Origin of seismic reflectors within carbonate-rich sediments from northeastern Australian margin

We use continuous velocity records from 12 sites and continuous density records from 9 sites from Ocean Drilling Program (ODP) Leg 133 to examine both direct and indirect controls on seismic reflectors and depth-to-time conversion for the northeastern Australian margin. In these carbonate-rich sediments, the character of both velocity and density appears to be responding primarily to porosity variations and indirectly to lithologic or diagenetic variations. However, we find no consistent empirical relationship between velocity and porosity because of intrasite and intersite variations in style of cementation. Impedance, the product of velocity and density, is dominated by porosity change throughout the depth range studied, but the mechanism of this control changes: porosity influence on density is an important impedance control in the top 100 meters below seafloor (mbsf). while porosity influence on velocity is most important to deeper impedance variations. This difference arises from the relative insensitivity of velocity to porosity change at the high porosities of the uppermost 50 to 100 mbsf. Because velocity dominates most impedance variations and because velocity and density are locally well correlated, density can be omitted entirely from the calculation of most ODP synthetic seismograms without adverse effects. Depth-to-lime conversion can be based either on matching a synthetic seismogram to the seismic section or on a plot of two-way time vs. depth. Synthetic seismogram character is sensitive not only to the character of velocity variations caused by changes in porosity and diagenesis, but also to wavelet uncertainties and impedance interference patterns. Plots of two-way time vs. depth for these carbonate-dominated sediments are remarkably similar down to 250 mbsf and moderately predictable down to at least 450 mbsf.