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doi:10.2204/iodp.pr.320T.2009

Methods

We have included a brief methodology section in the Preliminary Report because a Proceedings volume will not be produced for Expedition 320T.

Lithostratigraphy

This section outlines the procedures used to document the lithology of the sediment material recovered during Expedition 320T, including visual core description, smear slide description, digital color imaging, and color spectrophotometry. It is modeled after the Methods chapter for IODP Expedition 306 Sites U1312 and U1313 (Expedition 306 Scientists, 2006) with only general procedures outlined, highlighting new systems and equipment used and evaluated during the transit.

Overview of new core description process and integration of databases

A new core description process was implemented and assessed during Expedition 320T using the DESClogik application. The use of visual core description forms (VCDs) for hand-written descriptions has been supplanted by direct entry of descriptive and interpretive information into the DESClogik program through the Tabular Data Capture (TDC) mode. Prior to core description, a spreadsheet template was constructed in TDC (Fig. F3). Tabs and columns established within this program were customized to include many of the former VCD form information categories (e.g., lithology, drilling disturbance, and bioturbation), as well as other information that allows for complete documentation of the observations made during core description in the new database format. A second template was developed specifically for recording smear slide data (Fig. F4), which contains category columns similar to the former smear slide forms to record texture and relative abundance of biogenic and minerals components.

Visual core description

Lithology

Sediments recovered during Expedition 320T are composed of >60% pelagic components and described as pelagic sediments using the classification scheme of Mazzullo et al. (1988). These sediments comprise the skeletal debris of open-marine calcareous and siliceous microfauna (e.g., foraminifers and radiolarians, respectively) and microflora (e.g., calcareous nannofossils and diatoms, respectively) and associated organisms. The lithologic names assigned to these sediments consist of a principal name based on composition, degree of lithification, and/or texture as determined from visual description of the cores and from smear slide observations. For sediment that is a mixture of components, the principal name is preceded by major modifiers (in order of increasing abundance) that refer to components making up ≥25% of the sediment. Minor components that represent between 10% and 25% of the sediment follow the principal name (after a "with") in order of increasing abundance. In the Expedition 320T classification of biogenic carbonate oozes, only one category was used: nannofossil ooze (content of nannofossils = >50%). The term ooze (rather than chalk) was applied because all the sediment lithification was soft (i.e., could be easily deformed by touch).

Sediment color was determined qualitatively for core intervals using Munsell Color Charts (Munsell Color Company, Inc., 1991, 1994). Toothpick samples were taken at selected intervals in the core and used to create smear slides according to the method outlined in Mazzullo et al. (1988). Note that owing to the absence of a reticulated eyepiece on the microscope in the core description area during core description, no determinations of texture were made (4 μm silt/clay boundary was difficult to estimate). The petrographic determinations of the proportion of sediment components as tabulated in DESClogik were used along with visual descriptions to determine sediment lithology. The DESClogik database included percentages of biogenic and mineral components, as well as whether the sample represents a major or a minor lithology in the core.

Sedimentary structures and diagenetic and other features

The locations and types of bedding features, bedding styles, and other sedimentary structures, as well as diagenetic features, visible on the prepared surfaces of the split cores were recorded by core interval in the DESClogik spreadsheet. Those observed in the Expedition 320T cores are listed in Figure F5, along with the symbol used to depict them in the summary (barrel) sheets created using the Strater application (see "Summary (barrel) sheets").

The bioturbation classification scheme employed during Expedition 320T defined three levels (moderate, slight, and absent) of bioturbation intensity modified from Mazzullo et al. (1988). Note, however, that in a homogeneous uniformly colored sediment section it may not be possible to differentiate "absent" from "homogenized" by bioturbation.

Intervals of core ~10 cm or more exhibiting drilling-related sediment disturbance were recorded in the DESClogik spreadsheet with the type and degree of disturbance described using the following categories (modified from Mazzullo et al., 1988): biscuited, slight, moderate, mousselike, very disturbed, soupy, and fractured.

