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doi:10.2204/iodp.proc.309312.102.2006

Underway geophysics

While transiting between Panama and Site 1256 during Expedition 309, 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, no additional surveys were conducted.

Navigation data were acquired throughout the expedition on an Ashtech GG24 GPS receiver. The antenna was mounted on the starboard stack 46.33 m aft of the moonpool (Fig. F23). The datum is the moonpool. GPS fixes were recorded by WinFrog navigation software every 60 s. Generic Mapping Tools software (Wessel and Smith, 1995) on Sun Sparc 10 Unix workstations was used to process and display the navigation data.

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 the 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 an EG&G Geometrics Marine Proton Magnetometer (model G-886) towed 500 m behind the ship. Values of total field intensity were acquired every 60 s using the WinFrog navigation software on a Windows PC.

Underway 3.5 and 12 kHz echo sounder data were acquired using an EDO Model 515A-250 transceiver mounted in a sonar dome located 45.52 m forward of the moonpool. Data were processed in real time using a Raytheon CESP III (Correlator Echo Sounder Processor). Underway echo sounder data were not digitized but were recorded on an EPC 9802 analog linescan thermal paper recorder. The ship’s speed, heading, and position were annotated on the EPC recorders and logged each minute on WinFrog. The paper recorder was set to a 1 s sweep, and uncorrected depths were logged at 5 min intervals by hand.

Uncorrected depths convert traveltime to nominal depth assuming a velocity of 1500 m/s. A corrected depth (using Matthews’s tables to allow for varying sound speed with depth and location in the ocean [Carter, 1980]) was computed by hand upon arrival at the site.

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. Figure F24 is a schematic of the ship that summarizes some key dimensions used in computing depth to various datums.

A program called U/W Watch was developed by ODP to configure acquisition sequences for the echo sounder and the magnetometer and to display and annotate echo sounder output on the EPC graphic recorders. This program is written in the National Instruments LabView language and runs on a Compaq Deskpro Workstation.

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