HAWAII MR1 : Overview
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The HAWAII MR1 seafloor imaging system is a wide swath side-scanning sonar instrument that measures bathymetry and acoustic backscatter (sidescan sonar) in full ocean depths. MR1 was designed and built by the Hawai`i Mapping Research Group at the University of Hawaii in 1991, and has since been deployed aboard 12 different vessels on 14 separate research surveys. The MR1 system is available as a shared-use research tool for U.S. and international oceanographic institutions, and for commercial seafloor surveys. |
MR1 System MR1 is a portable side-scanning seafloor imaging system that simultaneously acquires digital bathymetry (swath width > 3.4 times water depth) and sidescan sonar imagery (swath width > 7.5 times water depth). The system's sonar transducers are housed in a 5-metre-long vehicle that is towed beneath the surface mixed layer (60 to 100 m) at ship speeds of 8 to 10 knots. The MR1 towfish is extremely stable due to its multi-body towing configuration and large righting moment. As a result, MR1 has successfully operated in rough sea conditions (up to sea state 6) that typically cause performance degradation in hull-mounted systems due to bubble masking and violent ship motion.
MR1 Bathymetry MR1 is a sidescan bathymetry system that uses the phase difference method to measure the angle of returning echoes on either side of the towfish. Flexible data processing software allows power, ping rate and beam spacing to vary in order to maximize the swath width and data quality with changing seafloor depth and acoustic properties. MR1 accounts for ray bending due to acoustic velocity gradients in the water by applying an empirical transfer function based on data collected during a calibration test at the beginning of each survey, and the system can be re-calibrated at any time during the survey to account for changing water column properties. The bathymetric precision for MR1 is within 1.5% of Sea Beam Classic, based on a quantitative comparison between Sea Beam and the MR1 sister-system SEAMAP, which is operated by the U.S. Naval Oceanographic Office.
MR1 Electronics The MR1 towfish uses separate transducer arrays on its port and starboard sides, which operate at different frequencies (11 and 12 kHz, respectively) to minimize crosstalk. Each array contains two rows of elements spaced one-half wavelength apart, and both rows are driven by 10 kW amplifiers to transmit acoustic pulses 1-10 msec long. This high power capability results in a high signal to noise ratio, which improves the quality of bathymetric measurements and widens the swath of seafloor over which sidescan data are collected. MR1 uses an advanced network of digital signal processors on the towfish and in the shipboard data acquisition electronics to control the operation of the sonars, and to process acoustic data.
Shipboard Operations The MR1 system was designed for portability and ease of use on different ships. The launch and recovery of the MR1 towfish is accommodated by a hydraulic Launch and Recovery System (LRS) that can be mounted on the fantail of any suitable ship. The LRS features a hydraulic tilt-bed assembly that supports the towfish, and can be extended aft and tilted up to deploy or capture the tow vehicle. For data acquisition, the system is lowered into the water and attached to a 50 m-long umbilical cable that in turn is attached to a one-ton depressor weight. The weight is required because the towfish is slightly positively buoyant (so that the towfish will return to the surface if the tow cable breaks), and because the resulting two-body towing configuration decouples the towfish from the heave of the ship. The depressor weight is attached to the ship by a cable that passes through a tow point on the LRS to a winch mounted forward of the LRS. In addition to mechanically supporting the towfish, the tow cable carries power down to the towfish and data up to the ship.
MR1 Data Acquistion and Processing As bathymetry and acoustic data are collected, the real-time images of each are displayed on the acquisition computer monitor. Data acquisition and display are controlled by a Sun Microsystems Sparc2 workstation, and raw acoustic data are redundantly archived on dual Exabyte 8mm data cartridges. A complete backup acquisition computer system stands ready to mitigate data loss due to computer failure. After archival, bathymetry and imagery data are merged with navigation and further processed to maximize data quality according to the specific environmental characteristics of the survey area. Sidescan image processing tools include several amplitude normalization, destriping and despeckling routines that can be applied interactively or in batch-processing mode. Sidescan and bathymetry mosaics are generated in near-real time at sea, and can be displayed on computer monitors, output to printers, or exported as digital data that can be easily imported into other display programs (such as the
Generic Mapping Tools) or geographic information systems. HMRG software is freely available (although copyright is retained by the University of Hawaii), and MR1 users are encouraged to learn and participate in data processing at sea, and to install and use HMRG processing software at their research institutions.