Poroelastic analysis of frequency-dependent amplitude-versus-offset variations

Using analytic equations and numerical modeling, we have investigated characteristics of the frequency-dependent amplitude versus incident angle at an interface between a nondispersive medium and a patchy-saturated dispersive medium. For acoustically hard rocks, at normal incidence and smaller incident angles, the reflection magnitude increases when frequency increases, whereas in the amplitude versus incident-angle domain, the amplitude decreases with increasing incident angle (offset). For acoustically moderate to slightly hard rocks, phase reversal may occur when frequency increases from low to high. This type of response can happen in traditional amplitude-versus-offset class I and II reservoirs, but the frequency-domain phase reversal will be in different incident-angle ranges. For acoustically soft reservoirs, in amplitude versus incident-angle domain, the reflection magnitude increases with increasing incident angle. However, in amplitude-versus-frequency domain, the reflection magnitude increases when frequency decreases, which occurs in all investigated frequencies.