Summary (barrel) sheets

DESClogik includes a graphic display (GD) mode for core data (e.g., digital images of section halves and measurement data) that can be used to augment core description. The end product of DESClogik, a data spreadsheet, must be manipulated using the LIMS2Excel application before importing the data into another program, Strater, to produce a publication-quality, simplified, annotated standard graphic report of the core, similar to the "barrel sheets" created by AppleCORE on prior expeditions/legs. During Expedition 320T, Strater reports were created at the core level and included descriptive, graphical interpretation, core image, and biostratigraphic zone data. Lithologies and sedimentary structures of the core intervals recovered are represented on barrel sheets by graphic patterns in the Graphic Lithology column using the symbols illustrated in Figure F5.

Digital color imaging

The Section-Half Imaging Logger (SHIL) captures continuous high-resolution images of section-half surfaces for analysis, description, and reporting of recovered core material. The system uses a commercial line scan camera, a specially assembled light emitting diode (LED)–based illumination subsystem for maximum image quality, and an optical bar code scanner to identify the section halves from the physical label on the core liner. Capturing images is made very simple for the user. With minimum user interaction, the image logger control software saves the original high-resolution image with gray scale and ruler as well as a cropped and reduced image for easy handling and display in many applications.

Spectrophotometry (color reflectance)

The Section-Half Multisensor Logger (SHMSL) integrates multiple sensors for the measurement of bulk physical properties in a motorized and computer-controlled core section logging machine. The sensors included in the SHMSL are reflectance spectroscopy and colorimetry (RSC) and magnetic susceptibility. We attempted to use this equipment, but technical issues prevented us from collecting meaningful data. We were, however, able to assess its use in the context of core flow.

Sampling plan

The sampling plan for cores from Hole U1330A was to take two interstitial water samples and two moisture and density (MAD) samples per core. Samples for whole-rock analysis were taken on an as-needed basis (i.e., to determine chemical variations within and between cores). Headspace gas analysis was conducted on each core. The sampling plan for Hole U1330B was similar, except we took one interstitial water and MAD sample per core.

X-ray diffraction analysis

No X-ray diffraction analyses were conducted during the expedition owing to equipment nonfunction.

Biostratigraphy

A standard, simplified process for micropaleontological analyses was carried out during Expedition 320T. Core catcher (CC) samples were soaked in water and briefly placed on a hot plate with a small volume of hydrogen peroxide added to disaggregate the nannofossil ooze. Samples were then washed over a 63 μm sieve and dried in a warm oven. Washed residues from all core catchers were examined by stereo binocular microscopy. The relative abundances of most taxa present in the assemblages of planktonic foraminifers were estimated and tabulated using the DESClogik application. Relative abundance values are as follows:

A = abundant (>25%).

C = common (11%–25%).

F = few (6%–10%).

R = rare (1%–5%).

VR = very rare (<1%).

The planktonic foraminifer biostratigraphy and zonal scheme are based on Berggren et al. (1995) and the astrochronologically tuned datums of Chaisson and Pearson (1997). FO = first occurrence datum, and LO = last occurrence datum.

Geochemistry

During Expedition 320T, the following geochemical analyses were performed on cores taken from Holes U1330A and U1330B:

• Alkalinity on interstitial waters,

• Elemental concentrations of interstitial water samples,

• Headspace gas composition, and

• Elemental whole-rock compositions.

Methods for each of these analyses are given below. The analytical methods followed the procedures outlined in the user guides for the various pieces of analytical equipment. Carbonate content was not determined because of a lack of silver nitrate reagent on board.

