We used densities and compressional wave velocity from index properties and MST measurements (Fig. F83) to synthesize seismograms at Site 1138, as no downhole logs were collected. A detailed comparison of velocities from samples and logs is given in "Downhole Measurements" in the "Site 1137" chapter. Based on the coherence analysis of velocity data from Site 1137 (Fig. F86 in the "Site 1137" chapter), we filtered the sample velocities from Site 1138 using a filter width of 5 m. The data were resampled simultaneously with filtering at an increment of 2.5 m. A more closely spaced sampling rate results in the inclusion of noise in the filtered profiles. We used a robust mode filter (i.e., a maximum likelihood probability estimator) that calculates the mode within the given data window. In addition, the median of the filtered data set was computed during filtering, and outliers, whose values exceed 2.5 times the L1 scale, were replaced with the median, as described in "Downhole Measurements" in the "Site 1137" chapter.
Velocity data show little scatter within nannofossil ooze in Units I and II (Fig. F83). Velocities in the underlying sedimentary units show much larger variations, including a well-resolved inversion from 475-505 mbsf, corresponding to the lower part of Subunit IIIA (i.e., foraminifer-bearing nannofossil chalk) and its transition to Subunit IIIB (foraminifer-bearing chalk) (Fig. F83). Six other velocity inversions are present at 625-660, 665-670, 710-720, 780-785, 792.5-795, and 810-835 mbsf (Fig. F83), all of which result in reflections that can be linked to the MCS reflection data (Figs. F84, F85).
GRAPE and discrete sample densities agree well at depths above ~290 mbsf, except for one instance at 118 mbsf where GRAPE densities are substantially higher than a calculated sample density (see "Physical Properties"). At greater depths, all GRAPE density measurements are lower than those from discrete samples. All major velocity anomalies are reflected in density anomalies based on discrete samples, illustrating a good agreement between the two profiles.
We synthesized seismograms for Site 1138 (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 (TWT), and we 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. Reflection coefficients with and without multiples and transmission losses show distinct differences at this site. The upper Miocene-Pliocene section shows three interbed multiple reflections (MUL1-MUL3) based on reflection coefficients including multiples (red trace) (Fig. F84F). In the lower part of the section, amplitudes are reduced by up to a factor of 2 (red trace) (Fig. F84F), if transmission losses within the basement are accounted for, compared with the synthetic seismogram excluding multiples and transmission losses (black trace) (Fig. F84F).
We find substantial differences between the two synthetic seismograms shown in Figure F84F (Figs. F84F, F85A, F85B). Both synthetic traces match the MCS data, except for those reflections resulting from multiples. The MCS data show three reflections in the upper Miocene to upper Pleistocene part of the section, which are not present on the synthetic trace (Fig. F85A); however, these emerge as prominent reflections if we include interbed multiples (MUL1-MUL3) (Fig. F85B). This leaves little doubt that the three corresponding MCS reflections are products of positive interference of interbed multiples.
The Miocene section shows five distinct reflections labeled M1-M5. M1, M2, M4, and M5 are distinct reflections in both synthetic seismograms (Figs. F84, F85A, F85B). However, M3, the most prominent Miocene reflection in the MCS data, correlates with a broad low-amplitude reflection in the synthetic seismogram excluding multiples, whereas the alternative synthetic trace, including multiples, shows a distinct short-wavelength, high-amplitude reflection (Fig. F85B). This demonstrates the potential importance of identifying and including multiples when trying to determine the origin of MCS reflections. Reflections M4 and M2 correspond roughly to the boundaries between Unit I and Subunit IIA, and Subunits IIA and IIB, respectively. M5 is a product of density variations within Unit I (Fig. F84).
Five late Maastrichtian to late Oligocene reflections (labeled MO1-MO5) result largely from velocity variations within Subunit IIIA of nannofossil chalk. The largest amplitude reflection in this section, both in the synthetic seismograms and in the MCS data, is MO4. It is associated with an increase in both velocity and density within Subunit IIIA (Figs. F83, F84). MO4 shows no obvious link with a change in lithology and, thus, may represent a diagenetic effect. MO3 cannot be linked to any MCS reflection, and both MO1 and MO2 correspond to a broad MCS reflection at 2.05 s TWT (Fig. F85A, F85B).
Two reflections labeled CM1 and CM2 from the mid-Campanian-upper Maastrichtian section (Subunit IIIB) in the synthetic seismogram may correspond to low-amplitude MCS reflections between 2.1 and 2.15 s TWT. We correlate the prominent MCS reflection labeled C to the upper part of the Cenomanian-Turonian-mid-Campanian Unit IV (Fig. F84). Reflection C is caused by the velocity increase from foraminifer-bearing chalk (Subunit IIIB) to chalk interbedded with claystone in Unit IV. Synthetic reflection C is deeper than the equivalent MCS reflection (Fig. F85), indicating cumulative errors in transit time summation based on incomplete core data. Reflection TS (Turonian-Santonian) marks the boundary between Units IV and V, accompanied by a marked increase in velocity from chalk/claystone to glauconite-bearing sandstone/claystone. Similar lithologic units, found at other sites overlying acoustic basement (e.g., 1135 and 1137), typically result in large-amplitude reflections.
The basement reflection B3 constitutes the most prominent reflection of the entire section on the synthetic and can be tied to the MCS data unambiguously (Fig. F85A, F85B). However, its relative amplitude in the synthetic trace scales more realistically if transmission losses are considered (Fig. F85B). The large negative peak underlying the positive basement reflection is caused by a major velocity inversion at 720-740 mbsf (Fig. F83). Reflection B2 is associated with the top of relatively unweathered basalt flows, and B1 is caused by a velocity inversion resulting from the transition from pahoehoe basalt flows to underlying aa flows and transitional/rubbly units at ~795 mbsf (see "Igneous Petrology"). Reflections B1 and B2 interfere in the MCS data to form a broad reflection that cannot be clearly separated into two distinct reflections at Site 1138 (Fig. F85B). However, the distance between these two reflections increases southeast of Site 1138, where they diverge to form two separate MCS reflections. The basement units in the vicinity of Site 1138 show an apparent dip to the southwest on the MCS data. The MCS data indicate no faults; thus, this dip appears to represent the geometry of local basalt-flow emplacement.