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doi:10.2204/iodp.proc.330.101.2012

Logging and downhole measurement strategy

Downhole logging aided in achieving the objectives of Expedition 330 by assisting in lithologic identification and recognition of structural characteristics (particularly cored volcanic basement sequences) and by providing detailed magnetic anomaly data that may allow monitoring of changes in magnetic properties and paleomagnetic directions within and between lava flows. Wireline logs provided a continuous record to aid in detecting lava flow boundaries, interlayered (baked) sediments, and alteration zones in the basement section and enabled evaluation of lava flow tilting. Determining the number of lava flow units has implications for how well geomagnetic secular variation has been sampled and hence the extent to which paleomagnetic paleolatitudes can be constrained most precisely. These logging measurements complement shipboard core measurements by recording the characteristics of lithologic units in intervals of poor core recovery and by allowing comparison of logging data to discrete sample analyses.

Combinations of four wireline logging tool strings were deployed during Expedition 330: (1) the triple combination (triple combo) tool string, (2) the Formation MicroScanner (FMS)-sonic tool string, (3) the Ultrasonic Borehole Imager (UBI), and (4) the third-party Göttingen Borehole Magnetometer (GBM). These tools and their applications are further described in the sections below; additional information is available at iodp.ldeo.columbia.edu/TOOLS_LABS/index.html.

Triple combo, FMS-sonic, and UBI tool strings

The triple combo tool string consists of several probes that record geophysical measurements of the penetrated formations and measures total and spectral natural gamma ray, density, porosity, and resistivity of the formation. These measurements enable assessment of changes in lithology and variations in alteration. The FMS-sonic tool string acquires oriented high-resolution electrical resistivity images of the borehole wall and measures compressional and shear wave forms. The high-resolution images allow small-scale fractures and lithologic variations to be detected, enable the tilt of lava flows to be evaluated, and may allow some core pieces to be reoriented. The sonic and density logs from this tool string can be used together to create synthetic seismograms, allowing correlation between the regional seismic data and lithologic units recovered from the boreholes. Two full uplog FMS runs were done for each logged hole, resulting in eight individual pad tracks, producing an electrical image that can cover >65% of the 360° borehole wall. In the case of Site U1374 the UBI was deployed as well, providing high-resolution acoustic amplitude images with 100% coverage of the borehole wall.

FMS images, in conjunction with the UBI, can provide a comprehensive overview of structure, virtual hardness, and variation in lithofacies. Images collected with the FMS-sonic tool string are extremely helpful in picking out formation features such as vesicles, breccia, and coherent units (Bartetzko et al., 2003). To most accurately reorient the core pieces using FMS and UBI images, these tool strings are run with a General Purpose Inclinometry Tool (GPIT). However, in formations characterized by a strong remanent magnetization these orientation data can be affected. By running the GBM (see below) and comparing its data with those collected with the GPIT, more accurate image orientation can be accomplished, which in turn allows for more accurate reorientation of core pieces using FMS and UBI images (Gaillot et al., 2004).

Göttingen Borehole Magnetometer (third-party tool)

High-quality paleolatitude data are required to document the motion of the Louisville hotspot and to compare it with the 15° shift observed for the Hawaiian hotspot over the same time interval. These data, in concert with detailed radiometric ages, will provide the basis for calibrating and testing various geodynamic models. The most robust paleolatitude information will be derived from detailed demagnetization studies of the cored—but azimuthally unoriented—basaltic basement samples. However, the GBM (Fig. F10) provides valuable complementary data that can significantly enhance these paleomagnetic directional studies that are critical expedition objectives.

The GBM allows fully oriented component magnetic anomalies to be determined by measuring three orthogonal components of the magnetic field (Steveling et al., 2003). The GBM includes three optical gyroscopes, which record the tool’s overall rotation since the start of measurement, allowing the cumulative rotation of the tool to be compensated for in the data using a Matlab software program (S. Ehmann, Technische Universität, Braunschweig, Germany). This allows for independent determination of the intensity, inclination, and declination of the magnetization in the lava flow formations. For optimum data quality the GBM requires centralization during logging and the addition of a nonmagnetic sinker bar just above the GBM.

The GBM was developed in Göttingen, Germany, by Drs. M. Leven and E. Steveling at the Institut für Geophysik of the Georg-August-Universität (e.g., Steveling et al., 2003) to measure all three magnetic components in the walls of a borehole during downhole logging runs. This tool is currently housed at the Technische Universität in Braunschweig, Germany, and is maintained and deployed under the supervision of Professor A. Hoerdt. The GBM was successfully deployed for the first time in ODP Hole 1203A (Leg 197) in the Emperor Seamounts, although the results were used primarily to test filtering algorithms for the fluxgate sensors on the GPIT (Gaillot et al., 2004). The GBM was subsequently deployed during IODP Expedition 304/305, but short-period oscillations in the gyro data prevented recovery of component anomaly data. More recently, the GBM was successfully deployed in the 2.5 km deep Outokumpu hole in eastern Finland of the International Continental Scientific Drilling Program (Virgil et al., 2010) and now during IODP Expedition 330.

The ability of the GBM to make in situ three-component determinations of the magnetic anomalous field in the borehole and estimate both the inclination and declination of penetrated lava flows allows for the following important improvements. First, magnetization declination data from logging using the GBM may aid in recognizing temporally distinct flow units in the core, usually judged by inclination differences alone. This is an important first-order application because the number of distinct flow units is critical in establishing the uncertainty in paleolatitude estimates as measured from discrete core samples (by averaging out secular variation). Second, high-quality determination of the vertical and horizontal magnetic anomalies will allow in situ magnetization inclinations to be determined, which will provide complementary in situ data that may help in determining the paleolatitude history of the Louisville hotspot. And, finally, even though the fluxgate magnetometers incorporated in the GPIT are sufficient for orienting the FMS system, these sensors have relatively poor sensitivity (50 nT) and, more importantly, show substantial (~1000 nT) offsets that limit interpretation of the data (e.g., Ito et al., 1995). The fluxgate magnetometers in the GBM have a better resolution (12 nT) and are well calibrated. For example, when the ambient field was measured with the GBM above the HSDP-2 drill hole on Hawaii (Steveling et al., 2003), the inclination of 36.5° compared well with the International Geomagnetic Reference Field inclination of 36.6°, and the measured total field was compatible with that determined by aeromagnetic surveys.

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