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AVO and Poisson's ratio

University of Calgary, Calgary, Alberta, Canada

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

It remains disturbing that AVO analysts still cling to the term Poisson's ratio (see e.g., Dutta, TLE February 2002).

In 1990 (TLE) Leon Thomsen pointed out that it is both unnecessary and confusing to approach the V_P/V_S ratio (my PS ratio) by way of Poisson's ratio. Gretener (TLE, 1994) showed that it is also erroneous resulting in values that from a mechanical perspective are unacceptable, if not ludicrous. Domenico's (TLE, 1995) and Thomsen's (TLE, 1996) replies remain unconvincing and have never been challenged.

The dispute centers on the so-called dynamic determination of Poisson's ratio (v_q)

(1)

The original static definition of Poisson's ratio as given by Poisson (v_s) is:

(2)

where: _h lateral strain, _z axial strain in a specimen subjected to uniaxial stress. For linearly elastic materials, such as metals, we have:

(3)

Unfortunately rocks are elastic but not linearly elastic and thus we have:

(4)

Domenico's value of 0.1 for a dry sandpack (GEOPHYSICS, 1976, 1977 and TLE, 1995) and Gregory's values (GEOPHYSICS, 1976) reaching into the negative realm (sic) make no sense. The reason: A fundamental assumption of equation (1), linear elasticity, is not fulfilled. In Poisson's time, attention was focused on metals which are linearly elastic.

Poisson's ratio has been in the AVO literature ever since the Muskat and Meres paper (GEOPHYSICS, 1940). To abandon the term is not devastating to AVO analysis as stated by Domenico (TLE, 1995). AVO analysts merely use it to define the PS ratio. The name . . . [Full Text of this Article]