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doi:10.2204/iodp.proc.310.103.2007

Downhole logging

Downhole geophysical logs provide continuous information on physical, chemical, textural, and structural properties of geological formations penetrated by a borehole. In intervals of low or disturbed core recovery, downhole geophysical logs provide the only way to characterize the borehole section. This is especially true when recovery is poor and when comparable measurements or observations are obtained from the core, as downhole geophysical logs allow precise depth positioning of core pieces by visual (borehole images) or petrophysical correlation.

All measurements were performed under open borehole conditions (no casing) with the exception of a few of the spectral gamma ray logs (see the “Downhole logging” sections in the individual site chapters). After coring was completed, the drill string was pulled and the coring bit was changed to an open shoe casing to provide borehole stability in unstable sections and allow smooth exit and entry of logging tools. In addition, a wiper trip was performed with fresh seawater (no drilling mud was used). Borehole conditions were extremely hostile, and very often boreholes had to be logged in intervals where the drill string was used as a temporary casing. To be able to record ultra high–resolution geophysical downhole logging data, acquisition was done in the rooster box, which in the piggyback drilling system is heave compensated.

Wireline logging measurements are recorded in depth (meters below seafloor) while the tools are pulled to the surface. The logging speed depends on the physics of the specific tool deployed. For example, to obtain spectral gamma radioactivity data without excessive statistical variations, the tool needs to record a certain number of counts per second. Therefore, more time is needed between sampling points when logging low-gamma formations such as carbonates than when logging high-gamma clays and shales. During Expedition 310, not all tools were deployed in each borehole (see the “Downhole logging” sections in the individual site chapters for details of logging tools deployed). With a shallow depth of penetration below seafloor (<100 m) and small-diameter holes drilled (near 100 mm), slimline downhole instruments were used during Expedition 310. These slimline tools were run individually (no logging tool strings).

Borehole geophysical instruments

The set of borehole geophysical instruments was constrained by the scientific objectives and the geological setting of the expedition. A suite of downhole geophysical methods was chosen to obtain high-resolution images of the borehole wall, characterize the fluid nature in the borehole, measure borehole size, and measure or derive petrophysical or geochemical properties of the formation such as porosity, electrical resistivity, acoustic velocities, and natural gamma radioactivity. Because of environmental constraints, no nuclear tools were deployed during Expedition 310. Most probes were run with Advanced Logging Technologies, Ltd. (ALT) surface recording systems, others with Mount Sopris (Mount Sopris Instrument Company, Inc.) recording systems (see Table T3; also see the “Downhole logging” sections in the individual site chapters for details on the systems deployed).

Optical borehole televiewer (OBI40)

The ALT OBI40 (Fig. F7) produces a millimeter-scale, high-resolution image of the borehole wall, similar to an endoscope. A reflection cone placed at the bottom of the tool enables a vertical charge-coupled device (CCD) camera located inside the tool to image the borehole wall directly. A 360° image is collected and recorded simultaneously. An incorporated lighting system illuminates the borehole wall, and centralizers center the tool for optimal image precision. The resolution of the image is user defined. The highest quality images are obtained with a vertical sampling of 1 mm and 720 pixels taken around the borehole (every 0.5°). A set of triaxial magnetometers allows positioning of measurements relative to magnetic north. Data quality is monitored in real time from a computer display. The precision of measured inclination is 0.5°, and the precision measured azimuth is 1.5°.

Subsequent processing can improve color contrast, and interpolation may be necessary over data transmission errors. The resulting continuous digital image (in RGB color, calibrated from a Kodak reference plate) of the borehole is essential for geological facies recognition and precise core-depth positioning. Consequently, it can be used for sedimentological and structural interpretation, as well as for meso- to macro-scale porosity quantification.

The downhole measurement spacing interval selected was 2 mm × 360 pixels per circumference.

Acoustic borehole televiewer (ABI40)

The ALT ABI40 (Fig. F8) produces millimeter-scale, high-resolution images of the borehole surface and can be directly used for sedimentological and structural interpretation, as well as for meso- to macro-scale porosity quantification. It is less affected by suspended sediments in the borehole fluid than the OBI40.

A voltage is applied in the piezoelectric ceramic to produce acoustic waves (1.2 MHz) at regular intervals. On hitting a focalizing mirror, these waves are deflected perpendicularly to the wave source and toward the borehole wall. The focal point corresponds to the point of maximum energy, giving an ~4 mm diameter footprint on the wall in a 100 mm diameter borehole. To obtain a 360° image, the mirror pivots on a central axis. The resolution is user defined and depends on the number of measurements made in one rotation and on the vertical sampling interval. The highest quality images are obtained with a vertical sampling of 2 mm and a radial sampling with 288 shots per circumference.

