Ocean Drilling Program: Leg 197 Scientific Prospectus

LOGGING PLAN

Downhole logging will be used during Leg 197 to address issues concerning possible deviation of holes from vertical, in situ basalt magnetizations, core orientation, volcanic stratigraphy, and eruptive morphology. Whereas core recovery is often biased and incomplete in lithologies such as alternating pillows and massive flows, logging data are continuous and therefore provide useful information over intervals of low core recovery. During Leg 197, we are particularly interested in determining the number of flow units, which has implications for how well geomagnetic secular variation has been sampled, and hence, how well paleomagnetic paleolatitudes can be constrained. If time permits, we also plan to use logging data to create synthetic seismograms, which will then lead to improved correlation between the seismic records and the lithologic units recovered from the boreholes. As shown in the Operations Schedule (Table 1), we tentatively plan to log at each site. Subject to time constraints, this logging plan may be adjusted at the discretion of the co-chiefs within the guidelines of the normal JOIDES logging policy.

To achieve the Leg 197 scientific objectives, the proposed sites will be logged with the standard logging tool strings (triple combination [triple combo] and Formation MicroScanner [FMS]). Specifically the logging plan includes one triple combo and one FMS run along the whole drilled section and a second FMS run in the basalt intervals. A triple combo run and one FMS run may be extended into the sedimentary section depending on time constraints. The characteristics of these logging tool strings can be found at the Borehole Research Group web site at http://www.ldeo.columbia.edu/BRG and are briefly described in the next paragraph.

The triple combo tool string consists of several probes recording geophysical measurements of the penetrated formations. It consists of the accelerator porosity sonde (APS), which gives the porosity from epithermal neutron measurements, and the hostile environment lithodensity sonde (HLDS), which measures bulk density and photoelectric absorption through the interaction of gamma rays with electrons in the formation.The hostile environment natural gamma ray sonde (HNGS) and natural gamma ray tool are used to measure the natural radioactivity of the drilled formation.The resistivity tool should aid in identification of lava flows in the upper oceanic crust. The resistivity of the basaltic rocks is rather high and will likely exceed 2000 Ohm-m, so the dual laterolog (DLL) will be used in combination with the triple combo to ensure that reliable resistivity data are collected.
The FMS provides high-resolution electrical images of the penetrated formations. We will attempt to azimuthally reorient the cores by identifying fractures and veins in the FMS images that can be correlated with their counterparts on the recovered core. The FMS tool string also includes the general purpose inclinometry cartridge (GPIT), which provides accelerometer and magnetometer data to allow determination of the tool position and spatial orientation of the images. The GPIT will also be used to constrain the deviation of the hole from vertical, which is an important factor in evaluating possible biases in paleolatitude estimates derived from paleomagnetic measurements on the core.
If time allows, the dipole sonic imager (DSI) will be used to measure the compressional and shear wave velocities. The natural gamma ray tool (NGT) is run in combination with the previous probe to achieve depth matching between the different logging runs. Importantly, the DSI will be run separately from the FMS/GPIT because it will otherwise introduce a large spurious magnetic signal in the GPIT data. The GPIT data and their use in the calculating hole deviation are of highest priority in terms of leg objectives.

In addition, we plan to log one site with a magnetic logging tool being developed by the Geophysical Institute of the University of Göttingen, Federal Republic of Germany (referred to as the Göttingen Borehole Magnetometer [GBM]), and with a third-party magnetic susceptibility borehole tool (referred to as the SUSLOG 403-D). The GBM has three fluxgate sensors that measure three orthogonal components of the magnetic field, whereas the SUSLOG 403-D obtains an estimate of bulk magnetic susceptibility useful for modeling the contributions of induced magnetizations to the total magnetic signal recorded by the GBM. The effectiveness of these tools will be evaluated during the leg, and, time allowing, they may be run at additional sites.

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