Sea Technology

JUN 2018

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Page 14 of 52

14 ST | June 2018 F or many years, extreme shallow water areas (0 to 5 m) were considered to be so dynamic that mapping them, given the survey inefficiencies, navigation hazards and operational costs, was prohibitive. The maps are obsolete before the boat makes it back to the dock, was the com- monly heard maxim. However, nearshore shallow-water environments are invalu- able coastal resources; estuaries in particular are among the world's most productive ecosystems, and mapping the sea- floor can provide useful information with regard to benthic habitats, eco- system state, aquaculture status, sediment transport and other biological, chemical and physical characteristics and pro- cesses. These data, while critical to other scientists, modelers and engineers, are becoming increasingly central to coastal resource man- agers. The combination of extreme shallow-draft survey platforms and phase-measuring side scan sonars (PMSS) allows for mapping of the seafloor in areas otherwise dif- ficult to obtain synoptic, quantitative 2D and 3D data. Further, acoustic methods have long been used to map in environments inhospitable to optical methods such as LiDAR, vertical aerial photography and other multispec- tral data. Turbid waters in many nearshore environments prevent optimal performance due to limited penetration through the water column. In addition, LiDAR, as of yet, provides only elevation information, though exper- imental data layers similar to side scan imagery are in the early stages of development. A PMSS allows for both high-resolution side scan imagery and bathymetric data in extreme shallow and/or turbid waters. The Coastal Processes and Ecosystems (CAPE) Lab, a joint research laboratory between the School for the Envi- ronment at University of Massachusetts, Boston and Sea- floor Mapping Program at the Center for Coastal Studies in Provincetown, Massachusetts, is working on projects in extreme shallow waters, mainly 0- to 5-m depth. Mapping Seafloor Shallows Vessel-Based, Nearshore Surveys Using Phase-Measuring Side Scan By Dr. Mark Borrelli • Bryan Legare • Theresa Smith Terminology The primary acoustic instrument for shallow-water seafloor mapping in the CAPE Lab is the EdgeTech 6205, a phase-measuring side scan sonar, which has been in use since 2014. Given the evolving nature of the industry, terminology continues to evolve and has led to confusion when referring to this newer technology. The term "in- terferometric sonar," while prevalent, refers to an older method of measuring the phase difference of the return- ing acoustic energy. The term "phase-differencing bathy- metric sonar" could be construed as redundant, as all phase-differencing sonars yield bathymetry, but perhaps Shallow-draft survey vessels. Left: two-person, custom-built, pontoon boat (15-cm draft). Below: customized, 8.2- m Eastern. A bow-mount is preferred to reduce noise from hull and to maximize distance from engine. Both vessels pic- tured are using the EdgeTech 6205 .

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