While transiting from Panama to Site 1243 and from Site 1243 to Victoria, we carried out 3.5-kHz echo sounding and magnetometer surveys. In addition, as we approached Site 1243, we collected a series of single-channel seismic reflection lines to define a precise location for drilling. The survey was quite successful, and the data were used primarily to measure sediment thickness and water depth.
A site survey for Leg 138 had also been conducted during the previous century (Mayer et al., 1992), but the density of seismic and bathymetric lines was inadequate to place a long-term observatory with respect to the bottom topography.
Navigation data were acquired throughout the leg on an Ashtech GG24 Global Positioning System (GPS) receiver. The antenna was mounted on the starboard stack 46.33 m aft of the moonpool (Fig. F18), although the datum is the moonpool itself. GPS fixes were recorded by WINFROG navigation software every 60 s, except during water gun shooting, when they were recorded at each shot instant (9 s). Generic Mapping Tools (GMT) software (Wessel and Smith, 1995) was used to process and display the navigation data on SUN workstations.
The time datum for all underway geophysics activities is Universal Time Coordinated (UTC) as provided from the Ashtech receiver. 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 the UTC time many times per second, but the internal clocks of the data acquisition systems are not accurately or continuously synchronized to UTC. During this cruise, we used the WINFROG navigation system to send a trigger pulse to the water gun based on WINFROG's internal clock. The time stamp on the seismic data comes from the SUN workstation internal clock, which was synchronized manually with UTC. The underway geophysics data were acquired using the A2D package (University of Hawaii software running on a SUN workstation). A2D has the time referenced approximately to UTC, whereas the trigger pulses from WINFROG are based on the WINFROG clock, which may not be precisely synchronized to UTC. We did note that A2D might have dropped a few shots during the survey because of a differential shot count between WINFROG triggers (larger count) and A2D records (smaller count). The greatest possible cumulative error in timing, as a result of the discrepancy in counts, is ~90 s over the 6-hr survey period. At a survey speed of 5 kt, this is equivalent to a 225-m uncertainty over ~22 km of track line, or 1%.
The magnetic data were acquired with an EG&G Geometrics Marine proton precession Magnetometer (model G-886) towed 500 m astern. Values of total field intensity were acquired every 60 s using the WINFROG navigation software on a a PC with a Microsoft windows operating system.
The underway 3.5-kHz echo sounder data were acquired using an EDO model 515A-250 transducer mounted in a sonar dome located 45.52 m forward of the moonpool. The 3.5-kHz transceiver is a Raytheon model PTR-105 with a maximum 2-kW output. The data were processed in real time using a Raytheon Correlator echo sounder processor (CESP III). Uncorrected depths were recorded on an EPC 8082 analog line-scan recorder. These were read visually from the recorders every 5 min and were entered into a Microsoft Excel spreadsheet by the underway technician. The line-scan recorder was automatically annotated with ship's speed and heading every 5 min and ship's position every 30 min. These annotations were also logged by WINFROG. Uncorrected depths convert traveltime to nominal depth assuming a sound velocity in seawater of 1500 m/s. Corrected depths (using Matthews' Tables to allow for the varying sound speed with depth and location in the ocean [Carter, 1980]) were computed by hand for each site.
The transducer element is 0.91 m below the keel of the ship and 18.39 m below the dual elevator system (the reference datum for drilling activities). Water depth relative to sea level was obtained by adding 0.91 m and the mean draft (typically, 6.5 sounder depth. Figure F19 is a schematic of the ship, which summarizes some of the key dimensions used in computing depth to various locations.
The acoustic source consisted of a single 80-in3 water gun developed by Seismic Systems Inc. (SSI) (Hutchinson and Detrick, 1984). In general, the SSI water gun was fired at a rate faster than is typical in order to maximize horizontal resolution. A 60-phone 100-m-long Teledyne oil-filled streamer recorded the water gun shots. The streamer was towed ~15 m deep, ~225 m astern (Fig. F20). The midpoint between the water gun and the active streamer's midpoint was 188 m astern the ship's recorded GPS position. Streamer output was split to enable it to be used for real-time analog displays on EPC facsimile recorders and for digitizing and recording of SEG-Y files on 4-mm DAT tape and 8-mm EXABYTE tape using SUN workstations. Digital data were subsequently processed using SIOSEIS software (Henkart, 1992).
We conducted a test of the seismic system on 7 June, with the result that one water gun was found to be inoperable, one gun failed in the water, and the streamer was insensitive. Following repairs, the tests were repeated on 8 June with excellent results. The survey itself (10 June) was conducted with outstanding results, although one water gun failed during the survey and had to be replaced.