Interstitial water analysis

Interstitial water samples were obtained by sampling whole-round pieces (~10 cm long) of the core and taking them immediately to the Chemistry Laboratory for processing. Processing involved scraping away the outer few centimeters of the core using a plastic spatula (to remove any smeared sediment and contamination). From both APC and RCB cores, sediment was removed until the core consistency appeared to be firm, or at least significantly firmer than the outside. After removing the potentially contaminated material, the remaining sediment was placed into a sediment squeezer with a titanium case/pressure plate/mesh with a stainless piston and base, modified after the stainless steel squeezer of Manheim and Sayles (1974). In most cases, gauge pressures up to 20 MPa were applied using a laboratory hydraulic press to extract interstitial water. The interstitial water was filtered through a prewashed Whatman Number 1 filter fitted above a titanium screen, filtered through a 0.45 μm Whatman polyethersulfone disposable filter, and subsequently extruded into a precleaned (10% HCl) 100 mL plastic syringe attached to the bottom of the squeezer assembly.

Interstitial water samples were analyzed for alkalinity, pH, and elemental composition. Alkalinity and pH were measured by Gran titration with a Brinkman pH electrode and a Metrohm 702 SM autotitrator using 0.1 N HCl. The alkalinity analysis is a measure of how much acid it takes to lower the pH of the water sample enough to convert all bicarbonate (HCO₃^–) and carbonate (CO₃^2–) to carbonic acid (H₂CO₃), given that almost all alkalinity (~97%) in seawater is due to carbonate. The electrode was calibrated at pH 4, 7, and 10 prior to the expedition.

For inductively coupled plasma–atomic emission spectroscopy (ICP-AES) analyses, interstitial water samples were immediately acidified using clean (trace metal grade) concentrated nitric acid (10 μL of acid per 1 mL of sample) to prevent Fe precipitation. Samples were diluted to 1:10 (1 mL sample + 9 mL nannopure water) immediately prior to being analyzed and placed in scintillation vials reserved for ICP pore water analysis. Samples were run on a Leeman Teledyne Labs Prodigy high-efficiency ICP-AES that allowed simultaneous detection of different wavelengths and detection in both axial and radial positions. Samples were run in two batches: major elements (Na, K, Mg, and Ca), where the samples underwent a 100-fold dilution, and minor elements (Li, B, Sr, Mn, and Fe), where the samples underwent a 25-fold dilution. All analyses were conducted in 5% HNO₃ acid. Major elements were standardized using International Association for the Physical Sciences of the Ocean (IAPSO) seawater (at dilutions of 0, 1, 3, 5, 50, 100, and 120). Minor elements in interstitial water samples were only run in test mode and are not reported here.

Whole rock analyses

ICP-AES analysis of whole rock samples quantified the major (Al, Ca, Fe, K, Mg, Mn, Na, P, Si, and Ti) and trace (Ba, Cr, Ni, Sc, Sr, V, Y, and Zr) elements. Samples were prepared by flux-fusion using a lithium metaborate flux (from rock to bead) and then dissolution of the bead (from bead to solution). An aliquot of 100 mg of the ignited sample powder was weighed and added to a vial containing lithium metaborate flux that was preweighed on shore to 400 mg. The combination of sample powder and flux was then fused to make a glass bead that was dissolved in nitric acid (via a bench-top hand shaker). The resulting fluid was analyzed by ICP. Weighing the ignited sample is a fairly critical step. The weight should be as close to 100 mg as possible. Inaccuracies in the weight will affect the analytical results. The sample bead was dissolved in 50 g (50 mL) of nitric acid to yield a dilution factor of 100 for the sample plus flux and a dilution factor of 500 for the sample. This solution was further diluted in a 2.5 mL sample (17.5 mL 10% HNO₃).

Headspace gas

Concentrations of methane through propane hydrocarbon gases were monitored at intervals of one sample per core. The standard gas analysis program for safety and pollution prevention purposes was conducted (Kvenvolden and McDonald, 1986). Samples for headspace analysis were collected with interstitial water samples to integrate the interstitial water and gas data sets. For the required safety analysis, a 3 cm³ bulk sediment sample from a freshly exposed end of a core section was collected upon core removal using a brass boring tool or plastic syringe and then extruded into a 20 mL headspace vial and immediately capped with a silicone/polytetrafluoroethylene (PTFE) septum, which was sealed with an aluminum crimp cap. The vial was then heated to 80°C for ~30 min prior to analysis.

