specific time horizon is extracted. Remembering the 2-term Shuey Approximation, the amplitude of each trace is plotted against sin^2 of its offset and the relationship becomes linear, as seen in the diagram. Using linear regression, a line of best fit can now be calculated that describes how the reflection amplitude varies with offset using just 2 parameters: the intersect, P, and the gradient, G. As per the Shuey approximation, the intersect P corresponds to R(0), the reflection amplitude at zero-offset, and the gradient G describes the behaviour at non-normal offset, a value known as the AVO gradient. Plotting P (or R(0)) against G for every time sample in every CMP gather produces an AVO crossplot and can be interpreted in a number of ways. Interpretation 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. Increasing AVO is usually present in oil-bearing sediments with at least 10% gas saturation, but is especially pronounced in porous, low-density gas-bearing sediments with little to no oil. Particularly important examples are those seen in Middle Tertiary gas sands of the coastal counties of Southeast Texas, 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 External links http://sepwww.stanford.edu/public/docs/sep73/carlos2/paper_html/node5.html Seismology measurement Petroleum geology