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Downhole logging

Downhole geophysical logging can provide continuous data throughout the length of a borehole, giving information on the physical, chemical, textural, and structural properties of the geological formations encountered. Where core recovery is poor, downhole geophysical logs may provide the only method of fully characterizing a borehole. In addition, downhole logs can allow more precise depth positioning of individual core pieces by visual (borehole images) or petrophysical correlation.

Usually, logging measurements are recorded in depth as the tools are pulled from the bottom of the borehole to the surface. Logging speeds are dependent on the physics of the specific tool that is deployed. For example, to obtain spectral gamma radioactivity data without excessive statistical variations, the tool must record a certain level of counts per second (cps). Therefore, where low radiating formations, such as carbonates, are being logged, higher count times at sampling points are required and logging speed is slower. Conversely, when logging high radiating formations such as clays and shales, a lower count time can be used and logging speed is therefore quicker. During Expedition 325, not all tools were deployed in each borehole (refer to individual transect chapters for details of logging tools deployed). With a shallow depth of penetration below seafloor (<50 m) and small-diameter holes (near 100 mm [HQ]–200 mm [American Petroleum Institute]), slimline downhole instruments were used. These slimline tools were run individually (no multiple-tool logging strings).

Borehole geophysical instruments

The set of logging tools used during Expedition 325 was decided based on the scientific objectives of the expedition and the limitations posed by the geological setting. The tool suite includes high-resolution borehole surface imaging tools, a borehole fluid sampling instrument, and a series of tools that allow determination of the borehole size and derivation of petrophysical or geochemical properties of the formations. These formation properties include porosity, electrical resistivity, acoustic velocities, magnetic susceptibility, and natural gamma radioactivity. Owing to the environmental constraints in place in the Great Barrier Reef Marine Park, no nuclear tools were deployed during Expedition 325. Most probes were run with Advanced Logging Technologies Ltd. (ALT) surface recording systems and combined ALT/ Mount Sopris Instrument Company Inc. (Mount Sopris) recording systems.

Optical Borehole Televiewer (OBI40)

The ALT Optical Borehole Televiewer (OBI40) (Fig. F14A) produces a millimeter-scale, high-resolution optical image of the borehole surface. A reflection cone placed at the bottom of the tool enables a vertical charge-coupled device camera located inside the tool to image the borehole surface directly. A 360° image of the borehole is captured. The tool has a built-in lighting system that illuminates the borehole surface, along with centralizers to optimize image precision. Image resolution is user-defined, with the highest quality images having a vertical sampling interval of 1 mm and 720 pixels taken around the borehole (every half-degree). Positioning of the images relative to magnetic north is possible using a set of triaxial magnetometers integrated in the tool. Data quality for OBI40 measurements is monitored by the logging engineer in real time from a computer display. Minor processing can improve color contrast, and interpolation may be necessary over data transmission errors. The resulting continuous digital image (in real color, calibrated from a Kodak reference plate) of the borehole can be used as an efficient tool for lithologic recognition and precise core-depth positioning. Consequently, it can be used directly for sedimentological and structural interpretation, as well as for meso- to macro-scale porosity quantification.

Acoustic Borehole Televiewer (ABI40)

The ALT Acoustic Borehole Televiewer (ABI40) (Fig. F14B) generates centimeter-scale, high-resolution acoustic images of borehole surfaces. Similar to the OBI40 measurements, resulting data can be used directly for sedimentological and structural interpretation of a borehole, as well as for meso- to macro-scale porosity quantification. The ABI40 differs from the OBI40 in that it is unaffected by possible murkiness of the borehole fluid or drilling mud.

A voltage is applied to the piezoelectric ceramic to produce acoustic waves (1.2 MHz) at regular intervals. On reaching the focalizing mirror, the acoustic waves are deflected perpendicularly to the wave source and toward the borehole surface. The focal point corresponds to the point of maximum energy. In a 100 mm diameter borehole, this gives a ~4 mm diameter footprint on the borehole wall. To obtain a 360° image, the mirror pivots on a central axis. Similar to the ABI40, the resolution is user-defined, with the highest quality images obtained while using a vertical sampling interval of 2 mm and a radial sampling of 288 shots per circumference.