Gas chromatography (GC) analyses of headspace samples was performed in the following way. A 5 mL volume of headspace gas was extracted from the sealed sample vial using a standard gas syringe and directly injected into the GC. The headspace gas samples were analyzed using the GC3 chromatograph, an Agilent 6890 GC equipped with an 8 ft x 1/8 inch stainless steel column packed with HayeSep R (80–100 mesh) and a flame ionization detector (FID). Some samples were analyzed for higher molecular weight hydrocarbons by injection into the natural gas analyzer, a modified Hewlett Packard 5890 II Plus GC with an FID and a thermal conductivity detector. Concentrations of methane, ethane, ethene, propane, and propene were obtained. The carrier gas was helium, and the GC oven was programmed from 100°C (5.5 min hold) to 140°C (4 min hold) at a rate of 50°C/min. Data were processed using Agilent Chemstation software.

The concentration of dissolved methane, both in the safety and refined protocols, was derived from the headspace concentration by the following equation:

where

V_H = volume of the sample vial headspace,

V_S = volume of the whole sediment sample,

χ_M = molar fraction of methane in the headspace gas (obtained from GC analysis),

χ_bkg = molar fraction of methane in headspace gas because of background,

P_atm = pressure in the vial headspace (assumed to be the measured atmospheric pressure when the vials were sealed),

R = the universal gas constant,

T = temperature of the vial headspace in degrees Kelvin, and

ϕ = sediment porosity (determined either from MAD measurements on adjacent samples or from porosity estimates derived from gamma ray attenuation [GRA] data representative of the sampled interval).

Physical properties

Physical properties were measured on core material recovered during Expedition 320T to test the new installation of the shipboard measurement systems and the ability of the users to upload and retrieve data from the Laboratory Information Management System (LIMS).

Magnetic susceptibility, GRA bulk density, compressional wave velocity (V_P), and natural gamma ray activity were measured on the whole-core samples. Thermal conductivity was not measured because of problems with the new system (see "Thermal conductivity"). Split-core measurements on the working half of the core included V_P with the IODP P-wave sensor system (PWS-3; three measurement directions in soft sediment), sediment strength with the automated vane shear (AVS) system, and MAD. A comprehensive description of the methodologies and calculations used in the JOIDES Resolution physical properties laboratory can be found in Blum (1997). Manuals and quick start guides are available upon request from the USIO.

Multisensor track sampling strategy

Magnetic susceptibility, compressional wave velocity, and GRA bulk density were measured nondestructively with the Whole-Round Multisensor Logger (WRMSL) on all whole-round core sections. To optimize WRMSL performance, sampling intervals and measurement residence times were the same for all sensors for any one core. Sampling intervals were therefore set at 5 cm so that a 9.5 m long core would take ~1 h to pass through the WRMSL with a residence time of 3 s for each measurement. These sampling intervals are common denominators of the distances between the sensors installed on the WRMSL (30–50 cm) and allow truly simultaneous measurements and optimal use of total measurement times.

Magnetic susceptibility

Magnetic susceptibility is a measure of the degree to which a material can be magnetized by an external magnetic field. It provides information on the magnetic composition of the sediments that often can be related to mineralogical composition (e.g., terrigenous versus biogenic materials) and/or diagenetic overprinting. Magnetite and a few other iron oxides with ferromagnetic characteristics have a specific magnetic susceptibility several orders of magnitude higher than clay, which has paramagnetic properties. Carbonate, silica, water, and plastics (core liner) have small negative values of magnetic susceptibility. Sediments rich in biogenic carbonate and opal therefore have generally low magnetic susceptibility, even negative values, if practically no clay or magnetite is present. In such cases, measured values approach the detection limit of magnetic susceptibility meters.

