Sea Technology

JUN 2017

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Page 13 of 72 June 2017 / st 13 vertical grazing angles. Separate scans were performed to synthetically populate the array over its entire vertical ex- tent, enabling 3D beamforming. Successful 3D beamform- ing from the initial phase of the project served as the im- petus for progression to the field test phase, which used an AUV rather than a rail system. Initial 3D volumetric algorithms were imported from the prior phases of the project (i.e., circular rail system) and ap- plied to AUV field tests at a test site in the southeast United States in December 2014. The test platform was a Hydroid REMUS-600 12.75-in.-diameter AUV developed by various stakeholders: ONR, Pennsylvania State University Applied Research Laboratory (PSU ARL), Woods Hole Oceano- graphic Institution (WHOI) and NSWC PCD. The first se- ries of multipass CSAS scans were conducted in about 20 m of water with a soft homogenous fine- to medium-grained quartz sand seabed. The data presented in this article comes from a high- frequency band with a center frequency in the hundreds of kilohertz. The first primary multipass pattern (multialtitude constant radius) fixed the radii at about 30 m and varied the altitudes from about 4.5 to 6.9 m in steps of about 0.3 m. The secondary multipass trajectory (multiradii constant altitude) fixed the altitude at about 6.0 m and varied the radii from about 27 to 35 m in about 1-m steps. Comparison of the two scan patterns revealed that the scan composed of vertically stacked circles enables more efficient beam- forming algorithms to be applied to the data, however both techniques are viable approaches for generating volumetric images. Follow-on trials were then conducted in July 2015 using the same AUV platform but in a relatively complex environ- ment compared to the first trial. The total water depth was about 12 m, the water was relatively choppy due to inclem- ent weather, and the seafloor was complex, composed of rock, outcroppings, patches of sand and coral. The purpose of this field trial was two-fold: first, to test the integration between the various advanced sensors instrumented on the AUV by the various stakeholders and performers, ensuring the technologies were working well together to achieve the system objectives, and second, to test the robustness of the 3D volumetric imaging algorithms on AUV platforms op- erating in very different environments. For these measure- ments, the multialtitude fixed radius scan methodology was employed. More recently, in July 2016, field exercises were com- pleted at Buzzards Bay, Massachusetts, using the AUV plat- Think our transponders are too big to fit your vehicle? You might need to think again. Meet Nano, our smallest ever 6G-enabled positioning transponder. At 155 millimetres long and weighing 200 grams in water, it's the perfect size to keep track of your divers, small AUVs and micro ROVs. Use it with our Mini- Ranger 2 USBL and you can track multiple targets simultaneously. The system is quick to install and easy-to-use from small boats. You can get to work in just a few simple steps - saving you valuable time on your operations. To see more, search Sonardyne Nano Tracking "Volume images, as opposed to the 3D point clouds generated by interferometric SAS or side scan systems, are composed of truly three- dimensional pixels, or 'voxels.'"

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