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doi:10.2204/iodp.proc.311.106.2006

Downhole logging

Logging while drilling

Operations

Hole U1328A was spudded at 1279 mbrf water depth (drillers depth) at 0455 h on 24 September 2005. Hole U1328A was drilled after completing Hole U1327A by pulling the drill string clear of the seafloor and moving the ship in DP mode. LWD tools included the GeoVISION resistivity tool, the EcoScope tool, the SonicVISION tool, the TeleScope MWD tool, the ProVISION NMR tool, and the ADNVISION tool. For details on each tool and the measurements it makes, see "Downhole logging" in the "Methods" chapter.

Drilling in Hole U1328A began after a VIT camera survey established that no living chemosynthetic communities were present at the seafloor. Because massive gas hydrate was suspected near the surface, Hole U1328A was initially drilled more slowly than the other LWD holes of Expedition 311. The first 10 m of Hole U1328 was drilled with a rotation rate of 10 rpm, a pump flow rate of 100 gpm, and an ROP of 10–15 m/h. We then increased the rotation rate to 40 rpm while keeping the low pumping rate of 100 gpm until 30 mbsf, and then to 60 rpm and 270 gpm (to start the MWD telemetry), keeping the instantaneous ROP below 50 m/h with depth. The target depth of 300 mbsf (1633 mbrf) was reached at 0415 h on 25 September. The LWD tool string was then brought back to the surface, and rig down and data download for Holes U1326A, U1327A, and U1328A was completed by 1050 h on 25 September 25.

Gas monitoring with real time logging-while-drilling/ measurement-while-drilling data

The LWD logs were acquired to plan coring and pressure coring operations in subsequent holes at Site U1328. As Hole U1328A was drilled without coring, the LWD data had to be monitored for safety to detect gas entering the borehole. As explained in "Downhole logging" in the "Methods" chapter, the primary measurement we used in gas monitoring was annular pressure while drilling (APWD) measured by the EcoScope tool in the borehole annulus. We looked for sudden decreases of >100 psi in the annular pressure, which could be caused by low-density gas entering the borehole. We also monitored pressure increases of the same magnitude, which could be the result of fluid acceleration caused by a gas kick (Aldred et al., 1998).

Figure F56 shows the measured borehole fluid pressure profile in Hole U1328A after subtraction of the best-fit linear trend. The borehole fluid pressure shows only small fluctuations over the trend, except for two negative pressure anomalies of almost 20 psi at 245–255 mbsf and ~10 psi at 263–270 mbsf. These fluid pressure anomalies were well below the 100 psi level that would have required preventive action.

We also monitored the coherence of the sonic waveforms acquired by the SonicVISION tool, focusing on the velocity of fluid in the borehole. Loss of coherence in the waveforms and a slower velocity for the drilling fluid indicate the presence of gas. The sonic waveform coherence image in Figure F56 shows a generally well defined fluid arrival with a slowness of ~200 µs/ft, which corresponds to the expected fluid velocity of ~1500 m/s. There is an anomalous interval at 222–240 mbsf, which is probably below the GHSZ at this site, and if excess gas was present in the formation it had to be free gas. The sonic waveform coherence suggests that the fluid velocity decreased in this interval and that the slowness was higher than the upper bound of 240 µs/ft used in the processing. The ultrasonic caliper shows a smaller hole diameter at 233–250 mbsf, roughly coinciding with the interval of negative fluid pressure anomalies. The apparent offset between the pressure anomaly and the low-coherence interval could be caused by the spacing between the pressure and sonic sensors, with both reacting at the same time to the release of free gas in the borehole. These results suggest that the fluid velocity measured while drilling may indicate layers containing free gas and warrant further study. Even if the low coherence and low drilling fluid velocities were caused by free gas it is important to stress that the fluid pressure anomalies were small and that if gas was in the drilling fluid at ~223–250 mbsf, it had to be in low concentrations.

