Dictionary:Amplitude variation with angle/offset (AVA/AVO)

The variation in the amplitude of a seismic reflection with angle of incidence or source-geophone distance. Depends on changes in velocity, density, and Poisson's ratio. Often used as a hydrocarbon gas indicator because gas generally decreases Poisson’s ratio and often increases amplitude with incident angle/offset. Other conditions can produce similar effects. The amplitude of an event is often plotted (Figure A-12) against sin2θ (or sin2x), where θ is incidence angle (and x is offset), and the slope (gradient) of a best-fit line is measured as the indicator:

$A(\theta) = A + B \sin^2\theta \$

Because measurements have to be made with prestack data, the noise level is usually large. The gradient is often determined by the ratios of amplitudes of large-offset to short-offset stacks. Also amplitude versus angle/offset. Class 1 reservoirs have higher impedance than the surrounding rocks, class 2 are those with very small, either positive or negative, impedance contrast, and class 3 are low-impedance reservoirs. In Tertiary clastic sections, class 1 reservoirs often yield dim spots, class 3 bright spots, and class 2 reservoirs are difficult to see unless they have appreciable increase of amplitude with offset. Class 4 are low-impedance reservoirs where the magnitude decreases with offset. See also Figure A-12, Castagna and Backus (1993)[1], Allen and Peddy (1993)[2], Rutherford and Williams (1989)[3], and Hilterman (2001)[4].

Amplitude variation with angle. The Rutherford-Williams classification (q.v.) based predominantly on normal reflectivity is also indicated. (From Castagna et al., 1998.)[5]

References

1. Castagna, J. P. and Backus, M. M., Eds., 1993, Offset-dependent reflectivity–Theory and practice of AVO analysis: Soc. Expl. Geophys.
2. Allen, J. L. and Peddy, C. P., 1993, Amplitude variation with offset Gulf Coast case studies: Soc. Expl. Geophys.
3. Rutherford, S. R. and R. H. Williams, 1989, Amplitude-versus-offset variations in gas sands: Geophysics, 54, 680–688.
4. Hilterman, F. J., 2001, Seismic amplitude interpretation. SEG-EAEG Distinguished Instructor Series #4: Soc. Expl. Geophys.
5. Castagna, J. P., Swan, H. W., and Foster, D. J., 1998, Framework for AVO gradient and intercept interpretation: Geophysics, 63, 948–956.