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doi:10.2204/iodp.proc.324.108.2010

Methods

Navigation

The GPS navigation system was used throughout Expedition 324. A Trimble DSM232 GPS receiver was used as the primary navigation device. GPS positions were continuously updated at 1 s intervals and subsampled at 1 min intervals with a WINFROG software system. Subsequent processing and display of navigation data were performed using the Generic Mapping Tools software package (Wessel and Smith, 1995).

CHIRP/echo-sounder

A 3.5 kHz CHIRP/echo-sounder was used to acquire bathymetric data as well as high-resolution subbottom seismic reflection data. The 3.5 kHz system uses a SyQwest Bathy-2010 echo-sounder system driven by a single EDO-type 323c transducer. The transducer is mounted in a sonar dome located 45.5 m forward of the ship's moonpool. Digital bathymetry and Society of Exploration Geophysicists (file format "Y") subbottom seismic data were recorded on the SyQwest Bathy-2010 echo-sounder system during all transits.

Marine magnetometer

Total intensity measurements of the Earth's magnetic field were obtained with a Geometrics Model G-886 proton precession marine magnetometer towed ~300 m astern. Magnetic data were recorded at 3 s intervals and then reduced to 1 min intervals with navigation data produced by the WINFROG navigation software. In order to measure the effect of the ship's magnetic field with heading, a circular survey (~7 km in diameter) was conducted while in transit from Site U1349 to Site U1350 between Universal Time Coordinated (UTC) 1055 h and 1215 h on 12 October 2009 (Fig. F2). This method was proposed by Bullard and Mason (1961). The equation for predicting the effect of the ship's magnetic field is:

where θ is the ship's heading measured clockwise from north, F_Q is the total field at location Q, F is the ambient magnetic field, and C₀, C₁, C₂, S₁, and S₂ are constants dependent on the ship's magnetic properties (Bullard and Mason, 1961). For a symmetrical ship, the sine terms are negligible compared with the cosine terms; therefore, we set S₁ and S₂ to 0. To minimize diurnal effects, the survey calibration circle was conducted at night (time difference between local time and UTC is +10 h). The circular survey was conducted over a relatively flat portion of seafloor (depth variation between 3242 and 3342 m) (Fig. F3). Even though the International Geomagnetic Reference Field (IGRF) values around the circular survey show only 40 nT differences (from 41,851 to 41,950 nT), the maximum differences observed are as much as 100 nT (from 41,723 to 41,823 nT). The measured magnetic data were plotted versus magnetic heading, and a best fit curve was computed (Fig. F4). The computed heading correction constants C₀, C₁, and C₂ of the R/V JOIDES Resolution from 300 m astern are

Thus, the magnetic field errors generated by the ship (F_H) are expressed as

The obtained constants C₀ and C₁ are higher than previously reported results (C₀ = –3.2 to 5.5; C₁ = –12.9 to –3.0) measured from other research vessels (Bullard and Mason, 1961; Buchanan et al., 1996). Higher constants are expected because of the length of the JOIDES Resolution compared to the lengths of other research ships. Even though the magnetometer was towed 300 m astern, this distance is not sufficient to avoid the ship's magnetic effect. The acquired total fields were reduced to magnetic anomalies using the 10th generation IGRF coefficients (McLean et al., 2004; Maus et al., 2005). Before correction of the ship's heading effect, the maximum track crossover errors are as much as 48 nT (root mean square [RMS] = 20 nT) at track crossings. After the ship's heading effect was corrected, the crossovers are reduced to 13 nT (RMS = 9 nT). Herein, we provide the results from both before and after the ship's heading correction.

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