Magnetic susceptibility was measured with the Bartington Instruments MS2C system on the WRMSL. The output of the magnetic susceptibility sensors can be set to centimeter-gram-second (cgs) units or SI units. The IODP standard is the SI setting. However, to actually obtain the dimensionless SI volume-specific magnetic susceptibility values, the instrument units stored in the IODP database must be multiplied by a correction factor to compensate for instrument scaling and the geometric ratio between core and loop dimensions, as described above.

Gamma ray attenuation bulk density

Bulk density reflects the combined effect of variations in porosity, grain density (dominant mineralogy), and coring disturbance. Porosity is mainly controlled by lithology and texture, compaction, and cementation (controlled by both mechanical and chemical processes).

The GRA densitometer uses a 10 mCi ¹³⁷Cs capsule as the gamma ray source (with the principal energy peak at 0.662 MeV) and a scintillation detector. The narrow collimated peak is attenuated as it passes through the center of the core. Incident photons are scattered by the electrons of the sediment material by Compton scattering.

The attenuation of the incident intensity (I0) is directly related to the electron density in the sediment core of diameter (D), which can be related to bulk density given the average attenuation coefficient (in micrometers) of the sediment (Evans, 1965; Harms and Choquette, 1965). Because the attenuation coefficient is similar for most common minerals and aluminum, bulk density is obtained through direct calibration of the densitometer using aluminum rods of different diameters mounted in a core liner that is filled with distilled water. The GRA densitometer has a spatial resolution of <1 cm.

Natural gamma radiation

The Natural Gamma Radiation Multisensor Logger (NGRL) was designed and built at the Texas A&M University IODP facility from 2006 to 2008. The NGRL measures gamma rays emitted from whole-round core sections, which arise primarily because of the decay of uranium, thorium, and potassium isotopes. Data generated from this instrument are used to augment geologic interpretations.

The main NGRL detector unit consists of 8 sodium iodide (NaI) scintillator detectors, 7 plastic scintillator detectors, 22 photomultipliers, and passive lead shielding. The NaI detectors are covered by at least 8 cm of lead shielding. In addition, lead separators (~7 cm of low-background lead) are positioned between the NaI detectors. Half of the lead shielding closest to the NaI detectors is composed of low-background lead, and the outer half is composed of regular (virgin) lead. In addition to this passive lead shielding, the NGRL employs a plastic scintillator to suppress the high-energy gamma and muon components of cosmic radiation by producing a veto signal when charged particles from cosmic radiation pass through the plastic scintillator.

A core section measurement run consists of two positions on each core section counted for at least 5 min each for a total of 16 measurements per section. Complete spectra for each measurement are uploaded to the LIMS.

Thermal conductivity

Thermal conductivity was not measured at Site U1330. We tried to use the Teka TK04 measurement system, which employs the transient linear heat source method with a needle probe that is inserted into the soft sediment. The TK04 uses an automated routine to find the conductivity by least-squares fitting to the measured temperature time series. No calibration is required for this system because each probe is calibrated prior to leaving the factory. The heating curves displayed on the computer screen while being collected appeared reasonable, but the software failed to calculate a value of thermal conductivity. We suspect that the parameters set to accept a measurement are perhaps set too narrowly. Measurements on a standard were satisfactory, so the system seems to be functioning.

P-wave velocity

P-wave velocity in marine sediments varies with lithology, porosity, and bulk density, state of stress (such as lithostatic pressure), fabric or degree of fracturing, degree of consolidation and lithification, occurrence and abundance of free gas and gas hydrate, and other properties. P-wave velocity was measured with two systems during Expedition 320T: the WRMSL-mounted P-wave logger (PWL) on whole-round cores and the PWS3 on every section of the split cores. All IODP P-wave piezoelectric transducers transmit a 500 kHz compressional wave pulse through the core at a repetition rate of 1 kHz.

