Amplitude variations with offset (AVO)

AVO has been a prominent technique in detecting hydrocarbons and reducing drilling risk. AVO can detect hydrocarbons because AVO shows the variation of the amplitude of the offset, which represents the amplitude of the wave energy as it passes through the layer which is influenced by the parameters of the speed and density of the coating, so that the density of the layer can be analyzed by analyzing the reflection coefficient. AVO means that amplitude change with offset caused by lithology of fluid. AVO is also known as AVA (amplitude variation with angle) because this phenomenon is based on the relationship between the reflection coefficient and the angle of incidence. But since the angle of incidence affecting the offset and the offset itself can be varied in order to change the angle of incidence (fig.1), so it is commonly known as AVO.

The incident angle is directly proportional to the offset (www.wikiwand.com)

Variation of reflection coefficient is the primary thing in analyzing AVO. It starts when the P-Waves hit the interface, if it is normal incidence (fig.2), the coefficient reflections will be the difference of acoustic impedance (product of density and P velocity) over the sum.