Coastal Services Center

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Multibeam Sonar

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Summary | Data Products | Applied Uses | Specifications | Common Types | Data Ordering Details | Frequently Asked Questions | For More Information

Summary

Multibeam sonar (sound navigation and ranging) sensors are active sensors that utilize acoustic energy to collect measurements of seafloor depth and character. Multibeam sensors pulse the bottom with a series of soundings normal to the track of the vessel and record the reflected echoes in an orientation parallel to the vessel track. This produces a swath of data that, depending on specific sensor and mission requirements, is normally several times the water depth. Like other acoustic sensors, multibeam sonars normally collect data in a series of transect lines that allow sufficient sidelap to avoid gaps in coverage. As a rule, the deeper the water, the wider the swath of data collected. Since the swath width is strongly influenced by water depth, some planning of transect spacing is needed to ensure that no gaps occur where water depth decreases.

A unique aspect of multibeam sonar is the integration of vessel attitude into the collection of the data. This is accomplished through a Global Positioning System and data from an Inertial Motion Unit (IMU). The IMU makes very precise measurements of vessel attitude many times per second. By integrating attitude measurements with the timing of the sonar echo, an accurate bathymetric record can be produced regardless of the echo path through the water. This aspect of multibeam technology makes it the most complicated sonar system and the most expensive to operate.

Because of its ability to produce highly accurate depth measurements over a wide swath, multibeam sonar is most often used for bathymetric surveying. Recently, the backscatter intensity of the acoustic return has been used to derive bottom characteristics from multibeam data that can be used for habitat mapping.

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Data Products

Point to the names below to view the different data products.

Color-Ramped Bathymetry Bottom Classification

Acoustic Backscatter Sun-Illuminated Bathymetry

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Applied Uses

The following example shows how multibeam sonar data are being used:

Nearshore Habitat Mapping Coastal resource managers use multibeam sonar sensors and aerial digital multispectral systems that allow analysts to infer the distributions and extents of different habitats.

Other Potential Uses:

• Mapping Seafloor Geology
• Bathymetric Surveying
• Pipeline and Cable Routing
• Disaster Recovery and Salvage
• Benthic Habitat Studies
• Shellfish Harvest Planning and Management

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Common Types of Multibeam Acoustic Sonar Sensors Used for Benthic Mapping

All multibeam sonar sensors share similar characteristics and system components. Each system consists of a transducer to generate acoustic pulses and receive the echoes, a GPS unit to determine vessel location and speed, an IMU which records vessel attitude at the time of each pulse, a signal processing system to convert the echoes into bathymetric and backscatter values, and a data processing computer to compile a series of pulses into sea floor information.

There are two major types of multibeam sensors:

Pole-mounted sensors – Pole-mounted transducers are normally used on smaller vessels that will be temporarily dedicated to acoustic surveying. The poles used to hold the transducers range in size from about 6 feet long to tens of meters long, require cranes or vessel hydraulics to which deploy. The transducer pole is usually mounted along the vessel hull near the stern or working deck. The positional surveying of system components required of a through-the-hull sensor is also needed for pole-mounted systems.

Through-the-hull sensors – Through-the-hull sensors are those integrated with the vessel's bottom. These are a stable configuration but expensive to install. Through-the-hull systems are usually found on large boats and ships specifically dedicated to bathymetric surveying. Installing and configuring a multibeam system is a complicated and expensive task. The exact physical location of each system component and the distances between them must be surveyed with great precision. This information is used to correct for vessel motion and assure that the spatial accuracy of the bathymetry or backscatter information is of the highest possible quality. With through-the-hull systems, this only needs to be done once with a check up procedure needed only if there is damage to the system or other major alterations to the ship's structure.

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Specifications

The following information outlines the major specifications for multibeam sonar sensors. The specifications were collected from a variety of systems and may not apply to all sensors.

Signal frequency - Varies based on the transducer and model, generally between 12 and 450 kilohertz (kHz). The frequency determines the depth capabilities of the system.

