We used densities and compressional wave velocity from index properties data and downhole logs (Fig. F49 ) to synthesize seismograms at Site 1140. A detailed comparison of velocities from samples and logs is given in the "Downhole Measurements" in the "Site 1137" chapter. Based on coherence analysis of velocity data from Site 1137 (Fig. F90 in the "Site 1137" chapter), we filtered the sample velocities and densities from Site 1140 using a 5-m-long filter. The data were resampled simultaneously every 2 m, and data gaps >5 m were linearly interpolated. We used a robust mode filter (i.e., a maximum likelihood probability estimator) that calculated the mode within the given data window, as described in "Seismic Stratigraphy" in the "Explanatory Notes" chapter.
Velocities from downhole logs and discrete samples agree well above 235 mbsf (Fig. F49). In the basement section, below 235 mbsf, discrete samples yield higher velocities than log values. The majority of the low-velocity sediments intercalated with pillow basalts, as documented in the FMS images (see "Downhole Measurements"), were not recovered, resulting in the observed bias in the discrete sample velocities. Within basement, low recovery hampers the spatial resolution of sample density data. High-frequency variations in density and velocity data within basement are fairly coherent and likely represent real downhole variations of physical properties, rather than noise.
The deployment of the WST at this site offered an opportunity to test the integrity of velocities from the downhole log and samples by comparing relationships between depth and two-way traveltime (TWT) derived from the latter two data sets and TWT derived from WST one-way traveltime (Fig. F50A; Table T15). The comparison illustrates that TWT from sonic velocities and the WST agree well. Above basement (~235 mbsf) TWT derived from sample velocities agree with the other two data sets as well. Within the basement, use of discrete sample velocities results in underestimating TWT because the discrete samples are biased toward higher values (Fig. F50B).
The depth-TWT relationship for Site 1139 juxtaposes TWT derived from the sonic velocity log and from discrete samples (Fig. F50). The two functions differ by >100 ms near the bottom of the hole. Comparison of data from Sites 1139 and 1140 emphasizes that (1) TWT based on high-quality sonic logs are accurate, (2) TWT based on velocities from discrete samples are fairly reliable if core recovery is consistently high, and (3) only the WST can resolve differences between transit times from logs and samples.
We used velocity and density data from physical properties measurements (0-85 and below 312 mbsf) and from downhole logs (85-312 mbsf) to synthesize seismograms for linking cores and logs with MCS reflection data. We constructed a composite TWT-depth array using TWT from the sonic log above 109 mbsf and WST data below this depth, which corresponds to the shallowest deployment depth of the WST.
We resampled the TWT array linearly using a sampling interval of 0.1 ms, resulting in oversampling to avoid aliasing. We then resampled velocities and densities using the resampled TWT array, obtained impedance from the product of velocity and density, and computed reflection coefficients from impedance contrasts (Fig. F51). We constructed synthetic seismograms with and without multiples and transmission losses (see "Seismic Stratigraphy" in the "Explanatory Notes" chapter). However, at this site we used a Ricker wavelet with a peak frequency of 30 Hz to synthesize seismograms, as the use of higher frequencies results in peaks that are not observed in the MCS data. Comparison of the two synthetic seismograms (Fig. F51F) shows that the phase of large-amplitude peaks is nearly identical. However, transmission losses reduce amplitudes in the basement section (Fig. F51F, red trace). The sedimentary section does not include any high-amplitude reflections caused by a lack of substantial impedance contrasts (Fig. F51C).
The synthetic seismogram, including transmission losses, matches the MCS data extremely well (Fig. F52). Igneous basement is represented by reflection B2, which corresponds to pillow basalts underlying dolomite (see Fig. F15). Reflection B1 represents the transition from the massive sheet flow of basement Unit 5 to pillow basalts of basement Unit 6 (Fig. F15).