AVO & AVAZ
AVOAVO describes the effect of variation in reflection amplitude with offset as a result of differences in rock properties either side of an interface. The relative change in reflection coefficient is particularly significant when Poisson's ratio differs greatly either side of an interface. It is this phenomenon which allows AVO effects to be used in the detection of gas.
For a complete study of AVO character, details of the rock properties present are ideally required. If a well log is available then parameters such as compressional velocity and bulk density can be derived. Forward modelling can be done and synthetic gathers derived. These can then be compared to real gathers at the well location.
Spectrum's batch AVO programs offer a cost-effective method of examining large volumes of data for AVO anomalies. They use a qualitative approach designed to highlight those areas where further in-depth studies such as modelling or inversion may be appropriate.
Using the robust method of Walden (1990), Spectrum offers AVOQC which uses a statistical method to calculate the intercept and gradient values for a straight line fit of amplitude against angle of incidence. The straight line assumption is tested and where this breaks down, the gradient values are edited, giving the interpreter more confidence in the AVO results. These are then presented as pseudo-seismic sections, allowing correlation with conventional data.
Data can be stacked by corridors, offset ranges or incidence angle ranges – this way the amplitude anomalies are enhanced and can be studied in greater detail. An option is also available to transform normal X-T gathers to incident angle (θ)-T gathers for further analysis.
These AVO programs can produce various AVO attribute volumes to aid the interpreter such as pseudo Poisson’s ratio, Pseudo S wave, Fluid Factor or various derivatives and combinations from intercept (I) and gradient (G) stacks such as I x G, I – G, Standard Deviation of I & G. A wide variety of cross plotting is also available.
References: Waldon, A. T. 1991, Making AVO sections more robust. Geophysical Prospecting 39 916-942
AVAZSpectrum offers a practical method of evaluating true AVAZ (AVO as a function of azimuth) effects in 3D seismic.
Seismic data may be recorded with different offsets and azimuths at each midpoint, and there are geologic factors which cause the AVO response to vary with respect to azimuth. Conventionally extracted AVO attributes are incorrect if the azimuth dependencies are not considered. AVAZ behaviour is measured, together with the provision of a statistical significance attribute which adds a level of confidence in the analysis and interpretation of the results.
Spectrum’s software development group have developed software which measures amplitude variation with azimuth (AVAZ). This work was presented at the 1999 SEG conference and is published in the Expanded Abstracts. (Skoyles et al). If rock properties have azimuthal dependence then P wave reflection amplitude will vary with azimuth. 3D AVAZ is achieved by surface amplitude fitting to an azimuth gather analysis of intercept and gradient. If the AVO slope is the same at all azimuths then the surface fit is a cone with circular cross section. If the AVO slope varies with azimuth then the cone will have an elliptical cross section and the ratio of the major to minor axis is a measure of the variation of AVO (eccentricity). The phase is the direction of maximum AVO. These properties can be used to get an idea of azimuthal anisotropy in the reservoir.
The above method can be performed on Spectrum’s PSTM outputs as our software has the capability of outputting independently migrated azimuth sectors for subsequent recombination and/or analysis.
References: Skolyes D., DeWildt J., Erickson J.E., Innemee H., Monk D, (1999) Azimuthally dependent Amplitude vs Offset in 3D seismic: An automated statistical measurement and case study.
