DOWNHOLE MEASUREMENTS

Logging Procedures and Logging Data

Introduction

Downhole logging on board the JOIDES Resolution is provided by Lamont-Doherty Earth Observatory Borehole Research Group (LDEO-BRG) in conjunction with Leicester University Borehole Research (LUBR), the Laboratoire de Mesures en Forage (LMF), and Schlumberger Well Logging Services. During Leg 182, the high-resolution temperature/acceleration/pressure tool (LDEO-TAP) replaced the Lamont temperature tool (LDEO-TLT) as the borehole temperature tool.

Borehole Preparation

Before logging, most holes were first flushed of debris by circulating heavy viscous drilling fluid (sepiolite mud and seawater) through the hole. The bottom-hole assembly (BHA) was then pulled up to 80-110 mbsf and run down the hole again to ream out borehole irregularities and stabilize borehole walls. The hole was then filled with a sepiolite mud pill, and the BHA was raised to 80-110 mbsf.

Data Recording and Processing

Data for each logging run were recorded and stored digitally and monitored in real time using the Schlumberger Multitask Acquisition and Imaging System (MAXIS 500). Upon the completion of logging at each hole, data were transferred to the shipboard Downhole Measurements Laboratory for preliminary processing and interpretation. Formation MicroScanner (FMS) image data were interpreted using Schlumberger's GeoFrame 3.1.2 software package.

Logging data were transmitted for processing to LDEO-BRG using a satellite high-speed data link after each hole was logged. Data processing at LDEO-BRG included (1) depth-shifting of all logs relative to a common datum (i.e., mbsf), (2) corrections specific to individual tools, and (3) quality control and rejection of unrealistic or spurious values. Once processed at LDEO-BRG, log data were transmitted back to the ship. Log curves of LDEO-BRG processed data were then replotted on board and used to refine interpretations. Further postcruise processing of the log data from the FMS and the geologic high-resolution magnetic tool (GHMT) is performed at LDEO-BRG, at LMF in Aix-en-Provence, France.

Postcruise-processed acoustic, caliper, density, gamma-ray, magnetic, neutron porosity, resistivity, and temperature data in ASCII format are available (see the "Related Leg Data" contents list).

Log Data Quality

A major factor influencing the quality of log data is the condition of the borehole. If the borehole diameter is variable over short intervals, resulting from washouts during drilling, clay swelling, or borehole wall collapse, there may be data acquisition problems for tools that require good contact with the wall (i.e., FMS, density, and porosity logging tools). Measurements that do not require contact with the borehole wall, such as resistivity and sonic velocity, are generally less sensitive to borehole conditions. The quality of boreholes was improved by minimizing the circulation of drilling fluid, flushing the borehole to remove debris, and logging as soon as possible after drilling and conditioning were completed. Factors affecting data quality for each tool are discussed in the following section.

Log Depth Scales

The depth of each logged measurement is calculated from the length of the logging cable, minus the cable length to the seafloor (seafloor is identified by an abrupt reduction in gamma-ray counts at the water/sediment interface). Discrepancies between the core depth and the log depth may occur because of core expansion, incomplete core recovery, and drill-pipe stretch in the case of core depth, and incomplete heave compensation, cable stretch (1 m/km), and cable slip in the case of log depth. Tidal changes in sea level may also have an effect. Thus, there may be significant differences between drill-pipe depth and cable depth that should be considered when using the logs.

Logging Tools and Tool Strings

Logging Tool Strings

During Leg 182 individual logging tools were combined into the following four different logging strings (Fig. F10; Table T8):

The triple combination (porosity, density, and resistivity) tool string, composed of the phasor dual induction-spherically focused resistivity tool (DITE-SFL), the hostile environment lithodensity sonde (HLDS), the accelerator porosity sonde (APS), and the hostile environment natural gamma-ray sonde (HNGS). The LDEO-TAP was attached to the base of this tool string.
The FMS/sonic tool string, composed of the FMS, the general-purpose inclinometer tool (GPIT), and the sonic digital tool (SDT). The natural gamma-ray tool (NGT) was included at the top of this tool string.
The GHMT, composed of the high-sensitivity total magnetic field sensor (nuclear magnetic resonance sonde [NMRS]) and the susceptibility magnetic sonde (SUMS). The NGT was again positioned at the top of this tool string. During Leg 182 the NMRS was not operating and total magnetic field was not measured.
The well seismic tool (WST).