Traveltime is determined by the software, which automatically picks the arrival of the first wavelet to a precision of 50 ns. It is a challenge for an automated routine to pick the first arrival of a potentially weak signal with significant background noise. The search method skips the first positive amplitude and finds the second positive amplitude using a detection threshold limit, typically set to 30% of the maximum amplitude of the signal. Then it finds the preceding zero crossing and subtracts one period to determine the first arrival. To avoid extremely weak signals, minimum signal strength can be set (typically to 0.02 V) and weaker signals ignored. To avoid cross-talk signals at the beginning of the record from the receiver, a delay (typically set to 0.01 ms) can be set to force the amplitude search to begin in the quiet interval preceding the first arrival. In addition, a trigger (typically 4 V) is selected to initiate the arrival search process, and the number of waveforms to be stacked (typically 5) can also be set. Linear voltage differential transducers determine length of the travel path.

The P-wave velocity systems require two types of calibration, one for the displacement of the transducers and one for the time offset. For the displacement calibration, five acrylic standards of different thickness are measured and the linear voltage-distance relationship determined using least-squares analyses. For the time offset calibration, room temperature water in a plastic bag is measured multiple times with different transducer displacements. The inverse of the regression slope is equal to the velocity of sound in water, and the intercept represents the delay in the transducers.

Moisture content and density

Samples of 10 cm³ were taken from the working-half sections with a piston minicorer and transferred into previously calibrated 10 mL glass vials. Usually one sample from the middle of Section 3 or 4 was taken. Wet and dry weights were determined with twin Mettler Toledo electronic balances, which compensate for the effect of the ship's motion on the balance and give a precision better than 1%. Samples were dried in a convection oven at a temperature of 105° ± 5°C for a period of 24 h.

Dry volume was measured in a helium-displacement penta-pycnometer with an uncertainty of 0.02 cm³. This equipment allowed the simultaneous analysis of four different samples and a calibration sphere and took ~15–20 min. Three measurements were averaged per sample. The calibration sphere was cycled from cell to cell of the pycnometer during each batch so that all cells could be checked for accuracy at least once every five runs.

Automated vane shear test

The Giesa Automated Vane System (AVS) consists of a controller and a gantry for shear vane insertion. A four-bladed vane is inserted into the split core and rotated at a constant rate (determined by the user) to determine the torque required to cause a cylindrical surface (having a diameter equal to the overall width of the vane) to be sheared by the vane. This destructive measurement is done in the working half, with the rotation axis parallel to the bedding plane. The torque required to shear the sediment along the vertical and horizontal edges of the vane is a relatively direct measure of the shear strength. Typical sampling rates are one per core section until the sediment becomes too firm for insertion of the vane.

Downhole measurements

The downhole logging program during Expedition 320T was specifically designed to determine the efficiency of the new WHC system on the JOIDES Resolution by using uphole (surface) and downhole acceleration data to evaluate the efficiency of the heave compensation and by comparing logging data collected on Leg 130 from Site 807 to data collected onboard. In addition the MSS, a new third-party LDEO tool, needed testing in readiness for deployment during Expedition 320. Several wireline logging tools were deployed as described below.

Wireline logging tools and tool strings

Individual logging tools were joined together into tool strings so that several measurements could be made during each logging run. Tool strings were lowered to the bottom of the borehole and data were logged as the tool string was pulled back up the hole. Repeat runs were made to confirm the accuracy of log data. The following tool strings were deployed in Hole U1330A (Fig. F6):

1. The trial combination string (gamma ray, acceleration, and resistivity measurements), which consisted of the HNGS, HLDS (without radioactive source, used for caliper measurement only), GPIT, and DIT-E.

