), and vertical compressional wave velocity (V) are related by the following expression:
In ideal circumstances, logging data provide the critical link between cores and seismic reflection profiles by measuring the in situ geophysical properties of borehole lithologies at a scale intermediate between the seismic and the core data. This linkage is essential because it allows detailed information obtained by core analysis to be extended to the entire seismic reflection data set. The seismic data can then provide constraints on the regional extent and nature of lithologic and age boundaries observed at the drill site.
Logging data could not be collected at Site 1276 because of poor hole conditions. In lieu of sonic and density downhole logs, laboratory measurements of velocity and density taken on discrete, representative samples from each core section were used to generate a time-depth relationship and to create synthetic seismograms to link the seismic reflection and core data. Excellent core recovery throughout Site 1276 allowed us to construct a robust time-depth relationship from 800 to 1730 mbsf. This made it possible to track our progress toward drilling targets identified on time-migrated seismic reflection sections and to correlate major lithologic units with the seismic data. However, to understand how geologic features that are encountered during drilling are manifested in seismic data as reflections, we took one additional step and generated synthetic seismograms.
Measured density and velocity vary systematically between primary lithologic units, particularly between Units 1 and 2 (Figs. F159, F162A, F162B). Because velocity and density contrasts exert primary control on the amplitudes of seismic reflections, we anticipated that the creation of synthetic seismograms from the laboratory data would facilitate correlation of lithologic units with reflections in the seismic data. In the upper 1000 m of Site 1276, reflections predicted in synthetic seismograms can be readily correlated with observed reflections in Study of Continental Rifting and Extension on the Eastern Canadian Shelf (SCREECH) line 2MCS and line 303, which cross Site 1276 (Fig. F170). However, predicted reflection patterns near the U reflection do not match observed patterns, suggesting that the velocity structure created by the alternating high and low velocities of interlayered igneous sills and sediments, respectively, might result in complex tuning effects in seismic reflection data. Further postcruise modeling will be required to understand the manner in which sills and sediments are manifested in the site survey seismic reflection dataset.
Shipboard physical property measurements were used as input into this procedure. As described in "Physical Properties" horizontal and vertical velocities were measured on representative sediment and rock samples (~8 cm3) from each section of core. We chose to use the vertical (z) component of velocity measurements to build a time-depth relationship because they most closely approximate the path of seismic waves recorded in multichannel seismic (MCS) reflection data sets.
Two types of density measurements were used to calculate synthetic seismograms; each of these has limitations for this application. These measurements are described below.
GRA bulk density data represent evenly spaced measurements of density obtained over the entirety of each core prior to the core being split. Although this procedure provides continuous density measurements, the lithified rocks recovered using RCB drilling typically fracture when they are recovered, leading to breaks in the core and thus to significant artificial variations in the density data. Additionally, GRA bulk density measurements are systematically low because recovered cores do not completely fill the core liner.
Bulk density (MAD) was also measured on discrete samples taken from every section of each core. Because the samples analyzed were small and hand selected, they are not influenced by the fractures that degrade the GRA density. However, these samples do not contain regularly sampled and relatively unbiased information on the distribution of lithologies (and corresponding changes in density and velocity) that would be encompassed in logging data sets. Furthermore, MAD density is obtained from the same samples used for velocity measurements, so any sampling biases that exist in the velocity data are also present in the density measurements.
The use of physical property data instead of traditional sonic and density logs requires several additional considerations to ensure that velocity and density measurements are representative of overall core properties. Some of these steps were conducted in the following analysis, and others will be addressed in postcruise work. First, velocity measured on samples from intervals described as "concretions" by sedimentologists was removed from the data set. This included eight measurements with vertical component velocity ranging from 2126 to 4625 m/s. The exclusion of these data points does not affect the time-depth relationship, which comprises 553 other data points (Fig. F171). However, their presence could affect synthetic seismograms, which are sensitive to small velocity perturbations.
Second, additional data were collected by the physical property group to characterize velocity variations over specific lithologic intervals likely to display important velocity characteristics (e.g., turbidites throughout the sedimentary section and the sediment/sill contact preserved in Section 210-1276A-87R-6). As described fully in "Physical Properties" measurements of x-direction (horizontal) velocity taken every 2-10 cm over several turbidites revealed that turbidites commonly display consistent velocity trends. Velocity tends to increase from the muddy turbidite tops downward, with maximum velocity occurring in well-cemented, fine-grained sandstones. Below, velocity decreases in the coarse turbidite base. This velocity distribution can be used to predict velocity surrounding routine measurements taken within turbidites.
Detailed velocity measurements made across the contact between the upper sill and the overlying sediments show that the change from sedimentary velocity (2300 m/s) to diabase velocity (~5500 m/s) occurs over a very short interval despite a broad zone of contact metamorphism, thus creating a sharp velocity contrast rather than a gradational boundary. Including a boundary where velocity increases from 2300 to 5500 m/s over tens of centimeters rather than over 3 m (spacing between routine physical property samples) yields a substantially higher reflection coefficient.
