Hydrographic Survey
Side Scan Sonar
The Office of Coast Survey conducts hydrographic surveys first and foremost with side scan and multibeam sonar. SONAR (Sound Navigation and Ranging) uses sound waves to locate and identify objects in the water and determine water depth.
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 generally is made up of three rudimentary aspects: a towfish, a transmission cable and the topside processing unit. In a side scan the transmitted energy is formed into the shape of a fan that sweeps the seafloor from directly under the towfish to either side, generally to a distance of 100 meters.
The strength of the return echo is continuously recorded developing a "picture" of the ocean bottom. As an example, objects that protrude from the bottom create a light area (strong return) and shadows from these objects are dark areas (little or no return) as in the image at left above or vise versa depending on operator preference. While the shape of the seafloor and objects on it can be well- depicted, most side scan systems can not provide any depth information.
NOAA hydrographic survey units use side scan sonar systems for both object detection and object recognition. Side scan sonar is generally used in concert with a sole beam or multibeam sonar system to meet full bottom coverage specifications for OCS surveys. NOAA field units use several models of side scan sonar in both hull mounted and towed configurations for hydrographic survey operations. Any side scan sonar system employed must meet the specifications outlined in the “NOS Hydrographic Surveys Specifications and Deliverables”.
Multibeam Echo Sounders
The Office of Coast Survey conducts hydrographic surveys first and foremost with side scan and multibeam sonars. SONAR (Sound Navigation and Ranging) uses sound waves to locate and identify objects in the water and determine water depth.
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 create 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 make sure of full coverage of an area. The coverage area on the seafloor is dependent on the depth of the water, generally two to four times the water depth.
Many MBES systems are capable of recording acoustic backscatter data. Multibeam backscatter is intensity data that can be procedureed to develop low resolution imagery. Backscatter is co-registered with the bathymetry data and is usually used to assist with bathymetric data interpretation and post-processing.
NOAA hydrographic survey units use mutlibeam echo sounder systems to acquire full- and partial- bottom bathymetric coverage throughout a survey area, to determine least depths over crucial stuff such as wrecks, obstructions, and dangers-to-navigation, and for general object detection. NOAA field units use several models of swath-type multibeam systems both hull and pole mounted for hydrographic survey operations. Mutlibeam echo sounder systems employed must meet specifications outlined in the “NOS Hydrographic Surveys Specifications and Deliverables".
LIDAR
LIght Detection And Ranging or LIDAR is is a way used by NOAA to measure distance or depth by analyzing pulses of laser light reflected off an object. These survey systems are generally aircraft-mounted and provide seamless coverage between land and sea. Bathymetric LIDAR aludes to its use to determine water depth.
Bathymetric LIDAR systems use laser pulses inherited at two frequencies. Water depths are determined by measuring the time delay between the transmission of a pulse and its return signal detecting the seafloor. A lower frequency infrared pulse is reflected off the sea surface, while a higher frequency green laser penetrates through the water column and reflects off the bottom. Uptightyses of these two distinct pulses are used to establish water depths and shoreline elevations. Depending on water clarity, these systems can reach depths of 50 meters.
Bathymetric LIDAR is used to acquire data in areas with complicated and rough shorelines. Surface vessels usually cannot operate efectively or safely in these areas because of rocks, kelp or breaking surf. NOAA uses LIDAR to collect close to shore bathymetry in Alaska, the North Atlantic Coast and the Caribbean. Future developments include improving object detection capabilities to better identify close to shore hazards to navigation.