2. The FMS tool string, composed of the HNGS and FMS with the GPIT.

3. The MSS tool string, made of the HNGS, HLDS (without source, used for caliper measurement only), GPIT, and MSS.
   

Principles and uses of the wireline logging tools

The properties measured by each tool, sampling intervals, and vertical resolutions are summarized in Table T3. Explanations of tool name acronyms and their measurement units are summarized in Table T4. More detailed descriptions of individual logging tools and their geological applications can be found in Ellis (1987), Goldberg (1997), Rider (1996), Schlumberger (1989, 1994), Serra (1984, 1986, 1989), and the LDEO-Borehole Research Group (BRG) Logging Services Manual (LSM) (2001).

Hostile Environment Gamma Ray Sonde

The HNGS measures natural gamma radiation from isotopes of potassium, thorium, and uranium and uses a five-window spectroscopic analysis to determine concentrations of radioactive potassium (in weight percent), thorium (in parts per million), and uranium (in parts per million). The HNGS uses two bismuth germanate scintillation detectors for gamma ray detection with full spectral processing. The HNGS also provides a measure of the total gamma ray emission and uranium-free or computed gamma ray (CGR) emission that are measured in American Petroleum Institute units (gAPI). The HNGS response is influenced by the borehole diameter. HNGS data are corrected for borehole diameter variations during acquisition.

Hostile Environment Litho-Density Sonde

The HLDS normally consists of a radioactive cesium (¹³⁷Cs) gamma ray source (622 keV) and far- and near-gamma ray detectors mounted on a shielded skid, which is pressed against the borehole wall by a hydraulically activated eccentralizing arm. Radioactive sources were not used on this expedition, but the tool was used for its eccentralizing arm (or caliper) to measure the diameter of the borehole and to aid with pressing the MSS tool sensor against the borehole wall. In standard deployment, gamma rays emitted by the source experience both Compton scattering and photoelectric absorption. The number of scattered gamma rays that reach the detectors is directly related to the number of electrons in the formation, which is related to bulk density. Porosity may also be derived from this bulk density if the matrix density is known. The HLDS also measures the photoelectric effect factor (PEF) caused by absorption of low-energy gamma rays. Photoelectric absorption occurs when gamma rays reach <150 keV after being repeatedly scattered by electrons in the formation. PEF depends on the atomic number of the elements in the formation; thus, the PEF varies according to the chemical composition of the formation. Coupling between the tool and borehole wall is essential for high-quality HLDS logs. Poor contact results in underestimation of density values. Both density correction and caliper measurement of the hole are used to check the contact quality.

Phasor Dual Induction Spherically Focused Resistivity Tool

The DIT-E provides three different measurements of electrical resistivity, each with a different depth of penetration into the formation. Two induction devices (deep and medium resistivity) transmit high-frequency alternating currents through transmitter coils, creating magnetic fields that induce secondary (Foucault) currents in the formation. These ground-loop currents produce new inductive signals, proportional to the conductivity of the formation, which are measured by the receiving coils. The measured conductivities are then converted to resistivity. A third device, a spherically focused resistivity instrument, gives higher vertical resolution, as it measures the current necessary to maintain a constant voltage drop across a fixed interval.

Formation MicroScanner

The FMS produces high-resolution images of borehole wall microresistivity that can be used for detailed lithostratigraphic or structural interpretation. This tool has four orthogonally oriented pads, each with 16 button electrodes that are pressed against the borehole walls. Good contact with the borehole wall is necessary for acquiring good-quality data. Approximately 30% of a borehole with a diameter of 25 cm is imaged during a single pass. Coverage may be increased by a second run. The vertical resolution of FMS images is ~5 mm. Resistivity measurements are converted to color or gray-scale images for display. Local contrast in FMS images was improved by applying dynamic normalization to the FMS data. A linear gain is applied, which keeps a constant mean and standard deviation within a sliding window of 2 m. FMS images are oriented to magnetic north using the GPIT, assuming that the GPIT can locate magnetic north in magnetite-rich environments (see "General Purpose Inclinometer Tool" for more details). This method allows the dip and strike of interpreted geological features intersecting the hole to be measured from processed FMS images.

