In the first section of this book, the philosophy of “Geophysical data can’t be interpreted without knowing the answer” was introduced. If one doesn’t have past experience or rules-of-thumb, the most sophisticated interpretation workstation will be of no help. A short story will illustrate the goals of this section.
You’re exploring a new area. You find a bright spot and map it. The amplitude map does not conform to the structure. You don’t want to condemn the prospect yet. You question the reason for the bright spot. Is it a pore-fluid effect or lithologic change? You examine available well-log curves to determine the local rock types and their properties. What curves have you selected? Most likely, you have chosen the sonic curve to determine if there are large velocity contrasts. You find one. Is the contrast at approximately the right depth? Is the high-velocity bed thick enough to yield the bright spot found on the seismic?
In this little scenario, most interpreters wouldn’t stop to compute the normal-incident reflection coefficient. For now, knowing the velocity contrast is sufficient … there is no need to include density and normalize by the sum of the acoustic impedances. Having found the depth on the log, equating the depth in meters to the time in milliseconds is adequate. There is no need to waste time with check-shot corrections. Quickly, rules-of-thumb have been applied and new avenues for explaining the bright spot are investigated.
Can this concept of rules-of thumb be expanded to AVO interpretation? Or is it necessary to rely upon time-consuming petrophysical analyses and Zoeppritz solutions? Hopefully, answers to these questions will be addressed in this section.