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:
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), Allen and Peddy (1993), Rutherford and Williams (1989), and Hilterman (2001).