Amplitude versus offset

In geophysics, amplitude versus offset (AVO) or amplitude variation with offset is a variation in seismic reflection amplitude with change in distance between shotpoint and receiver. It is also referred as AVA (amplitude variation with angle). As AVO studies are being done on CMP data, the offset increases with the angle.

An AVO anomaly is most commonly expressed as increasing (rising) AVO in a sedimentary section, 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 a lower rate than the surrounding reflective events.

Applications in the oil and gas industry

The most important application of AVO is the detection of hydrocarbon reservoirs. Rising AVO is typically pronounced in oil-bearing sediments, and even more so in gas-bearing sediments. Particularly important examples are those seen in deepwater turbidite sands such as the Late Tertiary deltaic sediments in the Gulf of Mexico (especially during the 1980s-1990s), West Africa, and other major deltas around the world. Most 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.

AVO is not fail-safe

An important caveat is that the existence of abnormally rising or falling amplitudes can sometimes be caused by other factors, such as alternative lithologies and residual hydrocarbons in a breached gas column. Not all oil and gas fields are associated with an obvious AVO anomaly (e.g. most of the oil found in the Gulf of Mexico in the last decade), and AVO analysis is by no means a panacea for gas and oil exploration.

References

* [http://www.glossary.oilfield.slb.com/Display.cfm?Term=amplitude%20variation%20with%20offset Schlumberger Oilfield Glossary]
*http://sepwww.stanford.edu/public/docs/sep73/carlos2/paper_html/node5.html