Upon completion of the second pass of the triple combo, control switched to the downhole measurements lab, and the tool string was again lowered to the bottom of the hole. The formation was then logged up hole for two passes, with the logging conducted in the "Lamont mode" for high-resolution MGT data acquisition. At the end of the second pass, the tool string was retrieved to the rig floor.

The FMS-sonic was rigged up and run to the bottom of the hole. The dipole shear sonic imager (DSI) was run in primary (compressional, [P-wave] and secondary [shear, S-wave]) modes, monopole and dipole shear modes and also in first motion detection (FMD) mode. As a result of the slowness of the formation and the large hole size, the P- and S-wave velocity logs are of low quality, but the FMD log appears to provide a better P-wave velocity (see below). The caliper readings from the FMS suggest that the hole conditions are better than those indicated by the triple combo caliper. The FMS-sonic tool string was then retrieved to the rig floor, and logging operations were completed by 1200 hr on 26 November. The weather was variable throughout logging operations, and heave conditions were good, typically <2 m. Consequently, the wireline heave compensator (WHC) experienced no problems during logging operations.

The triple combo caliper indicated that the hole conditions were good in the bottom 20 m of the borehole and from 206 to 146 mbsf, with widths <16 in (Fig. F40). The worst section of the hole is between 226 and 206 mbsf, with a hole diameter >18 in (beyond the maximum extension of the caliper arms). From 146 to 82 mbsf, the hole condition is variable, with a diameter of 16-18 in. The FMS caliper logs indicate that the hole is close to, but just below, the maximum (15 in) diameter of the FMS caliper arms (Fig. F40). On FMS-sonic pass 1, neither caliper extended beyond 14.1 in. On pass 2, hole conditions appear slightly different with caliper 2 extended to 15 in for 17 m of the hole. Close inspection of the caliper data, however, indicates that the caliper 2 (C2) curves (for both passes) are "overly smooth" (compared to C1 curves), suggesting poor contact with the borehole wall. Thus, the triple combo caliper data are thought to be reliable. The differences between the triple combo and FMS caliper logs may also result from an elliptically shaped borehole.

It appears that only one FMS caliper (C1) (each caliper has two pads) made good contact with the borehole wall for most of the log, which is indicated by the rugosity differences between C1 and C2 (Fig. F40). Despite the apparently poor hole conditions, the processed images are reasonably good for most of the borehole length on both passes. The slow sonic velocity of the sediments combined with the similarity in velocity between the drilling mud (typically 189 µs/ft) and the formation led to problems in processing DSI data. The wave labeling algorithm in the Schlumberger software could not differentiate between the fluid wave from the drilling mud and the compressional P-wave from the formation. Thus, for significant portions of the log the compressional P-wave remains unidentified (Fig. F41). In FMD mode, the wave labeling algorithm identifies the first good signal that it receives as the P-wave velocity of the formation. A comparison between the FMD data and core velocity measurements is presented in Figure F42.

Computed gamma ray measurements from each pass show excellent correlation (Fig. F40), indicating the high quality of the data.

Continuous wavelet analysis provides an automatic localization of specific behavior, such as cyclic patterns or discontinuities, both in time and frequency (e.g., Torrence and Compo, 1998). In contrast to classical Fourier or windowed Fourier transform, which decompose the original signal on the basis of an infinite periodic function depending on a unique parameter (space frequency), the wavelet transform (WT) allows a "depth-scale" representation that depends on a scale parameter and a translation parameter. The accelerometer data integrate the effects of heave, borehole wall contact, and wireline stretch on the tool string, and WT of the acceleration data allows the multiscale components of the tool acceleration to be deciphered. This analysis provides a quality control check on the logging data. Figure F43 presents the analyses from one pass each of the MGT and FMS-sonic logging runs. The MGT pass displays a very smooth wavelet spectrum with the only significant frequency band operating at a depth scale of <1.6 m (Fig. F43A). This frequency band relates to the heave motion of the ship (despite the damping effect of the WHC). Thus, for the MGT passes the data quality can be assumed to be excellent. The wavelet spectrum from pass 2 of the FMS-sonic tool string is shown in Figure F43B. Visually, the acceleration data are more complex, and this is interpreted to reflect the greater degree of contact between the FMS-sonic tool string and the borehole wall than is the case for the triple combo. The four FMS pads and two DSI centralizers contact the borehole wall, resulting in greater frictional resistance during logging compared to the MGT, which only has one bow spring. The wavelet spectrum shows the same high-frequency heave component but also three lower-frequency bands operating at depth scales of around 4.5, 15, and 25 m, as seen in the Blackman-Tukey spectrum (Fig. F43B). The 25-m frequency is present downhole to a depth of ~200 mbsf, whereas the two shorter frequency bands only appear above 150 mbsf (i.e., in the carbonates). The FMS-sonic accelerometer data indicate good quality, with the longer frequency depth-scale cycles being interpreted to represent formation parameters.

