SEPTEMBER 2017 • FOGHORN 15 FOGHORNFOCUS: TECHNOLOGY mission. Think of hydrographic surveying as “looking” into the ocean to see what the seafloor looks like. A survey supports a variety of activi- ties: nautical charting, port and harbor maintenance (dredging), coastal en- gineering (beach erosion and re- plenishment studies), coastal zone management, and offshore resource development. The primary information obtained by hydrographic surveys is water depth. They also help determine the nature of the sea floor material (i.e. sand, mud, rock), which is important for anchoring, dredging, structure con- struction, pipeline and cable routing and fisheries habitat. Planning and prioritization for where we conduct these surveys is based on a number of factors, such as: • Marine traffic volume, type and size • Chart Vintage • Dynamic seafloor changes • Local Input from partners like the USCG, Maritime Pilots, Ports The advances in hydrographic surveying technologies provide a much clearer picture than we’ve ever seen in the past. With these tools we’re able to create charts that can better assist vessel operators navigate safely. Long-time mariners may recall when the only tool available was the lead line which gave a very coarse un- derstanding of what was beneath a vessel. It was adequate but left a lot of uncertainty because it was incomplete and easily missed hazards. The single beam was a vast im- provement and provided a better picture of what was below a vessel. Although better, there was room for improvement. Today, Coast Survey conducts hy- drographic surveys primarily with multibeam and side scan. Multibeam echo sounders (MBES), like other sonar systems, transmit sound energy and analyze the return signal (echo) that has bounced off the seafloor or other objects. Multibeam sonars emit sound waves from directly beneath a ship’s hull to produce fan-shaped coverage of the seafloor. These systems measure and record the time for the acoustic signal to travel from the transmitter (transducer) to the seafloor (or object) and back to the receiver. Multibeam sonars produce a “swath” of soundings (i.e., depths) to ensure full coverage of an area. The coverage area on the seafloor is dependent on the depth of the water, typically two to four times the water depth. And, side scan sonar is yet another advancement in surveying the seabed and has proved helpful for scanning submerged natural and manmade objects, such as shipwrecks. Side scan sonar is a specialized sonar system for searching and detecting objects on the seafloor. Like other sonars, a side scan transmits sound energy and analyzes the return signal (echo) that has bounced off the seafloor or other objects. Side scan sonar typically consists of three basic components: a towfish, a transmission cable and the topside processing unit. In a side