Physical properties of sediments and basalts from the Argo and Gascoyne Abyssal Plains in the Indian Ocean

The Wyllie time average equation has, for many years, been universally applied to predict porosities from compressional wave velocities, or visa-versa. However, it has long been recognized that the Wyllie equation does not adequately describe the actual relationship between these two parameters, and there have been many attempts to improve upon it. These have included the use of a simplified Wood equation, the concept of acoustic formation factor, and a wide range of empirical relationships. In many cases these models have been derived by testing them against a set of data representing a relatively narrow range of porosity values and, similarly, the use of the Wyllie equation has often been justified by virtue of a pseudo-linear relationship over a narrow range of porosity values. During the Ocean Drilling Program - Leg 123 two sites were drilled in the deep Indian Ocean. Continuous coring at Site 765 recovered over 930m succession of soft Quaternary through Lower Cretaceous sediments and a further 271m of oceanic basement with relatively fresh, glass-bearing pillow lava and massive basalt. Soon after core recovery, measurements were made of: saturated bulk density, grain density, water content, porosity, and compressional wave velocity. The porosity ranged from 89%, close to the sea floor, to 1.6% for the dense basalts. This self consistent set of measurements made on fresh samples, with a wide range of values, has enabled some of the descriptive models to be tested more rigorously. Some of' the limitations of the time average equation were also recognised by Wyllie and his co-workers who amended the Wood emulsion equation to partially take account of the rigidity of the materials. Further modifications to this Wyllie-Wood equation have been shown here to not only describe the relationship between porosity and velocity more closely than the time average equation, but also more closely than some of the alternative proposals suggested by contemporaries of Wyllie and since. Indeed, bearing in mind the Wyl1ie-Wood equation was discussed in the sam~ paper the time average equation was first proposed, it is somewhat curious that the time average equation has been adhered to for so long. A semi-empirical acoustic impedance relationship has been developed which is shown to provide a more accurate porosity-velocity transform than has hitherto been possible using realistic material parameters.