• Higher frequency (100 to 450 kHz) for shallower waters (<10 to 300 meters)
• Medium frequencies (30 to 100 kHz) for medium-depth waters (300 to 3,000 meters)
• Low frequencies (12 to 18 kHz) for very deep waters (6000+ meters)

Footprint (on seafloor) - Each individual sounding in a multibeam pulse is generally small, but when integrated with others in the cross-vessel sweep, the swath is up to eight times the water depth.

Spatial Resolution - Spatial resolution is the area represented by individual sound pulse in the sweep. The horizontal resolution of each pixel is a function of the vessel speed and the pulse rate on the sonar. The vertical resolution of the pixel value is a function of the frequency of the sonar. Therefore, the higher the frequency, the greater the vertical resolution.

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Data Ordering Details

Data Acquisition	Multibeam data are usually collected by large marine survey/engineering companies or government agencies because of the technical and financial resources necessary to configure and maintain a system. Data are usually processed by these same groups, who normally deliver a bathymetric grid or a backscatter image. Development of habitat maps from either of these two products is usually accomplished by scientists or other technical users. more info
Timing	Multibeam sonar data can be viewed in real-time and/or processed after data collection. Timing of data delivery depends on several factors: survey speed (and state and local conditions) the size of the area surveyed and the amount of analysis required. Occasionally, a survey might require collection at certain tidal states. Times when salinity fronts or large thermoclines exist may affect the ability to conduct a survey or influence results.
Cost	The costs of collecting and analyzing multibeam data are highly variable. Factors that may influence collection costs include Contracting data collection vs. purchasing unit Transect line spacing Survey point density Manufacturer and distributor Type of data being collected (bathymetric, backscatter, or classified seafloor) Time needed to collect data (see "Timing" section above)
Data Formats/Software Needed	Bathymetric data collected from a multibeam sensor can be provided in a variety of formats depending on the collection software package. However, the *.xtf format has emerged as an industry standard. Backscatter imagery is usually delivered in a GeoTiff or Windows bitmap format. Certain software exists that allows Web display of backscatter data. The formats used in these applications are usually *.gif, and *.jpg, which produce smaller files suitable for Web browsing.
Projections	Sonar data are generally collected in the reference plane of the Global Positioning System (GPS) unit or as specified by the manufacturer's hardware and software. However, data can be delivered in cartesian projections, such as state plane and universal transverse mercator, according to the user's needs.
Licensing	Licensing agreements may exist for contracted data.

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Frequently Asked Questions

Data Acquisition

Where can I get the data?
Multibeam sonar data are usually collected for a specific project and the ability to obtain this data is dependent on the terms of a contract. Data collected under contracts for national mapping efforts are generally more commonly available than data collected for private engineering projects since they're publicly funded. Bathymetric grids are the most commonly distributed multibeam data. Normally the native bathymetry and backscatter data are not distributed due to large file sizes. The NOAA National Environmental Satellite, Data, and Information Service distributes bathymetric data from multibeam sonar and other sources that are available to the public. These data can be accessed at www.ngdc.noaa.gov/mgg/bathymetry/.

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Collecting Data

What technical expertise is needed to collect the data?
Configuring a multibeam sensor and collecting the data is a very specialized technical skill. Some of the major software packages have made strides in creating a user-friendly interface for actual data logging; nevertheless, collecting data successfully involves system calibration and is best performed by those with training and expertise in this area. Multibeam acoustic data are often collected by a survey company that works with the user to determine mission specifications. The company will then supervise the installation of the system onto either a permanent or temporary platform and collect and post-process the data before delivering it to the user.

The skills needed may include:

• good knowledge of acoustics
• good knowledge of electronics (to install and power the system, as well as troubleshoot issues such as signal interference)
• ability to operate and manage GPS data and data formats
• knowledge of the hardware being employed and the ability to manage several data streams and their serial connections

In addition, if backscatter imagery is being collected, the user must be able to collect appropriate field data (towed video, core or grab sampling, diver transects, etc.) and ensure that it can be related back to the multibeam data for signature development or thematic validation of final products. In general, it is not advisable for inexperienced technicians to collect multibeam data or design missions. There are hydrographer training courses available for those seeking to enter this field, but for the average user, it is most effective to contract the data collection and post-processing tasks.

