Synthetic Seismograms at Site 866: Origin of Reflections and Implications for Recognizing the Limestone-Basalt Transition in Cretaceous Mid-Pacific Guyots

Abstract

Synthetic seismograms at Hole 866 were derived from sonic velocity and neutron density logs and compared to the lithology and seismic reflection data. In addition, logs of neutron density, neutron porosity, resistivity, gamma-ray, and discrete measurements were used to generate pseudo-velocity logs as input for synthetic seismograms to evaluate their potential as proxy for sonic velocity. The experiment has several implications for the study of seismic reflection profiles over Cretaceous Mid-Pacific guyots. High-amplitude reflections in the synthetic seismograms derived from log velocity and density are generated at lithologic boundaries, possibly related to changes in sea level or Oceanographic events, at diagenetic boundaries (dolomitization), and by interference processes. The generally poor correlation between the synthetic seismogram and reflection seismic profile may be related to quality of recording, the quality of the well logs, rapid lateral changes in lithology or diagenetic overprinting, or changing interference patterns. The precise cause remains unclear, but the synthetic seismograms suggest that it is difficult to identify reflections in conventional seismic reflection profiles that are related to changes in sea level or Oceanographic events and to correlate these to other guyots. In addition, as the impedance contrasts between limestone and basalt in these guyots is smaller, or equal to, the impedance contrasts within the limestone succession, it is nearly impossible to identify a basement reflection based on amplitudes alone. Synthetic seismograms calculated from sonic velocity, neutron density, neutron porosity, and resistivity logs produce similar results. Gamma-ray data are poorly correlated to impedance because the clay-content is not the primary source of gamma-ray activity. Resistivity, however, is an unexpectedly good proxy for impedance. Density shows a useful correlation because Gardner's equation works well for pure limestone, whereas Wyllie's equation underestimates velocity.