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.
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
Return to top