Two logging runs (triple combo and FMS-sonic) (see Fig. F10 in the "Explanatory Notes" chapter) were planned for Hole 1218A (after basement was reached at 0645 hr on 16 November) following a combination of APC (~190 mbsf) and XCB (190-280 mbsf) coring. The hole was conditioned with a wiper trip of the drill string (up to 78 mbsf and back to basement), and 2 m of fill was flushed from the bottom of the hole. The hole was displaced with 110 bbl of 8.9 lb/gal sepiolite mud, and the bit was withdrawn to 80 mbsf in preparation for logging. Tool rig-up was begun at 1130 hr and, after some technical problems, was completed by 1645 hr. The tool was run to the bottom of the hole (5118.5 mbrf), and logging began at 2210 hr on 16 November. Seasoning of the new wireline (installed during port call) limited the maximum speed to run into and out of pipe to 5000 ft/hr (~1500 m/hr), leading to an approximate doubling of the times normally expected.
The classic triple combo configuration with the Lamont-Doherty Earth Observatory high-resolution MGT on top was run first, followed by the FMS-sonic tool string (see Fig. F10 in the "Explanatory Notes" chapter). Pump pressure was required to get the triple combo through the bit into open hole. Initially, this problem was thought to be the result of a mud/clay plug in the bottom-hole assembly (BHA), but upon inspection of the tool at the end of the run, a bent caliper arm was probably the cause. No serious damage was sustained by the tool. A total of five passes were made (providing ~200 m of logged section per pass) with no bridges encountered, and the bottom of the hole was reached on all passes (Fig. F34). A breakdown of the chronology of logging operations is provided in Table T24, along with details of the tools used.
The triple combo caliper log indicated good conditions for most of the hole (Fig. F35). After 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. Logging was conducted in the "Lamont mode" for high-resolution MGT data acquisition on a single pass. Upon completion of this pass, the tool string was retrieved and the FMS-sonic tool string was rigged up and run to the bottom of the hole. The dipole shear sonic imager was run in primary (compressional; P-wave) and secondary (shear; S-wave) monopole and dipole shear modes. On the first pass, a lower frequency was used, which resulted in better dipole S-wave measurements, and on the second pass a higher frequency was used, which resulted in better monopole P-wave measurements. However, because of the slowness of the formation, the P- and S-wave velocity logs are incomplete and of low quality. During both passes, there were two brief telemetry and power losses to the tool string, but this had little effect upon the log acquisition. The drill pipe was pulled upward toward completion of the second pass, providing an extra 20 m of logged formation. The tool string was then retrieved, and logging operations were completed by 1900 hr on 17 November. The weather was variable throughout the logging operation with heave typically ~2 to 2.5 m. The wireline heave compensator stroked out on only one occasion during the whole logging operation.
The hole conditions were excellent in the lower 90 m of the borehole with widths averaging 13.3 in, as recorded by the triple combo caliper (Fig. F35). Above this lower interval, the hole opens out to an average of 16.7 in for the next 55 m, before increasing to an average of 18.2 in for the remainder of the hole. A hole diameter of this magnitude may degrade the quality of the data from tools that require contact with the borehole wall (e.g., the density or porosity logs). However, there appears to be little evidence of deterioration of data quality uphole in these logs. The FMS caliper readings suggest a smaller hole (the maximum extension of the FMS calipers is only 15 in compared to 18 in for the triple combo caliper) (see Fig. F35). These observations suggest that damage sustained by the triple combo caliper arm when entering the open hole caused an overestimation in the recorded hole size. Problems were encountered during data downloading from the Lamont-Doherty Earth Observatory temperature/acceleration/pressure tool, and no data were obtained.
Tool string accelerometer data from the MGT and the FMS-sonic indicate that hole conditions and stick-slip of the tool remained at low levels until the cable head entered the BHA. The FMS recorded good data most of the way up the formation, and there is good correlation between the two passes. The sonic logs are of low quality because of the slowness of the formation. Computed gamma ray measurements from each pass show excellent correlation (Fig. F35), indicating the high quality of the data. More detailed gamma ray logs are displayed on Figure F36, with further good correlation between passes. Density and porosity logs show good repeatability, and as expected, core density and porosity values are lower than those recorded by logging. This difference between log and core data sets 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 log data will thus provide the baseline for depth shifting and compressing the core-derived composite-depth scale (see "Composite Depths"). The second full pass of the same tool string provided a continuous log of high quality data. As a further check on data quality, a wavelet analysis of the accelerometer data from the FMS-sonic tool string passes was undertaken. 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 transform 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 WT analysis of the acceleration data allows the multiscale components of the tool acceleration to be deciphered (Fig. F37). The full record (80-276.8 mbsf) is characterized by acceleration/deceleration mainly over a range of <1 m. The bottom of the hole is characterized by localized stick-slip displacement over intervals of intermediate scale (~ 4-7 m) (Fig. F37B). The upper part of the hole displays a more complicated pattern with components ranging over intervals from 4 to 100 m. Whereas, the high-frequency component may be explained by incomplete heave compensation, the other components are directly linked to hole conditions. A comparison of wavelet representation and hole shape shows that the intervals where tool acceleration changes correspond to changes in hole diameter (Fig. F37). Increases in hole diameter are connected to increases in the scale of the stick-slip intervals and vice versa.
