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doi:10.2204/iodp.pr.340T.2012

Downhole logging

Downhole logs are measurements of physical, chemical, and structural properties of the formation surrounding a borehole. The data are continuous with depth (at vertical sampling intervals ranging from 2.5 mm to 15 cm) and are measured in situ. The sampling is intermediate in scale between laboratory measurements on core samples and geophysical surveys and provides a necessary link for the integrated understanding of physical properties on all scales.

Logs in “hard rock” such as at Site U1309 can be interpreted in terms of the lithology, mineralogy, and geochemical composition of the penetrated formation. They also provide information on the status and size of the borehole and on possible deformations induced by drilling or formation stress. When core recovery is incomplete or disturbed, log data may provide the only way to characterize the formation. These data can be used to determine the actual thickness of individual units or lithologies when contacts are not recovered, to pinpoint the true depth of features in cores with incomplete recovery, and to identify intervals that were not recovered.

Logs are recorded with a variety of tools combined into several tool strings, which are run downhole. For deep holes, logging runs may be made to intermediate depths between coring phases to obtain wall rock measurements before multiple bit runs risk the possibility of hole degradation. Four tool strings were used during Expedition 340T in Hole U1309D, which had previously been cored and partially logged to 1415 mbsf (see Fig. F4; Table T1):

1. A modified triple combination (triple combo) tool string (gamma ray, density, resistivity, and borehole temperature),

2. A sonic string (gamma ray and sonic velocity),

3. The Versatile Seismic Imager (VSI) tool string (vertical seismic profile and gamma ray), and

4. The Magnetic Susceptibility Sonde (MSS) tool string (gamma ray, magnetic susceptibility, and temperature).

After reentering Hole U1309D, the logging bit at the end of the bottom-hole assembly (BHA) was set below the casing shoe located at ~20 mbsf, below an interval where prior experience suggested potential obstacles. Bit depth varied throughout the operations based on hole conditions. See Table T2 for details of logging operations.

Each tool string deployment is a logging “run,” starting with the assembly of the tool string and the necessary calibrations. The tool string is then sent down to the bottom of the hole while recording a partial set of data, and, except for the VSI, is pulled up at a constant speed, typically 300–500 m/h, to record the main data. The VSI is clamped against the borehole wall at regularly spaced depths while shooting the seismic source and pulled up between each station. During each run, tool strings can be lowered down and pulled up the hole several times for control of repeatability or to try to improve the quality of the data. Each lowering or hauling up of the tool string while collecting data constitutes a “pass.” During each pass the incoming data are recorded and monitored in real time on the surface system. A logging run is complete once the tool string has been brought to the rig floor and disassembled.

Logged properties and tool measurement principles

The main logs recorded during Expedition 340T are listed in Table T3. More detailed information on individual tools and their geological applications may be found in Ellis and Singer (2007), Goldberg (1997), Lovell et al. (1998), Rider (1996), Schlumberger (1989), and Serra (1984, 1986, 1989). A complete online list of acronyms for the Schlumberger tools and measurement curves is available at www.slb.com/modules/mnemonics/index.aspx.

Borehole temperature

The triple combo tool string deployed during Expedition 340T included the Modular Temperature Tool (MTT) to measure the borehole fluid temperature. The MTT was designed at Lamont-Doherty Earth Observatory (LDEO) to resolve centimeter-scale temperature variations at typical logging speeds of 300–500 m/h. It uses two temperature sensors: a fast responding thermocouple and a highly accurate resistance-temperature device. The sonde also contains an accelerometer for depth correction and is combined with a specially designed cartridge to allow data transmission through the Schlumberger tool string and wireline.

Additional temperature measurements were made during each logging run by a sensor in the logging equipment head–mud temperature (LEH-MT) cablehead and processed by the Enhanced Digital Telemetry Cartridge (EDTC) (see “Telemetry cartridges”).

Natural radioactivity

The EDTC (see “Telemetry cartridges”), which is used primarily to communicate data to the surface, also includes a sodium iodide scintillation detector to measure the total natural gamma ray emission. It is not a spectral tool but it provides high-resolution total gamma ray for each pass that allows for precise depth-match processing between logging runs and passes.

Density

Formation density was measured with the Hostile Environment Litho-Density Sonde (HLDS). The sonde contains 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 an eccentralizing arm. Gamma rays emitted by the source undergo Compton scattering, whereby gamma rays are scattered by electrons in the formation. The number of scattered gamma rays that reach the detectors is proportional to the density of electrons in the formation, which is in turn related to bulk density. Porosity may also be derived from this bulk density if the matrix (grain) density is known. Good contact between the tool and borehole wall is essential for good HLDS logs; poor contact results in underestimation of density values.

