A comprehensive theory of the elastic behaviour of cracked and porous solids has been derived by O'Connell and Budiansky. A summary has been given of those portions of the theory that are useful in the interpretation of seismic velocities, measured both in the laboratory and in the field , in ternis of crack or fracture densities and fluid content. Numerical estimates of these crack parameters and the associated errors have been derived from both field and laboratory data. The results emphasize the importance of being able to measure reliable S wave velocities. The relevance of the theory to the study of potential radioactive waste disposal sites has been discussed, and a strategy for future studies has been described.

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