What hardware and software are needed to collect multibeam sonar data?
Like other acoustic sensors the hardware and software required to collect the data vary based on the type of system being used; however, all systems require a transducer, signal processor, a GPS unit, a IMU, power source, computer for logging data, and system specific software. The hardware used for Multibeam sonar processing must support large file sizes and high rates of data transfer.

Are there limitations when comparing data collected with a multibeam sonar sensor for the same area with different dates?
As with other acoustic sensors, environmental factors may greatly influence the repeatability of sampling. Physical characteristics, especially temperature and salinity, strongly affect the speed and path of sound through water. Thus, surveys conducted on different days may face very different environmental conditions. Fortunately, as part of the mission calibration process, multibeam sonar sensors have the capacity to record and calculate the speed of sound through water before surveying begins. This allows data to be produced that is comparable to earlier or subsequent surveys. During long surveys or in areas where water quality may vary considerably, it is not uncommon to measure speed of sound through water multiple times. If the speed of sound through water is adequately characterized by testing, multibeam data acquired on different dates should be comparable.

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Analyzing Data

What technical expertise is needed to analyze the data?
A high level of expertise and skill is required to perform the immediate post-processing of multibeam data, much of which involves correcting for anomalous bathymetric records. The end user will most likely use the bathymetric grid or conduct an analysis of the backscatter data set. The bathymetric grid can be treated like any other grid product. Users normally will color code the grid to ease visual interpretation of the data. In cases where a particular isobath is important to a study, the user may contour the data. For overall visualization of the bottom topography, a sun-illumination raster is most useful. The tools to produce these derived products are available in ESRI's ArcGIS and several other GIS and image processing software programs.

The most common way to analyze backscatter data remains visual interpretation. Knowledge of the seascape is most important for this process to be successful. It is essential to know what types of habitats are present in the study area and how these features will respond to acoustic energy at either high or low frequencies. In certain cases, region-growing tools may be useful for analyzing the backscatter. These are most effective when the feature of interest has a consistent acoustic signature that is distinct from the surrounding seascape. In general, the normal set of skills possessed by an experienced spatial analyst is sufficient to begin analyzing multibeam data.

As in any remote sensing effort, field signature development and validation data are essential to project success. Signature development should be carried out during the survey period. One of the most commonly used tools for signature development is underwater videography. There are several types of towed video units that allow the camera to follow the boat track as it is actually collecting multibeam data over the same area. By comparing the two data sets, a better interpretation of the multibeam backscatter can be accomplished. When comparing videography to other remotely sensed data, it is important to be aware of the scale factor between the two. Videography typically produces very high amounts of detail over a very small viewing area. This can be difficult to relate to coarser data at a smaller scale.

What hardware and software are needed to analyze multibeam sonar data?
The bathymetric data in a multibeam data stream can be managed, edited, and post-processed in the system manufacturer's software. In addition, some softwares have modules that allow analysis of the backscatter data. The tools available in these modules usually contain some kind of spectral "region-growing" tool and tools for conducting manual delineations of similar features that can be exported in an ESRI shapefile format. The region-growing tools allow the user to select a set of pixels in the backscatter image that represent a feature of interest. The tool then locates all pixels of similar spectral characteristics in the image. This approach is most effective if the focus is on one particular type of bottom or feature. For example, the region-growing tool would be useful in locating all hardbottom areas in an otherwise sandy bottom. The tool would be less useful in a diverse benthic environment or if the objective was to produce a comprehensive map of the bottom. In these cases, the user might be better advised to do a spectral clustering over the entire image. Whatever approach is taken, it is advisable to have a computer with sufficient CPU speed and RAM to manage large files in a timely way. Two commonly used softwares that are used for analysis of Multibeam backscatter data are CARIS and ISIS packages.