The HLDS also measures photoelectric absorption as the photoelectric effect (PEF). Photoelectric absorption of the gamma rays occurs when their energy is reduced below 150 keV after being repeatedly scattered by electrons in the formation. Because PEF depends on the atomic number of the elements encountered, it varies with the chemical composition of the minerals present and can be used for the identification of some minerals (Bartetzko et al., 2003; Blackman, Ildefonse, John, Ohara, Miller, MacLeod, and the Expedition 304/305 Scientists, 2006).

Electrical resistivity

The High-Resolution Laterolog Array (HRLA) tool provides six resistivity measurements with different depths of investigation (including the borehole fluid, or mud, resistivity and five measurements of formation resistivity with increasing penetration into the formation). The sonde sends a focused current beam into the formation and measures the intensity necessary to maintain a constant drop in voltage across a fixed interval, providing direct resistivity measurement. The array has one central (source) electrode and six electrodes above and below it, which serve alternately as focusing and returning current electrodes. By rapidly changing the role of these electrodes, a simultaneous resistivity measurement at six penetration depths is achieved. The tool is designed to ensure that all signals are measured at exactly the same time and same tool position, and to reduce the sensitivity to shoulder bed effects. The design, which eliminates the need for a surface reference electrode, improves formation resistivity evaluation compared to the traditional dual laterolog sonde that was used in previous expeditions to Hole U1309D.

Typically, igneous minerals found in crustal rocks are electrical insulators, whereas sulfide and oxide minerals as well as ionic solutions like pore water are conductors. In most rocks, electrical conduction occurs primarily by ion transport through pore fluids and, thus, is strongly dependent on porosity. Electrical resistivity can hence be used to estimate porosity, alteration, and fluid salinity.

Acoustic velocity

The Dipole Sonic Imager (DSI) generates acoustic pulses from various sonic transmitters and records the full waveforms with an array of eight receivers. The waveforms are then used to calculate the sonic velocity in the formation. The omnidirectional monopole transmitter emits high-frequency (5–15 kHz) pulses to extract the compressional velocity (V_P) of the formation, as well as the shear velocity (V_S) when it is faster than the sound velocity in the borehole fluid. The same transmitter can be fired in sequence at a lower frequency (0.5–1 kHz) to generate Stoneley waves that are sensitive to fractures and variations in permeability. The DSI also has two dipole transmitters, which allow an additional measurement of shear wave velocity in “slow” formations, where V_S is slower than the velocity in the borehole fluid. However, in formations such as the basement penetrated in Hole U1309D, V_S was primarily measured from the monopole waveforms. These higher frequency waveforms usually provide a sharper shear arrival and more accurate estimate of V_S than either of the dipole sources. The two shear velocities measured from the two orthogonal dipole transducers can be used to identify sonic anisotropy that can be associated with the local stress regime.

During acquisition, V_P and V_S are extracted from the recorded waveforms using a slowness/time coherence processing algorithm (Kimball and Marzetta, 1984). In the process, a semblance function is calculated for a fixed time window across the receiver array, varying traveltimes and velocity within a predefined range to identify peaks in semblance corresponding to individual mode arrivals. Acquisition parameters were configured for the velocity range expected in the deepest part of Hole U1309D (V_P = 6–7 km/s).

Accelerometry and magnetic field measurement

The traditional purpose of the General Purpose Inclinometry Tool (GPIT), which includes a three-component accelerometer and a three-component magnetometer, is to determine the acceleration and orientation of the imaging tools. During Expedition 340T, the GPIT was used primarily to provide orientation for the cross-dipole data from the DSI.

Vertical seismic profile

In a VSP experiment, a borehole seismic tool is anchored against the borehole wall at regularly spaced intervals and records the acoustic waves generated by a seismic source positioned just below the sea surface. The first purpose of these measurements is to provide a direct measurement of the time necessary for seismic waves to travel from the surface to a given depth, to tie the observations in the well, recorded as a function of depth, to the reflections observed in the seismic survey data, recorded as a function of time. In addition, analysis of the full waveforms can be used to characterize seismic reflectivity beyond the borehole, which could help document the structure within Atlantis Massif core complex.

The seismic source for the VSP was a Sercel G-gun parallel cluster, composed of two 250 in³ air guns separated by 1 m. It was positioned by one of the ship cranes on the port side of the ship at distance of 27.4 m from the centerline of the ship. The set-back along the centerline from the top of the wellhead was 35.5 m, and the total horizontal (diagonal) offset from the wellhead was 44.7 m. The air guns were suspended from a float at a water depth of ~7 m, corresponding to a notch frequency of 107 Hz. The average firing pressure was 1950 psi but noted as varying by up to ±50 psi over the course of operations. Minimum firing interval during VSP operations was 18 s, and pressure recovery after firing was ~5 s, so shot-to-shot variations in pressure were minimal. The bubble pulse interval was ~125 ms.

