We used densities and compressional wave velocity from index properties and MST measurements to create synthetic seismograms at Site 1136 because no downhole logs were collected. Neither density nor velocity data at this site appear to show much variation within the sedimentary or basement section, except for noise (Fig. F44). Densities determined from core samples and the GRAPE data agree well in the sedimentary section, but not in the basement, where the core liner was not entirely filled with core. Also, core recovery between 80 and 120 mbsf was extremely low. For synthesizing a seismogram, we used densities from discrete samples, as they display much less scattering than GRAPE densities. We used densities from GRAPE data above 315 mbsf and data from discrete samples below this depth. We filtered all density and velocity data identically (Fig. F44).
We synthesized seismograms for Site 1136 (see "Seismic Stratigraphy" in the "Explanatory Notes" chapter). We resampled both densities and velocities every 0.5 ms as a function of two-way traveltime and created profiles for impedance, reflection coefficients, and a seismic trace. The seismic trace is based on convolution with a Ricker wavelet with a peak frequency of 40 Hz. Only reflection coefficients without multiples and transmission losses were constructed for this site, given that its depth below seafloor does not exceed 160 m and that the data are fairly poor (Fig. F45).
The synthetic seismogram shows only two high-amplitude reflections below the seafloor (Fig. F45). The upper reflection (C) correlates with Unit III, dated as mid-Campanian, and the lower reflection (B) corresponds to the top of acoustic basement. In Figure F45, Reflection C is 5 m above acoustic basement, rather than at the basement contact. This is entirely an aliasing effect introduced by poor core recovery and subsequent interpolation. Both correlate with reflections in the MCS data (Fig. F46). However, a prominent reflection observed at ~40-50 ms below the seafloor in the MCS data is not reproduced by the synthetic seismogram. This reflection was apparently produced by the Flexichoc source used on the Marion Dufresne (Fig. F46).