Seismic reflection profiles across Site 1096, collected by the Osservatorio Geofisico Sperimentale of Trieste (see "Seismic Stratigraphy" in the "Explanatory Notes" chapter; also see "Appendix" and Fig. AF1, both in the "Leg 178 Summary" chapter), have been compared with data from Site 1096 to establish the seismic stratigraphy described below. Physical properties measurements from cores and data from downhole logging facilitated the construction of a synthetic seismogram for comparison with the seismic section.
Bulk densities for the formation were derived from the MST GRAPE measurements (2-cm separation downcore), index properties samples (averaging 3.7-m separation), and lithodensity logging (HLDS, spatial resolution ~15 cm) for the depth interval 360 to 540 mbsf (Fig. F55). The index properties measurements and the MST data are in better agreement than at Site 1095 (Fig. F55). However, above 174 mbsf, the index properties density is slightly lower than the MST density; below 174 mbsf, it is an average of 0.04 g/cm3 higher. This can be explained by the final change from the APC to the XCB coring method at this depth. Two major negative MST density excursions at 200 and 250 mbsf are not represented within the index properties data. Zones of core disturbance at these depth intervals are probably responsible for this phenomenon. The limited available in situ densities from the downhole logging are within the range of the MST and index properties data. For a first modeling approach, only the index properties densities have been used.
Two differently derived velocities are available. The MST P-wave logger provided continuous data (4-cm spatial resolution) down to 125 mbsf (within the APC-cored part of the hole) but no data between 125 and 580 mbsf. Single Hamilton Frame (PWS3) measurements (one every section) provide high-quality data from 40 to 580 mbsf. The SV longitudinal and transverse data sets (PWS1 and PWS2) for the upper 40 m are combined in the apparent absence of velocity anisotropy. A comparison of the "cleaned" MST data (see "Physical Properties" in the "Explanatory Notes" chapter) with the combined SV velocity measurements is shown in Figure F56. Most of the MST-derived P-wave velocities are higher than SV values. This may be a result of the influence of the core liner. For a first modeling approach, only the SV velocities have been used.
The digital far-field signal of the Generator Injector air gun and a seafloor reflection have been used for convolution with the reflectivity coefficient series (see "Seismic Stratigraphy" in the "Site 1095" chapter).
Both velocity data sets have been used to produce time-depth curves (Fig. F57). The SV and MST time-depth relationships are compared with the Carlson et al. (1986) standard time-depth relationship and with the polynomial regression of Site 1095 vertical seismic profile data. In spite of sedimentation rates about three times higher at Site 1096, there is a substantial similarity between the three data sets, which support a model in which the velocity-depth relationship is dominated by compaction. Nevertheless, a less-pronounced curvature of the time-depth curve of Site 1096 may reflect undercompaction (see also "Physical Properties").
The raw velocity and density data were reviewed and cleaned. Only four and one data points, respectively, were removed from the SV and index properties data sets. The generation of a complete synthetic seismic trace using index properties densities and SV velocities for Site 1096 is depicted in Figure F58. In general, there is a positive correlation between the density and velocity curves. The two data sets were adjusted to the same length and sample interval using a linear interpolation. After applying a 5-point moving average filter, the data were again subsampled with a 1.5-m spacing to provide better contrast between adjacent impedance values. Two unfiltered synthetic traces (one for the far-field wavelet and one for the seafloor reflection) are displayed in Figure F58.
All four calculated synthetic traces were interpolated to a 1-ms resolution (1000 Hz) and subsequently filtered using a high-order, zero-phase equi-ripple band-pass filter (pass band = 20-110 Hz; attenuation = 60-70 dB; filter order = 170-220). Five of these traces are shown together with 42 traces of the field seismic profile in Figures F59 and F60. A zero-phase Butterworth band-pass filter (pass band = 10-110 Hz) and a 150-ms automatic gain recovery window were applied to the sorted and stacked data set. Additionally, the total time window and the delay were reduced, and the 2-ms field data were interpolated to 1 ms to fit the time resolution of the synthetic trace. The optimal filter parameters were adjusted to values used during the actual processing of the seismic survey data. Different amounts of time-invariant gain were applied to equalize the overall amplitude appearance of the field and synthetic traces.
The synthetic and survey traces correlate only in the lower and middle parts. Correlation of the synthetic and the survey traces allows the time equivalent of the total penetration depth to be determined (700 ms TWT below seafloor). A unique zone of reflectors occurs at ~150 ms, which is not represented in the synthetic traces. At its base lie three closely spaced wiggles with positive amplitudes and a 50-ms transparent interval. Timewise, this section could be correlated with an upper BSR (Fig. F61) visible below the scarp slope of the drift to the southwest. However, this reflector cannot be associated with a gas hydrate stability zone (see "Physical Properties"). In Figure F61, the boundaries of the major seismostratigraphic units are correlated with the major lithostratigraphic units.
