The DSOG operates split-beam sonar systems designed for
deeply-towed, near-bottom seafloor imaging and swath phase-bathymetric mapping.
The sonars were developed by WHOI-DSL in conjunction with the Applied Physics
Laboratory (APL) of the University of Washington and Acoustic Marine Systems,
Inc. of Redmond, Washington, and have been developed as modular subsystems
capable of operation from the Jason ROV or other deep-sea platforms.
Transmit state, repetition rate, pulse length, and gain of the systems are operator selectable. Other significant features include digitization at the receiver, multiple channels of quadrature-detected data, and a high-bandwidth optical fiber link to the surface. For each receiver channel (two per beam), a surface-selectable step gain and/or time varying gain (TVG) is applied and the antialiased signal is quadrature sampled at 10 kHz. The digital signal is Manchester encoded at a 2 MHz clock rate and transmitted to the surface over a fiberoptic serial link (eight 16-bit numbers per sampling interval). All system characteristics/settings are recorded on Exabyte tape.
The dual-receiver design also provides phase information that can be used to generate high-resolution swath bathymetry. System calibration includes phase-center separation and acoustic axis offset so that accurate bathymetry can be extracted without resort to empirical techniques. At present real time phase-bathymetric data processing is not available, but all raw phase data are recorded and supplied to the science party for post-cruise processing either using their programs or via separate agreement with DSOG staff.
Digital data from the 120 kHz and 200 kHz sonars are transmitted up the cable, and in conjunction with all gain settings and sonar system parameters, are recorded to Exabyte tape media while raw backscatter images are displayed on an EPC dry-paper recorder. Simultaneously, the data are processed to include first-order slant-range and speed corrections and are displayed on a Raytheon thermal paper recorder and on a soft-copy display system. The processed records are used by the science party and DSOG field personnel to construct shipboard mosaics of seafloor surveyed. Data products include: tape copies of all raw sonar data in user readable format, speed and slant range corrected processed data as digital image files, EPC paper rolls of raw backscatter data, and Raytheon paper copies of processed data and shipboard mosaics compiled by DSOG and science party using the Raytheon records.
A software package called Enduser for plotting DSL-120A sonar grids on UNIX platforms has been developed by Dan Scheirer at Brown Univ. It is available via anonymous FTP as a compressed tar file, Enduser.v1.0.tar.Z, from emma.geo.brown.edu, directory /pub/scheirer.
The 4.5 kHz subbottom profiler on the 120 kHz sonar system produces a high-resolution profile of seafloor and shallow subbottom displayed on an EPC dry-paper recorder. Data products include: digital data recorded on Exabyte tape with correlative time/navigation/ attitude data string; and paper EPC rolls of real-time output. Any further processing or manipulation of the 4.5 kHz data will be done by the science party post-cruise.