The ABI40 produces two distinct images of the borehole wall. First, an acoustic impedance image (from the contrast between the borehole fluid and wall) is derived from the reflected wave amplitude obtained around the hole. The amplitude ratio between the emitted wave and the reflected wave provides information on the formation’s capacity of absorption (low returned amplitude corresponds to a high capacity of absorption, i.e., soft formation). Second, a traveltime image is derived from the reflected wave traveltime from the ceramic transducer to the borehole wall and back. The traveltime is directly proportional to the distance between the borehole wall and the probe. Like the OBI40, the tool is equipped with magnetometers and accelerometers for tool orientation with respect to north. The precision of measured inclination is 0.5°, and the precision of the measured azimuth is 1.5°. For each of the two images, a set of false colors is assigned. From the measurements, a virtual image of the borehole wall depth is produced. This image is displayed as an unfolded representation of the 360° view. The FAC40 tool of ALT is used as a backup for the ABI40. The ABI40 is a second-generation FAC-40 tool. The operating principles are the same; only the output parameters of the FAC are slightly different. The downhole measurement spacing interval selected was 4 mm × 288 pixels per circumference.

Hydrogeological probe (IDRONAUT)

The ALT IDRONAUT (Fig. F9) measures hydrogeological properties of borehole fluid, which are explained below.

Borehole fluid pressure and temperature

Analysis of fluid temperature could help to locate inflows of water coming from the main island into the borehole and is necessary to derive salinity of the fluid from electrical conductivity. Fluid pressure provides an indirect assessment of the tool’s progress down and up the hole. It can also be integrated to obtain fluid density in the hole. 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 is 0.004°C.

Electrical conductivity (C and C₂₀)

Conductivity is measured using seven platinum electrodes grouped within a cell, a central electrode that emits an alternating current, and six peripheral electrodes for current return and potential measurements. Electrical conductivity provides a means to identify different fluid types and derive fluid salinity. In the case of freshwater circulation through the reef and into the borehole, it may record lower values than background seawater (where C_w = 50 mS/cm on average, at 20°C). The precision of measured conductivity is 0.004 mS/cm.

Hydrogen concentration (pH)

The pH is obtained using two electrodes, one of which is a reference. An electrical current is created between the electrodes. The current is a function of the number of H⁺ ions in the water. The resulting value is then amplified to acquire a precise signal. The precision of measured pH is 0.01 units.

Oxydo-reduction potential (Eh)

This sensor functions in the same way as the pH sensor with a two-electrode setup. The reference electrode is the same electrode as for pH. The measured potential is that of the redox couple located between the two electrodes. The precision of measured Eh is 1 mV.

The tool as a whole was calibrated by the manufacturer prior to the expedition. On site, temperature was checked using a thermometer, and electrical conductivity, hydrogen concentration, and oxydo-reduction were checked using special reference liquids. Finally, the oxygen concentration was checked in air. The downhole measurement spacing interval selected was 0.1 mm.

Spectral natural gamma probe (ASGR)

Unlike other slimline instruments recording total gamma ray emissions, the ANTARES ASGR (ANTARES Datensysteme GmbH) (Fig. F10) allows identification of 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 (Ar and Ca), and a characteristic energy of 1.46 MeV is released. U and Th decay into unstable daughter elements. In nature, U and Th decay chains contain many radioactive elements of which the final daughter elements are stable isotopes of Pb. For both U and Th decay, a characteristic energy is also produced. The most prominent of the gamma rays in the uranium series originate from decay of ²¹⁴Bi and in the Th series from decay of ²⁰⁸Tl. Because there is an equilibrium relationship between the daughter product and parent, it is possible to compute the quantity (concentration) of parent ²³⁸U and 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 (BGO) scintillation crystal optically coupled to a photomultiplier. The BGO detector has an absorption potential eight times greater than a more classic NaI crystal. As most of the spectral discrimination is performed in the high-energy range, only instruments equipped with BGO detectors prove to be reliable in the slimline tool domain.

As the probe moves up the borehole, gamma rays are sorted according to their emitted energy spectrum (the tool has 512 reference spectra in memory) and the number of counts in each of three preselected energy intervals are recorded. These intervals are centered on the peak values of ⁴⁰K, ²¹⁴Bi, and ²⁰⁸Tl. Tool output comprises K, U, and Th in becquerel per kilogram, and total gamma ray (GR) counts in counts per second (cps). 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 provides a means to identify the presence of clays (usually K and/or Th rich) from terrestrial (nearby island) erosion and organic matter by the U band (or Th/U ratios).