Logging quality

Figure F56 also shows the quality control logs for Hole U1328A. The ROP was generally 60 m/h or less in the interval from the seafloor to TD. This is sufficient to record one measurement every 4 cm (~25 measurements/m) with the GeoVISION resistivity tool, and no significant resolution loss was observed with variation in ROP. The ultrasonic caliper log, which is a direct measurement of the borehole diameter recorded by the EcoScope tool, is our best indicator of borehole size. Most of the hole had a diameter slightly >10 inches (25 cm), with larger washouts slightly above 11 inches (28 cm) restricted to the uppermost 70 m of the hole. The density correction, calculated from the difference between the short- and long-spaced density measurements, generally varies from 0 to 0.2 g/cm³ (Fig. F56), showing the good quality of the density measurements. Figure F57 is a summary of the main LWD logs with density and porosity measurements from cores from Holes U1328B, U1328C, and U1328E superimposed (see "Physical properties").

The depth relative to the seafloor was fixed for all LWD logs by identifying the step change in the gamma ray log associated with the seafloor. For Hole U1328A, the gamma ray logging pick for the seafloor was at a depth of 1279 mbrf.

Wireline logging

Operations

Hole U1328C was continuously cored to a depth of 300 mbsf, which was reached at 1000 h on 14 October 2005. A short wiper trip was made in the bottom 40 m, and the hole was displaced with 10.5 ppg mud. Wireline logging operations in Hole U1328C began with the deployment of the triple combo tool string (resistivity, density, and porosity measurements), which consists of the Hostile Environment Gamma Ray Sonde, the Dual Induction Tool, the Hostile Environment Litho-Density Tool, the Accelerator Porosity Sonde (APS), and the Lamont-Doherty Earth Observatory high-resolution Temperature/Acceleration/Pressure (TAP) tool. For details on the different tools, see "Downhole logging" in the "Methods" chapter. Rig up of the triple combo tool string started at 1415 h and was completed at 1605 h. The tool string reached the bottom of the hole at 1573 mbrf (294 mbsf) without difficulty by 1735 h, and we started to log up. During the run, the minitron (neutron generator) on the APS tool started to malfunction at ~1380 mbrf (101 mbsf). Consequently, when the tool reached the seafloor we decided not to run a second pass. The triple combo tool string was returned to the rig floor at 1930 h. When the TAP tool was brought back to the Downhole Logging laboratory to download the data, we discovered that it had not functioned properly, and there are no downhole temperature data available for Site U1328.

After rig down of the triple combo tool string was complete, rig up of the FMS-sonic tool string began and was finished at 2110 h on 14 October. The FMS-sonic tool string consists of the FMS, the General Purpose Inclinometer Tool, the Scintillation Gamma Ray Tool, and the Dipole Sonic Imager (DSI). The tool string was lowered into the hole and reached the bottom (1574 mbrf; 295 mbsf) at 2318 h when the first FMS pass started. After reaching the seafloor, we lowered the tool string to the bottom of the hole for a second pass, which began at 0030 h on 15 October. The second pass ended at 1346 mbrf (67 mbsf) and was followed by a third, short pass between 1480 and 1450 mbrf (201–171 mbsf) to test the Schlumberger heave compensator, which worked well. Rig down of the FMS-sonic tool string was completed at 0315 h, when rigging for the VSP operations started (see "Vertical seismic profile").

Logging quality

Wireline logging data from the triple combo and FMS-sonic tool string runs in Hole U1328C were compromised to some extent by poor hole conditions typical of these unconsolidated formations (Fig. F58). The two FMS calipers measure the hole diameter in two orthogonal directions, and Figure F58 shows that the hole was particularly elongated with a consistent difference of ~3 inches between the two calipers. The larger measurement was beyond the maximum range of 15 inches (38 cm) in most of the hole. As a consequence, there was often poor contact between some of the FMS pads and the borehole wall, which degraded the quality of the resistivity images.

