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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 using the amplitudes of prestack seismic data. The basics of the AVO method will be explained here using the two geological models shown in Figure 1:
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 using compressional waves, or P-waves, which move
through the earth by alternately, compressing and expanding the rocks in
their direction of propagation. However, there is a second type of There are several important differences between P- and S-waves:
Figures
3 and 4 show the P- Figures 3 and 4 also show how the amplitudes are created. First, we multiply the velocity times the density to get the P or S impedance. Then, we calculate the difference between the impedances divided by their sum, which gives us a reflection coefficient (reflectivity) at each interface. Finally, we superimpose the seismic response, or wavelet, on the reflection coefficients to get the synthetic seismic traces shown at the far right of both figures. The
P and S synthetics for the wet model are almost identical, but for the
gas model the S- 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-
The AVO Method
Figure
6, which shows a typical prestack seismic raypath, records that the
incident 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- Gradient = the P- To
illustrate this point, the amplitudes from a small portion of one of the
gathers in Figure 7 are shown in
Figure
8, with a straight line fit
superimposed. Notice that the top of the sand has a negative intercept
(a trough) and a negative gradient, and the base of the sand has a
positive intercept (peak) and a positive gradient. When we perform this
analysis at every sample, on every gather, we create two sections, or
volumes. The intercept section is similar to the conventional
stack--except that it represents a better estimate of the vertical
P- 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- These
displays are shown in Figures 9 and
10 for our real example. Notice that
the intercept (P- 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- References
Ostrander,
W.J., 1984, Plane- 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. |