave), whereas the effect of the random component of noise occurs equally in the determination of the velocities and density. The effect is controlled by the noise enhancement factor 1/A, which is determined by a combination of stack angles. Based on the results of the analysis, we show an innovative method of decomposition incorporating rock-physics bounds as constraints for the analysis. The method is applied to an actual data set from an offshore oilfield; we demonstrate the result of analysis for sandbody detection.
I N T R O D U C T I O NElastic impedance (EI) is the apparent seismic impedance at specific angles of incidence, which was derived by Connolly (1998, 1999) and Mukerji et al. (1998). The elastic impedance is a function of P-wave velocity (V P ), S-wave velocity (V S ), density (ρ) and the angle of incidence (θ), based on the approximations of Zoeppritz's equations (1919) by Aki and Richards (1980) and Shuey (1985). Therefore, it is possible, in principle, to back-calculate V P , V S and ρ from the three elastic impedances for different stack angles of the same seismic data. Very often, the maximum stack angle of seismic data is limited to less than 30 • , which is not wide enough to obtain stable calculation results. Hence, except for long-offset seismic data with a maximum offset angle greater than approximately 60 • , it is impractical to calculate V P , V S and ρ separately from the seismic data. * Instead of calculating V P , V S and ρ directly from elastic impedances, the required properties are most often estimated by establishing empirical relationships between the impedance and the rock properties at well locations using well-log data. Whitcombe et al. (2002) introduced extended elastic impedance in a modified form of elastic impedance using tanχ as an alternative for sin 2 θ, where χ is an arbitrary angle. They then extended the linear trend of the impedance change as a function of tan χ , from negative infinity at χ = -π/2 to positive infinity at χ = π/2, in place of a limited extension of the horizontal axis f...