The ABI40 produces two distinct images of the borehole surface, an acoustic impedance image and a distance (or traveltime) image. The acoustic impedance image (between the borehole fluid and borehole wall) is derived from the reflected wave amplitude obtained 72, 144, or 288 times around the hole. The amplitude ratio between the emitted wave and the reflected wave provides information on the formation’s absorption capacity (low returned amplitude corresponds to a high capacity of absorption, i.e., soft formation). The distance image is derived from the reflected wave time of flight between the ceramic transducer to the borehole surface and back. This traveltime is directly proportional to the distance between the borehole wall and the probe. Similar to the OBI40 sonde, the ABI40 is equipped with magnetometers, as well as accelerometers for tool orientation with respect to magnetic north. The precision of measured inclination is 0.5°, and the precision measured azimuth is 1.5°. For each of the images, a set of false colors is assigned and a virtual image of the borehole wall depth is produced. This image is displayed as an unfolded representation in 360°. The magnetometers are factory calibrated in Luxembourg. Therefore, a postcruise recalibration file will be provided on return and recalibration.

Hydrogeological probe

The ALT hydrogeological probe (IDRONAUT) (Fig. F14C) measures the hydrogeological properties of the borehole fluid, including the following.

Borehole fluid pressure and temperature

Fluid temperature can aid in the identification of inflow of water into the borehole and is also necessary to derive an accurate borehole fluid salinity from electrical conductivity measurements. Fluid pressure provides an indirect assessment of tool progress down and up the borehole, but it can also be integrated to obtain fluid density in the borehole. The tool was calibrated for temperature by the manufacturer and checked on site using a thermometer. The precision of measured pressure is 0.01 dbar and the precision of measured temperature values is 0.004°C.

Electrical conductivity

Conductivity is measured using seven platinum electrodes grouped within a cell. One central electrode emits an alternating current and six peripheral electrodes provide for current return and potential measurements. Electrical conductivity provides a means of identification of different fluid types in a borehole. Ultimately it can be used to derive borehole fluid salinity. The precision of measured electrical conductivity is 0.004 mS/cm.

Hydrogen concentration

The hydrogen concentration (pH) is obtained using two electrodes, of which one is a reference. An electrical current is created between the electrodes. This current is a function of the amount of H+ ions in the fluid. The resulting value is then amplified to acquire a precise signal. The precision of measured pH is 0.01 pH.

The tool was calibrated by the manufacturer prior to Expedition 325. Prior to each deployment of the tool, temperature was checked using a thermometer, and electrical conductivity, pH, and oxydo-reduction were checked using special reference liquids. Finally, the oxygen concentration was measured in air.

Spectral Natural Gamma Probe

Unlike other slimline instruments recording total gamma ray emissions, the ANTARES Spectral Natural Gamma Probe (ASGR) Datensysteme GmbH (Fig. F14D) allows identification of the individual elements that emit gamma rays. Naturally occurring radioactive elements such as K, U, and Th emit gamma rays with a characteristic energy. K decays into two stable isotopes (argon and calcium), and a characteristic energy of 1.46 MeV is released. U and Th decay into unstable daughter elements also producing characteristic energies. In nature, U and Th decay chains contain many radioactive elements of which the final daughter elements are stable isotopes of lead. The most prominent of the gamma rays in the uranium series originate from the decay of 214Bi (bismuth) and in the thorium series from the decay of 208Tl (thallium). Because of the equilibrium relationship between the daughter product and parent, it is possible to compute the concentration of parent uranium (238U) and thorium (232Th) in the decay series by counting gamma rays from 214Bi and 208Tl, respectively, if the probe has been properly calibrated.

The ASGR detector for gamma rays is a bismuth germanate (Bi4Ge3O12, also referred to as BGO) scintillation crystal which is optically coupled to a photo-multiplier. The BGO detector has an absorption potential eight times greater than a more classic sodium iodide (NaI) crystal. As most of the spectral discrimination is performed in the high-energy range, only instruments equipped with BGO detectors prove to be sufficiently reliable for use in slimline downhole logging tools.