On the other hand, the density tool caliper measured less than its maximum range of 20 inches (51 cm) in most of the hole, and the density pad was generally in good contact with the formation. The density log in Figure F58 is of good quality and shows only a few intervals where the logged densities are erroneously low (below ~1.5 g/cm³) because of hole enlargements. The excellent quality of the density log and the derived porosities is also supported by the close fit with core measurements, which is especially good at 128–200 mbsf where resistivity was low and uniform. This suggests that little or no gas hydrate was present, and that formation disturbance after coring caused by hydrate dissociation had to be minimal. It should also be stressed that the core measurements plotted in Figure F58 were obtained on samples taken from the same hole that was logged, and that the comparatively poor fit between the LWD density log and the core data (Fig. F57) may be due at least in part to lateral heterogeneity.

Porosities measured by the neutron log are close to those computed from the density log at 100–130 mbsf, but are markedly higher than the density porosities at depths >128 mbsf. This difference is probably caused by sediments with a greater clay content below 128 mbsf because the neutron log measures hydrogen abundance and it tends to overestimate porosity in clay-rich formations where hydrogen in clay minerals is counted as porosity. The induction log resistivities are generally not affected by hole conditions as much as the nuclear logs, and the resistivity logs in Figure F58 are of very good quality. The only exception is at depths above 92 mbsf, where the logged resistivities are variable and locally anomalously low. We have no caliper information in this interval, but hole enlargements are probably the reason for the poor measurement quality.

The acoustic data measured in Hole U1328C appear to be of excellent quality. The acoustic waveforms and slowness-time coherence projection are shown in Figure F59. The velocity profiles acquired are very robust, and almost no additional processing was required to derive reliable compressional (V_P) and shear (V_S) wave velocities.

The depth relative to the seafloor for all wireline logs was fixed by identifying the step change in the gamma ray log associated with the seafloor. The gamma ray pick for the seafloor was at 1279 mbrf in Hole U1328C.

Logging-while-drilling and wireline logging comparison

Figure F60 shows a comparison of LWD (Hole U1328A) and wireline (Hole U1328C) data using the gamma ray, neutron porosity, density, and resistivity logs. In general, the LWD and wireline data from each hole match relatively well, exhibiting similar curve shapes and absolute logging values. For example, the LWD and wireline density logs give generally similar values except for a few intervals where the hole is enlarged and the wireline density log gives anomalously low readings (compare the wireline density log to the wireline caliper track in Fig. F60).

A notable difference between LWD and wireline measurements is the gamma ray log, where the LWD log gives higher readings (100 gAPI on average) than the wireline log (50 gAPI on average). The LWD and wireline gamma ray curves have similar character and correlate well in some intervals (e.g., 95–115 mbsf), but in much of the drilled sequence there is no obvious correlation between the two. The cause of the discrepancy is not fully understood; however, the wireline data are consistent with the logs recorded during Leg 146.

Except for some anomalies caused by the enlarged hole, LWD and wireline neutron porosities are similar between and 102 and 128 mbsf. Below 128 mbsf, however, the neutron wireline log gives higher porosities than the LWD log and the core measurements. As noted above, this is most likely caused by an increase in the clay content of the formation, which is supported by a small increase in both the LWD and wireline gamma ray readings below 128 mbsf. The LWD neutron porosity shown in Figure F60 is not affected by this increase in clay content below 128 mbsf because it is the "best thermal neutron porosity" measured by the EcoScope tool, which has been corrected for the effect of clay so that it is only slightly higher than the density porosity.

Comparison of LWD and wireline resistivities shows a similar overall trend. The low resistivities measured by the wireline logs in the shallow interval above 92 mbsf are probably the result of washouts in Hole U1328C. There are differences between LWD and wireline logs, however, in resistivity peaks that may be caused by the presence of gas hydrate. The LWD resistivities show peaks where resistivity reaches 2–3 m at 90–98, 125, 160, 170–178, 183–191, and 195–206 mbsf. The wireline resistivities do not contain any of these LWD peaks but instead display two other peaks at 217 and 231–237 mbsf that are not seen in the LWD resistivity logs.

