Downhole logs reveal the physical, chemical, and structural properties of formations penetrated by a borehole. A variety of geophysical tools make rapid, continuous in situ measurements as a function of depth after the hole has been drilled that can be used to interpret the lithology of the penetrated formation. Where core recovery is good, core data are used to calibrate the geophysical signature of the rocks. In intervals of low core recovery or disturbed cores, log data may provide the only way to characterize the borehole section. Geophysical well logs can aid in characterizing sedimentary sequences and stratal stacking patterns when integrated with core and seismic reflection data.
Individual logging tools are joined together into tool strings (Fig. F10) so that several measurements can be made during each logging run (Table T7). The tool strings are lowered to the bottom of the borehole on a wireline cable, and data are logged as the tool string is pulled back up the hole. Repeat runs are made in some holes to improve coverage and confirm the accuracy of log data. Not all tool strings are run in each hole; refer to individual site chapters for details of logging strings deployed at each site.
During ODP Leg 194, the following three different logging strings were deployed (Fig. F10 and Table T7):
The parameter measured by each tool, the sample intervals used, and the vertical resolution, are summarized in Table T7. Explanations of tool name acronyms and their measurement units are summarized in Table T8.
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, 1994), Serra (1984, 1986, 1989), and the Lamont-Doherty Earth Observatory-Borehole Research Group (LDEO-BRG) Wireline Logging Services Guide (1994).
The HNGS and the NGT measure the natural gamma radiation from isotopes of potassium (K), thorium (Th), and uranium (U). The NGT uses a sodium-iodide (NaI) scintillation detector to measure the natural gamma ray emission and a five-window spectroscopy to determine concentrations of radioactive K (in weight percent), Th (in parts per million), and U (in parts per million). The HNGS uses a measurement principle similar to that of the NGT, but 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 improving measurement accuracy. The HNGS generates the same output as the NGT and also estimates the average borehole potassium contribution to the total potassium signal. Shipboard corrections to the HNGS account for variability in borehole size and borehole potassium concentrations. The NGT and HNGS also provide a measure of the total gamma ray signature SGR (gAPI [American Petroleum Institute] units) and uranium-free or computed gamma ray CGR (gAPI units).
The NGT response is influenced by the 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 freshwater clays from swelling and forming obstructions. All of these effects are corrected during processing of NGT data at LDEO-BRG.
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, which is pressed against the borehole wall by a hydraulically activated eccentralizing arm. Gamma rays emitted by the source experience 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 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 factor (PEF) caused by absorption of low-energy gamma rays. Photoelectric absorption occurs when gamma rays reach <150 keV after being repeatedly scattered by electrons in the formation. As PEF depends on the atomic number of elements in formation, it is essentially independent of porosity. Thus, PEF varies according to the chemical composition of the sediment. Some examples of PEF are: 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. Coupling between the tool and borehole wall is essential for good HLDS logs. Poor contact results in underestimation of density values. Both density correction and caliper measurement of the hole are used to check the contact quality.
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 high-energy (fast) neutrons are slowed down by collisions with atoms. The amount of energy lost per collision depends on the relative mass of the nucleus with which the neutron collides. A lot of energy loss occurs when the neutron strikes a nucleus of equal mass such as hydrogen, which is mainly present in pore water. Degrading to thermal energies (0.025 eV), the neutrons are captured by the nuclei of silicon, chlorine, and boron (Si, Cl, and B) 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 the neutron arrival times, which act as a measure of formation porosity. However, hydrogen bound in minerals such as clays or in hydrocarbons also contributes to the measurement, so the raw porosity value is often an overestimate.
The DITE provides three different measurements of electrical resistivities, each with a different depth of penetration into the formation. Two induction devices (deep and medium resistivity) transmit high-frequency alternating currents through transmitter coils, creating magnetic fields that induce secondary (Foucault) currents in the formation. These ground-loop currents produce new inductive signals, proportional to the conductivity of the formation, which are measured by the receiving coils. The measured conductivities are then converted to resistivity. A third device, a spherically focused resistivity instrument that gives higher vertical resolution, measures the current necessary to maintain a constant voltage drop across a fixed interval.
The newly developed MGT was tested as part of the triple combo tool string (Fig. F10). The major advantage of the new tool is improved vertical resolution, comparable with the resolution of MST core measurements (Table T7). This is achieved by real-time stacking of natural gamma spectral data from four independent small-sized scintillation detectors positioned at 0.64-m spacing in the measurement module (Fig. F11). The tool provides 256-channel spectral analysis of each detector signal in the 0.2- to 3.0-MeV energy range. The full spectra are later combined into five- or three-window spectral data for compatibility with the older tools. The total gamma (gAPI) and concentrations of K (in weight percent), Th (in parts per million), and U (in parts per million) are calculated in real time either from spectral data of individual detectors or from stacked data. Postcruise processing will correct for borehole size and tool sticking, which is assessed by accelerator data recorded in the MGT.
The 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). Data acquisition electronics are dependent on the purpose and required precision of logging data. During Leg 194, the 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.
Temperatures determined using the TAP do not necessarily represent in situ formation temperatures because water circulation during drilling will have disturbed temperature conditions in the borehole. However, from the spatial temperature gradient, abrupt temperature changes can be identified that may represent localized fluid flows into the borehole, indicating fluid pathways and fracturing and/or breaks in the temperature gradient that may correspond to contrasts in permeability at lithologic boundaries.
The LSS measures the compressional wave velocity of the formation. The LSS is configured with two acoustic sources and two receivers, each spaced 61 cm apart. The spacing between the upper receiver pair and the transmitter pair is 2.44 m. The tool measures traveltime in microseconds over a certain distance in the formation. The configuration of the tool allows eight different traveltime measurements that compensate for irregular borehole walls. The velocity data together with the formation density can be used to generate a synthetic seismogram.