Seismic units defined at Site 1095 have been traced across the MCS profiles to Site 1096 in a preliminary way. Changes in the seismic character and thickness of the different units from the distal (Site 1095) to the proximal (Site 1096) parts of the sediment drift are noted below. Drilling at Site 1096 reached a depth of 607 mbsf and probably penetrated seismic Unit I and Subunits IIa, IIb, and IIc, previously defined at Site 1095.
In the MCS profiles, the top 80 m of seismic Unit I consists of parallel and subparallel high-amplitude and continuous reflectors (Fig. F61). The sub-bottom 3.5-kHz seismic profile, obtained during the site approach, penetrated the upper 60 m of seismic Unit I. This profile shows reflector packages with a marked cyclic variation of amplitude (Fig. F4). The lower part of seismic Unit I, as imaged in MCS profiles, consists of lower amplitude continuous reflectors that alternate with disrupted and transparent reflectors (Fig. F61). Although the overall external geometry of seismic Unit I at Site 1096 is of a sheet drape, changes in thickness occur in directions parallel and perpendicular to the margin. From the distal part of the drift at Site 1095 to the proximal part at Site 1096, there is an increase in thickness of this younger sequence of ~50 m. In a direction parallel to the continental margin, thickness increases from 100 m at the crest of the drift to 150 m at the base of the gentler flank.
At Site 1096, it seems likely that only three of the five seismic subunits defined at Site 1095 were drilled. Subunit IIa (110-316 mbsf) is characterized by parallel and subparallel low-amplitude to transparent reflectors (Fig. F61). The acoustic character corresponds to low-amplitude traces in the synthetic seismogram (Fig. F59). Thickness of Subunit IIa increases considerably from Site 1095 to Site 1096. In a direction parallel to the margin, thickness decreases toward the base of the gentler flank of the drift where reflectors are partly truncated by channel cutting. The base of Subunit IIa is marked by a reflector with moderate amplitude, which correlates with a higher amplitude reflector of the synthetic seismogram (Fig. F59). Subunit IIb (316-519 mbsf) consists of a package of high-amplitude and continuous reflectors that correspond to a series of high-amplitude reflectors in the synthetic seismogram (Fig. F59). The thickness of Subunit IIb appears to change from 40 m at Site 1095 to ~200 m at Site 1096. In a direction parallel to the margin, Subunit IIb increases in thickness to the southwest, where reflectors diverge below the crest of the drift. Subunit IIc (519- >607 mbsf) is the lowest seismic unit penetrated at this site. This subunit is bounded by two high-amplitude reflectors. Reflectors within this subunit are rare at the site but are present below the crest of the drift where the subunit thickens (Fig. F61).
Several general observations can be made from the seismic profile across Site 1096.
Seismic Unit I comprises lithostratigraphic Unit I and the upper half of lithostratigraphic Unit II (see "Lithostratigraphy"). Lithostratigraphic Unit I (0-33 mbsf) is characterized by a well-defined alternation of biogenic-rich and terrigenous horizons. This cyclic pattern can be recognized in the sub-bottom 3.5 kHz profile (Fig. F4). Lithostratigraphic Unit II (33-172 mbsf) is characterized by a repetitive succession of laminated and massive facies. Seismic Subunits IIa, IIb, and IIc include the lower half of lithostratigraphic Unit II and the whole of lithostratigraphic Unit III. Lithostratigraphic Unit III (172 to 607.7 mbsf) is an alternation of laminated turbidite contourite facies and bioturbated hemipelagic facies. A possible BSR at 170 ms TWT (~125 mbsf) near shotpoint 1710 on profile IT92-109 (Fig. F61) cannot be attributed to the base of the hydrate stability zone, which lies at ~340 m (see "Physical Properties").
The increase in thickness of units between Site 1095 and Site 1096 indicates an expanded section in Site 1096, in accord with the higher sedimentation rates (see "Sedimentation Rates"). External geometry (mainly sheet drape) and regular reflection patterns of the seismic units at Site 1096 suggest sedimentation from turbidity currents, weak bottom currents, and hemipelagic deposition. The observed change in thickness of seismic Unit I toward the northeast channel is compatible with its being a major sediment source during a "drift maintenance" stage (Rebesco et al., 1996, 1997). The increased thickness toward the southwest of sediments below Unit IIb is a feature of the earlier "drift growth" stage. The considerable thickening of sedimentary units from Site 1095 to 1096 supports the view that the terrigenous component originates at the nearby continental margin.
The ages assigned to the seismic units recognized at Site 1096 (Table T35; Fig. F61) are based on preliminary paleomagnetic data from drilled cores (see "Paleomagnetism"). Seismic Unit I is Pleistocene in age (0-1.4 Ma). The rest of the drilled sediments are early Pleistocene and Pliocene in age (1.4-4.7 Ma).