Given the evidence for lateral heterogeneity at nearby Site U1327 (see "Downhole logging" in the "Site U1327" chapter), the observed differences between the LWD and wireline resistivity logs at Site U1328 are consistent with similar lateral variability in gas hydrate distribution and/or lithology. This implies that one should be cautious when comparing measurements taken in different holes (e.g., LWD logs from Hole U1328A and core analyses from Hole U1328B or U1328C).

Logging units

The logged section in Holes U1328A and U1328C can be divided into three logging units based on obvious changes in LWD and wireline gamma ray, density, electrical resistivity, and acoustic measurements (Figs. F57, F58, F59). Except for minor differences in the boundary depths (3–5 m), these three logging units correspond to lithostratigraphic Units I, II, and III (see "Lithostratigraphy").

Logging Unit 1 (0–128 mbsf) is characterized by a well-defined increase in density with depth (from 1.7 g/cm³ near the seafloor to 2 g/cm³ at 128 mbsf), and a decrease in porosity (from 70% near the seafloor to 45% at 128 mbsf). The P-wave velocities average ~1550 m/s. The most striking features are seen in the LWD logs between the seafloor and 46 mbsf, where high resistivities (>25 m) alternate with intervals of much lower resistivity (1–2 m). These high resistivities are not associated with high densities (at most 1.7 g/cm³) and are, therefore, likely to indicate gas hydrate and not cemented sands or carbonates. The low resistivities are closer to the background values observed in the rest of the section, and Unit 1 is likely to contain layers with high gas hydrate content intercalated with sediments that have little or no gas hydrate in the 0–45 mbsf interval.

Logging Unit 2 (128–200 mbsf) is marked by a clear decrease in density (from 2 to 1.75 g/cm³) at the top, compared to the bottom of Unit 1. Unit 2 is also characterized by a constant background resistivity value just above 1 m and no resistivity peaks attributable to gas hydrate; however, an exception may be at 160 mbsf in the LWD resistivity logs. Porosity and V_P average ~50% and ~1600 m/s, respectively.

Logging Unit 3 (200–300 mbsf) displays a small increase in background resistivity and a markedly higher variability in resistivity (1–4 m) and V_P (1500–1800 m/s) compared to Unit 2. Density and porosity are otherwise similar to those in Unit 2.

Logging-while-drilling borehole images

The GeoVISION, ADNVISION, and EcoScope LWD tools generate high-resolution images of borehole log data. The ADNVISION and EcoScope tools produce images of density and hole radius (computed on the basis of the density correction, which depends on the borehole standoff). The GeoVISION tool produces a gamma ray image and shallow, medium, and deep depth of investigation resistivity images.

Figure F61 shows some of the LWD images collected by the EcoScope and GeoVISION tools. It should be noted that the display in Figure F61 is highly compressed in the vertical direction. The unwrapped images are ~90 cm wide (for an 11 inch diameter borehole), and the vertical scale is compressed by a factor of ~37:1. These high-resolution images can be used for detailed sedimentological and structural interpretations and to image gas hydrate distribution in sediments (e.g., in layers, nodules, or fractures). Gas hydrate–bearing sediments exhibit high resistivities within intervals of uniform or low bulk density. Layers with high resistivities and high densities are likely to be low-porosity, compacted, or carbonate-rich sediments.

High concentrations of gas hydrate is evident from 0 to 46 mbsf in Hole U1328A, where densities are generally low and high-resistivity, gas hydrate–rich layers (bright in the resistivity images of Fig. F61) alternate with low-resistivity layers (dark in the images of Fig. F61) that are likely to contain little or no gas hydrate. Other points of interest in the resistivity images are at 25–45 and 92–97 mbsf, where roughly sinusoidal, bright features can be interpreted as dipping fractures containing gas hydrate (Fig. F62). These fractures have steep dips. For example, the top and bottom of the fracture at 92–97 mbsf are 3 m apart vertically, and for a borehole of 10.5 inch diameter (27 cm) this implies a dip of 85°. These near-vertical fractures may act as gas-migration conduits that feed the gas hydrate accumulation observed near the seafloor at Site U1328.