Data from the NGT or HNGS placed at the top of all but the WST tool string provides a common basis for correlating between logging runs and depth shifting of logs.

Logging Tools

Brief descriptions of individual logging tools used during Leg 182, including their geological applications and the controls on data quality, are given below. Properties of the formation measured by each tool, sample intervals used, and precision of the measurements made (including the vertical resolution and depth of investigation at typical logging speeds) are summarized in Table T8. Explanations of tool name acronyms, the acronyms by which the log data generated by the different tools are referred, and their units of measurement are summarized in Table T9.

More detailed descriptions of individual logging tools and their geological applications can be found in Ellis (1987), Goldberg (1997), Lovell et al. (1998), Rider (1996), Schlumberger (1989, 1994a, 1994b, 1995), Serra (1984, 1986, 1989), Timur and Toksöz (1985) and the LDEO-BRG Wireline Logging Services Guide (1994).

Natural Gamma-Ray Tool (NGT)

The NGT uses a sodium iodine (NaI) scintillation detector to measure total formation NGR emissions (K + U + Th in American Petroleum Institute [API] units) and utilizes five-window spectroscopy to determine concentrations of radioactive K, Th, and U. The NGT also provides a measure of the uranium-free or computed gamma ray (K + Th in API units).

The NGT response is influenced by borehole diameter and the weight and concentration of bentonite or KCl present in the drilling mud. KCl may be added to the drilling mud to prevent fresh-water clays from swelling and forming obstructions. All these effects are corrected for during processing of NGT data at LDEO-BRG.

Hostile Environment Natural Gamma-Ray Sonde (HNGS)

The HNGS uses a measurement principle similar to that of the NGT described above. However, the HNGS uses two bismuth germanate scintillation detectors for gamma-ray detection with full spectral processing, significantly improving tool precision compared to the NGT. The spectral analysis filters out gamma-ray energies below 500 keV, eliminating sensitivity to bentonite or KCl in the drilling mud, and improves measurement accuracy. The HNGS generates the same output as the NGT, as well as estimating the average borehole potassium contribution to the total potassium signal. Log data from the HNGS are corrected for variability in borehole size and borehole potassium concentrations on board ship.

Hostile Environment Lithodensity Sonde (HLDS)

The HLDS consists of a radioactive cesium (¹³⁷Cs) gamma-ray source (622 keV) and far and near gamma-ray detectors mounted on a shielded skid that is pressed against the borehole wall by a hydraulically activated eccentralizing arm. Gamma radiation emitted by the source undergoes both Compton scattering and photoelectric absorption. Compton scattering involves the transfer of energy from gamma rays to the electrons in the formation via elastic collision. The number of scattered gamma rays that reach the detectors is directly related to the number 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 density is known.

The HLDS also measures the photoelectric effect (PEF) caused by absorption of low-energy gamma rays. Photoelectric absorption occurs when gamma rays have energies <150 keV after being repeatedly scattered by the electrons in the formation. Because PEF depends on the atomic number of elements in the formation, it is essentially independent of porosity. Thus, PEF varies according to the chemical composition of the sediment. For example, the PEF of pure calcite = 5.08 barn/e^-; illite = 3.03 barn/e^-; quartz = 1.81 barn/e^-; and kaolinite = 1.49 barn/e^-. The PEF values can be used in combination with NGT curves to identify different types of clay minerals. Failure to maintain good contact between the tool and borehole wall is essential for good HLDS logs as poor contact will result in an underestimation of density values.

