Sea Technology

JUN 2018

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www.sea-technology.com June 2018 | ST 15 more importantly does not em- phasize that a "true" side scan sonar is the primary instru- ment. We propose to use the term "side scan sonar" and the modifier "phase-measuring," yielding: "phase-measuring side scan sonar." "Phase-differ- encing side scan sonar" is also used, and seems synonymous with PMSS, but we prefer the latter. EdgeTech refers to these instruments as multiphase echosounders (MPES). Setup The operating frequencies for our instrument are 550 and 1,600 kHz for backscat- ter (collected simultaneously) and 550 kHz for bathymetry. The side scan sonar range res- olution is 1 cm, and the hor- izontal beam width is 0.5° at 550 kHz. The corresponding quantities at 1,600 kHz are 0.6 cm and 0.2°. The bathymetric range and vertical resolution are both 1 cm. The respective bandwidths at 550 and 1,600 kHz are 67 and 145 kHz. A Teledyne TSS DMS-05 mo- tion reference unit (MRU) mounted on the sonar col- lects data on heave, pitch and roll, measuring heave to 5 cm and roll and pitch to 0.05°. A Hemisphere GNSS V110 vector sensor is used to measure heading. Two differential GPS receivers spaced 2 m apart yield heading accuracies of less than 0.10° RMS. A Trim- ble GNSS receiver utilizing real-time-kinematic GPS (RTK-GPS) is used for positioning and tide correction for vessel-based surveys. Ideally, both GPS units are mounted onto the survey pipe that holds the sonar and MRU. The Cape Lab has multiple research vessels from which to conduct acoustic surveys, ranging from medi- um-sized, shallow-draft vessels (e.g., 8.2 m loa, 0.5-m draft) to an extreme shallow-draft pontoon boat (4.3 ft. loa, draft 0.15 cm). Projects A tidal restoration project for the Herring River in Wellfleet, Massachusetts, is in the design and analysis stage. Salinity in this small tidal estuary ranges from 0 ppt at the head of the river to 32 ppt at the tidal restriction that controls flow between the estuary and Wellfleet Har- bor. The estuary is largely within Cape Cod National Sea- shore, is very shallow and has no boat ramps or access points. The pontoon boat was deployed and retrieved via a low sloping hillside into the water. The survey was funded by the Massachusetts Division of Ecological Restoration (Mass-DER) and conducted on board the pontoon boat June 2016. More than 9 km of (Top) Data from Herring River Estuary, Wellfleet, Massa- chusetts. Left: raw side scan imagery (1,600 kHz). Height above bottom ranges from 10 to 50 cm. Upper right: still imagery taken from underwater video of widgeon grass, approximate location denoted by arrow. Lower right: single line of bathymetry from same area shown in side scan im- agery to the left. (Bottom) Upper: side scan imagery (550 kHz) of lobster pot trawl. Lower: same area with single line of bathymetry overlain. Line can be seen in bathymetry, de- noted by arrows.

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