While transiting from Site 1223 to Site 1224 and from Site 1224 to San Diego, we conducted 3.5-kHz echo sounding and magnetometer surveys. In addition, as we approached and departed Site 1224, we shot single-channel seismic reflection lines within 66 km of the site.
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. F15). The datum is the moonpool. GPS fixes were recorded by WinFrog navigation software every 60 s except during water gun shooting, when they were recorded at each shot instant. Generic Mapping Tools (GMT) software (Wessel and Smith, 1995) on Sun Sparc 10 UNIX workstations was used to process and display the navigation data.
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 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 clock is not being 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 blast phone pulse is then used to start the acquisition in the A2D software from the University of Hawaii. The time stamp on the seismic data comes from the Sun internal clock, which is manually synchronized with UTC. So data acquired in A2D have time referenced approximately to UTC. The trigger pulses from WinFrog, however, are based on the WinFrog clock, which may not be precisely UTC. Time constraints did not permit calibrating the various time bases against UTC during the cruise.
The 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.
The underway 3.5-kHz echo sounder data were acquired using the hull-mounted EDO transceiver with a multielement 10-kW transducer as both the source and receiver. The 3.5-kHz transducer consists of six elements mounted on the ship's hull 45.5 m forward of the moonpool and is described in more detail in "Onsite Geophysics". The data were processed in real time using a Raytheon CESP III (Correlator Echo Sounder Processor). The underway echo sounder data were not digitized but were recorded on an EPC 9802 analog line scan thermal paper recorder. The ship's speed, heading, and position were annotated on the EPC recorders and were logged each minute on WinFrog. The paper recorder was gated to a depth range of 4500-5250 uncorrected meters, 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. 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. For example, an uncorrected depth of 4967 m at the H2O site (area 51 in Matthews' Tables) corresponds to a corrected depth of 4979 m. An uncorrected depth of 4250 m at the Nuuanu site (area 52 in Matthews' Tables) corresponds to a corrected depth of 4250 m (i.e., no change).
The 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 F16 is a schematic of the ship that summarizes some of the key dimensions used in computing depth to various datums. The shipboard 12-kHz system had poor signal-to-noise characteristics and was not used during this leg.
A program called "U/W Watch" was developed by ODP to configure the acquisition sequences for the echo sounder and the magnetometer and to display and annotate the 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.
A Seismic Systems Inc. "T Water Gun" (Type S80 model 1; 80 in3) was fired at 10-s intervals as a source for the single-channel seismic reflection studies as well as for shallow refraction surveys to the H2O broadband seafloor seismometer. Water gun depth was estimated at 4 m (Fig. F17). Shot instants and navigation were acquired on the WinFrog navigation system.
Data from the H2O seismometer (Chave et al., 1997; Duennebier et al., 2000) were continuously acquired in real time at the University of Hawaii, and files were downloaded to the ship daily. The data can be analyzed in a similar fashion to data from radio sonobuoys (Knott and Hoskins, 1975).
The single-channel streamer used was a Teledyne Exploration model 178 hydrophone string; it consisted of 60 elements at a spacing of 1.45 m and was towed 225 m (to the midpoint) behind the ship (Fig. F17). Streamer depth was estimated as 15 m. Data were acquired to 4-mm DAT tape and 8-mm Exabyte tape in SEG-Y format and were displayed in real time using the SIOSEIS software package (Henkart, 1992) running on a Sun UNIX work station. The hydrophone streamer malfunctioned during the approach survey because of a corroded connector.