Logging porosities

Sediment porosities can be determined from analyses of recovered cores and from downhole measurements (see "Physical properties" and "Downhole logging" in the "Methods" chapter). Data from the LWD density and neutron logs were used to calculate sediment porosities from Hole U1328A. Core-derived physical property data, including porosities (see "Physical properties"), were used to calibrate and evaluate the log-derived sediment porosities.

The LWD log–derived density measurements were used to calculate sediment porosities () using the standard density-porosity relation

= (_g – _b)/(_g – _w).

We used a constant water density (_w) of 1.03 g/cm³ and a grain/matrix density (_g) of 2.76 g/cm³, which is the average grain density measured in the core samples (see "Physical properties"). The density log–derived porosities range from ~70% at 10 mbsf to ~45% at 300 mbsf (Fig. F63).

The LWD neutron porosity log (Fig. F63) yielded sediment porosities ranging from an average value of ~70% at 20 mbsf to ~45% at 300 mbsf. As noted earlier, porosities measured by the neutron log are expected to be higher than those computed from the density log in clay-rich sediments because the neutron log essentially measures hydrogen abundance, and hydrogen in clay minerals is counted as porosity. The neutron porosity measured by the EcoScope tool (Fig. F63) has been corrected for the effect of clay, so that it is only slightly higher than the density porosity.

The comparison of core- and LWD-derived porosities in Figure F63 reveals that the log-derived porosities agree with the core-derived values throughout the logged interval, with the density porosities being slightly lower and the neutron porosities slightly higher than the core porosities.

Gas hydrate and free gas occurrence

As previously discussed (see "Downhole logging" in the "Methods" chapter), the presence of gas hydrate is generally characterized by increases in electrical resistivity and acoustic velocity that are not accompanied by a corresponding decrease in porosity. A decrease in porosity alone in a water-saturated sediment can result in an increase in resistivity and acoustic velocity. Resistivities logged in Holes U1328A and U1328C show a number of positive anomalies over a general increase of resistivity with depth without a corresponding decrease in porosity (Figs. F57, F58), suggesting that there are several intervals where gas hydrate may be present.

Water saturations from Archie's equation

To estimate the amount of gas hydrate at Site U1328, we used the Archie relation (e.g., Collett and Ladd, 2000)

S_w = [(a x R_w)/(^m x R_t)]^1/n,

where

• S_w = water saturation,
• a = tortuosity coefficient,
• R_w = formation water resistivity,
• = density porosity computed from the adnVISION enhanced resolution bulk density,
• m = cementation coefficient,
• R_t = GeoVISION high-resolution button deep average resistivity, and
• n = saturation coefficient.

We use the button deep resistivity instead of the ring resistivity because the button deep resistivity matches the resistivities with the greatest depth of investigation measured by the LWD EcoScope tool (except for having a higher resolution), whereas the ring resistivity is generally lower than the EcoScope deep resistivities.

Gas hydrate saturation (S_h) is the percentage of pore space in sediment occupied by gas hydrate, which is the complement of the water saturation S_w

S_h = 1 – S_w.

The procedure followed to estimate S_w with Archie's relation is illustrated in Figure F64. We first computed porosity from the density log as described above with a water density _w of 1.03 g/cm³ and a grain/matrix density _g of 2.76 g/cm³ (see "Physical properties").

To estimate the formation water resistivity (R_w), we started by constructing a salinity versus depth function based on the IW salinity measurements (see "Interstitial water geochemistry"). This salinity versus depth function consists of three linear segments fitted to the data (0–110, 110–230, and below 230 mbsf) plus a Gaussian peak centered at 12 mbsf that reproduces a shallow high-salinity anomaly. At every logging depth, we combined the salinity value with a formation temperature obtained from the geothermal gradient estimated from the downhole formation temperature measurements (see "In situ temperature profile"), and used the formulas of Fofonoff (1985) to obtain the corresponding value of the water resistivity R_w.