What are some limitations of analyzing data collected with a multibeam sonar sensor?
There are few limitations on using this technology for either bathymetric surveying or backscatter imaging because of the capability of a multibeam system to calibrate for water column characteristics and compensate for vessel attitude during data collection. One limitation of the backscatter data from multibeam is that most of the signal processing and bandwidth are dedicated to making a very accurate depth determination. As a result, the quality of the backscatter has generally been poorer than that produced by side-scan sonar. New systems are improving on this. Another potential limitation from a user's perspective is the large size of the files produced by multibeam sonars; however, with proper data management this problem can be mitigated.

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Sensor Variations

What are some other ways that multibeam sonar sensors vary?
Most of the variation in multibeam sensors lies with the quality of the system components, robustness, and intuitiveness of the collection software and skills of the technicians operating the system. Perhaps the most critical element of a multibeam system is the quality of the IMU. High-quality units are stable, reliable, and record precise measurements at rapid sampling rates. In addition, transducers and systems may vary on range of frequencies they can collect and analyze.

How does the frequency of the transducer influence the type of data I will collect?
With the same amount of power, lower frequency acoustic signals travel farther in water than higher frequency signals. Therefore, transducers with lower frequency are better for mapping in deeper water. Higher frequencies suffer more attenuation in the water column and are thus better suited for shallower areas. The size of features that can be detected is dependent on the frequency of the system, so while they don't penetrate the water column as effectively, the higher frequency systems capture much more detail than the lower frequency systems.

Are there ways to improve the performance of a multibeam system?
Most multibeam systems have some capability to optimize the acoustic pulse as well as compensate for excessive vessel motion.

The acoustic pulses can be "focused" to produce the narrowest beam at the point at which the bottom is expected. This optimizes the detail of the returning backscatter image.

Multibeam systems often have the capacity to aim the beam when vessel roll or pitch becomes excessive. In these cases the system points the beam toward the nadir below the boat. This system prevents gaps that might exist during especially significant boat roll or change in attitude.

Can I increase the power to my high-frequency transducer so it will travel as far as a low-frequency signal?
This is not recommended because increasing the power may result in the water molecules being split and oxygen becoming trapped on the transducer's head, which interferes with its ability to detect the returns.

What are some differences between single-beam, multibeam, and side-scan sonars?

• Single-beam sonars have a narrow footprint on the seafloor and do not provide continuous coverage of the seafloor. The data resolution is determined by the footprint size, sampling interval, sampling speed, and transect spacing. Depending on the sensor type, single-beam sensors may collect bathymetry data or be calibrated to identify seafloor habitats or subsurface sediments. Many of the systems are designed to be easily transported and deployed from small boats that can access shallow areas. Transportability makes it a good choice for agencies with multiple users and needs, or that do not have a vessel to devote to acoustic data collection.
• Multibeam sonars are used to measure bathymetry and can be used in extremely deep waters (up to 11,000 meters). These sensors emit multiple beams that cover large overlapping swaths of the seafloor, enabling continuous coverage of the seafloor. Multibeam systems are often larger and less transportable than single-beam sensors and produce very large data streams. Users are now exploring the use of the multibeam's backscatter signals for potential in habitat or feature mapping.
• Side-scan sonars point their beams toward the seafloor at oblique angles and cover large overlapping swaths of the seafloor, enabling continuous coverage. Side-scan sonars only measure features on the seafloor and cannot collect bathymetry data. However, they can be deployed in waters up to 11,000 meters deep and provide data with a resolution of a few centimeters. These sonars are primarily deployed on a towed unit (a tow-fish) and produce very large data streams. Side-scan sonar systems are often larger and less transportable than single-beam sensors, although the technology is advancing.

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Environmental Variables

How does the physical environment affect data collection?

