Coastal Services Center

---

Single Beam Sonar

---

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

Summary

Single beam sonar sensors utilize sonar (sound navigation and ranging) technology to collect measurements of the seafloor. These sensors collect point or raster data derived from the strength and time of the acoustic return. Single beam sensors consist of a transducer, mounted on or towed by a boat, that feeds into a signal processor and display device. The transducer emits a single sound pulse with a narrow footprint into the water column at specific intervals directly below the transducer. The sound wave bounces off the seafloor and the return is captured by the transducer.

Single beam sonar data are collected along transect lines and typically cannot provide continuous coverage of the seafloor. The output resolution of the data are determined by the footprint size, sampling interval, sampling speed, and distance between transects.

Different systems analyze the returns of the sound waves very differently - some systems only analyze the first return to measure bathymetry, while others use one or more returns to derive classifications of the seafloor or subsurface sediments. Because single beam systems can be portable and relatively inexpensive, many users own, share, or rent their systems.

---

Different Data Products

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

Bathymetric Data Points
Interpolated Bathymetric Grid
Classified Data Points
Interpolated Bottom Classification Grid

Return to top

---

Applied Uses

Examples of how single beam sonar data are being used:

	Managing a Nuisance Aquatic Plant Every summer in Delaware's inland bays, a species of algae experiences massive die-off events that deplete the water of oxygen and emit a noxious odor. Managers in Delaware are using a single beam sonar seafloor classifier to map algae populations and target harvesting efforts.
	Mapping Past and Present Oyster Reefs Researchers are using a side-scan sonar and single beam sonar subbottom profiler to assess the current status of oyster beds and find substrates suitable for oyster restoration projects.

Other Potential Uses:

• Mapping Seafloor Geology
• Field Verifying Other Remotely Sensed Data Sets
• Navigation (Depth Finding)
• Benthic Habitat Studies
• Shellfish Restoration Siting

Return to top

---

Common Types of Single Beam Sonar Sensors Used for Benthic Mapping

Echosounder - An echosounder system measures the length of time it takes for a sound wave to return to the transducer. The time is analyzed to provide a measure of depth or bathymetry.

Subbottom profiler - A subbottom profiler system emits lower frequency sound waves that can penetrate up to 50 meters into the seafloor, depending on seafloor type and water conditions (e.g., turbidity, salinity). The sensor is most effective in soft sediments. Some subbottom profilers are referred to as "chirp" sonars because of the high-pitched clicking sound the transmitter emits. This sonar system requires field validation to calibrate the system.

Seafloor classification systems - Seafloor classification systems are used to classify features of the seafloor, such as vegetation or surface type. Some seafloor classification systems analyze the first return of a sound wave, while others analyze the first and second returns of a sound wave. Seafloor classification systems require field validation to calibrate the system.

Return to Top

---

Specifications

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

Signal frequency – Varies based on the transducer and manufacturer, generally between 12 and 710 kilohertz (kHz)

• Higher frequency (200 kHz) for shallower waters (1 to 50 meters)
• Lower frequencies (12 to 50 kHz) for deeper waters (up to 11,000 meters)
• Some transducers are available with dual frequencies (high and low)

Depth – < 1 to 3000 meters (dependent on frequency)

Footprint (on seafloor) – Generally small - dependent on depth and transducer (covers a smaller area in shallower water, a larger area in deeper water)

Return to top

---

Data Ordering Details

Data Acquisition	In most cases the single beam sonar data are collected and analyzed by a user that owns, shares, or rents the sensor. Raw data are generally unavailable and are not typically the desired end product. Derived products such as bathymetry or habitat maps may be available through a variety of sources. There are also private companies, academic institutions, and government agencies that collect, or can be contracted to collect, single beam sonar data. more info
Timing	Single beam sonar data can be analyzed in real-time or post processed after data collection. Timing of data delivery depends on several factors: survey speed (depends on sea state and local conditions); the size of the area surveyed; the density of data collected; and the data analyses.
Cost	The costs of collecting and analyzing acoustic data are highly variable. Factors that may influence collection costs include Contracted data collection vs. purchasing unit Transect line spacing Survey point density Manufacturer and distributor Type of data being collected (bathymetric, subbottom, or classified seafloor) Time needed to collect data (see "Timing" section above)
Data Formats/Software Needed	There are no standard formats for data collected with single beam sonar. The format is determined by the system and software used to collect and process the data. Bathymetric and seafloor classification data sets are typically point features with date, time, location, and depth data (x, y, z), as well as additional attributes. The data may be directly analyzed within the proprietary software and/or exported as a text or database file for use in a geographic information system (GIS) or statistical package. Subbottom profiler data may be output directly to a graphic recorder (printer) as a hard copy printout, or saved in a digital format. The data may be directly analyzed within the proprietary software and/or exported for use in an image processing software package.
Projections	Acoustic 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.
Licensing	Licensing agreements may exist for contracted data.