To estimate the water saturation S_w, we also need to choose values for the Archie coefficients a and m. One way to do this is to choose a logged interval where the sediments can be assumed to be water saturated and fit a and m to a plot of measured resistivity versus porosity, known as a "Pickett plot" (e.g., Doveton, 1994). In marine sediments, however, the range of porosity is relatively small and it is not possible to obtain a robust estimate of both a and m. We prefer to set a = 1, which is physically the most realistic value because it gives a resistivity equal to the formation water resistivity when the porosity is 100%. We then compute a log of "estimated m" given by

m_est = –log(F)/log(),

where F = (R_t/R_w) is the formation factor. This m_est curve should give the appropriate value to be used in Archie's law in water-saturated intervals and will give anomalously high values in intervals that contain hydrocarbons. A reasonable value of m = 2.6 can be chosen from the baseline trend of the m_est curve in Figure F64.

The next step is to compute the resistivity R₀ predicted by Archie's equation for a water-saturated formation of a given porosity, which is given by:

R₀ = (a x R_w)/^m.

Using Archie coefficients of a = 1 and m = 2.6, we computed an R₀ curve that generally follows the measured resistivity R_t, but is significantly lower than R_t in several intervals of logging Unit 1 (Fig. F64). Finally, we computed the water saturation S_w using a saturation coefficient n = 2.

The most notable intervals in logging Unit 1 that show a measured resistivity R_t greater than the resistivity R₀ predicted for water-saturated conditions are between the seafloor and 46 mbsf. In the first 15 m below the seafloor, porosities are 75% or higher and the inferred water saturations S_w are as low as 5%. Archie's relation suggests that where the measured resistivities are highest at 0–15 mbsf, most of the formation is composed of gas hydrate, with small amounts of sediment and pore water. These layers of high hydrate concentration alternate with layers of much higher water saturations, which are predicted to contain only small amounts of gas hydrate. Another interval where R_t is greater than the predicted water-saturated resistivity R₀ is at 92–97 mbsf, which is where the LWD resistivity images show a steeply dipping fracture that contains gas hydrate (see "Logging-while-drilling borehole images").

Comparison with infrared images

The IR core images taken on the catwalk measure the core liner temperature, and cold anomalies in the IR images caused by gas hydrate dissociation give an independent "map" of gas hydrate concentrations (see "Physical properties"). Figure F65 shows a comparison of the LWD and wireline resistivity logs with the IR image results. There are a number of correlations between temperature anomalies and layers of high resistivity. For example, the IR images show a cold interval at 215–222 mbsf that corresponds to a high-resistivity interval both on the LWD and wireline logs. There is also a 4 m high-resistivity peak at 217 mbsf in the wireline spherically focused resistivity log. This peak is unlikely to correspond to gas hydrate, however, because it is matched by a high-density peak (2.3 g/cm³; Fig. F58). The thin layer at 217 mbsf may be authigenic carbonate, which is occasionally found in gas hydrate–rich intervals (e.g., Medioli et al., 2005).

On the other hand, the layer with high resistivity and low water saturation from Archie's relationship observed in the Hole U1328A LWD logs at 90–98 mbsf is not seen on either the IR images or the wireline logs, both from Hole U1328C. The IR data may have missed this interval because of poor core recovery, but the lack of a high-resistivity layer in the wireline log points to horizontal variation. This is not unexpected, as the high-resistivity layer in the LWD image contains a steeply dipping fracture that is unlikely to be intersected at the same depth by Hole U1328C.