The FMS produces high-resolution images of borehole wall microresistivity that can be used for detailed sedimentologic or structural interpretation. This tool has four orthogonally oriented pads, each with 16 button electrodes that are pressed against the borehole walls. Good contact with the borehole wall is necessary for acquiring good-quality data. Approximately 30% of a borehole with a diameter of 25 cm is imaged during a single pass. Coverage may be increased by a second run. The vertical resolution of FMS images is ~5 mm, allowing features such as burrows, thin beds, fractures, veins, and vesicles to be imaged. The resistivity measurements are converted to color or grayscale images for display. In site chapters in this volume, local contrasts in all FMS figures were improved by applying dynamic normalization to the FMS data. A linear gain is applied, which keeps a constant mean and standard deviation within a sliding window of 1 m. When dynamic normalization is used, the values of color indicate relative changes in resistivity. Furthermore, the hole diameter was reduced artificially from 25.1 to 15.2 cm to reduce the blank space between the pad tracks and enlarge the FMS images proportionally.
FMS images are oriented to magnetic north using the GPIT (see "General Purpose Inclinometer Tool"). 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 the core to ascertain the orientations of bedding, fracture patterns, and sedimentary structures and to identify stacking patterns. FMS images have proved particularly valuable in interpreting sedimentary structures, and they have been used to identify cyclical stacking patterns in carbonates (Eberli, Swart, Malone, et al., 1997; Williams and Pirmez, 1999; Kroon et al., 2000), turbidite deposits (Lovell et al., 1998), and facies changes (Serra, 1989).
The GPIT is 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.
The WST is used to produce a zero-offset vertical seismic profile and/or 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 194, a 300-in3 air gun, positioned at a water depth of ~7 m with a borehole offset of 49 m on the port side of the JOIDES Resolution, was used as the seismic source. The WST was clamped against the borehole wall at 30- to 50-m intervals, and the air gun was typically fired between 5 and 15 times at each station. The recorded waveforms were stacked and a one-way traveltime was determined from the median of the first breaks for each station, thus providing check shots for calibration of the integrated transit time calculated from sonic logs. Check shot calibration is required for the core-seismic correlation because P-wave velocities derived from the sonic log may differ significantly from true formation velocity because of (1) frequency dispersion (the sonic tool operates at 10-20 kHz, with seismic data in the 50-200 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, so the traveltime down to the uppermost logging point has to be estimated by other means.
The drill string acceleration (DSA) tool is a modular downhole tool designed to acquire acceleration and pressure data near the bit. The DSA tool contains a single-axis high-sensitivity accelerometer for heave measurements, a three-axial high-frequency accelerometer for drill bit vibrations, and a high-resolution pressure sensor. For ease of deployment, the DSA tool has been designed as a removable extension of the APC/XCB/RCB and HYACE core barrels. Using standard threaded connections, the DSA tool was attached to the top of the rotary HYACE core barrel, without affect coring activities.
On each run, the DSA tool begins data acquisition at a predetermined depth as programmed by the LDEO logger. In addition to drilling vibration data and downhole pipe motion data, information about core barrel landing, pressure up, and other operational events are recorded by the DSA tool to assist in development of the HYACE core barrels. Upon DSA/core barrel retrieval, the DSA tool is disconnected and the data downloaded to the third party data acquisition system in the DHML for immediate analysis.
Logging data quality may be seriously degraded by rapid changes in the hole diameter and in sections where the borehole diameter greatly decreases or is washed out.
Deep-investigation measurements such as resistivity and sonic velocity are least sensitive to borehole conditions. Nuclear measurements (density and neutron porosity) are more sensitive because of their shallower depth of investigation and the effect of drilling fluid volume on neutron and gamma ray attenuation. Corrections can be applied to the original data in order to reduce these effects. Very large washouts, however, cannot be corrected for. HNGS and NGT data provide a depth correlation between logging runs. Logs from different tool strings may, however, still have minor depth mismatches caused by either cable stretch or ship heave during recording. Ship heave is minimized by a hydraulic wireline heave compensator designed to adjust for rig motion during logging operations.
Data for each logging run were recorded and stored digitally and monitored in real time using the Schlumberger MAXIS 500 system. After logging at each hole, data were transferred to the shipboard downhole measurements laboratory for preliminary processing and interpretation. FMS image data were interpreted using Schlumberger's Geoframe 3.8 software package. Well seismic, sonic, and density data were used for calculation of synthetic seismograms with GeoQuest's IESX software package to establish the seismic to borehole tie.
Log data were also transmitted to LDEO-BRG using a FFASTEST satellite high-speed data link for processing soon after each hole was logged. Data processing at LDEO-BRG consists of (1) depth-shifting 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 providing near real-time data processing. Log curves of LDEO-BRG-processed data were then replotted on board (see the "Downhole Measurements" section in each site chapter). Further postcruise processing of the log data from the FMS is performed at LDEO-BRG.
Postcruise-processed acoustic, caliper, density, gamma ray, neutron porosity, resistivity, and temperature data in ASCII format are available directly from the LDEO-BRG Internet Web site at http://www.ldeo.columbia.edu/BRG/ODP/DATABASE/. A summary of "logging highlights" is posted on the LDEO-BRG Web site at the end of each leg.
Downhole logging aboard the JOIDES Resolution is provided by LDEO-BRG in conjunction with Leicester University Borehole Research, the Laboratoire de Mesures en Forages Montpellier, University of Aachen, University of Tokyo, and Schlumberger Well Logging Services.