Logging units were differentiated mainly using variations in the gamma ray logs (Fig. F36). The static normalized FMS images confirm the logging units as do the density, porosity, and resistivity logs, but to a lesser extent (Fig. F38). The formation is divided into three logging units: (1) an upper unit characterized by low gamma ray counts with cyclicity at a number of depth scales, (2) a middle unit where significantly higher gamma ray counts are recorded, and (3) a lower unit characterized by a return to low gamma ray counts (this unit is defined solely on the basis of the FMS NGR tool [NGT] data). The middle logging unit is further subdivided into two subunits, based on a reduction in gamma ray counts.
Logging Unit 1 is characterized by low total gamma ray counts (Fig. F38). Porosity values are initially high but drop from 129 mbsf to a minimum at ~157 mbsf and then rise again to values similar to those seen at the top of the log. An opposite trend is observed in the density data (i.e., increasing toward a plateau centered around 160 mbsf and then decreasing down to the base of the unit). The density-porosity relationship seen in the logs reflects carbonate variations downhole (see "Physical Properties" and "Geochemistry"). Resistivity data remain at uniform levels throughout the unit. At higher resolution, the MGT and FMS image/microresistivity logs show cyclicity on a scale of 1-1.5 m (Fig. F39). The FMS image/microresistivity logs also provide a record of shorter-period cyclicity within the 1- to 1.5-m cycles. Figure F39 shows a detailed section of this logging unit at 177 mbsf, with a plot of the microresistivity recorded by a single button on FMS pass 1, which highlights the high- and low-resistivity areas. This logging unit correlates with lithostratigraphic Unit II (nannofossil ooze) (see "Unit II" in "Lithostratigraphy").
The upper boundary of logging Unit 2 is defined by a step in gamma ray values (Figs. F35, F36). A concomitant decrease in density, resistivity, and sonic velocity and an increase in porosity also occur at this point (Fig. F38). Potassium levels most consistently exemplify this change in formation properties (Fig. F36). The whole unit is characterized by greater amplitude variability in the gamma ray, density, porosity, and resistivity data than logging Unit 1 (Figs. F36, F38). The large peaks in density and resistivity at ~224.5 and 238.5 mbsf are associated with bands of chert. Figure F40 shows a portion of both FMS passes associated with the peak around 225 mbsf. The high-resistivity chert bands are easily visible in the image, and the microresistivity log shows the record of "relative resistivity." These two peaks in density and resistivity correlate with a massive reduction in calcium and an increase in silica (see "Solid-Phase Geochemistry" in "Geochemistry"). Based on the gamma ray data (Fig. F36), a two-unit subdivision is made. From 214 to 231 mbsf, Subunit 2a has larger-amplitude fluctuations (Figs. F36, F41), whereas lower Subunit 2b has lower-amplitude, longer frequency variations (Fig. F41). The whole of logging Unit 2 correlates with lithostratigraphic Unit III (radiolarite) (see "Unit III" in "Lithostratigraphy").
The NGT (see Fig. F10 in the "Explanatory Notes" chapter) was located just above the FMS on the second tool string and logged the formation to a depth of 270 mbsf, recording the deepest gamma ray data (Fig. F35). The NGT log records a decrease in the gamma ray counts at a depth of 255 mbsf, to the levels observed in logging Unit 1. Density and resistivity increase toward the bottom of the formation (Fig. F38). This unit is correlative with lithostratigraphic Unit IV (nannofossil chalk and dolomite) (see "Unit IV" in "Lithostratigraphy").
