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doi:10.2204/iodp.proc.330.109.2012

Methods

Navigation

The GPS navigation system and gyrocompass were used for the duration of Expedition 330. The R/V JOIDES Resolution is equipped with two Ashtech GG24 GPS+GLONASS receivers with antennas and a Scandinavian Micro Systems LR40DC Digital Gyro Repeater. GPS position was continually updated at 1 s intervals, and integrated navigational software subsampled these data to align with bathymetric and magnetic sampling intervals. Loss of GPS information during Expedition 330 was rare, generally affecting no more than a few consecutive data points, with the exception of ~100 km during Transit L7T, where no positioning data were available for the magnetic anomaly data.

Bathymetry

A SyQwest Bathy 2010 CHIRP Subbottom Profiler and Bathymetric Echo Sounder system were used to acquire bathymetric data as well as high-resolution subbottom seismic reflection data. The SyQwest Bathy 2010 system consists of a 3.5 kHz transducer array using 12 TR-109 transducers driven by a 10 kW power amplifier and mounted in a sonar dome 45.5 m forward of the moonpool on the JOIDES Resolution. Bathymetric and seismic data were recorded for all transits except Transit L5T.

After collection, the bathymetric data were filtered to remove individual bad depth readings or small clusters of unreliable depths. This filtering was based on the fourth derivative of the data, which is very sensitive to the very narrow, sharp spikes characteristic of a few isolated unreliable data points. Two filtering passes were done, each removing data points with a fourth derivative value of >1000 m/(sample distance)⁴. The data were then projected onto the great circle that best fit the transit path and aligned with the magnetic anomaly data for plotting.

Magnetic anomaly

Total intensity measurements of the Earth’s magnetic field were collected using a Marine Magnetics SeaSPY Overhauser magnetometer towed ~500 m astern. The SeaSPY magnetometer has an accuracy of 0.1 nT, with a repeatability of 0.01 nT. Kang et al. (2010) investigated the effect of the magnetic field of the JOIDES Resolution on magnetometer readings made 300 m astern, finding an effect as high as 100 nT. However, up to 40 nT of this may be due to real magnetic variation according to the International Geomagnetic Reference Field (IGRF), leaving an effect of 60–100 nT. The greater 500 m tow length used in this survey further reduces the ship’s magnetic effect by approximately two-thirds, for a final effect of 20–30 nT. The magnetic effect of the JOIDES Resolution is thus negligible compared to the typical observed anomalies of 250–500 nT, and no correction for the ship’s magnetic field has been made. However, the position information logged with the magnetometer data has been corrected for the cable length.

Magnetic data were recorded at 3 s intervals, and these total intensity measurements were then reduced to magnetic anomalies using the 10th version of the IGRF (MacMillan and Maus, 2005) and filtered using the same fourth-derivative filtering technique used for the bathymetric data (as discussed above). However, for the magnetics data, only a single filtering pass was used, rejecting data points with a fourth derivative value of >50 nT/(sampling distance)⁴. The data were then projected onto the same best-fitting great circle used for bathymetry.

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