ACOUSTIC AND ELASTIC PROPERTIES OF CALCAREOUS SEDIMENTS ACROSS A SILICEOUS DIAGENETIC FRONT ON THE EASTERN US CONTINENTAL-SLOPE

The Ocean Drilling Program drilled Hole 904A on the upper continental slope offshore New Jersey through 230 m of Eocene chalks and an opal-A /opal-CT diagenetic front. In addition to a suite of standard logging tools, a dipole sonic tool was deployed to determine the in situ shear sonic velocity and the elastic properties of these slow, highly porou s, sediments. While porosity decreases by 20% across the diagenetic fr ont, the compressional velocity and the density are observed to increa se by 30%, the shear sonic velocity and the bulk modulus by about 60%, and the shear modulus by almost 300%. It is shown that shear and comp ressional velocities are both controlled by porosity and consolidation , and that existing models for the prediction of elastic properties an d shear velocity from standard logs are still valid through the diagen etic front. The strong effect of diagenesis on rigidity makes the shea r velocity log a highly sensitive indicator of pore shape and pore fil ling transformations.