The instrument was master-calibrated by the manufacturer. On site, the stability of the sensor was checked using a volume of purest potassium. The downhole measurement spacing interval selected was 0.1 mm.

Induction resistivity probe (DIL 45)

The ALT DIL 45 (Fig. F11) provides measurements of electrical conductivity. Variations in electrical conductivity correspond to variations in, among other things, lithology (composition and texture), formation porosity and saturation, and interstitial fluid nature. It can, for example, be used to derive porosity from borehole fluid identification when the pore fluid nature is known independently.

An oscillator sends an alternating current through an emitting coil. The resulting alternating electromagnetic field induces Foucault currents in the formation. These induced currents then generate their own electromagnetic fields, which are detected by a solenoid (receiving coaxial coil). This secondary field induces an electromotive force proportional to the flow running through the coil. The alternating current is of constant amplitude and frequency; thus, the Foucault current is proportional to the formation conductivity and to the electromagnetic field induced in the solenoid.

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 it is 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 by 90° with the emitting current.

A phase sensitive detector (PSD) enables the elimination of the “reactive” signal and therefore 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). Their characteristics and position vary from tool to tool and are selected to reduce the effect of formations and drilling mud located above and below the probe.

The output of the tool comprises two logs:

• Induction electrical conductivity of medium investigation depth (ILM, 0.57 m), and
• Induction electrical conductivity of greater investigation depth (ILD, 0.83 m).

Measured conductivity is finally converted into electrical resistivity. The instrument was calibrated against a Wenner array in a reference hole located in Campos, Mallorca (Spain). Stability of this calibration is checked on site using a reference coil. The downhole measurement spacing interval selected was 0.05 mm.

Full waveform sonic probe (SONIC)

The Mount Sopris 2PSA-1000 sonic probe (Fig. F12) measures compressional wave velocities of the formation. When bulk density is known (from core), elastic properties (bulk and shear moduli) and an estimate of porosity can be derived from sonic measurements. In addition, the analysis of surface waves in the borehole (i.e., Stoneley waves) can be indicative of formation permeability.

This downhole instrument is composed of an acoustic transmitter and four receivers. The transmitter transmits an acoustic signal that propagates through the borehole fluid to the rock interface where some of the energy is critically refracted along the borehole wall. As a result of wavefront spreading (Huygens principle), some of the refracted energy is transmitted back into the borehole. At some point, energy will be transmitted back into the borehole adjacent to a receiver. Each receiver picks up the signal, amplifies it, digitizes it, and then sends the digitized signal to the surface. The recorded waveforms are then examined and wave arrival times are selected (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), fluid velocity, and borehole diameter, the sonic velocity of the rock is calculated. Consequently, three interval velocities values are generated at each sampling point. In specific configurations P-wave (10 kHz monopole survey), S-wave (10 kHz dipole survey), and Stoneley wave (1 kHz monopole survey) were recorded. Calibration of the tool is performed either in water (1500 m/s for P-wave; freshwater at 28°C) or into a steel pipe (5440 m/s) while running downhole. 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 to ensure good quality data. Where no clear arrivals in the waveform were present in at least two receivers, a value of zero was entered in the database. The precision of acoustic traveltime measurements is ~5%. The downhole measurement spacing interval selected was 0.05 mm.

Caliper probe (CAL3)

The Mount Sopris 2PCA-100 (Fig. F13) is a three-arm caliper tool that measures borehole diameter. The caliper measurement is made with the three arms attached to a mechanical assembly that drives a linear potentiometer. Because the three arms are linked mechanically, only a minimum diameter value is obtained for a particular 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 done using two cylindrical rings with known diameters before and after a logging run. The precision of the measurement is ~1 mm. The caliper log is essential for the processing of other logs and can be directly used in sedimentological and structural interpretation of the formation. The downhole measurement spacing selected was 1 cm.

Data recording, processing, and quality

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 T3 summarizes the acquisition system for each tool. WellCAD software was used for processing, visualization, and plotting the data. Sonic data were processed using LogCruncher (Mercury Geophysics) software. 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 to the conductor pipe (coupled with the seafloor via the DART) and the upper sleeve to the heave-compensated rooster box. Additional processing on the accelerometer data of the image tools allowed (by a double integration of the acceleration), whenever needed, a detailed repositioning of the recorded image data to its correct position with respect to the seafloor.

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