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Hydrographic Survey Equipment
 
Side Scan Sonar
Side scan sonar picture of USS Monitor     Graphic showing side scan sonar towfish eminating acoustic waves that reflect off the ocean bottom.  Acoustic shadows are created where the waves are blocked.

Side scan sonar image of USS Monitor

  Side scan sonar towfish emanating acoustic waves.  Acoustic shadows are created where the waves are blocked by an object.

The Office of Coast Survey conducts hydrographic surveys primarily with side scan and multibeam sonar. SONAR (Sound Navigation and Ranging) uses sound waves to find 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 typically consists of three basic components: 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, typically to a distance of 100 meters.

The strength of the return echo is continuously recorded creating a "picture" of the ocean bottom.  For 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 typically used in conjunction with a single beam or multibeam sonar system to meet full bottom coverage specifications for OCS surveys. NOAA field units use various 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

Digital Terrain Model (DTM) created from multibeam sonar draped over a nautical chart of Quicks Hole Mass.

  3-D of NOAA Ship using multibeam echo sounder to survey the ocean bottom.
Digital Terrain Model (DTM) created from multibeam sonar data draped on top of a nautical chart of Quicks Hole, MA.    3-D image of NOAA ship using multbeam sonar to survey the ocean bottom.


The Office of Coast Survey conducts hydrographic surveys primarily with side scan and multibeam sonars. SONAR (Sound Navigation and Ranging) uses sound waves to find 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 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.

Many MBES systems are capable of recording acoustic backscatter data. Multibeam backscatter is intensity data that can be processed to create low resolution imagery. Backscatter is co-registered with the bathymetry data and is often 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 critical items such as wrecks, obstructions, and dangers-to-navigation, and for general object detection. NOAA field units use various 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

LIDAR picture showing underwater and land terrain.
LIDAR image showing terrain and bathymetry. 

LIght Detection And Ranging or LIDAR is is a method used by NOAA to measure distance or depth by analyzing pulses of laser light reflected off an object. These survey systems are typically aircraft-mounted and provide seamless coverage between land and sea. Bathymetric LIDAR refers to its use to determine water depth.

Bathymetric LIDAR systems use laser pulses received 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.

Analyses 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 complex and rugged shorelines. Surface vessels often cannot operate efficiently or safely in these areas due to rocks, kelp or breaking surf. NOAA uses LIDAR to collect near shore bathymetry in Alaska, the North Atlantic Coast and the Caribbean. Future developments include improving object detection capabilities to better identify near shore hazards to navigation.

RESOURCES
NOAA Coastal Services Center LIDAR Page  

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