Return to top

---

Frequently Asked Questions

Data Acquisition

Where do I get the data?
Data collected by the owner of a single beam sonar may be freely distributed at the user's discretion. Data may also be acquired from private companies or through data collection contracts. On-line searches may help users find data collectors or providers.

The NOAA Coastal Services Center provides processed seafloor classification data collected using single beam sonar data for several study sites on the on-line benthic data Web page.

Return to top

Collecting Data

What technical expertise is needed to collect the data?
Single beam sonar data are often collected by a user who owns, shares, or rents a sensor. The user should be able to install the necessary software and know how to set up, power, and run the system, as well as troubleshoot potential problems. These skills may include the following:

• basic knowledge of acoustics
• basic knowledge of electronics (to install and power the system, as well as troubleshoot issues such as signal interference)
• ability to operate and manage Global Positioning System (GPS) data and data formats
• knowledge of the hardware being employed and the ability to manage several data streams and their serial connections
• ability to operate software associated with the sensor, GPS, and hyperterminal (for serial connection troubleshooting)

In addition, if seafloor classification data or subbottom profiling data are being collected, the user must be able to collect appropriate field validation data (towed video, core or grab sampling, diver transects, etc.) and know how to calibrate the classification system. Other expertise includes the ability to drive a boat and to troubleshoot all the components if necessary.

What hardware and software are needed to collect single beam sonar data?
The hardware and software required to collect the data vary based on the type of system being used. At a minimum, most systems require a transducer, a signal processor, a GPS unit, a laptop or portable field computer, and the system's specific software. The hardware used for single beam sonar processing must support the file size and software memory requirements. For subbottom profiling systems that output to hard copy printouts, a graphic recorder and the necessary components will also be required.

Users will also need an appropriate method of field verifying the data (i.e., grab or core sampling for subbottom profiling vs. video or diver for habitat data).

Are there limitations when comparing data collected with a single beam sonar for the same area with different dates?
Environmental factors may greatly influence the repeatability of sampling for certain types of single beam sonar. For example, when classifying the seafloor, users may collect field validation data while sampling to calibrate the classification system. Sometimes during the course of a sampling period (a day or a week), water temperature rises or dissolved oxygen levels increase. This may change the value of acoustic returns recorded later during the survey period, and the earlier calibration may need to be adjusted.

Return to top

Analyzing Data

What technical expertise is needed to analyze the data?
In many cases, the data can be post-processed (e.g., cleaned to remove anomalous values) and analyzed within the system's software. Point data sets derived from echosounders and seafloor classifiers can be exported as a text or database file that can be imported into a GIS or statistical software. Digital subbottom profiler imagery can be exported for use in image processing software.

Single beam sonar data are collected along a transect line at intervals specified by the user. This type of data does not typically provide continuous coverage of the seafloor. The output resolution is determined by the footprint size, sampling interval, sampling speed, and distance between transects. Prior to starting a sampling effort, users should determine what resolution of output data are required, and what sampling strategy will provide the appropriate resolution.

The user should also have an understanding of the heterogeneity of the habitat being surveyed and the footprint size. This is important because the return value represents the entire footprint of the seafloor being sampled. For example, if several small objects are within the entire footprint, they are averaged into one return value which may differ from the values the small objects or the substrate alone.

The data derived from a single beam sonar can be interpolated to create a continuous coverage. However, this can create misleading results if inappropriate methods are used. An interpolated data set creates data for the entire study area - including areas that were not sampled in the field. The user should carefully determine the interpolation method to be used and understand the necessary inputs, parameters and outputs. In addition, if interpolated data products are to be distributed (i.e., interpolated maps of seafloor habitat), the methods used should be clearly defined to prevent misinterpretation.

