During ODP Leg 165, sonic and density logs were collected. The depth-corrected logs were then obtained from the Borehole Research Group at the Lamont-Doherty Earth Observatory. After parsing and reformatting the logs, each log was examined individually, looking for obviously erroneous values in velocity or density that would create artificial acoustic impedance contrasts and could result in the creation of false reflections on the synthetic seismograms. The far-spaced sonic log was used at Sites 1000 and 998, but the near-spaced sonic had to be used at Site 1001 because of the poor quality of the far-spaced sonic log. Density logs of reasonably good quality were available for all three sites.
In general, the downhole sonic and density log measurements matched the values obtained from physical properties core measurements (Fig. 2, Fig. 3, Fig. 4, Fig. 5, Fig. 6.). The only major variance was in the Site 1001 sonic measurements (Fig. 7). The measurements recorded in this log were all significantly lower than those measured in physical properties. The trend of the log values does mimic the general trend of the physical properties measurements. The sonic log could not be used with the physical properties data because the gap between the two data sets would create an artificial impedance contrast at the data merge point. To obtain a proper merge of the physical properties and sonic log data, the physical properties data set needed to be divided by a constant factor of 1.125. The resultant data shows significantly better agreement between the sonic log data and the physical properties velocity data (Fig. 8). Thus, modified physical properties data were used as part of the velocity profile in creating the synthetic seismogram. Synthetic seismograms created using this velocity log correlate well with the seismic data over the site.
Because of the possibility that the physical properties measurements yielded more accurate values than the sonic log measurements, a second velocity profile was created in which the sonic log values were multiplied by 1.125 rather than dividing the physical properties data by 1.125. Synthetic seismograms were also created using this second velocity profile as well. This second synthetic seismogram did not tie well with the SCS profile at Site 1001, and resulted in synthetic horizons that appear significantly shallower in the synthetic than in the seismic data. A probable reason for the mistie between the synthetic seismogram and the seismic data is that the velocities used in this time-depth conversion (standardized to shipboard velocity measurements) were too high. The mistie between the synthetic seismogram and the seismic data using this method suggests that the physical properties velocity measurements for this site are not representative of in situ conditions and that the sonic log measurements should be used in constructing the synthetics.
The discrepancy between the laboratory velocity measurements and sonic log measurements at Site 1001 is problematic. Previous studies have shown that shipboard laboratory velocity measurements are often lower than sonic log measurements because of removal of overburden pressure (Hamilton, 1979; Fulthorpe et al., 1989; Urmos et al., 1993). It is unclear exactly why the laboratory velocity measurements would be higher than the in situ sonic log measurements. One possible reason for laboratory measurements being higher than log measurements could be the result of a sampling bias. In some cases, the laboratory velocity measurements were taken on core intervals having more consolidated sediment. This sampling bias favoring measurements of more consolidated sediment may occur when shipboard scientists attempt to avoid sampling a core interval that appears to be contaminated by mudcake. The more consolidated intervals may not be representative of the gross lithology and may cause the laboratory velocity measurements to be higher than those of the actual sedimentary section. The sonic log measurements are made through transducers and receivers separated by 0.91-3.66 m (Sigurdsson, Leckie, Acton, et al., 1997). Measurements made over this distance may more accurately represent the gross lithology and in situ conditions.
Downhole log measurements are not available for the top and bottom portions of the hole. To compensate for this, velocity and density profiles were created in which core-derived velocity and density measurements are spliced to the tops and bottoms of the sonic and density logs to construct the profiles. Synthetic seismograms are created from these velocity and density profiles that extend from the seafloor to near the base of the hole. The edited velocity and density profiles used for creating the synthetic seismograms are shown in Figure 9, Figure 10, Figure 11, Figure 12, Figure 13, and Figure 14.
Generation of the synthetic seismograms was performed using the Landmark "SynTool" module. In creating a synthetic seismogram, SynTool permits the interpreter to tie time data (the seismic data) to depth data (the well data) by integrating over the velocity profile. An impedance log and reflection coefficient are generated from the velocity and density profiles. The reflection coefficients are convolved with a seismic wavelet to produce a synthetic seismic trace. A band-pass filter (25-50-200-400 Hz) and 200-ms AGC are applied to the synthetic trace to mimic the processing of the SCS data. In this case, the seismic wavelet is obtained using a wavelet extraction from SCS data in each study area having a "clean" seafloor reflection. The synthetic seismograms are sampled at 1 ms to match the sample rate of the SCS data. The synthetic seismogram is then compared with the actual seismic traces at the drill site. The trace at the drill site was compared with adjacent traces to assure that it was representative of that part of the seismic section. Figure 15, Figure 16, and Figure 17 illustrate the relationship between the impedance logs, reflection coefficients, SCS traces, and synthetic traces for Sites 998, 1000, and 1001.