Good hole conditions combined with relatively calm sea conditions led to the acquisition of excellent logging data. The main objectives of acquisition of in situ, continuous, multiparameter logging data at this site were
The logging units described above correlate well with the designated lithostratigraphic units (see "Lithostratigraphy"). The lower Oligocene-lower Miocene nannofossil ooze and chalk 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. The density in logging Unit 1 gradually increases down to 160 mbsf and then gradually decreases toward the boundary with logging Unit 2. The porosity log covaries with the density log, with the minimum porosity more sharply defined than the maximum density. These porosity/density trends have been attributed to variations in clay content (opposite to carbonate levels) with depth (see "Lithostratigraphy," "Physical Properties," and "Geochemistry"). The gamma ray logs show no distinguishable shift in count levels, so if clay content is indeed the cause of the density and porosity changes, the percentage increase in clay must be below the MGT detection limits. The photoelectric factor (PEF), however, shows the same downhole trend as the density log (Fig. F36). The PEF is generally used as an indicator of lithology, with values toward six indicating carbonate and values toward one indicating silica. A peak in the PEF log at 224 mbsf correlates with similar peaks in density and resistivity that correspond with the first region of significant chert banding. Given this change in lithology (carbonate to silica), a shift toward lower values of PEF would be expected. However, the presence of iron-bearing minerals even in small quantities can also have a significant effect on the PEF (Rider, 1996). This peak in PEF is interpreted to result from the presence of iron oxides, as opposed to an enrichment of the carbonates resulting from silica migration toward the chert layers. This interpretation is corroborated by color descriptions of the sediment at this depth and by geochemical data (see "Lithostratigraphy" and "Geochemistry").
Passing downhole through the Oligocene-Eocene transition, the gamma ray counts increase significantly, indicating a higher quantity of clay in the formation. The density, porosity, and resistivity logs identify two zones where significant chert formation has occurred. The high-resolution FMS microresistivity images show that the high-resistivity, high-density/low-porosity zones are composed of multiple chert bands (e.g., Fig. F40). The zone between 218 and 229 mbsf is composed of two groups, an upper one comprising seven bands from 218 to 223 mbsf and a lower one comprising eight bands from 226 to 229 mbsf. The second major peak is composed of five bands between 238 and 241 mbsf. From 247 to 266 mbsf, there are a number of small groupings and single chert layers. A significant group of eight chert layers is located between 269 and 273 mbsf. In all of the locations mentioned above, the chert appears as layers. Layering width is highly variable, from detection limit up to a maximum of 15 cm at 227 mbsf. The chert layers are sinuous in the FMS images, indicating shallow dips. Below 218 mbsf, chert nodules are also observed throughout the formation, with a higher concentration in the interval 249-254 mbsf.
WT analysis was undertaken on the logging data to investigate the cyclicity in logging Unit I (Fig. F42). The density and porosity logs were used because they displayed cyclicity and were influenced by variable clay/carbonate content downhole. Both wavelet diagrams show a gradual increase in the depth scale of cyclicity toward a boundary around 166 mbsf, where a step change occurs to a larger depth scale in cyclicity. In order to better understand the mechanism of this shift, a cross-wavelet spectrum diagram was computed (Fig. F42C). The modulus diagram allows a comparison of the scale change with depth and shows a phase change occurring around 166 mbsf. The diagram shows the expected phase difference between the density and porosity logs. The scale change in cyclicity may be related to a shift in sedimentation rates during the deposition of this unit (i.e., rates ~1.5 times higher in the section above 166 mbsf).
The aim of creating a synthetic seismogram is to provide a means of matching up 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 chronological boundaries can be picked out as specific reflectors. If a synthetic seismogram can be generated for a number of sites, these data provide the basis for producing a regional seismic stratigraphy.
The P-wave labeling algorithm in the Schlumberger software had difficulty in identifying the P-wave of the formation from that of the drilling mud, making the velocity logs incomplete and of poor quality (Fig. F38). The PWL (whole core) velocity data only extend to 188 mbsf, but the P-wave sensor (PWS; split core) velocity data are available to 273 mbsf. Thus, the PWS velocity data were selected (the formation was assumed to be isotropic in order to obtain the longest possible data set; i.e., any axial velocity was used) and were resampled using a linear interpolation script run in GMT (Generic Mapping Tools; Wessel and Smith, 1999). The GRA density was spliced onto the top of the hostile environment litho-density tool (HLDT) density log to provide data for the entire formation. Because the core-derived densities and velocities differ from the in situ measured logging data (Mayer et al., 1985), a correction factor for the spliced density section was computed (i.e., core data were corrected to in situ values) and the raw PWS data were used. These full-depth data sets were then imported into the IESX module of the Schlumberger GeoQuest program Geoframe to calculate the synthetic seismogram. 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. F43).
The generated synthetic seismogram shows a good match with the seismic data. The first major reflector appears on the seismogram at 174 mbsf, which corresponds to a peak measured in the core velocities (Fig. F38). The upper chert bands appear between 218 and 229 mbsf as high-density and high-resistivity areas in the log and core data (Fig. F38). A large positive reflector is located around this depth on the seismic trace and is interpreted to represent the upper chert layers. Further analysis of the FMS data will allow the dip and dip directions of the chert layers to be determined, and these will be compared with the seismic data.