During operation, dynamic positioning (DP) maintained the wellhead over Hole U1309D, but the ship’s heading was changed between the 2 days of VSP operation from 335° on the first day to 040° on the second. This is significant because the slope of the seafloor is ~11°, shoaling most rapidly in a southwest direction. These headings placed the gun array on the updip side of Hole U1309D and increased the chances of recording nonvertical ray paths and polarizations at intermediate depths.

In accordance with the requirements of the National Environmental Policy Act (NEPA) and of the Endangered Species Act, all seismic activities were conducted during daytime and protected species observers (PSOs) kept watch for protected species during the entire duration of the zero-offset VSP. Any sight of protected species within the exclusion zone of 940 m (defined for water depths > 1000 m) would interrupt the survey for 60 min after the last sighting or until the protected species were seen leaving the exclusion zone. PSOs began observations 1 h prior to the use of the seismic source, which started with a 30 min ramp-up procedure, gradually increasing the operational pressure and firing rate to provide time for undetected protected species to vacate the area. The same ramp-up procedure would be used when resuming activity after any interruption due to the sighting of protected species or whenever the gun was not fired for more than 30 min.

The seismometer used was the VSI sensor, a three-axis geophone accelerometer, that was anchored against the borehole prior to recording by a caliper arm. The orientation of the horizontal components, x and y, varies due to sensor rotation during logging, but relative tool orientation during the run is recorded. Data were recorded in three sessions over 2 days. The first session was ended and the tool string recovered when it was determined that the VSI was no longer clamping due to a broken caliper. Recording conditions were very good for the second deployment, and the session ended only because of fading daylight. In contrast, conditions were extremely noisy the following day. The replacement caliper became slightly bent during operations, but noisy conditions were evident from the beginning of recording and persisted during the day; the reason is undiagnosed.

Data were recorded at 55 station depths between 1645 and 3005 meters below sea level (mbsl), or 0 and 1360 mbsf, corresponding to an average station spacing of 25 m. Stations at 2705, 2955, 2979, and 3005 mbsl were repeated due to failure of the caliper on the first deployment. Stations deeper than 3005 mbsl were not attempted as a precaution against damage to the instrument. A total of 659 shots were recorded; between 5 and 32 recordings were taken at each station with a median of 10.

VSP data format description

The data from the experiment are available in SEGY format for the individual shots as x-, y-, and z-components, the shot break hydrophone, and the automatic stack of traces produced during logging for individual stations. There are 659 traces in both raw data files and in the shot break file, although shot numbers, the third long integer entry in the SEGY header (i.e., 3L), ranges from 3 to 680 with some intermediate shots not recorded. Some unrecorded shots were fired manually between stations to maintain compliance with NEPA. The stacked files have 60 traces rather than 55 because the 4 repeated stations at 2705, 2955, 2979, and 3005 mbsl appear twice, and the station at 1795 mbsl appears twice, as it was reclamped during shooting.

Trace data are recorded in Institute of Electrical and Electronics Engineers (IEEE) floating point format with raw data amplitudes reaching almost 900 due to the noise; maximum signal amplitudes are typically <1. Start times for the raw traces are the source trigger times uncorrected for the source delay, which can be derived from the shot break records. Recorded maximum shot break amplitudes diminish around Shot 148, possibly due to slight misalignment of the guns; there was no indication of gun malfunction during operations.

The datum for elevation is the drill rig floor and the receiver group elevation with respect to sea level (Header 11L) is derived from the downhole depth (Header 14L) using a standard 11 m correction for rig floor. Estimates of the ship’s draft during the VSP put actual elevation of the rig floor at 11.5 m.

Magnetic Susceptibility Sonde

The MSS, a wireline tool designed by LDEO, measures the ease with which formations are magnetized when subjected to Earth’s magnetic field. 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. The sensor used during Expedition 340T was a dual-coil sensor providing deeper reading measurements with a vertical resolution of ~40 cm. The MSS was 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 tool temperature, z-axis acceleration, and low-resolution borehole conductivity.

The MSS used during Expedition 340T was run for the first time in seawater in Hole U1309D. Core sample data available from Expeditions 304/305 provide a means to test operational sensitivities of the deep-reading sensor as well as an opportunity to obtain constraints on absolute calibration parameters for the tool.