As the probe is pulled from the bottom of the hole to the surface, gamma rays are sorted according to their emitted energy spectrum (the tools available during Expedition 325 have 512 or 256 reference spectra in memory) and the number of counts in each of the three preselected energy intervals. These intervals are centered on the peak values of 40K, 214Bi, and 208Tl. Tool output comprises K, U, and Th in becquerel/kilogram, and total counts gamma ray in counts per second. K, U, and Th values can also be presented as percent K and parts per million U and Th values. The vertical resolution of the tool is ~15 cm.

In reefal carbonates, the ASGR sonde provides a means of identifying clays (usually K rich and/or Th rich) from nearby terrestrial erosion and organic matter by the U band (or Th/U ratios).

The instrument was master-calibrated by the manufacturer. Prior to each deployment, the stability of the sensor was checked using a known volume of purest potassium.

Induction Conductivity Probe

The ALT Induction Conductivity Probe (DIL45) (Fig. F15A) provides measurements of electrical conductivity. Variations of electrical conductivity correspond to variations in a number of factors including, but not limited to, lithology (composition and texture), formation porosity and saturation, and the nature of interstitial fluid.

An oscillator sends an alternating current of constant amplitude and frequency through an emitting coil. The electromagnetic field, created by the emitting coil, induces an alternating current as it runs through the receiving coil. This current is out of phase by 90°. The field, created by the Foucault currents, creates an alternating current when run through the receiving coil. This current has a phase in opposition with that of the emitting current. The Foucault currents are also out of phase with the emitting current by 90°. A phase sensitive detector enables the elimination of the “reactive” signal and hence only keeps the signal induced by the field linked to the Foucault currents. In addition to the main receiving and emitting coils, induction probes also possess other secondary emitting and receiving coils (focalization solenoids), which act to reduce the effect of formations and drilling mud. The Foucault current is proportional to the formation conductivity and to the electromagnetic field induced in the solenoid.

The output of the tool comprises two logs:

  1. Induction electrical conductivity of medium investigation depth (0.57 m) and

  2. Induction electrical conductivity of greater investigation depth (0.83 m).

The instrument was master-calibrated against a Wenner array in a reference hole located in Campos, Mallorca (Spain). Validity of this master-calibration is checked on site using a reference coil.

Full waveform sonic probe

The 2PSA-1000 sonic probe (SONIC) manufactured by Mount Sopris (Fig. F15B) measures compressional wave velocities of the formations encountered in the borehole. Used in combination with bulk density derived from core measurements, elastic properties, including bulk and shear moduli, as well as porosity can be derived from the sonic dataset. It is also possible to derive information on formation permeability from the analysis of surface waves in the hole (called Stoneley waves).

This sonic probe is composed of an acoustic transmitter and four receivers. The transmitter emits an acoustic signal that propagates through the borehole fluid to the fluid/rock interface, where some of the energy is critically refracted along the borehole wall. As a result of this wavefront spreading (Huygens principle), some of the refracted energy is transmitted back into the borehole. Each receiver picks up the transmitted signal, amplifies it, digitizes it, and sends the digitized signal to the surface via the wireline. The recorded waveforms are examined, and wave arrival times are selected (this process is known as “picking”). Arrival times are the transit times of the acoustic energy. By measuring the acoustic transit time and knowing the distance between receivers (1 ft), the fluid velocity, borehole diameter, and sonic velocity of the rock may be calculated. As a waveguide, the borehole propagates energy in many different modes, including the compressional and shear head waves, an infinite series of normal compressional and normal shear modes, and the Stoneley wave modes. All of these wave modes are excited when the source spectrum contains sufficiently high frequencies. The complexity of the received waveforms can be reduced by moderating the transmitted frequency band.

Normal modes (monopole surveys) are a result of constructive interference in the waveguides (borehole). For each normal mode, there exists a frequency below which the mode cannot be excited (cutoff frequency). The normal modes are highly dispersive, with their phase velocities approaching head wave velocities as frequency approaches the cutoff frequency. The optimal frequency band for producing head waves narrowly includes the cutoff frequencies for the first-order compressional and shear-normal modes. In this manner, unwanted modes are not excited and received head waves are high.

Calibration of the tool is performed either in water (1500 m/s for P-wave) or into a steel pipe (5440 m/s) while running downhole. The precision of acoustic traveltime measurements is ~5%.

