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Mapping Techniques: Acoustics

Bathymetry Surveys

Illustration

Illustration of a research vessel with a hull-mounted multibeam sonar and a towed side-scan sonar system.
Courtesy: NOAA Coast Survey

Bathymetry surveys use sound navigation and ranging (sonar) systems to collect depth information. These systems are based on the principle that when a sound signal is sent into the water it will be reflected back when it strikes an object. An instrument, called a transducer, sends a sound pulse straight down into the water. The pulse moves down through the water and bounces off the seafloor. The transducer also picks up the reflected sound. Computers precisely measure the time it takes for the sound pulse to reach the bottom and return. The water depth is calculated by knowing how fast sound travels in the water (approximately 1,500 meters per second). This method of seafloor mapping is called echosounding.

Echosounders can transmit a single beam or multiple beams of sound simultaneously. Single-beam sounders collect discrete data points along survey track lines. Multibeam sounders collect continuous high-resolution bathymetry data throughout the survey area.

Some multibeam systems can simultaneously measure the strength of the reflected sound, called backscatter. Hard acoustic returns indicate rock or gravel while softer returns indicate mud or silt. Acoustic backscatter maps can assist with determining seafloor composition. This technique has the potential to be a powerful characterization tool.

What does a backscatter image look like? View Image

Advantages and Limitations

Single-beam depth sounders have several advantages, since they are commonly available at a relatively low cost, and portable units can be easily deployed on small boats or ships of opportunity. In addition, they can be interfaced with acoustic substrate classifiers that estimate seabed composition (Acoustic Seafloor Classification Systems).

Multi-transducer systems have a series of single-beam transducers mounted along a boom. They provide greater bottom coverage than a single-beam system (their coverage is based on the length of the transducer boom). These systems are typically mounted semi-permanently on designated boats. They are far less portable than single-beam systems and may restrict boat maneuverability. Multi-transducer systems are primarily used for accurate navigation or dredging-related surveys in protected harbors and rivers.

Example of Topography Map

Seafloor topography map of Stellwagon Bank.
Courtesy: U.S. Geological Survey

Multibeam echosounding is a relatively new seafloor mapping technology. These systems collect bathymetric soundings across a swath of the seafloor using an array of acoustic beams. They acquire dense sounding data both along and between track lines. They provide accurate, high-resolution, 100 percent coverage of the seafloor over relatively large areas. Multibeam systems require additional navigation sensors including a vessel motion sensor. These systems are often installed on a semi-permanent basis, as temporary installations are not as simple as single-beam systems. The acquisition and processing of multibeam bathymetric data is far more complex than single-beam surveys. Multibeam bathymetric surveying generates orders of magnitude more data than single-beam surveying. This in turn requires more data editing, processing, and storage capacity.

What does high-resolution multibeam data look like? View Image

Single-beam vs. Multibeam Bathymetry

There are many factors to consider when deciding between using either single-beam or multibeam bathymetry. If the survey covers large and complex areas of the seafloor and requires complete bottom coverage, then multibeam systems may be the better option. Multibeam data will provide greater resolution and permit wider spacing of survey track lines. This could decrease the number of survey lines and reduce costly ship time. If lower resolution is acceptable, then single-beam bathymetry in conjunction with other technologies (such as side-scan sonar) may provide a lower cost alternative.

View Image Comparison View Image

Uses

Accurate bathymetry data are essential for nautical charting, dredging, and navigation safety. For much of the last 50 years, single-beam bathymetry has been the primary method used for mapping the seafloor. It is an accurate and relatively simple technique for collecting seafloor topography data. In single-beam surveys, the area between adjacent survey tracks is not surveyed; therefore, there are many gaps in the data. These surveys typically cover only 5 to 10 percent of the total seafloor area. The depth data must be interpolated between track lines when constructing contours or gridded maps. These maps may not have high enough resolution to detect small habitat features.

Multibeam data provides a powerful tool for examining benthic habitats. High-resolution multibeam systems can accurately detect and define bottom features as small as one meter in diameter. The U.S. Geological Survey is using multibeam sonar to map the seafloor and biological habitats of the Stellwagen Bank National Marine Sanctuary. These maps will assist scientists, policy makers, and managers to better understand and monitor this protected marine ecosystem. Additionally, scientists at the United States Geological Survey (USGS) are using detailed multibeam bathymetric and acoustic backscatter data to investigate sediment and pollution transport in San Francisco Bay.

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