Auxiliary logging equipment

Cable head

The Schlumberger logging equipment head (LEH), or cablehead, measures tension at the very top of the wireline tool string, which diagnoses difficulties in running the tool string up or down the borehole, or when exiting or entering the drill string or casing. The LEH-MT used during Expedition 340T also includes a thermal probe to measure the borehole fluid temperature. Several of the Expedition 340T runs encountered difficulty as the tool strings passed out of, or in to, the BHA, as documented by the LEH.

Telemetry cartridges

Telemetry cartridges are used in each tool string to allow the transmission of the data from the tools to the surface. The EDTC also includes a sodium iodide scintillation detector to measure the total natural gamma ray emission of the formation. This gamma ray log was used to match the depths between the different passes and runs. In addition, it includes an accelerometer, whose data can be used in real time to evaluate the efficiency of the wireline heave compensator (WHC). The temperature measurements from the LEH-MT are also processed by the EDTC before being sent to the surface for real-time monitoring.

Joints and adapters

Because the tool strings combine tools of different generations and with various designs, they include several adapters and joints between individual tools to allow communication, provide isolation, avoid interferences (mechanical and acoustic), terminate wirings, or to position the tool properly in the borehole. The knuckle joints in particular were used to allow some of the tools such as the HRLA to remain centralized in the borehole, while the overlying HLDS was simultaneously pressed against the borehole wall by an eccentralizing arm.

All these additions contribute to the total length of the tool strings and are included in Figure F4.

Log data quality

A principal factor in the quality of log data is the condition of the borehole wall. If the borehole diameter varies over short intervals because of washouts or ledges, the logs from tools that require good contact with the borehole wall (i.e., density tool in the Expedition 340T program) may be degraded. Deep investigation measurements such as gamma ray, resistivity, and sonic velocity, which do not require contact with the borehole wall, are generally less sensitive to borehole conditions. Very narrow (“bridged”) sections will also cause irregular log results. For the VSI, cable motion and/or ship noise can also be an important factor controlling data quality.

The quality of the logging depth determination depends on several factors. The depth of the logging measurements is determined from the length of the cable played out from the winch on the ship. Uncertainties in the depth of the core samples occur because of incomplete core recovery or incomplete heave compensation. Uncertainties in logging depth occur because of ship heave, cable stretch, cable slip, or even tidal changes. All these factors generate some discrepancy between core sample depths, logs, and individual logging passes. To minimize the effect of ship heave, a hydraulic wireline heave compensator is used to adjust the wireline length for rig motion during wireline logging operations.

Wireline heave compensator

Expedition 340T continued to evaluate the WHC system. It is designed to compensate for the vertical motion of the ship and maintain steady motion of the logging tools downhole. It uses vertical acceleration measurements made by a motion reference unit (MRU), located under the rig floor near the center of gravity of the ship, to calculate the vertical motion of the ship. It then adjusts the length of the wireline by varying the distance between two sets of pulleys through which the cable passes. Real-time measurements of uphole (surface) and downhole acceleration are made simultaneously by the MRU and by the EDTC tool, respectively. An 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 evaluation of the efficiency of the compensator. Observations during Expedition 340T neither confirm nor disprove the effectiveness of this system. The best signal-to-noise ratio VSI recordings were obtained when the WHC was turned off and stations were relatively shallow (shallower than 1810 meters below rig floor [mbrf] [150 mbsf]). However, sea conditions on a subsequent run when signal-to-noise ratio was poor and the WHC was in use were somewhat different so interpretation of the difference is difficult. Three stations recorded with the WHC off during that run did not result in a recognizable difference in noise levels on the VSI trace.

Logging data flow and processing

Data for each wireline logging run were monitored in real time and recorded using the Schlumberger MAXIS 500 system. They were then copied to the shipboard processing stations for preliminary processing. Each pass was depth matched to logs from Expeditions 304 and 305. After depth-match processing, all the logging depths were shifted to the seafloor. This is usually done by identifying the seafloor from a step in gamma radiation, but in this case the logs were shifted using the depth to seafloor established from previous expeditions to Hole U1309D. These data were made available to the science party within a day after their acquisition.

The downhole log data were also transferred onshore to LDEO for standardized data processing. The main part of the processing is depth matching to remove depth offsets between different logging passes, which results in a new depth scale: wireline matched depth below seafloor (WMSF). Also, additional corrections are made to certain tools and logs (e.g., speed and voltage corrections to resistivity images), documentation for the logs (with an assessment of log quality) is prepared, and the data are converted to ASCII for the conventional logs and to SEGY for the VSP data. Schlumberger GeoQuest’s GeoFrame software package is used for most of the processing.

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