Downhole logs are used to determine the physical, chemical, and structural properties of formations penetrated by drilling and to complement discrete core measurements. Where core recovery is incomplete, logging data may serve as a proxy for lab data for sedimentological and physical properties. Logging data offer advantages over core-based analyses in that they are collected rapidly and represent continuous, in situ measurements of the formation.
The Lamont-Doherty Earth Observatory Borehole Research Group (LDEO-BRG), in conjunction with Schlumberger Well Logging Services, provided the geophysical well logging aboard the JOIDES Resolution. Primarily designed for use in hydrocarbon exploration, logging tools have been adapted to meet ODP requirements and hole conditions. These modifications include a reduction in tool diameter to allow insertion into the 3.8-in drill string.
Individual logging tools were combined in four different tool strings during Leg 175: (1) seismostratigraphy tool string, comprising the sonic digital tool (SDT) and the phasor dual-induction-spherically focused resistivity tool (DIT-SFR); (2) lithodensity tool string, comprising the accelerator porosity sonde (APS) and the lithodensity sonde (LDS); (3) Formation MicroScanner tool string (FMS); and (4) geological high-sensitivity magnetic tool string (GHMT) comprising a high-sensitivity total magnetic field sensor and a susceptibility magnetic sonde. The natural gamma-ray spectroscopy tool (NGT) or the hostile environment natural gamma-ray sonde (HNGS) were placed at the top of all four tool strings to provide a common basis for log correlation and depth adjustment. The LDEO temperature logging tool (LDEO-TLT) was attached to the base of the two first tool strings to obtain borehole temperatures.
The detailed principles of operation of the various logging sensors and their geological applications can be found in Serra (1984), Timur and Toksöz (1985), Ellis (1987), Schlumberger (1989), and Rider (1996).
The NGT and HNGS measure the natural gamma radiation from isotopes of potassium (K), thorium (Th), and uranium (U) in the sediment surrounding the tool. High K and Th values indicate high clay concentrations, and high U values commonly indicate high abundances of organic matter.
The APS emits fast neutrons, which are slowed by hydrogen in the formation, and the energy of the rebounded neutrons is measured. Most hydrogen is in the pore water; hence, porosity may be derived. However, hydrogen bound in minerals such as clays also contributes to the measurement, so the raw porosity value is often an overestimate.
The LDS emits high-energy gamma rays, which are scattered by the electrons in the formation. The electron density, and hence the bulk gamma density, is derived from the energy of the returning gamma rays. Porosity may also be derived from this bulk gamma density, if the matrix density is known. In addition, the photoelectric effect is measured, and this varies according to the chemical composition of the sediment.
The DIT-SFR measures the formation resistivity at three different penetration depths by electromagnetic induction for the deep and medium resistivities and by current balancing for the shallow resistivity. Porosity, clay content, fluid salinity, grain size, and gas hydrate content all contribute to the resistivity.
The SDT measures the traveltime of sound waves along the borehole wall between two transmitters and two receivers, over distances of 2.4, 3.0, and 3.6 m. The sonic velocity increases with consolidation, lithification, and gas hydrate content. An impedance stratigraphy can be generated from the density and sonic logs and can be convolved with the appropriate wave sequence to produce a synthetic seismogram. In turn, this serves to provide the depth-traveltime tie-in between borehole data and seismic reflection profiles.
The LDEO-TLT is a high-precision, low-temperature tool for recording borehole temperature changes. However, the data recorded by the LDEO-TLT are unlikely to match equilibrated formation temperatures because drilling and circulation operations disturb the temperature conditions in the borehole. Accordingly, data developed from the Adara temperature probe (see "Physical Properties" sections, site chapters, this volume) is preferred for quantifying the true geothermal gradient. Nevertheless, the spatial temperature gradient from the LDEO-TLT is useful in identifying abrupt gradient changes, which commonly indicate localized fluid seepages from the formation.
The FMS produces high-resolution images of the microresistivity character of the borehole wall. The tool comprises four orthogonal pads, each having 16 button electrodes that are pressed against the borehole wall. Approximately 30% of a 25-cm diameter borehole is imaged. The vertical resolution is <1 cm, and features such as burrows, thin beds, slumps, laminations, fractures, veins, and high-frequency sedimentological changes can be imaged. The images are oriented so that directional structure can be obtained for the sediment fabric. FMS images can be used for detailed correlation of coring and logging depths, core orientation, and mapping of formation structures, as well as determination of strikes and dips of bedding planes.
The GHMT string comprises a high-sensitivity total magnetic field sensor (nuclear magnetic resonance sonde, or NMRS) coupled with a magnetic susceptibility sensor (susceptibility magnetic sonde, or SUMS). These two sensors are used to measure the vertical component of the total magnetic field to establish borehole magnetic polarity transitions and magnetic susceptibility variations. The NMRS measures the frequency of proton precession between a calibrated applied polarizing field and the Earth's magnetic field that is proportional to the total field intensity of the Earth. The SUMS sensor detects the mutual induction signal between two coils (0.8 m apart) caused by the surrounding borehole lithology. Logging data are recorded every 5 cm. Specifications of the probes, such as impulse response, calibration ratio, and geomagnetic location of the hole, are used to calculate the susceptibility effect on the scalar total-field magnetometer. From these data, the scalar remanent magnetization can be calculated. Results from borehole measurements of magnetic reversal sequence are useful in age dating when correlated to the geomagnetic polarity time scale.
Standard well-logging operations are as follows. After coring is completed, the holes are flushed of sediment fill by circulating heavy viscous drilling fluid (sepiolite mud with seawater) through the drill pipe into the hole. To clean the hole and stabilize the borehole walls for logging, the BHA is pulled up to ~60 to 100 mbsf, run down to the bottom of the hole, and finally pulled back up to near the seafloor. Tool strings comprising one or more combinations of sensors are then lowered downhole by a seven-conductor wireline cable. A newly refurbished wireline heave compensator minimizes the effect of the ship's motion on the tool position. Data from the tools are recorded in real time by the Schlumberger Multitask Acquisition and Imaging System (MAXIS 500) logging computers. After logging, data are processed for preliminary shipboard correction, correlation, and interpretation. Except for the FMS and GHMT, wireline logging data are typically recorded at 15-cm depth increments. Both the depth of investigation into the formation and the vertical borehole resolution are sensor dependent (Table 7).
Data quality is determined largely by the condition of the borehole wall. If it is irregular, wide, or if there are many washouts, there may be problems with those tools that require good contact with the wall (e.g., density, porosity, and FMS). Deep investigation measurements, such as resistivity and sonic velocity, are least sensitive to borehole conditions. The quality of the borehole is helped by minimizing the circulation of drilling fluid and by logging a young hole or a dedicated logging hole that has been drilled immediately before logging.