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doi:10.2204/iodp.sp.330.2010

Logging/downhole measurements strategy

Downhole logging will aid in achieving expedition objectives by assisting in lithologic identification and recognition of the structural characteristics of cored sequences (particularly in the volcanic basement) and providing detailed monitoring of changes in magnetic properties and paleomagnetic directions within and between lava flows. Wireline logs will provide a continuous record to aid the detection of lava flow boundaries, interlayered (baked) sediments, and alteration zones in the basement section and will enable the tilting of lava flows to be evaluated. Determination of 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 most precisely constrained. Logging measurements will complement core measurements by determining the characteristics of lithologic units in any intervals where core recovery is poor. In addition, wireline logging data can be compared to analyses of discrete core samples.

Combinations of four wireline logging tool strings will be deployed during Expedition 330: (1) the standard triple combination (triple combo), (2) the Formation MicroScanner (FMS)-sonic, (3) the Ultrasonic Borehole Imager (UBI), and potentially (4) the third-party Göttingen Borehole Magnetometer (GBM). Operational time estimates for each site can be found in Table T3. The tools and their applications are described below, and further information is available at iodp.ldeo.columbia.edu/TOOLS_LABS/index.html.

The triple combo tool string consists of several probes that record geophysical measurements of the penetrated formations and measure the total and spectral natural gamma radiation, density, porosity, and resistivity of the formation. These measurements will enable changes in lithology and variations in alteration to be assessed. The FMS-sonic tool string acquires oriented high-resolution electrical images of the borehole wall and measures compressional and shear wave forms. Velocity data can be used to determine preferred mineral and/or fracture orientations and fracture densities. The sonic and density logs can be used together to create synthetic seismograms, improving correlation between the seismic records and the lithologic units recovered from the boreholes. The UBI will be deployed at the first site (LOUI-1C), with data acquisition at subsequent sites depending on the time available and the quality of the logs acquired. The UBI provides high-resolution acoustic amplitude images with 100% borehole wall coverage. The high-resolution FMS and UBI images will allow detection of small-scale fractures and lithologic variations, enable the tilt of lava flows to be evaluated, and may allow reorientation of core pieces. The General Purpose Inclinometry Tool (GPIT) will be deployed with both image tools to collect accelerometer and magnetometer data, which will allow orientation of the images and provide information about borehole geometry. Constraining the deviation of the hole from vertical is a factor in evaluating possible biases in paleolatitude estimates derived from paleomagnetic measurements on the core.

The GBM was previously deployed as a third-party tool during Leg 197 and IODP Expedition 305 and is planned to be available during Expedition 330. This tool has three fluxgate sensors that measure three orthogonal components of the magnetic field (Steveling et al., 2003). The GBM includes a gyroscope that measures tool rotation during data acquisition and accordingly allows the orientation of the tool to be determined with respect to the borehole walls and thus with respect to magnetic north. Data from the magnetometer will be used to monitor changes in the magnetic properties of the lithologies as well as changes in paleomagnetic direction. The GBM requires a science party member/technician onboard who has had previous training and who will be responsible for operating the tool. For optimum data quality, the GBM requires centralization during logging, but GBM centralizers that fit through the bottom-hole assembly used on the JOIDES Resolution are not available at present. The final decision to deploy this tool on Expedition 330 will depend on whether an expedition participant can be trained and whether centralization of the tool can be successfully achieved.

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