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doi:10.2204/iodp.proc.340.105.2013

Downhole logging

Two tool strings were used to log a 120 m section of Hole U1395B over a period of ~12 h (Fig. F14). The borehole remained in good condition while downhole logging measurements—including gamma ray, density, electrical resistivity and images, and velocity—were obtained.

Operations

Downhole logging operations commenced on the morning of 16 March 2012 after sweeping and displacing Hole U1395B with heavy logging mud and pulling the pipe to a bit depth of 82.3 mbsf. The first tool string deployed was the triple combo-Magnetic Susceptibility Sonde (MSS) tool string, which included the Hostile Environment Natural Gamma Ray Sonde (HNGS), Hostile Environment Litho-Density Sonde (HLDS), High-Resolution Laterolog Array (HRLA), and MSS (see “Downhole logging” section in the “Methods” chapter [Expedition 340 Scientists, 2013a] for tool string details). Having experienced good hole conditions throughout the coring of both Holes U1395A and U1395B, the ¹³⁷Cs source was deployed with the HLDS sonde, allowing for acquisition of a density measurement. The tool string was lowered to a total depth of 204.8 mbsf, from where an initial uplog at 900 ft/h was completed to ~115 mbsf. This was followed by a repeat pass from total depth back through the bit at the same speed. The tools were rigged down by 1100 h. Unfortunately the MSS malfunctioned during this run despite tool diagnostics being normal, so resulting susceptibility data are unusable.

The FMS-sonic tool string (see Fig. F11 in the “Methods” chapter [Expedition 340 Scientists, 2013a]) was the second tool string to be run in this hole, for the purpose of acquiring acoustic velocity and electrical image data. Two uplogs were completed through the open hole section at a speed of 1800 ft/h, after which the tool string was pulled to the rig floor. Logging operations were completed at 1615 h.

Data processing and quality assessment

A depth shift to seafloor was applied to all logs, with the seafloor identified from the gamma ray data sets. A subtle inflection in the gamma ray (from the Enhanced Digital Telemetry Cartridge [EDTC] and HNGS) associated with the seafloor was observed at 1197.7 mbsl. Following the shift to seafloor, curves were depth-matched using the gamma ray from the HNGS recorded during the main pass of the triple combo as the reference curve. Logs from the other tool string passes (triple combo and FMS-sonic) were matched to this log through identification of common features through the logged section.

Caliper data from both tool strings (from the HLDS and FMS) indicate that Hole U1395B is relatively in gauge, with a similar profile to Hole U1394B. Hole diameter ranges from ~11 inches at the base of the hole to a slightly flared region (≤17 inches in diameter) just below the bit (Figs. F15, F16). This range in diameter and the fact the borehole profile is relatively smooth means that borehole conditions are reasonably favorable for the acquisition of good downhole measurements.

The quality of the logs can also be assessed by comparison with measurements made on cores from the same hole. Figure F15 shows a comparison of the gamma ray and density logs with the natural gamma ray track core logging data and with the moisture and density (MAD) measurements made on cores. The different sets of measurements display reasonably good agreement where core recovery was excellent. Between 119 and ~125 mbsf, logging data show higher values for gamma ray and density than the respective core data, and below 125 mbsf, there is very little core data to compare because of incomplete recovery. Because of the in situ nature of the gamma ray and density logs, their values should be equal or slightly higher than the same measurements made on the recovered core.

The repeatability of all of the logging measurements is very good between the first and second passes for both tool strings (Fig. F17). In addition, gamma ray measurements from both tool strings agree well (Fig. F16).

HRLA resistivity data are of generally very good quality, with logs from the different depths of investigation agreeing very well (Fig. F15). In the interval from ~99 to 112 mbsf, the log response indicates a high-frequency noise overprint in the data. This may be a function of the tool dynamics and the proximity to the pipe. The overall trend through this interval is reliable; however, the finer detail should be ignored.

Dipole Shear Sonic Imager (DSI) modes for this hole were the same as for Hole U1394B, with the monopole and upper dipole set to standard frequency and the lower dipole set to low frequency. Given the nature of the recovered cores, it was hoped this would optimize data quality. Compressional wave velocity (V_P) logs are of good quality, and values are highly coherent (Fig. F16). However, shear wave velocity (V_S) data for this hole will need some postcruise processing, particularly in the interval from 85 to 128 mbsf. V_S values between ~400 and 900 m/s in this interval are very coherent and can be trusted, but V_S values that exceed 900 m/s are anomalous and should not be interpreted.

FMS image quality relies on good contact between the tool’s pads and the borehole walls. This should have been achieved in this in-gauge hole, and the resulting images indicate that this was generally the case. Unfortunately, toward the top of the open hole section the cable tension on the wireline increased and the calipers were closed through a 5 m interval. As such, coverage of the 120 m section is not as extensive as planned.