General Purpose Inclinometer Tool

The GPIT is included in the FMS and MSS tool strings to calculate tool acceleration and orientation during logging. Tool orientation is defined by three parameters: tool deviation, tool azimuth, and relative bearing. The GPIT utilizes a three-axis inclinometer and a three-axis fluxgate magnetometer to record the orientation of the FMS images as the magnetometer records the magnetic field components (F_x, F_y, and F_z). Corrections for cable stretching and/or ship heave using acceleration data (A_x, A_y, and A_z) allow precise determinations of log depths. A hydraulic WHC designed to adjust for rig motion during logging operations minimizes ship heave.

Magnetic Susceptibility Sonde

The MSS, a new wireline tool designed by LDEO, measures the ease with which particular formations are magnetized when subjected to a magnetic field (in this case that of Earth). The ease of magnetization is ultimately related to the concentration and composition (size, shape, and mineralogy) of magnetizable material within the formation. These measurements provide one of the best methods for investigating stratigraphic changes in mineralogy and lithology because the measurement is quick, repeatable, and nondestructive and because different lithologies often have strongly contrasting susceptibilities. High-resolution susceptibility measurements aid significantly in paleoclimatic and paleoceanographic studies, where construction of an accurate and complete stratigraphic framework is critical to observe past climatic changes but core recovery is often imperfect. The MSS measures at two vertical resolutions and depths of investigation. A single-coil sensor provides high-resolution measurements (~10 cm) but is shallow-reading; therefore, bowsprings are used to eccentralize the tool (additionally the HLDS caliper arm can aid with forcing the tool against the borehole wall). A dual-coil sensor provides low-resolution (~40 cm), deeper-reading measurements, and because of its more robust nature acts as a quality control for the high-resolution readings. The MSS can be run as a component of a Schlumberger tool string, using a specially developed data translation cartridge, saving hours of operation time. For quality control and environmental correction, the MSS also measures internal temperature, z-axis acceleration, and low-resolution borehole conductivity. In future expeditions, the MSS may be run as part of the "Paleo-combo" tool string with the Multi-Sensor Spectral Gamma Ray Tool (MGT) and the HLDS.

Wireline Heave Compensator

Expedition 320T was the first time the new passive WHC system was used on the JOIDES Resolution. Using measurements made by a MRU (located under the rig floor near the center of gravity of the ship), it is designed to adjust the length of the wireline with its vertically moving flying head to compensate for the vertical motion of the ship. Real-time measurements of uphole (surface) and downhole acceleration are made simultaneously by the MRU and GPIT, respectively. A LDEO-developed software package allows these data to be analyzed and compared in real time, displaying the actual motion of the logging tool string and enabling the efficiency of the compensator to be evaluated. In addition to an improved design and smaller footprint compared to the previous system, its location, together with the winch unit, on the starboard side of the derrick contributed to a significant reduction in the time necessary to prepare for logging operations.

Logging data flow and processing

Data for each wireline logging run were recorded and stored digitally and monitored in real time using the Schlumberger MAXIS 500 system. After logging was completed, preliminary processing was performed by the shipboard logging team so that data could be utilized almost immediately by the science party. Standardized data processing took place onshore at LDEO, and the data were made available (in ASCII and DLIS formats) through the shipboard IODP logging database ~1 week after collection.

Wireline log data quality

Logging data quality may be seriously degraded by changes in hole diameter and in sections where the borehole diameter greatly decreases or is washed out. Deep-investigation measurements such as resistivity are the least sensitive to borehole conditions; however, measurements such as density and neutron porosity (not measured here) are more sensitive because of a shallower depth of investigation and the effect of drilling fluid volume on neutron and gamma ray attenuation. Corrections can be applied to the original data in order to reduce these effects. Gamma ray logs (e.g., HNGS) are generally used for depth correlations between logging runs. Logs from different tool strings may, however, still have depth mismatches caused by either cable stretch or ship heave during recording.

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