As mentioned above, the borehole width and wall rugosity (especially above 120 mbsf) may affect the quality of data from tools that require good tool-borehole wall contact (e.g., the hostile environment litho-density tool [HLDT]). The triple combo caliper indicates that the top of the borehole is especially rugose. The quality of these data was checked using WT analyses. The high-frequency range of the wavelet spectrum of both the density and the borehole wall are shown in Figure F44. The density displays no significant cyclicity in the formation below 150 mbsf (i.e., within the radiolarian ooze) but shows some cyclicity at a depth scale of ~8 m above (in the carbonates). The borehole-wall wavelet spectrum has no significant cyclicity operating at high frequencies, where borehole wall rugosity effects should be greatest. The cross-wavelet modulus spectrum shows no significant high-frequency cyclicity. The phase diagram perhaps sums up the data quality best. If data quality was being impaired it would be expected that, as the borehole wall widened, the density would decrease (i.e., greater influence of mud density) and the signals should be out of phase. However, the high-frequency spectrum shows that the borehole wall and density data are in phase, indicating no degradation of the data (Fig. F44F).

A similar analysis could be undertaken for the porosity data, but the relationship is not so simple (i.e., the two signals should be in phase whether they are degraded or nondegraded). However, given that the resolution of the porosity tool is less than the (unaffected) density tool's resolution and that there is good correlation between the porosity log and core porosity measurements, the porosity log is believed to be of excellent quality (Fig. F42). As expected, core density is lower than that recorded by logging. The difference between log and core density is assumed to result from the porosity increase in the cores caused by elastic strain recovery following unloading. Nevertheless, the correlation between data sets is good, and the porosity logging data will thus provide the baseline for depth shifting and compressing the core-derived composite depth scale (see "Composite Depths"). Gamma ray levels are low throughout the formation, but passes show good repeatability (Fig. F41).

Logging units were differentiated using a combination of gamma ray, density, porosity, resistivity, and velocity logs (Figs. F40, F42, F45) and the FMS static-normalized images. The formation is divided into three logging units: (1) an upper unit characterized by low gamma ray activity, porosity, and velocity, as well as high density and resistivity, with the density, porosity, and gamma ray logs displaying cyclicity at a number of depth scales; (2) a middle unit characterized by significantly higher gamma ray counts and porosities; and (3) a lower unit characterized by a zone of very high density and resistivity and higher-amplitude, lower-frequency gamma ray data. The middle unit is further subdivided into two subunits, based on an increase in density and a reduction in porosity and gamma ray counts.

Logging Unit 1: Base of Pipe (82 mbsf) to 151 mbsf

Logging Unit 1 is characterized by low gamma ray counts, low porosity, and high density (Figs. F42, F45). Density first increases then decreases downhole, most probably related to changes in carbonate levels (see "Solid-Phase Geochemistry" in "Geochemistry"). The porosity log displays cyclicity at a number of depth scales. For example, porosity has a frequency of variation at a depth scale of ~15 m (Fig. F42). The high-resolution MGT gamma ray log shows cyclicity down to the submeter scale. As a result of the hole diameter effects upon the FMS log, the resolution of the images is impaired, and small scale cyclicity is not easily identified. This logging unit correlates with lithostratigraphic Unit II (nannofossil ooze) (see "Unit II" in "Lithostratigraphy").

