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doi:10.2204/iodp.proc.301.105.2005

Wireline logging

The wireline logging program planned for Expedition 301 was specifically designed for assessing basement alteration, fine-scale lithostratigraphy, and fracture distribution; crustal layering, seismic velocity, and velocity anisotropy; and vertical and near-hole permeability distribution. Individual logging tools are joined together into tool strings to make several measurements during each logging run. The tool strings are lowered to the bottom of the borehole on a wireline, and data are logged as the tool string is pulled back up the hole. Repeat runs are made to improve coverage and confirm the accuracy of logging data.

Logging tools and tool strings

During Expedition 301, the following logging strings were deployed (Fig. F16; Table T23):

• The triple combination (triple combo) string, which consists of the Hostile Environment Gamma Ray Sonde (HNGS), the Accelerator Porosity Sonde (APS), the Hostile Environment Litho-Density Sonde (HLDS), and the SlimXtreme Array Induction Tool (QAIT);

• The Formation MicroScanner (FMS)-sonic tool string, which consists of the Dipole Sonic Imager (DSI), the Scintillation Gamma Ray Tool (SGT); the General Purpose Inclinometer Tool (GPIT), and the FMS;

• The Ultrasonic Borehole Imager (UBI) tool string, consisting of the SGT, the GPIT, and the UBI; and

• The Well Seismic Tool (WST).

Principles and uses of the logging tools

The properties measured by each tool, sampling intervals, and vertical resolutions are summarized in Table T23. Explanations of tool name acronyms and their measurement units are summarized in Table T24. More detailed descriptions of individual logging tools and their geological applications can be found in Ellis (1987), Goldberg (1997), Rider (1996), Schlumberger (1989, 1994), Serra (1984, 1986, 1989), and the Lamont-Doherty Earth Observatory–Borehole Research Group (LDEO-BRG) Wireline Logging Services Guide (2001).

SlimXtreme Array Induction Tool

Expedition 301 marks the first deployment in scientific ocean drilling of the QAIT, which is the slim and high-pressure/high-temperature version in the series of Schlumberger array induction tools. The QAIT features an outside diameter of 3 inches, is rated for operations in environments up to 260°C (500°F), and is pressure rated to 30,000 psi. The QAIT's cartridge and sonde weigh 500 lb. This sonde contains eight mutually balanced induction arrays, with spacings ranging from several inches to several feet. A single transmitter operates at one unique frequency (26.3 kHz). Thus, signal components are measured for each array at one frequency, providing 16 raw conductivity measurements acquired at 3 inch depth intervals.

As with other induction tools, primary calibration is done with a set of test loops. The QAIT continuously autocalibrates the tool while logging to minimize sensitivity changes with temperature. The 16-array conductivity measurements are corrected for borehole diameter and combined using functions that are weighted in both the radial and depth directions to produce a set of five logs. The logs have closely matched vertical resolution and median depths of investigation of 10, 20, 30, 60, and 90 inches from the center of the borehole.

Response functions are used to radially deconvolve the set of matched vertical resolution logs to produce a detailed record of the radial conductivity. This data can, in turn, be transformed into a number of computed color-coded images and discrete value log curves.

The QAIT provides the following list of major measurements:

• Five basic log resistivity curves with a median radial depth of investigation of 10, 20, 30, 60, and 90 inches;

• The basic log set with a 1, 2, and 4 ft vertical resolution;

• Flushed zone resistivity (R_xo), which can be used for determining water saturation (S_w) if porosity is known;

• Model-independent resistivity and two-dimensional quantitative imaging of formation (R_WA images);

• Model-dependent water saturation (S_w);

• Analog spontaneous potential log or a measure of the natural difference in electrical potential, in millivolts, between an electrode in the borehole and a fixed reference electrode on the surface; and

• Volume of mud filtrate estimation.

Logging data flow and processing

Data for each wireline logging run were recorded, stored digitally, and monitored in real time using the Schlumberger multitask, acquisition, and imaging system (MAXIS) 500 system located in the new Offshore Service Unit-F-model Modular Configuration MAXIS Electrical Capstan Capable (OSU-FMEC) winch unit. The OSU-FMEC is a full backup system to the main MAXIS 500 system commonly used during logging operations. During Expedition 301, the OSU-FMEC unit was used as the primary acquisition system because we were using new winch and heave compensator units that were at the same location (Fig. F17). Training and supervision of winch operators with the new system took place during deployments of the logging tools and the CORK remotely operated vehicle platforms.

The new heave compensation system is one of a kind and compensates via a hydraulic pump, which controls the motion of the wireline drum. The data are processed in the acquisition system and correlated with the compensated depth. The compensated distance measured in the winch control is applied to the acquisition system, and the measured depth is then corrected. This system also uses data from surface three-axis accelerometers to correct for tool motion at the drum. Ultrasonic transducers measure the radius of the drum to calculate tangential velocity and to get an estimate of the tool speed downhole.

After logging was completed in 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 (version 4.0.4.1) software package. Logging data were also transmitted to IODP-USIO Science Services, LDEO, using a satellite high-speed data link for processing soon after each hole was logged. Data processing at LDEO consists of (1) depth-shifting all logs relative to a common datum (i.e., in mbsf), (2) corrections specific to individual tools, and (3) quality control and rejection of unrealistic or spurious values. Once processed at LDEO, logging data were transmitted back to the ship, providing near-real time data processing. Processed data were then replotted on board (see "Wireline logging" in the "Site U1301" chapter). Postcruise-processed data in ASCII format are available directly from the IODP-USIO Science Services LDEO Web site at iodp.ldeo.columbia.edu/DATA/IODP/index.html. A summary of "logging highlights" is posted on the LDEO Web site at the end of each expedition.

Wireline logging data quality

Logging data quality may be seriously degraded by 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. HNGS and SGT data provide a depth correlation between logging runs. Logs from different tool strings may, however, still have depth mismatches caused by either cable stretch or ship heave during recording. FMS, DSI, and UBI data acquired during Expedition 301 were seriously degraded because of poor performance by the new wireline heave compensation software system (see "Wireline logging" in the "Site U1301" chapter).

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