One limitation of this technology is the difficulty in collecting multibeam data in shallow or complicated areas such as estuaries and bays. Despite swaths up to eight times the water depth, in shallow areas it is often not economical and sometimes technically difficult to produce comprehensive data. This is complicated by the fact that with the significant infrastructure requirements of multibeam sensors, often boats that can support the sensors have difficulty navigating in in-shore areas. Sampling speed is comparable to that required for other acoustic surveys with speeds averaging less than 10 knots and as slow as 5.

How does water depth influence the data?

Extremely shallow waters

In very shallow waters, the signal may return too quickly for the transducer to record it, so false depth values (or none at all) are recorded. False values may be indicated by depths that are twice what they are expected (signal bounces twice before recorded) or by abnormally high values in known shallow waters.

Footprint and signal intensity

	The swath of a multibeam system is determined by water depth. It is smaller in shallower water and larger in deeper water.
	In deeper waters, the footprint of a multibeam sonar is larger, up to eight times the water depth. Water depth, sonar frequency, and settings on the individual instrument determine how much the swath increases as the water deepens.

Does water temperature or salinity impact the signal?
Temperature and salinity change the density of water, which may speed up or alter the acoustic return. In many cases, changes from temperature and salinity are extremely subtle and undetectable. However, dramatic changes may be detected if a distinct thermocline or halocline (boundary between water due to temperature or salinity) exists in the water column. A thermocline or halocline may act as a barrier to sound waves and/or distort the return value.

Do gas bubbles impact the signal?
Small gas bubbles resulting from wind or currents mixing the surface water or released from plants can impact the return of the sound wave. Gas bubbles are strongly reflective to sound waves, especially at higher frequencies. They can produce a return that interferes with detecting the bottom or distorts the actual bottom return, hindering determination of habitat. In addition to forming near-surface turbulence, gas bubbles can occur when actively photosynthesizing submerged aquatic vegetation releases oxygen into the water column. The gas bubbles may collect on the leaves of the plants. The seagrass leaves, although not a hard feature, will reflect a signal that is interpreted as hard and the seagrasses may be misclassified as a result. Occasionally, under certain pressure and temperature conditions, methane gas in unconsolidated substrates can precipitate out and enter the water column.

What are some sources of noises in the water column and how do they influence the data?
Strong currents and waves, and boat engines are some sources of noise in the water column. Adjusting the frequency range of the signal and positioning the transducer appropriately can help eliminate these sources of noise. Noise is often displayed as reduction in signal intensity and the introduction of anomalous or false returns. Occasionally strong electrical or magnetic fields along the course of transducer and system wiring can also introduce noise into the process.

How does boat speed influence the data?
The density of data points is dependent on boat speed because the sensor emits pulses at specified intervals (e.g., 1 pulse every second or 1 pulse every 5 seconds). Slowing down or speeding up the boat changes the number of sweeps along the bottom collected. This manifests itself as higher spatial resolution and feature detection capability along the boat track. Excessive boat speed can result in gaps between sweep pulses that affect continuity of the data. In addition, if the boat is traveling too fast, an air pocket can form around the transducer that interferes with the transducer's ability to detect the returns.

At the other extreme are cases in which a boat is stationary or traveling extremely slow, making it more vulnerable to wave action and vessel roll. When a boat rolls, the transducer sends pulses at angles towards the seafloor and not directly below the transducer. This changes the area of the seafloor that the sensor is viewing and may alter the return signal.

How are multibeam sonar sensors affected by large fish or other suspended items in the water column?
Acoustic signals will bounce off solid objects in the water column. Depending on the size of the object, these returns may be filtered out as outlier or anomaly data points. Fish will be detected because their swim bladders contain air. The ability to detect fish is enhanced by the way multibeam data are collected in a wide swath below the vessel. There are a number of multibeam systems that have been specially designed to image fish in the water column and thus target them for harvest by mid-water trawling.

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For more Information

• Acoustical Society of America's Web Site
• NOAA Coastal Services Center Benthic Habitat Mapping Web Site

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