Analysis of sonic logging waveforms

The combined analysis of sonic velocities and waveform amplitudes (Fig. F59) can also help identify the occurrence of gas hydrate and/or free gas. In the interval between 210 and 220 mbsf, where resistivity data show a peak of ~2 m, V_P drops slightly while V_S derived from the upper dipole increases. This could be the result of the coexistence of free gas and gas hydrate in the vicinity of the BSR, which has been observed on the Blake Ridge (Guerin et al., 1999) and Hydrate Ridge (Tréhu, Bohrmann, Rack, Torres, et al., 2003). The presence of gas hydrate increases V_S, whereas V_P is affected by free gas in the pore fluid. The low amplitude of the high-frequency (~2.5 kHz) lower dipole waveforms over most of the interval above ~220 mbsf is similar to data recorded on the Blake Ridge and Hydrate Ridge, and could indicate the particularly strong effect of gas hydrate on shear attenuation in the higher dipole frequency range (Guerin and Goldberg, 2005).

Identification of free gas

The Archie-based analysis of Figure F64 does not show any clear evidence for free gas below the BSR (~219 mbsf) at Site U1328. There are some indications of free gas, however, if we integrate information from the LWD sonic coherence and fluid pressure, and from the wireline V_P , resistivity, density, and neutron logs. Three intervals (219–222, 230–236, and 260–265 mbsf) in the wireline sonic log show a low P-wave velocity of ~1500 m/s, and the shallower of these intervals corresponds to a peak of ~2 m in the wireline resistivity (Fig. F58). These two intervals coincide with negative fluid pressure anomalies in the LWD/MWD APWD log, and the shallower interval also displays a significant loss in coherence for the drilling fluid arrivals in the LWD/MWD sonic waveforms (Fig. F56). Moreover, in these two intervals the neutron porosity, which normally is well above the density porosity, decreases and converges to values near those of the density porosity (Fig. F58). This may be a manifestation of "neutron-density crossover," a gas effect well known in logging operations (e.g., Schlumberger, 1989). If the formation contains free gas, its bulk density will decrease and the density porosity calculated assuming water is in the pores will increase. At the same time, the porosity measured by the neutron log, which depends on hydrogen density, will decrease because gas has much less hydrogen per unit volume than water. The neutron porosity and the density porosity will cross over and clearly mark the free gas zone.

The evidence for free gas from the wireline logs at 219–222, 230–236, and 260–265 mbsf, however, is not conclusive. Neutron porosity does decrease as predicted by the gas effect, but the density porosity does not show a corresponding increase (Fig. F58). The convergence between neutron and density porosity may be caused by a decrease in clay content, which also affects the neutron log. In addition, the low V_P observed in these two intervals coincides with a low V_S, which should not be affected by free gas. Nevertheless, low V_P and V_S have been observed in well-defined, gas-bearing intervals and might be the result of loss of cohesion induced by drilling in a slightly overpressured interval. The most convincing evidence for free gas between 230 and 265 mbsf comes from the LWD drilling fluid pressure and sound velocity (Fig. F56).

Vertical seismic profile

Operations

VSP operations in Hole U1328C started with rig up of the WST, which was completed at 0335 h on 15 October 2005. After rig up, the WSTP was lowered in the drill pipe just above the mudline (~1280 mbrf), awaiting daybreak. The first GI air gun shots were fired at 0830 h, 1 h after the start of marine mammal observations, ramping up to the desired operational intensity over 30 min. After a short delay to troubleshoot the air gun triggering, the WSTP was lowered below the mudline at 0952 h, reaching a maximum depth of 1569 mbrf (290 mbsf) at 1009 h.

