Previous   |    Next

doi:10.2204/iodp.proc.335.102.2012

Underway geophysics

While transiting between Costa Rica, Site 1256, and Panama during Expedition 335, we collected 3.5 and 12 kHz echo sounder and magnetometer data. Because the exact location of Site 1256 is well established based on analysis of site survey data and drilling during Leg 206 and Expedition 309/312, no additional surveys were conducted.

Navigation data were acquired throughout the expedition with two Ashtec GG24 GPS+GLONASS receivers with antennas mounted on the starboard stack aft of the moonpool. The datum is the moonpool. GPS fixes were recorded by WinFrog navigation software.

The time datum for all underway geophysics activities is Universal Time Coordinated (UTC) (similar to Greenwich Mean Time), as provided from the Ashtech receivers. If communication between an Ashtech receiver and the satellite is interrupted, the receiver uses its own internal clock to maintain the time base. The WinFrog navigation system displays UTC time many times per second, but the internal clock is not synchronized to UTC.

Magnetic data were acquired with Marine Magnetics SeaSpy proton magnetometer towed 500 m behind the ship. Values of total field intensity were acquired approximately every 3 s using the SeaLINK program from Marine Magnetics.

Underway 3.5 and 12 kHz echo sounder data were acquired using a SyQwest Bathy 2010 CHIRP sub-bottom profiler and bathymetric echo sounder: the 3.5 kHz frequency was obtained with a 12 × TR-109 transducer array driven by a 10 kW power amplifier, and the 12 kHz frequency was obtained with an EDO-323C transducer. Transducers are housed in the forward sonar dome. Data were processed in real time using a Raytheon CESP III (Correlator Echo Sounder Processor). Underway echo sounder data were recorded digitally as uncorrected depths using the program Bathy from SyQwest.

Uncorrected depths convert traveltime to nominal depth assuming a sound velocity of 1500 m/s in seawater. Corrected depths (using Matthews’s tables to allow for varying sound speed with depth and location in the ocean (Carter, 1980) are computed by postprocessing.

Transducer elements are 0.9 m below the keel of the ship and 18.4 m below the dual elevator stool (the reference datum for drilling activities). Water depth relative to sea level was obtained by adding 0.9 m and the mean draft (typically 6.5 ± 0.6 m) to the corrected echo sounder depth.

Underway data are released to and archived by the National Geophysical Data Center at www.ngdc.noaa.gov/mgg/geodas/trackline.html.

Top of page   |    Previous   |    Next