Leg 197 was devoted to studying the motion of the Hawaiian hotspot from a paleomagnetic point of view (Tarduno, Duncan, Scholl, et al., 2002). Two tools were used to log magnetic data in Hole 1203A (see "Downhole Measurements" in Shipboard Scientific Party, 2002), the GPIT in combination with the classic FMS-sonic tool string (~40 m long) and the oriented GBM (Tarduno, Duncan, Scholl, et al., 2002).
The GBM was originally used to monitor magnetic field variations in a borehole continuously for several weeks and to compare these with field variations at depth (Steveling et al., 1991). The GBM tool consists of three fluxgate sensors that log the horizontal (Fx and Fy) and vertical (Fz) components of the magnetic flux density. The tool is equipped with an angular rate sensor (LITEF) to monitor the spin history around the z-axis and variations around the x- and y-axes during a logging run. The LITEF miniature fiber-optic rate sensor provides angular rate output. The rate sensor is an unconventional gyroscope, since it does not have a spinning wheel. It detects and measures angular rates by measuring the frequency difference between two counter-rotating light beams. When the gyro is at rest, the two beams have identical frequencies. When the gyro is subjected to an angular turning rate around an axis perpendicular to the plane of the two beams, one beam then has a greater optical path length than the other. Therefore, the two resonant frequencies change and the resulting differential frequency measured by optical means provides a direct digital output of the rotational component. This oriented magnetometer has an advantage over conventional (nonoriented) magnetometers because it is independent of the magnetic properties of the formation. We will use that property to test the effects of NRM and MS on the determination of the rotation of the tool string using nonoriented magnetometers.
Based on logging data, the penetrated formation at Site 1203 is composed of volcaniclastic sediments alternating with volcanic rocks (Fig. F3A). Rock magnetic measurements on sediments and basalts give MS values from 10–4 to 10–2 SI and 10–3 to 10–1 SI, respectively, and NRM values from 10–3 to 10–1 A/m and 1 to 10 A/m, respectively (Shipboard Scientific Party, 2002). The use of the GBM and the alternating sediments and basalts at this site provide a unique opportunity to test the proposed algorithm by comparing the tool string rotation obtained from filtering the magnetic tool declination record with the rotation deduced from the gyroscopic system.
The magnetic records from the GBM within the open hole (464–916 mbsf) correspond very well to those from the GPIT (Tarduno, Duncan, Scholl, et al., 2002) (Fig. F4A, F4B). Within sedimentary sections, the mean total field, F, is close to the expected 48,800 nT and the magnetic inclination derived from magnetic logging is ~63°, close to the expected 62° for this latitude according to IGRF2000 (Geomag version 4.0, available online at www.ngdc.noaa.gov/seg/geom_util/geomutil.shtml) (Fig. F3B, F3C). In the underlying basement sedimentary alternating sections, high and localized variations in the magnetic field are encountered that correlate well with lithology. Highly magnetized layers correlate with sequences of massive basalts and pillows and are interrupted by intervals of volcaniclastic sediments (Shipboard Scientific Party, 2002). The direct declination (Dec), the GBM tool declination (tdec) deduced from the raw magnetic records (Fx, Fy, and Fz), and the direct (optical-gyro based) rotation (Rot) are presented in Figure F3D, F3E, and F3G, respectively. Notations are summarized in Table T1. The mean declination is slightly and uniformly higher than the expected –1.5°, possibly due to a calibration or tool housing problem. The low-frequency trend (increase) in declination in the lower 40 m of the open hole is discussed in "Discussion and Conclusions". In general, the GBM rotated smoothly and at low wavelength during the run (mean rate of rotation = 1 rotation/~40 m), confirming our basic hypothesis on the frequency nature of rotation of downhole logging tools (Fig. F3G). As exemplified by the correlation (especially in the 550- to 750-mbsf depth interval) between the hole shape (Fig. F3F) and Rot (Fig. F3G), orientation of the tool is mainly guided by hole shape (Fig. F3F), which is strongly associated with lithology changes (Fig. F3A). An additional high-frequency component of weak amplitude due to (1) the small size and lack of a contact point between the tool and the borehole, (2) tool and cable friction on the borehole wall, as well as (3) vertical tool displacement (stick-slip and heave) is also present in the optical-gyro-based record of Rot (Fig. F3G).