A third means of conditioning the velocity function was attempted to better represent velocity changes in cores. A velocity function was constructed using the average velocity of all samples measured for every combination of lithology and grain size in each major lithologic unit. Information on the lithology and grain size for each physical property sample taken was obtained by linking visual core descriptions to samples (see "Physical Properties"). These average velocities were inserted where each combination of lithology and grain size was documented in the visual core descriptions. The problem with this approach, which is well illustrated in Figure F162A and F162B, is that velocity measured on samples with the same lithologic and grain size classification often varies substantially, and averaging masks actual velocity changes.
Another important consideration in using laboratory velocity rather than logging velocity is the difference in velocity expected at depth vs. that at atmospheric pressures. Throughout the sedimentary section of Site 1276, a notable anisotropy in horizontal vs. vertical velocities was measured, where z-direction velocity (vertical) is typically 200-300 m/s slower than the corresponding x- and y-direction velocities (horizontal). However, a sharp increase in observed seismic anisotropy occurs at ~1350 mbsf, with z-direction velocity often being >500 m/s slower than the equivalent horizontal components (Fig. F171). This might be explained by preferential fracturing of cores along horizontal bedding planes as they are unloaded or disrupted by drilling. Decompaction or a change in rock properties could also explain the change.
A critical piece of information missing from our analysis is a "checkshot" that normally would be provided by logging. By firing an air gun at the surface and measuring the traveltime to a geophone positioned in the hole, direct links between depth and two-way traveltime in seismic reflection data can be obtained. We lack this direct link, so we have to assume a tie between depth and the traveltime of some reflection. The reflection at ~6.98 s is the best candidate to make this link (Fig. F170). Seismic reflection character changes dramatically at this depth. Discontinuous low-amplitude reflections lie above this horizon, and continuous high-amplitude reflections are found below. The depth to this horizon, estimated from velocity obtained from semblance plots of MCS data, is ~900 mbsf. This is close (<4% error) to the depth at which fundamental changes in lithology are observed in the borehole between lithologic Units 1 and 2 (864 mbsf). Sediments change downhole from mudstones and claystones (lithologic Unit 1) to alternating layers of grainstones and claystones (lithologic Unit 2). The grainstones have exceptionally high velocity (up to 5.8 km/s) compared to the overlying mudrocks (~2-2.5 km/s) and thus would be expected to produce high-amplitude reflections. Because of these correlations, we chose this horizon as our checkshot. Below this level, the time-depth relationship was determined from the measured vertical compressional wave velocity.
Two-way traveltime and depth are related by velocity. To obtain a time-depth curve, z-component slownesses (inverse of velocity) for velocity samples were integrated over depth. The resulting time-depth curve is shown in Figure F172. Note the subtle steepening of this curve between ~850 and 920 mbsf, where high-velocity (~3500-5000 m/s), carbonate-cemented grainstones form a significant portion of the sediments. This curve is considered to be robust because of excellent core recovery and close spacing of velocity measurements. Figure F173 shows a histogram of z-direction velocity measured throughout Site 1276. This velocity distribution is clearly dominated by samples with velocities of 2100-2500 m/s. Although there are some outliers in the z-direction velocity data, the contribution of individual outliers has a minimal effect on the area obtained by integrating the time-depth curve.
To create a synthetic seismogram, the source wavelet of the real seismic data must be estimated and the reflection coefficient series of the formation must be determined. Density and velocity are the key variables in calculating the reflection coefficients, which in turn control the amplitudes of seismic reflections. For the interface between two layers (1 and 2), reflection coefficient (Rc), density (), and vertical compressional wave velocity (V) are related by the following expression:
2V2 - 1V1)/(2V2 + 1V1).
A series of reflection coefficients can be calculated for the depths of interest in this manner. This expression is valid only for a vertically incident seismic wave.
The source wavelet was extracted from the site survey reflection data by calculating the power spectrum (squared amplitude) of each trace in a user-defined window and averaging these power spectra. This assumes that over a sufficiently large window, the reflection-coefficient series is random and the signal-to-noise ratio is large enough so that
can be approximated by
where S2(f), W2(f), R2(f), and N2(f) are the power spectra of the seismogram, the source wavelet, the reflection coefficient series, and the noise, respectively, and f denotes that these variables are related as indicated in the frequency domain. After averaging the power spectra of the traces in the frequency domain, conversion back to an amplitude time series yields a source wavelet in the time domain. Once the reflection coefficient series and the source wavelet are determined, the two are convolved to create a synthetic seismogram. All of these steps are carried out in GeoFrame's IESX computer package, which we used in the shipboard downhole measurements laboratory.
The methods described above result in two sets of synthetic seismograms, with one created from each of the two types of density data (Fig. F174). From 800 to 1100 mbsf, the synthetic seismograms display distinct features that are very similar to those observed in the seismic reflection sections at Site 1276. This is true of synthetics produced from both GRA density and MAD bulk density. Below 1600 mbsf, however, correlations between the synthetic and the actual seismic data are less apparent, and they will require postcruise modeling to understand.
