9-3. SeaBEAM 2112

Modified by J.R. Smith and K. Satake
(originally written by K. Okino, T. Kodera and M. Takahashi)

System Description
Bathymetric data were collected by the SEA BEAM 2112 (Sea Beam Instruments, Inc.). The SEA BEAM 2112 is a multibeam survey system that generates data which allows the production of wide-swath contour maps and sidescan images of the seafloor. It transmits a sonar signal from projectors mounted along the keel of the ship. The sonar signal travels through the sea water to the sea floor and is reflected off the bottom. Hydrophones mounted across the bottom of the ship receive the reflected sonar signals. The system electronics process the signals, and based on the travel time of the received signals and signal intensity, calculates the bottom depth and other characteristics such as the S/N ratio for echoes received across the swath. Positioning of depths on the seafloor is based on GPS and ship motion input. The data is logged to the hard disk and 8 mm tape for post processing which allows for additional analysis. Plotters and a sidescan graphic recorder are also included with system for data recording and display.

The hardware system consists of two main subsystems, transmitter and receiver respectively. Figure Fig.SeaBeam on mac02_a shows a basic diagram of the system. The basic 12 kHz projector array is a 14-foot long linear array positioned fore and aft along the ship's keel. It forms a downward projected acoustic beam whose maximum response is in a plane perpendicular to its axis. The beam angle is narrow, 2�‹ in the fore/aft direction. The receiver array detects and processes the returning echoes through stabilized multiple narrow athwartship beams in a fan shape. The hydrophone array has a flat shape in the case of R/V KAIREI, although the standard SEA BEAM 2000 series system has a V-shaped array (Fig. Fig.SeaBeam on mac02_b). The system synthesizes 2�‹ X 2�‹ narrow beams at the interval of 1�‹, and the swath width varies from 120�‹ at depths from 1500 m to 4500 m, 100�‹ from 4500 m to 8500 m and 90�‹ deeper than 8500 m, as shown in Fig. Fig.SeaBeam on mac02_b. The transmit interval of the sonar signal ping cycle increases with water depth, for example about 20 seconds at 6500 m. So, the horizontal resolution of the bathymetry data depends on the depth and ship�fs speed. The accuracy of the depth measurement is reported as 0.5 % of the depth.

The software that controls the system is called SeaView, a product of SeaBeam Instruments, Inc., which includes quality control displays, SB-Logger to log data in a format used in post-processing, the SeaSurvey real-time gridding program, and the SeaSwath single swath plotting routine. SeaView is run under the UNIX operating system on Silicon Graphics Indy and Indigo2 workstations. The raw data files include data records for each ping (bathymetry, sidescan, position), navigation information, and correction parameters such as water velocity structure. Post-processing consists of editing the cross track and navigation data (deletion of bad data, correction of position, etc.), making grid data files, and various maps. The software used for post-processing includes MB-System ver. 4.5 (Caress and Chayes, 1996) and the GMT-System ver. 3.0 (Wessel and Smith, 1991, 1995). The post-processing file format is compatible with the MB-System software programs. It is a binary file which is named automatically based on the system clock (e.g., sb199809072331.mb41 was created on September 7, 1998 at 11:31 p.m. and is from a ship with a format matching MB-System format ID #41; i.e., SEA BEAM 2112).

Products from the SeaBeam system include standard contour maps, artificially illuminated bathymetry showing texture, beam amplitude, and sidescan data. The bathymetry data represents 120 data points per sonar ping, while the sidescan data contains 2000 pixels per ping. Both data types are included in the same binary SeaBeam file. The sidescan data is better at distinguishing between bare rock (recent lava flows) and sedimented areas, as well as highlighting small blocks, structural trends, fault scarps, and other steep slopes.

Additional Notes
One of the important parameters for depth and position accuracy of the bathymetric survey is the sound velocity profile. The SEA BEAM 2112 system uses the sound velocity data not only for calculating the depth and position of each beam during the ray tracing process, but also for the beam forming process. The temperature of the surface layer is most important in this regard and the system measures and uses surface temperature in real time. Except for the surface layer temperature, the user must input the temperature profile. Prior to beginning SEA BEAM operations on KR98-08, an Expendable BathyThermograph (XBT) was launched and the resulting profile was entered into the SEA BEAM system (Fig. _1_; Table _1_). Sound velocity profile (SVP) data for the KR98-09 cruise off the southeast coast of Hawaii Island were obtained during from the CTD instrument aboard the ROV Kaiko during its first dive which exceeded 5000 m (Fig. _2_; Table _2_).

The quality of the obtained bathymetry depends mostly on the sea state. After eliminating bad data, the data are gridded and various maps are produced. The grid size was typically 100-200 m, because the horizontal resolution of the raw data is about 30-150 m depending on the water depth (<1000m to >5000 m). Bathymetric data for all survey lines were processed manually onboard using the MB-System progam mbedit. The amplitude and sidescan data for the Nuuanu area were processed onboard during the Hilina/Loihi leg, while the Hilina/Loihi sidescan data were processed onshore at the University of Hawaii/SOEST following the cruise. Proper filtering of the sidescan data is a several step process, each of which is rather computationally intensive, time-consuming, and disk space exhaustive.

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