%0 Journal Article %1 ramos_castagna:2001 %A Ramos, Antonio C. B. %A Castagna, John P. %D 2001 %I SEG %J Geophysics %K geophysics seismics %N 6 %P 1721--1734 %R 10.1190/1.1487114 %T Useful approximations for converted-wave AVO %U http://dx.doi.org/10.1190/1.1487114 %V 66 %X Converted-wave amplitude versus offset (AVO) behavior may be fit with a cubic relationship between reflection coefficient and ray parameter. Attributes extracted using this form can be directly related to elastic parameters with low-contrast or high-contrast approximations to the Zoeppritz equations. The high-contrast approximation has the advantage of greater accuracy; the low-contrast approximation is analytically simpler. The two coefficients of the low-contrast approximation are a function of the average ratio of compressional-to-shear-wave velocity (alpha/beta) and the fractional changes in S-wave velocity and density (Deltabeta/beta and Deltarho/rho). Because of its simplicity, the low-contrast approximation is subject to errors, particularly for large positive contrasts in P-wave velocity associated with negative contrasts in S-wave velocity. However, for incidence angles up to 40 deg and models confined to Deltabeta/beta < 0.25, the errors in both coefficients are relatively small. Converted-wave AVO crossplotting of the coefficients of the low-contrast approximation is a useful interpretation technique. The background trend in this case has a negative slope and an intercept proportional to the alpha/beta ratio and the fractional change in S-wave velocity. For constant alpha/beta ratio, an attribute trace formed by the weighted sum of the coefficients of the low-contrast approximation provides useful estimates of the fractional change in S-wave velocity and density. Using synthetic examples, we investigate the sensitivity of these parameters to random noise. Integrated P-wave and converted-wave analysis may improve estimation of rock properties by combining extracted attributes to yield fractional contrasts in P-wave and S-wave velocities and density. Together, these parameters may provide improved direct hydrocarbon indication and can potentially be used to identify anomalies caused by low gas saturations.

@article{ramos_castagna:2001, abstract = {Converted-wave amplitude versus offset (AVO) behavior may be fit with a cubic relationship between reflection coefficient and ray parameter. Attributes extracted using this form can be directly related to elastic parameters with low-contrast or high-contrast approximations to the Zoeppritz equations. The high-contrast approximation has the advantage of greater accuracy; the low-contrast approximation is analytically simpler. The two coefficients of the low-contrast approximation are a function of the average ratio of compressional-to-shear-wave velocity (alpha/beta) and the fractional changes in S-wave velocity and density (Deltabeta/beta and Deltarho/rho). Because of its simplicity, the low-contrast approximation is subject to errors, particularly for large positive contrasts in P-wave velocity associated with negative contrasts in S-wave velocity. However, for incidence angles up to 40 deg and models confined to Deltabeta/beta < 0.25, the errors in both coefficients are relatively small. Converted-wave AVO crossplotting of the coefficients of the low-contrast approximation is a useful interpretation technique. The background trend in this case has a negative slope and an intercept proportional to the alpha/beta ratio and the fractional change in S-wave velocity. For constant alpha/beta ratio, an attribute trace formed by the weighted sum of the coefficients of the low-contrast approximation provides useful estimates of the fractional change in S-wave velocity and density. Using synthetic examples, we investigate the sensitivity of these parameters to random noise. Integrated P-wave and converted-wave analysis may improve estimation of rock properties by combining extracted attributes to yield fractional contrasts in P-wave and S-wave velocities and density. Together, these parameters may provide improved direct hydrocarbon indication and can potentially be used to identify anomalies caused by low gas saturations.}, added-at = {2012-09-01T13:08:21.000+0200}, author = {Ramos, Antonio C. B. and Castagna, John P.}, biburl = {https://www.bibsonomy.org/bibtex/2d91391d899dfdedeed72bfb7a1148401/nilsma}, day = 1, doi = {10.1190/1.1487114}, interhash = {2db8e81eee8dcd55001ad5dff1bfcd15}, intrahash = {d91391d899dfdedeed72bfb7a1148401}, journal = {Geophysics}, keywords = {geophysics seismics}, month = nov, number = 6, pages = {1721--1734}, publisher = {SEG}, timestamp = {2021-02-09T13:20:53.000+0100}, title = {Useful approximations for converted-wave AVO}, url = {http://dx.doi.org/10.1190/1.1487114}, volume = 66, year = 2001 }