AVO Slope and Intercept Attributes
Three linear approximations of Zoeppritz’s equation (Fig. 6.A.1) are commonly applied in AVO modeling. They have similar if not identical information content. However, the arrangement of terms in each equation emphasizes different AVO attributes as a function of incident angle. By making a few approximations, the interrelationship of the three equations is easy to visualize, and interpretational insight is gained.
The Shuey equation in Fig. 6.A.1 was discussed earlier. In 1987 Smith and Gidlow (and Gidlow et al, in 1992) introduced the linear-approximation equation shown in the middle of the figure. With this arrangement, Smith and Gidlow proposed a new attribute called the fluid factor that enhanced the visibility of hydrocarbon-saturated rocks. The lower equation is the more conventional one seen in the literature (Wiggins et al., 1983).
If the rock properties in Fig. 6.A.1 are replaced with the normal-incident reflection coefficient NIP, the variable B, and the last term in each equation is designated as a higher-order (H.O.) term, then the equations in Fig. 6.A.2 result. The similarity between the equations now becomes more obvious. The first term of each equation contains the normal-incident reflectivity modified by a different power of cosine-squared. The second term contains rock properties identified as the B reflectivities, which are modified by sine-squared. It should be noted that, unlike the other two, Shuey’s equation already has the assumption that background α/β = 2. This was necessary to obtain the cosine-squared term.
Figures & Tables
During the last 30 years, seismic interpreters have routinely applied bright spot and AVO technology for recognizing prospects and predicting lithology. New amplitude attributes were added to this technology as new exploration problems were defined. R&D continues in the field of amplitude interpretation, especially when E&P costs escalate as more severe environments are explored, such as the ultra-deepwater plays. With the high interest in reducing exploration risk, this course addresses the methodology of an amplitude interpretation and the subsequent benefits and limitations that one can expect in various rock-property settings. This book, originally produced for use with the fourth SEG∕EAGE Distinguished Instructor Short Course, begins with a review of relationships between rock properties and geophysical observations. Practical problems illustrate the assumptions and limitations of commonly used empirical transforms, and procedures for conducting and verifying fluid-substitution techniques are presented. The book identifies components of the seismic response best suited for differentiating pore fluid from lithologic effects. Field examples emphasize what combination of seismic signatures should be expected for different rock-property environments. To help select the best seismic attribute for calibrating amplitude to rock properties, rules of thumb are provided for predicting AVO responses and interpreting lithology from observed responses. A case history is also provided. The last part examines the numerous amplitude attributes that can be extracted from seismic data to quantify an interpretation. Benefits and limitations of these attributes in soft- to hard-rock environments are discussed with model data and in case histories.