Using the time-depth relationship given in Figure F172 (interpreted checkshot correlation of the lithologic Unit 1/2 boundary with the strong reflection at 6.98 s in SCREECH line 2MCS data) and the synthetic seismograms shown in Figure F174, we made initial correlations between major lithologic units and seismic reflection profiles. Table T20 gives the depths in the borehole and predicted two-way traveltimes derived for the boundaries between primary lithologic units. We summarize below the characteristics of the seismic reflection data (Figs. F170, F175, F176) and the synthetic seismograms for each lithologic unit. For a complete description of seismic facies, see "Geological Setting" in the "Leg 210 Summary" chapter.
Discontinuous, relatively weak reflections appear in the time interval corresponding to lithologic Unit 1 in both the seismic reflection data (Figs. F175, F176) and the synthetic seismogram (Fig. F174). The same seismic character continues above the first core (753 mbsf) for ~40 ms (50 m).
The boundary between Units 1 and 2 is marked by a sharp change in character in both the seismic reflection profile and the synthetic seismogram. Units 2-4 are represented by high-amplitude, laterally continuous reflections (Figs. F175, F176). In the synthetic seismogram, these bright reflections are generated by interlayered high-velocity carbonate-cemented grainstones and low-velocity claystones and mudstones in Unit 2 and, to a lesser extent, in Units 3 and 4.
The most obvious correlation between synthetic and actual seismic data from SCREECH line 2MCS occurs at a depth of 7.04 s, or ~60 ms beneath the checkshot tie point between seismic and core data described above, where a high-amplitude reflection is found in both the synthetic and actual seismic traces. Without applying any corrections to the time-depth relationship, this reflection is predicted at almost exactly the same time (within ~5 ms) as the actual reflection data.
The base of Unit 4 is predicted to be at 7.147 s. It most likely corresponds to a slightly deeper reflection with an onset at ~7.16 s in the MCS data. This reflection truncates underlying reflections and seismically represents an unconformity (Fig. F176). Lithostratigraphic and biostratigraphic data are consistent with a period of either very slow sedimentation or a hiatus near this boundary, although this is not well constrained because the cores are barren of microfossils (Fig. F141).
Lithologic Unit 5 is characterized by low reflection amplitudes in both the synthetic seismogram and actual seismic data. The lowest-amplitude reflections occur in the upper portion of Subunit 5B, where the sediments are comparatively homogeneous and where density and velocity patterns are relatively featureless (Figs. F159, F162A, 162B). At the base of Subunit 5B and the top of Subunit 5C, reflection amplitudes increase.
A significant discrepancy between the synthetic seismograms and actual seismic data occurs at depths of 7.335 and 7.375 s (Fig. F174), where notably strong reflections occur in the synthetic seismogram but not in the seismic data. At present, these mismatches are unexplained.
Correlation of the two sills cored at the base of Site 1276 with seismic reflection data is the most perplexing aspect of our seismic-core correlation. The upper sill is predicted at a two-way traveltime of 7.598 s, and the lower sill is predicted at a two-way traveltime of 7.681 s. These two predictions fall on either side of the U reflection, which has an onset at ~7.63 s. The proximity of both of the synthetic reflections to U and underlying high-amplitude reflections strongly suggests that the sills are at the level of U. However, our initial synthetic seismograms produce a high-amplitude, positive-negative couplet for the shallower reflection (Fig. F174) and it seems likely that this correlates with the similar U reflection in the SCREECH 2MCS record. The reflection created by the top of the lower sill in the synthetic seismogram also displays high amplitude and positive polarity; a large, positive impedance contrast here results from velocity and density differences between the diabase sill and overlying undercompacted sediments (see "Physical Properties"). This reflection is substantially deeper than the depth estimated for U and is probably within the U-basement sequence.
Velocity and density structure at the base of Site 1276 is considerably more complicated than in the remainder of the hole, with variations of >4500 m/s in velocity and 0.8 g/cm3 in density over tens of meters in depth. These extreme variations may cause seismic tuning effects, where the interference of reflections generated from closely spaced velocity interfaces combine to create a reflectivity pattern that cannot be readily correlated to lithology. Constraints provided by detailed analysis of the top of the upper sill have been included in the velocity function used to create synthetic seismograms, but similar constraints on the other boundaries between sills and sediments are absent because of incomplete core recovery at these interfaces. Postcruise analysis will consider different possible velocity models that might reproduce the observed seismic reflections.
A number of primary lithologic boundaries and characteristics observed in Site 1276 cores appear to correlate with seismic reflections on SCREECH line 2MCS. However, more work is required to understand the complicated velocity structure around the diabase sills at the bottom of the hole and to model this interval with synthetic seismograms. The interbedded high-velocity sills and low-velocity sediments may create tuning effects that complicate seismic interpretation of this interval.
