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USGS Sediment-Transport Investigators Calibrate Tripod-Mounted Underwater Sonars in a Large Tank at the University of New Hampshire
To assess and study the mechanisms of seafloor change in coastal regions, the Sediment Transport Group of the U.S. Geological Survey (USGS) Woods Hole Coastal and Marine Science Center (Woods Hole, Massachusetts) deploys instrumented tripods on the seafloor. Among the instruments commonly mounted on the tripods are sonars used to measure sand waves of various scales and document how they change over time. The data collected by these tripods are commonly employed in models that predict what conditions will cause sediment motion, as well as how much sediment is likely to be transported and in which direction. Coastal planners and environmental engineers can use such information to determine beach maintenance or enrichment needs, choose sites for barrier placement, and make various other management decisions.
Sonars work by emitting sound energy and detecting the resulting echoes, in this case, the reflections from features on the seafloor. The geometry of the sonar returns is complicated by transducer tilt, tripod movement during storms, and some calibration factors in the instrument settings. To better understand the sonar data, Chris Sherwood, Ellyn Montgomery, Marinna Martini, Dann Blackwood, and Michael Casso went to an engineering tank at the University of New Hampshire (UNH) Jere A. Chase Ocean Engineering Center.
The tank is 60 ft long by 40 ft wide, 20 ft deep, and filled with clear, cool New Hampshire groundwater. The indoor facility has power, cranes for moving equipment, and dive facilities. The sonars were calibrated in the tank by using targets of known location, size, and shape. The locations and orientations of the instruments on the tripod were carefully measured and mapped before the tripod was submerged in the tank. Target placement was designed to answer specific questions, and each target arrangement was measured and documented in the tank. Comparison between the target maps and the sonar images allowed us to verify calibration coefficients and check the geometric corrections.
We tested five arrangements of targets, each designed to quantify one or more aspects of sonar resolution. The divers (Blackwood and Casso, assisted by UNH student Sean Denny) placed targets around the tripod and surveyed their locations. Then we recorded sonar data, uploaded it from the datalogger used during autonomous deployments, converted it to image format, and assessed our ability to relate the images to the target pattern. This form of ground-truthing is very important for the correct interpretation of any sonar data we collect.
During the tests, the accuracy of the measurements of two types of sonars was assessed:
Our targets were inexpensive, acoustically reflective, and nonbuoyant: sheets of corrugated-metal roofing with various undulating patterns, and bricks. The divers created five different configurations in the tank, arranged so that the effects of distance from the sonar, the angle of incidence to the ridges in the roofing, and the tripod tilt could all be documented and the targets carefully measured for comparison with the data. The photograph below (labeled "A") shows the tripod in the tank from above, with the targets arranged for the first test. The graph below (labeled "B") shows the fan-beam sonar image from that arrangement—the fan-beam transducer is located at image center. Note that most of the targets are visible in the fan-beam sonar image, but ripples that are oriented perpendicular to the radial fan-beam scans are much more distinct, or better resolved, than those oriented parallel to them.
The piece of corrugated-metal roofing under the pencil-beam sonar (labeled "PB" in figure B) was oriented so the ridges and troughs were approximately parallel to the radial fan-beam scans (making the piece difficult to see in the fan-beam image) but perpendicular to the line sampled by the pencil-beam (line P-P'). The plot below, labeled "C," shows the ridges and troughs of the roofing and brick targets detected during the pencil-beam sweep. This image has not been corrected for a slight tilt to the sensor, resulting in the base not being parallel to the x-axis, but the heights of the features match what we expect from the measurements, validating that our software applies the correct calibrations.
Our final test focused on the fan-beam sonar, which is typically configured to make two sets of radial scans for every sample, one clockwise, then one counterclockwise. The two views should overlie, but our testing revealed a slight offset between the actual and reported head positions, resolving questions raised in interpreting field data. The photograph below shows divers after setting up the fifth arrangement of bricks, designed to help us quantify the offset between the clockwise and counterclockwise cycles and calculate appropriate corrections.
We have already improved our software, which converts the raw scan data into images. We will also be able to ensure that our programs properly account for sensor and seafloor tilts when we compare two configurations with differing tripod tilts. By using corrugated-metal targets to simulate ripples of two length scales, we also expect to improve the ability of our software to accurately determine ripple wavelength and direction. We're looking forward to using these enhanced methods to interpret the sonar data collected during several recent field programs.
We thank our wonderful hosts at the University of New Hampshire, particularly Andy McLeod and graduate student Gary Margelowsky. This huge and well-maintained facility provided the ideal venue for our experiment.
in this issue:
Investigators Calibrate Tripod-Mounted Underwater Sonars
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