Logging stratigraphy

Three logging units have been identified on the basis of characteristic features and trends in the downhole measurements data set. These units are identified in Figures F15, F16, and F18, and a description of their geophysical properties follows.

Logging Unit 1

Logging Unit 1, from 85 to 112 mbsf, is characterized by relatively consistent downhole profiles in density, resistivity, and V_P , all of which exhibit low-amplitude variability. Gamma ray data over this interval fluctuate around a mean value of 20 gAPI. Density appears to have a step increase at ~92 mbsf, but this is most likely a response to the closing of the eccentralizing caliper at this depth at the end of the logging run. Discarding density data from 85 to 92 mbsf gives a mean value of 1.8 g/cm³ in this logging unit. Resistivity in the upper part of this unit has an average value of 1.32 Ωm. At ~100 mbsf there is a subtle shift in the profile to lower resistivity values (mean = 1.26 Ωm). V_P demonstrates a slight increase with depth from ~1550 m/s at the top of the unit to ~1620 m/s at the base.

Logging Unit 2

Logging Unit 2 is distinguished from overlying logging Unit 1 at 112 mbsf on the basis of a sharp decrease in gamma ray, increases in density and resistivity, and a broad change in the character of the logging profiles. Gamma ray, density, resistivity, and V_P all show high-amplitude variations throughout this unit, relative to Unit 1. Logging Unit 2 is further divided based on the relative responses of the different geophysical properties.

Logging Subunit 2A

Gamma ray in logging Subunit 2A (112–137 mbsf) generally increases with depth and has a mean gamma ray value of 19 gAPI. The profile is punctuated by a series of intervals of lower gamma ray values (Fig. F15) that coincide with increases in density, resistivity, and V_P . These features may correspond to the turbidites observed in cores recovered from Holes U1395A and U1395B (see “Lithostratigraphy”). Resistivity values subtly increase with depth (average = 1.5 Ωm), a trend that is echoed by the density values (mean = 1.9 g/cm³). Both of these profiles are easily discerned from the overlying unit by some significant positive departures (notably at ~113 and ~119 mbsf) from these subtle trends.

Logging Subunit 2B

Logging Subunit 2B is distinguished from Subunit 2A for two main reasons. First, the profiles do not exhibit a net change with depth, but rather vary around a constant value. Second, the subunit is characterized by a series of gamma ray highs that do not coincide with changes in the other data sets in a consistent way. Broadly, there are two classes of gamma ray highs identified: (1) those that correspond to increases in density, resistivity and V_P (for example at ~140, 145, and 162 mbsf) and (2) those that coincide with decreases in density and resistivity and minor increases in V_P (for example at ~149 and 161 mbsf). Average values of gamma ray and V_P are somewhat elevated with respect to the overlying Subunit 2A (20 gAPI and ~1800 m/s, respectively), whereas mean density and resistivity values are slightly lower (1.82 g/cm³ and 1.4 Ωm, respectively).

Logging Unit 3

Logging Unit 3 (163–202 mbsf) is characterized by a return to lower frequency variations in all downhole logging measurements, similar to what is seen in logging Unit 1. Gamma ray values range from ~17 to 27 gAPI with an average similar to the overlying units (20 gAPI). Density exhibits very low amplitude variation around a mean value of 1.73 g/cm³. Resistivity is relatively invariant with depth in this logging unit, with a mean value of 1.2 Ωm. Because the HRLA tool is located lower in the tool string, resistivity measurements extend deeper than the other data types. A highly resistive layer centered on 188 mbsf may coincide with the recovery of a partial core (340-U1395B-24H) containing lithified material. V_P data in logging Unit 3 show a subtle increase with depth and are similar in character to that of logging Unit 1. However, the average value (~1800 m/s) is elevated in the lower unit, which is consistent with what might be expected with a classical compaction trend.

Formation MicroScanner images

Figure F19 gives some examples of FMS images acquired from Hole U1395B. Across the logged section a range of textures and features can be identified, including some dipping surfaces and some highly resistive layers. FMS image data are only available for most of logging Units 2 and 3, from ~112 to 202 mbsf. In the statically processed images (Figs. F16, F19), the range of resistivities encountered in logging Unit 2 is greater than that in logging Unit 3. The top half of Subunit 2A is characterized by sharply contrasting boundaries with some very resistive and also very conductive layers. In contrast, the lower half of the unit exhibits more subtle variations toward the more resistive end of the spectrum. Subunit 2B is marked by a shift back to more definite changes across boundaries and is punctuated by some very resistive layers, most notably at ~150–153 mbsf.

Logging Unit 3 is dominated by material with moderate to low resistivity. Boundaries between different layers range from gradational to sharp, in terms of resistivity.

Integrating the textures and structures seen on the FMS images with images of the core is difficult because of poor recovery through much of the logged section. However, using them in combination with the rest of the Hole U1395B data set may help to elucidate the nature of some of the missing core.

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