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 against sin²ß or sin²x (Figura), where ß is incidence angle and x is offset, and the slope gradient of a best-fit line is measured as the indicator:
A(ß)= A+B sin²ß
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 largeoffset to short-offset stacks. Also amplitude versus angleÕoffset. 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.