Chapter 3: Underway Geophysical Survey
(written by Barry W. Eakins, U.S. Geological Survey)
Five different geophysical instruments collected data during R/V Yokosuka cruise YK02-04, between July 13th and Sept 3rd, 2002. The primary instrument utilized was the hull-mounted SeaBeam 2112 multibeam sonar system, capable of recording bathymetry, beam amplitude and sidescan imagery in wide swaths across the seafloor. Other instruments included a shipboard gravimeter, shipboard 3-component magnetometer, intermittently towed proton magnetometer (usually over night), and occasionally towed single channel seismic system (during daylight hours on non-dive days).
SeaBeam 2112 multibeam
sonar
The SeaBeam 2112 is a state-of-the-art wide-swath multibeam sonar system with an array of transducers and hydrophones installed along and across the keel of R/V Yokosuka. The system transmits a 12-kHz sonar pulse and records the travel time and amplitude of the returning echoes (up to 121 bathymetry beams and 2000 sidescan pixels per gpingh). The highly automated system converts 2-way travel times to depths based on sound velocity and subsequently merges the data with shipfs navigation (GPS positioning and ship motion). It also adjusts system parameters (power output, signal gain and pulse length) to maximize data quality. Swath width is roughly three times bottom depth, or 10-15 km for typical Hawaiian depths (3000 to 5000 m). Bathymetry, amplitude and sidescan data are logged together in approximately hour long digital files, and post-processed using the MBSYSTEM4.6.10 [Caress and Chayes, 1995] and GMT 3.0 [Wessel and Smith, 1995] software packages; bathymetry data were edited manually (by the Shinkai Team) while the sidescan data were computer processed using the recommended steps in the MBSYSTEM manual [Caress and Chayes, 1995]. Sound velocity profiles (SVPs) were obtained from ship-launched XBTs (expendable bathythermographs) and from CTD (conductivity-temperature-depth) measurements taken by Shinkai during each dive, and were updated into the SeaBeam system.
Three previous JAMSTEC surveys (in 1998, 1999 and 2001) had illuminated significant portions of the submarine flanks of the Hawaiian volcanoes, and surrounding moat and arch. The 2002 Shinkai 6500 dive targets mostly lay within these surveyed areas so that the majority of the 2002 bathymetric survey work focused on filling in "holes" between previous years' track lines and extending map coverage onto shallower regions of the volcanoes. Each dive site was first resurveyed at slow speeds (<5 knots) to obtain an accurate bathymetric map before launching the Shinkai 6500. Most survey work occurred between 1800 and 0600 at speeds of 12 to 15 knots while transiting to the next dive site.
The SeaBeam 2112 sonar system installed on R/V Yokosuka is fairly robust (suffering only one incidence of system "hang up" through 14 survey days c so far), and retrieved high-quality bathymetry and sidescan data at the optimal 12-knot ship speed. We discovered during the first single channel seismic line, with ship speed at 5 knots, that SeaBeam data quality was severely degraded by excessive pitch and roll of the ship. Use of the ship's bow thrusters ameliorated this condition and dramatically improved data quality, and were therefore used throughout all subsequent slow-speed operations. Data quality was also poor during large (>30) course changes as JAMSTEC procedure calls for quick course changes, which prevents the SeaBeam system from accurately processing the returning sonar echoes.
Gravity Meter
Measurements of Earth's gravity field along R/V Yokosuka's ship track were taken continuously using a permanently installed LaCoste and Romberg Air-Sea Gravity Meter, which consists of a highly-damped spring type gravity sensor mounted on a gyro-stabilized platform (to nullify ship's pitch and roll). The gravimeter has a calibrated range of 12,000 mgals with an accuracy of ~1 mgal at sea, and samples the gravity field at 1 Hz. The digital output file records filtered 10-sec averages of the gravity field, plus Julian Day and time (Japan Standard Time), latitude and longitude, depth, ship's heading, course over ground (COG) and speed over ground (SOG), as well as various cross coupling corrections. Gravity ties were taken at the Honolulu pier, using a portable gravimeter to allow comparison with a known gravity value, for each port stop (before the beginning of Leg-1A, between Leg-1B and Leg-2A, and after Leg-2B) to measure instrument drift. Processing to account for ship's motion and removal of Earth's reference gravity field, and subsequent analysis (to investigate the deeper structure of various submarine rift zones) will be performed following the cruise.
Magnetometers
Earth's magnetic field was measured along ship track by two instruments: (1) a permanently installed 3-component magnetometer in the ship's interior, which continuously measured the vertical, north and east axes of the magnetic field, as well as ship's attitude and position, at 1 sec. intervals; (2) an intermittently towed proton magnetometer, which recorded the total magnetic field every 10 seconds. The proton magnetometer was deployed 300 meters aft, starboard side, following each Shinkai 6500 dive, and was recovered upon arrival at the next dive site. Local variations in Earth's magnetic field primarily reflect magnetization of the Cretaceous oceanic crust, and intrusive dikes and extrusive lavas of the much younger Hawaiian volcanoes. Such variations may be useful in constraining the eruptive histories of the volcanoes.
Single channel seismics
The SCS system is comprised of a generator-injector (g.i.) air gun ("source") towed ~30 m aft, mid-ship, at a depth of 5-10 m, and a mineral oil-filled, 65-m-long, 48-hydrophone streamer ("receiver") towed 135 m aft, port side, at a depth of 7-10 m. The system was operated at a ship's speed of ~5 knots, with "shot-points" occurring every 20 seconds (approximately every 50 m). The 245-cu in. generator creates the primary seismic pulse, while the 105-cu in. injector controls bubble oscillation. Associated air compressors (9-12 MPa - 1300-1740 psi) and electronics control the firing sequence and timing, as well as the digital processing and data logging (in SEG-Y format). The system was ably operated, maintained, and repaired by technicians S. Hosoya and K. Suzuki, who also post-processed the acquired digital seismic data.
The single channel seismics (SCS) subbottom profiler was deployed and operated during daylight hours on two non-dive days. Seismic line 8 was collected across Haleakala Volcano's submarine northeast flank. Line 9 was collected across the basal southern flank of Puna Ridge, the submarine extension of Kilauea Volcano's East Rift Zone.
References
Caress, D.W., and D.N. Chayes, New software for processing sidescan data from sidescan-capable multibeam sonars, in IEEE Oceans 95, pp. 997-1000, 1995.
Wessel, P., and W.H.F. Smith, New version of the generic mapping tools released, Eos, Transactions, American Geophysical Union, 76, 329, 1995.