Acoustic seabed classification is the partitioning of a
seabed
The seabed (also known as the seafloor, sea floor, ocean floor, and ocean bottom) is the bottom of the ocean. All floors of the ocean are known as 'seabeds'.
The structure of the seabed of the global ocean is governed by plate tectonics. Most of ...
acoustic
image
An image is a visual representation of something. It can be two-dimensional, three-dimensional, or somehow otherwise feed into the visual system to convey information. An image can be an artifact, such as a photograph or other two-dimensiona ...
into discrete physical entities or classes. This is a particularly active area of development in the field of
seabed mapping,
marine geophysics Marine geophysics is the scientific discipline that employs methods of geophysics to study the world's ocean basins and continental margins, particularly the solid earth beneath the ocean. It shares objectives with marine geology, which uses sedime ...
,
underwater acoustics
Underwater acoustics is the study of the propagation of sound in water and the interaction of the mechanical waves that constitute sound with the water, its contents and its boundaries. The water may be in the ocean, a lake, a river or a tank. Typ ...
and benthic habitat mapping. Seabed
classification Classification is a process related to categorization, the process in which ideas and objects are recognized, differentiated and understood.
Classification is the grouping of related facts into classes.
It may also refer to:
Business, organizat ...
is one route to characterizing the seabed and its
habitats
In ecology, the term habitat summarises the array of resources, physical and biotic factors that are present in an area, such as to support the survival and reproduction of a particular species. A species habitat can be seen as the physical ...
. Seabed characterization makes the link between the classified regions and the seabed physical, geological, chemical or biological properties. Acoustic seabed classification is possible using a wide range of
acoustic imaging An acoustic camera is an imaging device used to locate sound sources and to characterize them. It consists of a group of microphones, also called a microphone array, from which signals are simultaneously collected and processed to form a representat ...
systems including
multibeam echosounders,
sidescan sonar
Side-scan sonar (also sometimes called side scan sonar, sidescan sonar, side imaging sonar, side-imaging sonar and bottom classification sonar) is a category of sonar system that is used to efficiently create an image of large areas of the sea ...
,
single-beam echosounders,
interferometric
Interferometry is a technique which uses the ''interference'' of superimposed waves to extract information. Interferometry typically uses electromagnetic waves and is an important investigative technique in the fields of astronomy, fiber op ...
systems and
sub-bottom profilers. Seabed classification based on acoustic properties can be divided into two main categories; surficial seabed classification and sub-surface seabed classification. Sub-surface imaging technologies use lower
frequency
Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...
sound
In physics, sound is a vibration that propagates as an acoustic wave, through a transmission medium such as a gas, liquid or solid.
In human physiology and psychology, sound is the ''reception'' of such waves and their ''perception'' by the ...
to provide higher penetration, whereas surficial imaging technologies provide higher resolution imagery by utilizing higher frequencies (especially in shallow water).
Surficial seabed classification
Classification methods
Surficial seabed classification is concerned primarily with distinguishing marine
benthic
The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning "t ...
habitat characteristics (e.g. hard, soft, rough, smooth,
mud
A MUD (; originally multi-user dungeon, with later variants multi-user dimension and multi-user domain) is a Multiplayer video game, multiplayer Time-keeping systems in games#Real-time, real-time virtual world, usually Text-based game, text-bas ...
,
sand
Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural class of s ...
,
clay
Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4).
Clays develop plasticity when wet, due to a molecular film of water surrounding the clay par ...
,
cobble
Cobble may refer to:
* Cobble (geology), a designation of particle size for sediment or clastic rock
* Cobblestone, partially rounded rocks used for road paving
* Hammerstone, a prehistoric stone tool
* Tyringham Cobble, a nature reserve in Tyr ...
) of the surveyed area.
Multibeam echosounders,
sidescan sonar
Side-scan sonar (also sometimes called side scan sonar, sidescan sonar, side imaging sonar, side-imaging sonar and bottom classification sonar) is a category of sonar system that is used to efficiently create an image of large areas of the sea ...
systems an
acoustic ground discrimination systems (AGDS)are the most commonly used technologies. The use of optical sensors has been restricted to depths less than 40 m due to absorption of
electromagnetic radiation
In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic field, electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, inf ...
by water. Despite this limitation, processing tools have been developed to classify data acquired using airborne bathymetric
LiDAR
Lidar (, also LIDAR, or LiDAR; sometimes LADAR) is a method for determining ranges (variable distance) by targeting an object or a surface with a laser and measuring the time for the reflected light to return to the receiver. It can also be ...
systems.
[ Radiometric Calibration of Airborne LIDAR Intensity Data for Land Cover Classification](_blank)
by Wai Yeung Yan & Ahmed Shaker Nevertheless, acoustics remain the preferred method of imaging the seafloor because data can be acquired over a much larger area (than in-situ sampling) from almost any depth.
Multibeam systems acquire both
bathymetry
Bathymetry (; ) is the study of underwater depth of ocean floors (''seabed topography''), lake floors, or river floors. In other words, bathymetry is the underwater equivalent to hypsometry or topography. The first recorded evidence of water de ...