Magnetic susceptibility probe

The GEOVISTA EM51 (Fig. F15C) is an electromagnetic induction sonde designed to measure formation conductivity and magnetic susceptibility. The sonde includes two sets of two coils, one for conductivity measurement and the other for magnetic susceptibility measurement. Optimum operating conditions for the EM51 sonde are higher conductivity formations combined with low-conductivity borehole fluid (including air). Formation conductivity is typically measured in millisiemens per meter (or millimhos). Relationships between conductivity and resistivity are shown in Tables T12 and T13:

1 Ω = (1/S) → 1 Ωm = [1/(S/m)] → 1 S/m = (1/Ωm) →
1 mS/m = (10–3/Ωm) = (1 mmho).

A set of two calibration loops were used for calibration and testing of the electrical conductivity sensor. The equivalent conductivities of the calibration loops are 200 and 500 mS/m. A set of two calibration pieces was used for magnetic susceptibility sensor calibration and testing. The equivalent susceptibilities of the calibration pieces are 1.7 × 10–3 and 5 × 10–3 SI units. Prior to tool deployment, the calibration jigs must be placed over the middle of the respective TX-RX coil system. For conductivity, this is 100 cm from the bottom of the sonde. For magnetic susceptibility, the distance is 31 cm.

Magnetic susceptibility measures the degree of magnetization of a core in response to an applied magnetic field. It is a powerful tool for deciphering a formation’s sedimentary provenance and/or diagenetic environment, and data derived from this tool are also invaluable when used for borehole correlation. Processes including diagenesis of earlier coral reef generations (which can lead to coral dissolution) can result in a loss of magnetic susceptibility, and although these processes are poorly understood, often tracing where magnetic susceptibility is conserved allows sampling of well-preserved corals.

Caliper probe

The 2PCA-100 from Mount Sopris (Fig. F15D) is a three-arm caliper tool (CAL3) that measures borehole diameter. The caliper measurement is made with the three arms attached to a mechanical assembly that drives a linear potentiometer. The three arms are linked mechanically, and therefore only the minimum diameter value is obtained for hole size. A constant reference voltage is applied across the potentiometer. The direct-current output voltage from the wiper of the potentiometer is converted to frequency. A microprocessor applies a quadratic correction to this frequency so that the frequency is linearly related to borehole diameter.

Calibration of the caliper tool is conducted using two cylindrical rings of known diameters before and after a logging run. The precision of the measurement is ~1 mm. The caliper log is essential for processing other logs and can be directly used in sedimentological and structural interpretation of the formation.

Data quality

The quality of downhole logging data may be degraded by rapid changes in borehole diameter. Deep-investigation measurements such as induction resistivity are least sensitive to borehole conditions.

While deploying all the tools separately, a fixed zero depth position is maintained at the top of the drill pipe. Ship heave was minimized by attaching the winch (on the rooster box) to the drill pipe (placed into the seafloor), which is attached to a stabilization method depending on what logging was being conducted. When logging through API pipe, the drill pipe is attached to the heave-compensated part of the drill rig. When logging through HQ pipe, the HQ drill pipe is fixed in place in the top drive assembly, and so is attached to the heave-compensated part of the drill rig, and the API conductor pipe is clamped by the seabed template (Fig. F16).

Data recording and processing offshore

Downhole geophysical logging aboard the Greatship Maya was provided by EPC. Each logging run was recorded and stored digitally. Data flow was monitored for quality in real time using tool-specific acquisition boxes and software. Table T14 summarizes the acquisition system for each tool.

After logging each borehole, data were preliminarily processed. WellCAD was used for visualization and plotting of the data. Processing was carried out using the WellCAD software package. The processing procedure is described below for standard logs (natural gamma radioactivity [ASGR], induction [DIL45], and magnetic susceptibility logs [EM51]), image data (ABI40 and OBI40), and sonic data (2PSA).

Data processing onshore

Depth adjustments

One main processing task involved evaluating the depth of each log run and referencing the data to seafloor. While deploying all the tools separately in the same section, a fixed zero depth position (loggers’ zero) was maintained at the top of the drill pipe; hence, no depth shifting was necessary. Typical reasons for depth corrections include ship heave and tides, but as the logging for Expedition 325 was performed from a stable heave-compensated platform (see “Data quality”), no such corrections were necessary.