What hardware and software are needed to analyze single beam sonar data?
Most single beam sonar data can be analyzed and interpreted using the system manufacturer's software. Users can also export the data to a GIS, image processing software, or other spatial or statistical software package.

What are some limitations of analyzing data collected with single beam sonar?
A single beam sonar collects data as points or raster in a continuous stream at specified intervals directly below the transducer. Because the sensor has a narrow footprint, the raw data do not provide continuous coverage of the seafloor; only the area directly below the transducer is sampled. This can result in large portions of the seafloor going unsurveyed. For example, one type of transducer (a 6-degree transducer) has a footprint size that is one-tenth of the depth. When a user is collecting data with this transducer in 10 meters of water, each sound wave return will cover 1 meter of the seafloor.

The footprint width, sampling interval, sampling speed, and distance between transects determine the resolution of the output data. The density of data points depends on the specified interval (e.g., sensor emits 1 pulse every second or 1 pulse every 5 seconds) and boat speed. If the data are to be interpolated to create a continuous coverage, the user should be familiar with the application and use of interpolation methods, and plan the sampling strategy to ensure the data are collected at the appropriate density.

Slow sampling speeds may be a limitation of sampling with single beam sonar. Although some sensors mounted to the hull of a ship may tolerate faster speeds, in general, sampling is typically at 4 to 5 knots per hour. At higher speeds an air pocket can form around the transducer that interferes with the transducer's ability to detect the returns. In addition, the boat can be traveling too fast for the sensor to pick up the return.

Return to top

Sensor Variations

What are some other ways that single beam sonar vary?
Some single beam sonar are portable and designed to be easily set up and used on boats and vessels of opportunity. Other types of single beam sensors must be permanently mounted on a dedicated vessel.

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.

For subbottom profilers, very low frequency signals are able to penetrate much farther into the subsurface than higher frequency signals. Lower frequency signals can penetrate up to 50 meters of the seafloor (depending on substrate), while high frequency signals may only penetrate a few centimeters. Because higher frequency signals do not penetrate the seafloor well, they can provide higher resolution data of the seafloor surface. Dual frequency transducers are often used with subbottom profilers to provide the advantages of using both high and low frequencies.

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. In addition, the increased oxygen in the water will alter the signal.

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

• Single beam sonar 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 acoustic sensors 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 multiple beams at angles and cover large overlapping swaths of the seafloor, enabling continuous coverage of the seafloor. 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.

Return to top

Environmental Variables

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 footprint of a single beam transducer is determined by water depth. It is smaller in shallower water and larger in deeper water. For example, one type of transducer (a 6-degree transducer) has a footprint that is one-tenth the depth, so in water that is 10 meters deep, the sensor's footprint is approximately 1 meter.
	In deeper waters, the footprint of a single beam sonar is larger. Using the same example mentioned above, in 100 meters of water, the same transducer has a footprint of approximately 10 meters. However, the signal weakens as it is spread out over a larger area and the intensity of the return may be reduced.

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.

Does dissolved oxygen impact the signal?
Small oxygen bubbles resulting from wind or currents mixing the surface water or released from plants can impact the return of the sound wave.

For systems such as RoxAnn^™, which runs an algorithm to determine hardness of the seafloor, oxygen bubbles can alter the return of a sound wave off of a feature because oxygen bubbles reflect a signal that is interpreted as hard. For example, on a very sunny day, actively photosynthesizing seagrasses will release oxygen into the water column. The gas bubbles may collect on the leaves of the plant. 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.

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 reductions in signal intensity and the introduction of anomalous or false returns.

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 points collected along the transect.

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. In addition, the boat can be traveling too fast for the sensor to pick up the return.

If a boat is stationary or traveling extremely slow, it may be subject to wave action and the boat may rock from side to side. When a boat rocks, 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 is single beam sonar affected by large fish or other suspended items in the water column?
Acoustic signals will bounce off solid objects in the water column. Fish will be detected because their swim bladders are full of air. Depending on the size of the object, these returns may be filtered out as outlier or anomaly data points.

Return to top

---

For more Information

• Acoustical Society of America's Web site

Return to top