(depth) and
backscatter (intensity) data. Multibeam backscatter was previously considered to be a by-product of a multibeam survey, with bathymetry being the primary information. Recent advances in multibeam backscatter acquisition, processing and analysis methods have increased the range of applications for which multibeam systems can be used and now allow the collection of spatially and temporally coincident multispectral multibeam backscatter.
[4. Brown, C.J.; Beaudoin, J.; Brissette, M., and Gazzola, V., 2019, Multispectral multibeam echo sounder backscatter as a tool for improved seafloor characterization. Geosciences, 9(3)] New methods of analyzing backscatter data, have increased its potential for seabed characterization. Backscatter data resolution has also increased significantly with the introduction of snippet data. Snippet data is raw backscatter time-series data for each beam footprint and each pin
(Lockhart et al., 2007) These advances have allowed some multibeam backscatter data to achieve a quality comparable to that of sidescan sonar imagery.
Different classification approaches and algorithms can yield different results. These approaches include image-based seabed classification methods such as
texture analysis,
artificial neural networks
Artificial neural networks (ANNs), usually simply called neural networks (NNs) or neural nets, are computing systems inspired by the biological neural networks that constitute animal brains.
An ANN is based on a collection of connected unit ...
(ANN); and other methods, such as angular response characterizatio
(Hughes-Clarke et al., 1997) Image processing
An image is a visual representation of something. It can be two-dimensional, three-dimensional, or somehow otherwise feed into the visual system to convey information. An image can be an artifact, such as a photograph or other two-dimensiona ...
methods traditionally used in satellite
remote sensing
Remote sensing is the acquisition of information about an object or phenomenon without making physical contact with the object, in contrast to in situ or on-site observation. The term is applied especially to acquiring information about Earth ...
are often adapted to quantitatively analyze multibeam backscatter intensity data. After
image segmentation and classification, acoustic imagery can be used to discriminate between areas with different
morphological properties. No classification method produces a map that is 100% accurate and some attempt must always be made to assess the
accuracy
Accuracy and precision are two measures of ''observational error''.
''Accuracy'' is how close a given set of measurements (observations or readings) are to their ''true value'', while ''precision'' is how close the measurements are to each other ...
of classification results (e.g.
confusion matrix).
Seabed characterization
Classification maps are subject to ground-verification in order to identify the compositions and bottom type that characterize each class. The functionality of
Geographic Information Systems
A geographic information system (GIS) is a type of database containing geographic data (that is, descriptions of phenomena for which location is relevant), combined with software tools for managing, analyzing, and visualizing those data. In a br ...
(GIS) can be used to integrate data from different sources, including
ground truth
Ground truth is information that is known to be real or true, provided by direct observation and measurement (i.e. empirical evidence) as opposed to information provided by inference.
Etymology
The ''Oxford English Dictionary'' (s.v. "ground t ...
data. Such data may come from in-situ sediment grab sampling, the use of a dredge, trawl net, visual imagery or surveys using
Remotely Operated Vehicles (ROVs). The seabed classification map can be combined with other information about the area, such as fish distribution and abundance or vegetation characteristics, to establish habitat groups based on associations. This process allows classification maps derived from multibeam data to help characterize the seabed and more effectively manage its use.
Sub-surface seabed classification
Sub-surface seabed classification is commonly referred to as sub-bottom profiling and is generally used for
geological assessment of the sub-surface characteristics. Sub-bottom profiling can return information from tens to hundreds of meters below the seafloor, and is often used to complement
reflection seismology
Reflection seismology (or seismic reflection) is a method of exploration geophysics that uses the principles of seismology to estimate the properties of the Earth's subsurface from reflected seismic waves. The method requires a controlled seismi ...
. From sub-surface classifications, scientists and engineers can characterize rock and sediment types, as well as pore fluids. This information is used for many applications, such as slope failure analysis and
hydrocarbon exploration
Hydrocarbon exploration (or oil and gas exploration) is the search by petroleum geologists and geophysicists for deposits of hydrocarbons, particularly petroleum and natural gas, in the Earth using petroleum geology.
Exploration methods
Vis ...
.
References
External links
What is habitat mapping? (www.searchmesh.net)Lawrence & Bates, 2001, Acoustic ground discrimination systems (AGDS)Hughes-Clarke et al., 1997, Ocean Mapping Group, UNBApplied Acoustics- Special Issue:The Application of Underwater Acoustics for Seabed Habitat MappingAcoustic seabed classification: current practice and future directionsSediment Classification Software (2009)Acoustic Seabed Classification Systems (2001)Bottom ClassificationAcoustic Seabed Classification – Applications in Fisheries Science and Ecosystem StudiesSeabed classification
Resources - seabed surface:
Acoustic Seabed Classification BibliographyMaritime Way ScientificMapping European Seabed Habitats (MESH) ProjectRoxAnn Seabed Classification SystemICES Study Group on Acoustic Seabed ClassificationCase Studies and References for Lakes, Rivers & MarineECHOplus seabed discriminationGEOHAB - Marine Geological and Biological Habitat Mapping - Conferences
{{hydroacoustics
Oceanography