Using WellCAD, the original logs were depth adjusted to the seafloor (wireline log depth below seafloor [WSF]). This adjustment includes a couple of corrections:

  1. Difference in zero tool depth. Discrepancies in depths between initial zeroing and zeroing on removal of the tool were generally <1 m.

  2. Corrections specific to certain tools (e.g., matching down-logs to up-logs).

Logs were subsequently shifted to the seafloor (WSF) using the drillers depth from seafloor (DSF-A). Generally, discrepancies may exist between seafloor depths determined from the downhole logs and those determined by the drillers from the pipe length. This is because of the difficulty in obtaining an accurate seafloor from the “bottom felt” depth in soft sediment.

When necessary, logs have been manually adjusted by the log analyst to a reference log using distinctive peaks. In such cases, the gamma ray logs through pipe (or occasionally the induction logs) are taken as the reference logs (continuous). Generally, depth discrepancies between logs are <1.5 m. Matched log depths are referenced to seafloor and are referred to as wireline matched depth below seafloor (WMSF).

Quality control

Data quality is assessed in terms of reasonable values for the logged formation and in this case repeatability between the gamma ray curves taken through pipe and in open hole. Considering the challenging borehole conditions, the overall quality of downhole logging data is very good. Repeatability between data acquired on down and up acquisition of logs was checked by the log analyst and repeated well.

The quality of the ASGR spectral natural gamma data is good, even when logging through pipe and considering low counts. However, gamma ray logs recorded through drill pipe should only be used qualitatively because of attenuation of the incoming signal. Sections were acquired in open hole conditions in three holes (M0036A, M0042A, and M0054B) for through-pipe data calibration.

The quality of EM51 data is repeatable, with very low values recorded in each of the two logged holes (M0042A and M0054B).

The quality of DIL45 data is good. Induction resistivity is a deep-investigation measurement and is least sensitive to borehole conditions. Resistivity log values are within the expected range.

The quality of ABI40 varies from one hole to another. The distance to the borehole wall greatly affects the quality of this imaging log along with good centralization. Hole M0042A was an API diameter hole where it was not possible to both effectively centralize the tool in open hole and pass through the API bit. Additionally, this hole diameter is outside the normal operating range for this tool. These factors result in low-quality images containing dark lines oriented at 180°. For this reason, image data in these sections should be treated with great care. In Hole M0054B, acoustic images were acquired at high resolution and with HQ-appropriate centralizers, resulting in high-quality, high-resolution images.

The quality of the 2PSA (SONIC) data is variable. Measurements of compressional wave velocity are highly dependent on borehole conditions. Sonic measurements were taken in both API and HQ boreholes. The tool was not well centralized in the larger boreholes because of restrictions on deployment. Larger cavities cause the induced wave to scatter, and acoustic energy is lost more rapidly. The picked P-wave arrivals show variable values, and in Hole M0054B only 50% of the data were collected; it is unknown whether this collection rate was caused by borehole/formation factors or tool issues.

ABI40 and OBI40 image data processing

Images have been oriented with respect to magnetic north. Additionally, images have been enhanced by optimizing the amplitude range (ABI40) or altering the brightness and contrast (OBI40). Images provided have been depth corrected to the seafloor and matched (WMSF). They are displayed as an unwrapped borehole cylinder. As such, a dipping plane in the borehole appears as a sinusoid on the image, with the amplitude of this sinusoid proportional to the dip of the plane. Because of orientation of image logs with respect to magnetic north, the strike of dipping features can also be determined.

2PSA acoustic data

The 2PSA tool was run at a frequency of 15 kHz, and resultant logs can be used to calculate compressional wave velocities. The data were processed using the WellCAD logging package. For processing purposes, data were filtered (frequency filter) in such a way that only the energy around the induced frequency was analyzed. Waveform picking was done manually in the WellCAD logging package to ensure good quality data. Where no clear arrivals in the waveform were present in two receivers, a null value was entered in the dataset. Time picks were saved, and acoustic velocities were calculated. The precision of acoustic traveltime measurements is ~5%.