Accelerator Porosity Sonde (APS)

The APS consists of a minitron neutron generator that produces fast neutrons (14.4 MeV), and five neutron detectors (four epithermal and one thermal), positioned at different spacings along the tool. The tool is pressed against the borehole wall by an eccentralizing bow-spring. Emitted fast neutrons are slowed by collisions, especially with hydrogen nuclei (because of the large thermal neutron-capture cross sections of hydrogen nuclei), which are mainly present in the pore water. Upon degrading to thermal energies (0.025 eV), the neutrons are captured by the nuclei of Si, Cl, B, (silica, chloride, and boron), and other elements, resulting in a gamma-ray emission. The neutron detectors record both the numbers of neutrons arriving at various distances from the source and neutron arrival times, which act as a measure of formation porosity. If the concentration of water is low, as in low-porosity formations, neutrons travel further before being captured. This results in a high count rate. Data from the APS are used to derive (1) near and far epithermal neutron porosity (0.1-100 eV); (2) thermal (<0.025 eV) neutron measurement of the formation-capture cross section (sigma, f), which is a useful indicator of B, Cl, and rare-earth elements that may be present in shales and possibly dolomite; and (3) corrections for borehole irregularities and tool stand-off. The presence of clay minerals containing hydrogen may result in an overestimation of porosity with this tool. In addition, incomplete contact with the borehole wall causes the tool to detect borehole water, giving an overestimation of porosity. The HNGS, APS, and the HLDS are collectively called the integrated porosity-lithology tool.

Phasor Dual Induction-Spherically Focused Resistivity Tool (DITE-SFL)

The DITE-SFL provides measurements of three different resistivity values: (1) deep induction, (2) medium induction, and (3) shallow spherically focused resistivity (SFLU). Resistivity is controlled mainly by the conductivity of pore fluids and by the amount and connectivity of pores. Two induction devices transmit high-frequency alternating currents through transmitter coils, creating a magnetic field that induces secondary (Foucault) currents in the formation. These currents produce a new inductive signal proportional to formation conductivities that are recorded by the receiving coils. The measured conductivities are then converted to resistivity (m). The SFLU measures the current necessary to maintain a constant voltage drop across a fixed interval and gives a direct measurement of resistivity.

LDEO High-Resolution Temperature/Acceleration/Pressure Tool (LDEO-TAP)

The LDEO-TAP is a "dual application" logging tool (i.e., it can operate as either a wireline tool or as a memory tool using the same sensors and data acquisition electronics, depending on the purpose and required precision of logging data). The full specifications of this new tool are described in Table T10. During Leg 182 the LDEO-TAP was deployed as a memory tool in low-resolution mode, with the data being stored in the tool and then downloaded after the logging run was completed. The LDEO-TAP offers greater flexibility in logging operations, and it substantially improves the quality and resolution of data over an extended ambient temperature range (when used in the wireline mode) compared to the superceded LDEO-TLT.

The tool automatically starts recording after a preset pressure (depth) has been reached. Temperature, measured by high-precision thermistors, and pressure are measured every second. Tool acceleration is recorded four times per second. Data are recorded as a function of time and correlated to depth on the basis of a synchronized time-wireline cable depth record and pressure recordings. After logging, data are downloaded via a modem. Temperatures determined using the LDEO-TAP do not represent in situ formation temperatures because water circulation during drilling will have disturbed temperature conditions in the borehole. However, abrupt temperature changes superimposed on the overall measured gradient may represent localized fluid flow into the borehole (indicative of fluid pathways and fracturing) and/or changes in permeability at lithologic boundaries.

Sonic Digital Tool (SDT)

The SDT measures the time required for sound to travel through the formation between a transmitter and a receiver. As this tool averages replicate analyses, it provides direct measurements of sound velocity through the sediments that are relatively free from the effects of formation damage and borehole enlargement (Schlumberger, 1989).

