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Can P-wave AVO be quantitative?

CGG, Paris, France

Corresponding author: G. Cambois, gcambois@cgg.com

The first 20% of the full text of this article appears below.

At SEG's 1998 Annual Meeting in New Orleans, Ali Tura and I organized a workshop titled "Can P-wave AVO be quantitative or do we need multicomponent data?" At the end of the workshop, after an afternoon of talks and debates, we asked the audience to vote for either approach. Not surprisingly, the raised hands showed an almost perfect 50–50 split. This controversial issue is still not settled and, if this year's SEG convention is any indication, the debate is alive and well. The success of AVO and direct hydrocarbon indicators (DHIs) in finding oil and gas has been firmly established over the years and is well documented in numerous papers. However, the jury is still out on the quantitative aspects. I claim that achieving quantitative results from P-wave AVO may be possible, but it will take a lot more effort than is currently being done. Thus, my answer to the title question is a definite and resounding "maybe."

	Fundamentals of P-wave AVO

 
Under the small incidence angle approximation, Shuey showed in 1985 that the prestack behavior of P-wave reflection data follows the simple formula:

where is the angle of incidence. A and B, respectively called intercept and gradient, are related to elastic rock properties:

where R_P and R_S are respectively P- and S-reflectivity, I_P and I_S P- and S-impedance, V_P and V_S P- and S-velocity, and the density. Note that the second formula is only valid when the V_P/V_S ratio is 2. Using these two formulae, we can rewrite Shuey's equation as:

The first expression is the small angle approximation of Fatti's equation, and the second expression is equivalent to the formula derived by Verm and Hilterman in 1995. They called the second term on the right-hand-side Poisson's reflectivity (PR) because:

. . . [Full Text of this Article]