The first downhole VSP shots were fired at 1016 h, with a plan to try a station every 5 m and move along if there was no success in obtaining good coupling between the tool and the borehole wall. We fired a total of 305 shots and recorded 35 successful stations between 1565 and 1385 mbrf (286 and 106 mbsf), shooting seven times or more at each station for subsequent stacking. Only two 5 m intervals were missed (Table T25), where a good coupling could not be achieved because of enlarged borehole conditions (see the caliper log in Fig. F58). The ship's heave stayed between 2.5 and 3 m, and did not affect data acquisition. At depths shallower than 1385 mbrf (106 mbsf), the tool still achieved good coupling with the borehole wall, as shown by the tension decrease when the cable was slacked. Nevertheless, noise probably transmitted through the drill pipe (the bit was at 1339 mbrf or 60 mbsf), and/or the cable made it impossible to measure waveforms of acceptable quality. We decided to terminate VSP data acquisition at 1402 h, and brought the WSTP back to the rig floor at 1445 hr. Rig down was completed by 1545 h on 15 October 15.

Time versus depth relationship, interval velocity estimation, and depth to the bottom-simulating reflector

The VSP provides a direct measurement of the time versus depth relationship in the first break of the direct compressional wave arrival. Figure F66 shows the stacked waveforms at the 35 stations successfully recorded. Stacking was accomplished by taking the median of the seven or more waveforms recorded at each station. The first break times can be easily seen, and are marked by red crosses in Figure F66. Table T25 lists the first break times and the times corrected for the 50 m horizontal offset of the source, the depth of the gun 2 m below sea level, and the delay between triggering and firing. Corrected first break times are traveltimes from the sea surface to each receiver along a vertical path.

The corrected first break times are plotted in Figure F67 versus the depth of the receivers. The error bars show an uncertainty in the picked first break times of ±1 ms, which combines traveltime errors caused by uncertainties in the depth of the receiver and fluctuations in the vertical source position caused by waves (the latter errors are reduced but not eliminated by stacking). In a medium where the velocity is constant over the vertical range spanned by the receivers, the first break times follow a straight line whose slope gives the velocity. The first break times of Figure F67 follow a line within the ±1 ms error defining a velocity of 1645 m/s for all receivers between 106 and 286 mbsf. This uniform velocity is in contrast with the results from the Hole U1327D VSP, which clearly showed a high-velocity layer (1843 m/s) above a layer with low velocities characteristic of free gas (1281 m/s), separated by a discontinuity at 245–260 mbsf that defines the location of the BSR (see "Vertical seismic profile" in the "Site U1327" chapter). In the Hole U1328C VSP, there is no clear velocity contrast around the expected depth of the BSR (~219 mbsf).

To compare the VSP results to other velocity measurements, it is useful to translate the first break times into interval velocities. In principle, the vertical distance between two receivers divided by the difference between the two first break traveltimes should give an immediate estimate of the interval velocity. In practice, the first break picks are not exact, and small errors in time picking can translate to large errors in the estimated velocities. A reliable estimate of interval velocity requires an inversion that combines the first break measurements and some smoothness constraint. We use here a Bayesian inversion method where the smoothness of the final solution is determined by the data, whereas the standard deviation of the traveltime measurements is fixed to 1 ms (see above). For method details, see Malinverno and Briggs (2004). The inversion is applied to determine the interval velocity in a layered medium with 5 m thick layers.

The results are shown in Figure F68, which compares interval velocities from the VSP in Hole U1328C with P-wave velocities from the sonic log in the same hole. In the VSP interval velocity inversion (Fig. F68A), the solid line shows the best estimate of compressional wave velocity and the dashed lines show its uncertainty defined by the posterior standard deviation. The posterior standard deviation measures the uncertainty in the estimated velocity caused by uncertainty in the traveltime picks. Figure F68B compares the P-wave velocities measured in two passes of the DSI tool to the best estimate of P-wave velocity from the VSP first breaks. The sonic log and VSP velocities are generally similar, with the VSP measuring a slightly higher velocity than the sonic log between 120 and 155 mbsf. This could be the result of the VSP being affected by velocities farther away from the borehole compared to the shallow depth of investigation of the sonic tool. In any case, both the sonic log and VSP data agree in showing only small P-wave velocity variations between 105 and 290 mbsf, and no sign of the sizable 36% velocity decrease associated with free gas at Site U1327.

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