Logging Unit 2: 151 to 224.3 mbsf

The upper boundary of logging Unit 2 is defined by a rapid change in gamma ray, density, resistivity, and porosity (Figs. F40, F42, F45). The gamma ray counts for this unit are consistently the highest for the whole formation. The density log shows a marked decrease across the boundary and also a shift to more stable conditions (Fig. F42). Cyclicity occurs at ~0.5- and 0.1-m depth scales in the enhanced high-resolution density logs. The porosity shows much greater fluctuation than the density, with cyclicity displayed at a number of depth scales (Fig. F42). Resistivity fluctuates little throughout this unit, although there is a very subtle decrease downhole to 190 mbsf followed by a similarly subtle increase to the bottom of the unit (Fig. F42). The photoelectric factor (PEF) provides a qualitative representation of mineralogical change occurring at the upper boundary of this unit. The PEF is generally used as an indicator of lithology with values toward six indicating carbonate and values toward one indicating silica (Rider, 1996). The PEF values of logging Unit 2 are clearly lower than for logging Unit 1 above (Fig. F45), and the mineralogy change identified in the log matches that found in the cores (see "Solid-Phase Geochemistry" in "Geochemistry"). Logging Unit 2 is further subdivided into two subunits, with the boundary at 191 mbsf marked by a peak in density, minima in resistivity and porosity, and a shift in PEF (Figs. F42, F45). The boundary also marks the stabilization of gamma ray levels. Subunit 2a has high porosity and higher levels of gamma ray with greater variability (Figs. F42, F45). Subunit 2b has lower porosity and slightly higher density, but the amplitude and frequency of cyclicity appear little changed. PEF values increase at the top of Subunit 2b, indicating an increase in the carbonate content, which is substantiated by core measurements (see "Solid-Phase Geochemistry" in "Geochemistry"). Logging Unit 2 correlates with lithostratigraphic Subunit IIIA (radiolarian ooze) (see "Subunit IIIA" in "Lithostratigraphy").

Logging Unit 3: 224.3 mbsf to Bottom Of Logging Formation

This unit only has density, resistivity, and NGT gamma ray logs (Table T22). The top of the unit is marked by an increase in both gamma ray levels and the amplitude of fluctuation, with a clear periodicity operating at an ~10-m depth scale (Fig. F40). Density increases gradually from the boundary down to a depth of 230 mbsf, where there is a massive increase in density (Fig. F42). This high-density region also appears as a zone of very high resistivity (Fig. F42) and is related to the occurrence of 15 chert bands (see "Subunit IIIB" in "Lithostratigraphy"). Figure F46 shows the FMS images from this part of the formation. A number of chert layers are visible as light-colored, high-resistivity bands. The log of microresistivity from a single button highlights this region (Fig. F42). This logging unit correlates with lithostratigraphic Subunits IIIB (chert zeolitic clay) and IV (calcareous chalk) (see "Subunit IIIB" in "Lithostratigraphy").

Despite the borehole washout, the quality of the logging data appears good. The main objectives of acquisition of in situ, continuous, multiparameter logging data at this site were

1. To assess the physical, chemical, and structural characteristics of the formation, and to provide a baseline for depth matching the core-derived composite depth scale;
2. To perform cyclostratigraphic analysis of continuous Paleogene sequences;
3. To identify and characterize chert layers, which are usually poorly recovered in cores; and
4. To conduct a seismic integration (time-depth model and synthetic seismogram) allowing identification and dating of seismic reflectors at a regional scale.

The logging units described above correlate well with the designated lithostratigraphic units (see "Lithostratigraphy"). The high-resistivity region in logging Unit 3 correlates with the chert-rich zeolitic clay (lithostratigraphic Subunit IIIB) and can be easily mapped on the FMS images. The uppermost chert layer is located at 233 mbsf and is 0.2 m thick (the thickest of all the bands). The entire high-resistivity region is composed of at least 15 bands, down to 237 mbsf. Between 239 and 243 mbsf, a large number of chert nodules are visible, with some chert bands appearing again at 244 mbsf. Passing uphole into the Eocene section, the gamma ray counts increase significantly, indicating a higher quantity of clay in the formation. The Oligocene/Eocene boundary produces a very clear signal on most of the logs (Figs. F42, F43, F45). The PEF log provides a good approximation of the mineralogy changes occurring in the formation. The lower Oligocene nannofossil ooze is identified as a region with consistently low NGR activity, reflecting the low clay content of these sediments. However, the high-resolution MGT gamma ray data provide a detailed record of cyclicity in sedimentation down to the submeter level. The density data in logging Unit 1 show good agreement with the core data (Fig. F42). The porosity log generally covaries with the density log and displays cyclicity down to the meter scale.

