St. Petersburg Coastal and Marine Science Center
Bathymetric Sonar Systems
Echo sounding is a special application of sonar used to measure the distance from a vessel to the seafloor or lake bottom. Traditional 'soundings' were measured by lowering a lead line until it hit bottom. Modern soundings are obtained by sending acoustic signals from an echo sounder or fathometer to the seafloor. The outgoing signal is called a 'ping' and the return signal is called an 'echo'. Recreational and commercial boats are typically equipped with a single-beam fathometer or 'fish-finder'.
A conventional swath multibeam echosounder or 'backscatter' sonar system traverses the seafloor with a fan-shaped array of signals and returns. The returns produce multiple-depth measurements in a swath perpendicular to the vessel's track line. The swath bathymetric sonar system is attached to the vessel or platform instead of being towed. The system records the length of time for the acoustic signal to travel from the transmitter (transducer) to the seafloor or other object and back again to the receiver or hydrophones. Multibeam systems use a beam-forming method to obtain high spatial-resolution and to improve the signal to noise ratio with a vertical accuracy of about 5 cm. Seafloor coverage is usually 2-4x the water depth. These systems are well suited for mapping deep features (Pulley Ridge) and for highly detailed features in shallow water such as rock alignment of a jetty or pieces of a downed aircraft. The swath bathymetry system can be combined with sidescan sonar to map depth and bottom characteristics simultaneously.
The USGS uses a SeaSwath +H interferometric swath bathymetry system and a Knudsen 320BP 28/200 KHz single-beam bathy system.
An interferometric swath sonar system uses the phase difference between two or more receivers to measure the range and angle of the target return signal. The USGS uses a high-resolution interferometric bathymetric sonar system. These systems are effective for quick bathymetric measurements and have the advantage of greater seafloor coverage. Seafloor coverage is dependent on water depth, typically 20x depth. The trade-off is a decrease of vertical accuracy to approximately 15 cm.
USGS: Simrad multibeam echo sounder model EM 3000 Flower Gardens data: http://coastalmap.marine.usgs.gov/regional/contusa/gomex/centgulf/northwest/data.html