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An AVO Primer
By
Brian Russell1
Search and Discovery Article #40051 (2002)
*Adapted for online presentation from an article by the same author in AAPG Explorer (June, 1999), entitled “AVO Adds Flavor to Seismic Soup.” Appreciation is expressed to the author and to M. Ray Thomasson, former Chairman of the AAPG Geophysical Integration Committee, and Larry Nation, AAPG Communications Director, for their support of this online version.
1Hampson-Russell Software Services Ltd., Calgary, Canada (www.hampson-russell.com; [email protected]); past president of the Society of Exploration Geophysicists.
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General StatementAVO, which stands for Amplitude Variations
with Offset--or, more simply, Amplitude Versus Offset--is a seismic
technique that looks for direct hydrocarbon indicators
Wells have been drilled into each sand. To
understand the AVO effects of these two models, we must first discuss
seismic waves and the recording of seismic data. Traditional seismic
data are recorded There are several important differences between P- and S-waves:
Figures
3 and 4 show the P-wave
Figures
3 and 4
also show how the amplitudes are created. First, we multiply the
The P and S synthetics for the wet model are almost identical, but for the gas model the S-wave synthetic is the reverse of the P-wave synthetic and has lower amplitudes. The high amplitude reflections seen on the P-wave response of the gas model are called "bright-spots," and can be effective in the Gulf Coast and other areas in the search for gas sands. Figure 5 shows such a bright-spot reflection from a shallow Cretaceous play in Alberta at 640 msec. However, there are other geological situations that create 'bright-spots," such as coal seams or hard streaks. From this discussion, it is obvious that the P-wave response does not reveal the presence of gas unambiguously, and it needs to be supplemented with an S-wave recording. Unfortunately, S-wave recording is not that common. This leads us to the AVO method, which allows us to derive a similar result without actually recording an S-wave section. Return to top.
The AVO Method
Figure
6, which shows a typical prestack seismic raypath, records that the
incident wave displays both compressional and shear effects, since it
strikes the interface at an angle a. The reflected wave thus contains
the effects of both P- and S-waves. Although the mathematics of this
process has been known since the nineteenth century, it was only very
recently that we have recognized it on our seismic data. Ostrander
(1984) showed that, for the simple model of
Figure
1b, the amplitudes on
a prestack gather would increase with offset. This is shown in
Figure
7,
in which the reflections from the Not all gas sands show increasing AVO effects, since the result is dependent on the nature of the acoustic impedance change. The different types of AVO anomalies have been classified as classes 1, 2 and 3 by Rutherford and Williams (1989} In the present paper we are looking at a Class 3 example, in which the impedance of the sand is lower than the encasing shale. If we measure the amplitude of each reflection amplitude as a function of offset, and plot them on a graph as a function of the sine of angle of incidence squared, we observe a straight line. For any line, the intercept and gradient can be measured. By linearizing the complicated mathematics behind the AVO technique, Richards and Frasier (1976) and Wiggins et al. (1986) gave us the following physical interpretation of the intercept and gradient: Intercept = the P-wave reflection amplitude. Gradient = the P-wave reflection amplitude minus twice the S-wave reflection amplitude. To
illustrate this point, the amplitudes from a small portion of one of the
There
are many ways of displaying this information. As well as displaying the
intercept and gradient on their own, it is common to display the
difference and sum of the intercept and gradient. From the above
explanation it is obvious that the difference, after scaling, is the
approximate S-wave reflectivity. The sum of the intercept and the
gradient can be shown to represent the approximate Poisson's ratio
change, where Poisson's ratio is related to the square of the P-wave to
S-wave These displays are shown in Figures 9 and 10 for our real example. Notice that the intercept (P-wave) shows a strong "bright-spot," whereas the pseudo-S-wave (intercept minus gradient) does not show a "bright-spot," indicating the presence of a gas sand. As one final example, let us consider an example of the AVO technique applied to 3-D data. Figure 11 shows the sum of intercept and gradient, or pseudo-Poisson's ratio computed over the top of a channel sand in Alberta. The negative values on this plot indicate the possible presence of gas in the channel sand. This tutorial has reviewed the basic principles behind the AVO technique. We have concentrated on a single type of anomaly, the Class 3, in which the acoustic impedance of the gas sand drops with respect to the encasing shales. For a discussion of other types of anomalies refer to the papers by Rutherford and Williams (1989), Ross and Kinman (1995). and Verm and Hilterman (1995). The key thing to remember about the AVO method is that the AVO gradient responds to both P- and S-wave reflections from an interface, and this behavior can be used to locate gas charged reservoirs. Applied to 3-D seismic data, the AVO technique gives us a robust and inexpensive method for identifying potential reservoirs and is a technique that adds an extra dimension to studies done only with stacked seismic data. ReferencesOstrander, W.J., 1984, Plane-wave reflection coefficients for gas sands at nonnormal angles of incidence: Geophysics, v. 49, p. 1637-1648. Richards, P.G., and Frasier, C.W., 1976, Scattering of eleastic waves from depth-dependent inhomogeneities: Geophysics, v. 41, p. 441-458. Ross, C.P., and Kinman, D.L., 1995, Nonbright-spot AVO; two examples: Geophysics, v. 60, p. 1398-1408. Rutherford, S.R., and Williams, R.H., 1989, Amplitude-versus-offset variations in gas sands: Geophysics, v. 54, p. 680-688. Verm, R., and Hilterman, F., 1995, Lithology, color-coded siesmic sections; the calibration of AVO crossplotting to rock properties: Leading Edge, v. 14, p. 847-853. Wiggins, W., Ng, P., and Manzur, A., 1986, The relation between the VSP-CDP transformation and VSP migration (abstract): SEG Abstracts, v. 1, p. 565-568. |