The sonic velocity data in formations, such as those logged at this site, are invariably difficult to interpret because the velocity of the drilling mud is so close to that of the formation and the slowness of the formation reduces the quality of the return signal to the receivers (see "Synthetic Seismogram" in "Downhole Measurements" in the "Site 1218" chapter). The P-wave processing algorithm for the DSI tool has difficulty in resolving the P-wave of the formation from the fluid wave of the drilling mud (Fig. F41), leading to velocity data sets that are incomplete. As an experiment, an additional data collection mode (FMD) was run at this site. The wave-labeling algorithm used the first good signal it received this time and assumed this to be the formation P-wave. The results of this experiment are presented in Figure F41 and compare favorably with the core data. The FMS-sonic velocity data are shown along with the coherency plots of the stacked sonic waveforms, with the dashed lines representing algorithm-identified (P and S) waves (Fig. F41). Plotting the data, as shown in Figure F41, provides a method for qualitatively determining the data quality. The FMD data look good for the most part, except between 153 and 168 mbsf, where the coherency band is very narrow, indicating either mud velocity or equal mud/formation velocities. However, when compared to the density data (Fig. F44) and the core velocity data, the FMD data appear good. The use of the FMD mode, in conjunction with the P and S monopole mode, may prove to be a useful addition to the velocity measurements routinely made during ODP logging when investigating slow formations.

WT analyses were undertaken on the density and porosity logs from both passes of the triple combo (Fig. F47A, F47B). The porosity shows a dominant cyclicity through the formation at a depth scale of ~15 m (Fig. F47B). This frequency is most obvious in the nannofossil ooze (i.e., above 151 mbsf) but reappears downhole between 180 and 215 mbsf. As a first approximation, based on the LSRs calculated from the nannofossil, radiolarian, foraminifer, and paleomagnetic data (Fig. F22) ("Sedimentation and Accumulation Rates"), this frequency relates to an ~1-m.y. cyclicity. A localized frequency depth scale of ~8 m, which is equivalent to a timescale of ~0.5 m.y., appears between 90 and 115 mbsf. The density wavelet spectrum shows a very similar pattern, with the 15-m periodicity less apparent below 150 mbsf and the 8-m periodicity significantly better developed (Fig. F47C, 47D). The cross-wavelet spectrum modulus shows the dominant depth-scale frequency to be the 15-m one (Fig. F47E). Finally, the phase diagram confirms the expected covarying relations of density and porosity at this frequency (i.e., out of phase) (Fig. F47F).

The aim of creating a synthetic seismogram is to provide a means of matching the reflections expected from the formation (measured physical properties from log and core sources) with those in the seismic data. This allows the seismic data to be interpreted in terms of the measured formation properties; for example, lithologic or chronologic boundaries can be picked out as specific seismic horizons. If a synthetic seismogram can be generated for a number of sites, these data provide the basis for producing a regional seismic stratigraphy.

The GRA density and PWL (whole core) velocity data sets were spliced onto the top of the FMD sonic log and HLDT density logging data in order to provide data for the entire formation. Because the core densities and velocities differ from the in situ measured logging data, a correction factor for the spliced section was computed (i.e., core data were corrected to in situ values). These full-depth data sets were then imported into the IESX module of the Schlumberger GeoQuest program Geoframe to calculate the synthetic seismograms. The impedance (velocity x density) was calculated, and the impedance contrast between successive layers gave the reflection coefficient series. The source wavelet (obtained by extracting the seafloor reflection from the seismic data) was convolved with the reflection coefficient series to generate the synthetic seismogram (Fig. F48A).

The synthetic seismogram matched the seismic data well. The chert layers, which appear as a high-density and high-velocity region (Fig. F42), should be located at the base of the borehole and show up as a strong reflector. However, the first strong reflector appears 15 to 20 m lower in the section, suggesting that either the chert layers were discontinuous and the section cored was a very small patch higher than the main chert horizons, or the formation velocities are lower than those measured by the logging data. To check this latter possibility, the lower PWL velocities were used to calculate a second synthetic seismogram (Fig. F48B). The match between the synthetic seismogram and the seismic data is good. Generally it is expected that the in situ collected logging velocity data will better reflect the true formation velocity than the core velocity data (Mayer et al., 1985). Here, however, the opposite situation suggests that the FMD logging velocity measurements may have overestimated the true formation velocities, most likely as a result of problems with similarity in the mud and formation velocities.