Body-wave radiation patterns and AVO in transversely isotropic media

Technical Report · Tue Mar 01 04:00:00 EST 1994

DOI:https://doi.org/10.2172/10137856· OSTI ID:10137856

Tsvankin, I

It is well known that the angular dependence of reflection coefficients may be significantly distorted in the presence of elastic anisotropy. However, the influence of anisotropy on amplitude-versus-offset analysis (AVO) is not limited to reflection coefficients. AVO signatures (e.g., AVO gradient) in anisotropic media are also distorted by the redistribution of energy along the wavefront of the wave travelling down to the reflector and back up to the surface. Significant anisotropy above the target horizon may be rather typical of sand-shale sequences commonly encountered in AVO analysis. Here, I examine the influence of P- and S-wave radiation patterns on AVO in the most common anisotropic model - transversely isotropic media. A concise analytic solution, obtained in the weak-anisotropy approximation, provides a convenient way to estimate the impact of the distortions of the radiation patterns on AVO results. It is shown that the shape of the P-wave radiation pattern in the range of angles most important to AVO analysis (0 - 40{degrees}) is mostly dependent on the difference between Thomsen parameters {epsilon} and {beta}. For media with {epsilon} - {beta} > 0 (the most common case), the P-wave amplitude may drop substantially over the first 25{degrees} - 40{degrees} from vertical. There is no simple correlation between the strength of velocity anisotropy and angular amplitude variations: for instance, for models with a fixed positive {epsilon} - {beta} the amplitude distortions are less pronounced for larger anisotropies {epsilon} and {beta}. The distortions of the SV-wave radiation pattern are usually much more significant than those for the P-wave. The anisotropic directivity factor for the incident wave may be of equal or greater importance for AVO than the influence of anisotropy on the reflection coefficient.

• Research Organization: Colorado School of Mines, Golden, CO (United States). Center for Wave Phenomena
• Sponsoring Organization: USDOE, Washington, DC (United States)
• DOE Contract Number: FG02-89ER14079
• OSTI ID: 10137856
• Report Number(s): DOE/ER/14079--31; CWP--144; ON: DE94009214
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580000
AMPLITUDES
ANISOTROPY
GEOSCIENCES
MATHEMATICAL MODELS
P WAVES
S WAVES
TRANSVERSE ENERGY
WAVE PROPAGATION