In geophysics Amplitude Versus Offset (AVO) is a variation in seismic reflection amplitude with change in distance between the shotpoint and receiver.
An AVO anomaly is most commonly expressed as increasing (rising) AVO in a sedimentary section. This is often where the hydrocarbon reservoir is "softer" (lower acoustic impedance) than the surrounding shales. Typically amplitude decreases (falls) with offset due to geometrical spreading, attenuation and other factors. An AVO anomaly can also include examples where amplitude with offset falls at lower rates than the surrounding reflective events.
The most important application of AVO is the detection of hydrocarbon accumulations. Rising AVO is typically more pronounced in oil-bearing sediments (and more so in gas-bearing sediments). Particularly important examples are those seen in deepwater turbidite sands and other major clastic deltas around the world. Most hyrdocarbon filled sedimentary traps are tried to be vizualised and detected with AVO. Almost all major companies use AVO routinely as a tool to "de-risk" exploration targets and to better define the extent and the composition of existing hydrocarbon reservoirs.
An important thing to remember is that the existence of abnormal (rising or falling) amplitude anomalies can sometimes be caused by other factors, such as alternative lithologies and residual hydrocarbons in a breached gas column. Modeling of the petrophysical properties and good understanding of the sedimentary succession is paramount for succesful hydrocarbon detection using AVO. Not all oil and gas fields are associated with an obvious AVO anomaly and AVO analysis is by no means a failsafe method for gas and oil exploration.
Hope this made it clear!