R.A. Edwards2


Synthetic seismograms have been derived from downhole logging and physical properties measurements at Sites 959, 960, and 962. Logging data were not available at Site 961. The synthetic seismograms were calculated by convolving the source wavelet with a reflectivity series determined from the acoustic impedance. Seismic reflectors arise where changes in acoustic impedance are encountered because of increases or decreases in either, or both, sonic velocity and bulk density. Where sonic velocity or bulk density logs were not available, these parameters were calculated from resistivity or porosity logs. The agreement between calculated and observed velocities and bulk densities was good considering that large generalizations were made in assuming that the matrix velocity and density of the sediments remained constant throughout the section logged. The calculated synthetic seismograms generally show a good fit to the observed seismic data recorded over the sites. The best results were achieved for Hole 959D, where several individual reflectors could be determined. The most prominent reflector at Site 959, which separates seismically transparent claystones above the acoustic basement from more reflective facies above the reflector, was found to correlate with a decrease in clay content (increase in carbonate cement) at 760 meters below seafloor (mbsf) rather than a lithologic boundary identified from the cores. This reflector, within lithologic Subunit IIC, corresponds to a large increase in both sonic velocity and bulk density, and also represents the upper limit at which significant structural deformation is seen in the cores. The integration of different analyses (seismic, structural, and smear slide analyses) has helped to identify this significant boundary/event, which may otherwise have been missed from core descriptions alone.

Synthetic seismograms calculated for Holes 960A and 962D also modeled the observed seismic data fairly well. In these shallow holes no reflectors above acoustic basement could be identified. The acoustic basement reflection was found to represent at least two individual reflectors at both Sites 960 and 962, which could not be identified individually because of interference of the seismic waves.

The integration of core data to seismic data presented here will provide a good base for any further interpretation of the seismic stratigraphy over the Côte d’Ivoire-Ghana Transform Margin, and the Deep Ivorian Basin in particular.

1Mascle, J., Lohmann, G.P., and Moullade, M. (Eds.), 1998. Proc. ODP, Sci. Results, 159: College Station, TX (Ocean Drilling Program).
2Department of Earth Sciences, University of Cambridge, Bullard Laboratories, Madingley Road, Cambridge, CB3 0EZ, United Kingdom. edwards@esc.cam.ac.uk