The SDT contains two broadband piezoelectric ceramic transmitters and receivers spaced 61 cm apart, with the lower receiver located 91 cm above the upper transmitter. In addition, eight wideband ceramic receivers are arranged in an array 1.07 m long and 2.44 m above the upper transmitter at the base of the sonde. This configuration provides a total of eight different transit-time measurements. Interval transit times are converted to compressional wave velocities (km/s). Full waveforms are recorded by the tool, allowing shorebased postprocessing to estimate shear- and Stoneley-wave velocities, as well as amplitude attenuation. Logs are edited for cycle skipping and obviously spurious values.

Formation MicroScanner (FMS)

The FMS produces high-resolution images of borehole wall microresistivity. This tool has four orthogonally oriented pads, each having 16 button electrodes that are pressed against the borehole walls. Good contact with the borehole wall is necessary for high-quality data. During a single pass, ~30% of the borehole wall is imaged. Coverage may be increased by a second run. However, there is no active way of orienting the pads in the borehole; therefore, there is no control over differences in coverage between the first and second pass. The current-intensity measurements in each button are converted to variable-intensity color images that reflect microresistivity variations. The vertical resolution of FMS images is claimed by Schlumberger to be ~5 mm, but is probably lower (~1 cm) in practice.

FMS images are oriented to magnetic north using the GPIT. This allows the dip and strike of geological features intersecting the hole to be measured from processed FMS images. FMS images can be used to visually compare logs with core and ascertain the orientations of bedding, fracture patterns, and sedimentary structures. Comparatively small-scale features, such as burrows and vugs, can also be identified using FMS images.

FMS images have proved particularly valuable in the interpretation of sedimentary structures in previous ODP legs and have been used to identify cyclical stacking patterns in carbonates (Eberli, Swart, Malone, et al., 1997), soft sediment slumping (Norris, Kroon, Klaus, et al., 1998), turbidite deposits (Lovell et al., 1998), cross-beds (Hiscott et al., 1992), and facies changes (Serra, 1989). Detailed processing of FMS images in combination with other log and core data is performed postcruise at LDEO-BRG, LUBR, and LMF.

General-Purpose Inclinometer Tool (GPIT)

The GPIT was included in the FMS/sonic tool string to calculate tool acceleration and orientation during logging. The GPIT contains a triple-axis accelerometer and a triple-axis magnetometer. The GPIT records the orientation of the FMS images and allows more precise determination of log depths than can be determined from cable length, which may experience stretching and/or be affected by ship heave.

Geologic High-Resolution Magnetic Tool (GHMT)

The GHMT normally consists of a high-sensitivity total magnetic field sensor (NMRS) coupled with a MS sensor (SUMS). However, during Leg 182 the NMRS was not functioning. The SUMS measures MS by means of low-frequency induction in the surrounding sediment.

Well Seismic Tool (WST)

The WST was used to produce zero-offset check shots in the borehole. The WST consists of a single geophone used to record the full waveform of acoustic waves generated by a seismic source positioned just below the sea surface. During Leg 182 a 45/105-in³ generator-injector (GI) gun was used as the seismic source positioned at a water depth of 3 m and offset from the borehole by 50 m on the port side of the JOIDES Resolution. The WST was held against the borehole wall at variable intervals, and the GI gun was typically fired between 8 and 15 times at each station. The recorded waveforms were stacked and a one-way traveltime determined from the median of the first breaks measured; thus providing check shots for calibration of the integrated transit time calculated from sonic logs (see "Seismic Stratigraphy"). Check shot calibration is required for the borehole to seismic tie because P-wave velocities derived from the sonic log may differ significantly from seismic stacking velocities and velocities obtained via well seismic surveys. These differences result from (1) differential frequency dispersion (the sonic tool operates at 10-20 kHz, seismic data in the 50-100 Hz range), (2) difference in travel paths between well seismic and surface seismic surveys, and (3) borehole effects caused by formation alterations (Schlumberger, 1989). In addition, sonic logs cannot be measured through pipe and the traveltime to the uppermost logging point must be estimated by other means.