Downhole logging will be an essential component of Leg 206 scientific objectives by providing in situ information on the geophysical structure of the drilled basaltic formation. An extensive logging program is planned at proposed Site GUATB-03C to achieve objectives such as study of volcanic stratigraphy, eruptive morphology, variations in alteration, stress field, and seismic structure. Whereas core recovery is often biased and incomplete in igneous basement, downhole logging data are continuous and therefore provide information over intervals of low recovery.
As discussed in the "Drilling Strategy" section, we intend to log the entire section drilled, which requires logging the RCB pilot hole as well as the reentry hole. Specifically, the triple Combo and FMS/sonic tool strings and the BGR borehole magnetometer will be run both in the RCB pilot hole and the reentry hole. The WST-3 logging tool will be run in the reentry hole only because it is capable of providing seismic velocity data in the cased hole as well as the open hole. Similarly, we plan to run the UBI logging tool in the reentry hole and possibly in the RCB pilot hole, but use of this specialty tool is contingent on availability of funds, which will be determined prior to Leg 206. The characteristics of these logging tool strings can be found at the Lamont-Doherty Earth Observatory (LDEO) Borehole Research Group web site at http://www.ldeo.columbia.edu/BRG and are briefly described below.
The triple combo consists of several probes or sondes:
The FMS/sonic tool string has two main components:
The UBI measures the amplitude and transit time of an acoustic wave propagated into the formation. It provides high-resolution images with 100% borehole wall coverage, which allows detection of small-scale fractures. The GPIT is deployed with the UBI and allows orientation of the images; evaluation and orientation of fractures can provide information about the local stress field and borehole geometry even within the casing. An NGT is included in this tool string to allow correlation with other logging runs for establishing consistent depth estimates. Deployment of the UBI during Leg 206 is contingent on availability of funds.
The BGR borehole magnetometer is a third-party tool developed by Bundesanstalt für Geowissenschaften und Rohstoffe in Germany. It has been previously used in Holes 504B and 896A during ODP Leg 148 (Shipboard Scientific Party, 1993; Worm et al., 1996) and in the KTB (Kontinentales Tiefbohrprogramm Bundesrepublik) drill hole in Germany. It has three fluxgate sensors that measure three orthogonal components of the magnetic field. It can measure fields up to 100 mT with a resolution of 0.1 nT. The probe contains two inclinometers, aligned with the probe's x- and y-axes, that measure the tilt of the probe to 0.1°. The tool includes a gyroscope, which measures the tool rotation during data acquisition and allows the orientation of the tool to be determined. The data from the magnetometer will be used to monitor changes in the magnetic properties of the upper oceanic crust as well as changes in paleomagnetic direction that can aid in determination of the magnetic polarity. We are currently investigating whether the BGR borehole magnetometer will be available for Leg 206.
The WST-3 records acoustic waves generated by an air gun located near the sea surface. It provides a complete check-shot survey, a depth-traveltime plot, and accurate estimates of the drilling depth.
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