Acoustic Seabed Classification
Encyclopedia
Acoustic seabed classification is the partitioning of a seabed
acoustic image
into discrete physical entities or classes. This is a particularly active area of development in the field of seabed mapping, marine geophysics
, underwater acoustics
and benthic habitat mapping. Seabed classification
is one route to characterizing the seabed and its habitats. 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
systems including multibeam echosounders
, sidescan sonar, single-beam echosounders
, interferometric 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
sound
to provide higher penetration, whereas surficial imaging technologies provide higher resolution imagery by utilizing higher frequencies (especially in shallow water).
Surficial seabed classification is concerned primarily with distinguishing marine benthic habitat characteristics (e.g. hard, soft, rough, smooth, mud
, sand
, clay
, cobble
) of the surveyed area. Multibeam echosounders
, sidescan sonar systems and acoustic ground discrimination systems (AGDS) are the most commonly used technologies. Multibeam systems acquire both bathymetry
(depth) and backscatter
(intensity) data, with full bottom coverage possible. 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 processing and analysis methods have increased the range of applications for which multibeam systems can be used. 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 ping (Lockhart et. al., 2007). This has allowed multibeam backscatter data to be of a quality comparable to that of sidescan sonar imagery.
Different approaches to seabed classification can give different results depending on the algorithms used. These include image-based seabed classification methods such as texture
analysis, artificial neural networks (ANN); and others such as angular response characterization (Hughes-Clarke et al., 1997). One well established approach is to adapt image processing
methods traditionally used in satellite remote sensing
to quantitatively analyze the multibeam backscatter intensity data. After image segmentation and classification, acoustic imagery can be used to discriminate between areas with different morphological
properties. A fundamental point to note when using any classification method is that, no classification map is 100% accurate and some attempt must always be made to assess the accuracy (e.g. confusion matrix
).
The classification map needs to be subject to ground-verification in order to identify the class compositions and bottom type. The functionality of Geographic Information Systems (GIS) can be used to integrate data from different sources, for example, integrating with groundtruthing
data. This includes 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 acoustically
(or other method) derived information about the area, such as fish distribution and abundance or vegetation characteristics, to establish habitat groups based on the associations. This allows classification maps derived from multibeam data to help characterize the seabed and more effectively manage its use.
The use of optical sensors has been restricted to depths less than 40 m due to absorption of electromagnetic radiation
by water. Most recently, processing tools have been developed to classify data acquired using airborne bathymetric LiDAR
systems, (QTC, 2008). Nevertheless, acoustics remain the preferred method of imaging the seafloor as data can be acquired over a much larger area (than in-situ sampling) from almost any depth.
Sub-surface seabed classification is commonly referred to as sub-bottom profiling and is generally used for geological
assessment of the sub-surface characteristics, down to hundreds of meters, as necessary in the exploration for oil deposits.
Resources - seabed surface:
Seabed
The seabed is the bottom of the ocean.- Ocean structure :Most of the oceans have a common structure, created by common physical phenomena, mainly from tectonic movement, and sediment from various sources...
acoustic image
Image
An image is an artifact, for example a two-dimensional picture, that has a similar appearance to some subject—usually a physical object or a person.-Characteristics:...
into discrete physical entities or classes. This is a particularly active area of development in the field of seabed mapping, marine geophysics
Geophysics
Geophysics is the physics of the Earth and its environment in space; also the study of the Earth using quantitative physical methods. The term geophysics sometimes refers only to the geological applications: Earth's shape; its gravitational and magnetic fields; its internal structure and...
, underwater acoustics
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 and its boundaries. The water may be in the ocean, a lake or a tank. Typical frequencies associated with underwater acoustics are between 10 Hz and...
and benthic habitat mapping. Seabed classification
Classification in machine learning
In machine learning and pattern recognition, classification refers to an algorithmic procedure for assigning a given piece of input data into one of a given number of categories...
is one route to characterizing the seabed and its habitats. 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
Sonar
Sonar is a technique that uses sound propagation to navigate, communicate with or detect other vessels...
systems including multibeam echosounders
Multibeam echosounders
-History and Progression:Multibeam echosounders, also known as Swathe or Swath echosounders, originated in the late 1950s, originally for military applications...
, sidescan sonar, single-beam echosounders
Scientific echosounder
A scientific echosounder is a device which uses SONAR technology for the measurement of underwater physical and biological components—this device is also known as scientific SONAR...
, interferometric systems and sub-bottom profilers
Sonar
Sonar is a technique that uses sound propagation to navigate, communicate with or detect other vessels...
. 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
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
sound
Sound
Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...
to provide higher penetration, whereas surficial imaging technologies provide higher resolution imagery by utilizing higher frequencies (especially in shallow water).
Surficial seabed classification is concerned primarily with distinguishing marine benthic habitat characteristics (e.g. hard, soft, rough, smooth, mud
Mud
Mud is a mixture of water and some combination of soil, silt, and clay. Ancient mud deposits harden over geological time to form sedimentary rock such as shale or mudstone . When geological deposits of mud are formed in estuaries the resultant layers are termed bay muds...
, sand
Sand
Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...
, clay
Clay
Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...
, cobble
Cobble (geology)
A cobble is a clast of rock with a particle size of to based on the Krumbein phi scale of sedimentology. Cobbles are generally considered to be larger than pebbles and smaller than boulders . A rock made predominantly of cobbles is termed a conglomerate....
) of the surveyed area. Multibeam echosounders
Multibeam echosounders
-History and Progression:Multibeam echosounders, also known as Swathe or Swath echosounders, originated in the late 1950s, originally for military applications...
, sidescan sonar systems and acoustic ground discrimination systems (AGDS) are the most commonly used technologies. Multibeam systems acquire both bathymetry
Bathymetry
Bathymetry is the study of underwater depth of lake or ocean floors. In other words, bathymetry is the underwater equivalent to hypsometry. The name comes from Greek βαθύς , "deep", and μέτρον , "measure"...
(depth) and backscatter
Backscatter
In physics, backscatter is the reflection of waves, particles, or signals back to the direction they came from. It is a diffuse reflection due to scattering, as opposed to specular reflection like a mirror...
(intensity) data, with full bottom coverage possible. 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 processing and analysis methods have increased the range of applications for which multibeam systems can be used. 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 ping (Lockhart et. al., 2007). This has allowed multibeam backscatter data to be of a quality comparable to that of sidescan sonar imagery.
Different approaches to seabed classification can give different results depending on the algorithms used. These include image-based seabed classification methods such as texture
Texture
Texture may refer to:* Textures , album of Brian Eno* Textures , a metal band from the Netherlands* Texture , theoretical topological defect in the structure of spacetime...
analysis, artificial neural networks (ANN); and others such as angular response characterization (Hughes-Clarke et al., 1997). One well established approach is to adapt image processing
Image processing
In electrical engineering and computer science, image processing is any form of signal processing for which the input is an image, such as a photograph or video frame; the output of image processing may be either an image or, a set of characteristics or parameters related to the image...
methods traditionally used in satellite remote sensing
Remote sensing
Remote sensing is the acquisition of information about an object or phenomenon, without making physical contact with the object. In modern usage, the term generally refers to the use of aerial sensor technologies to detect and classify objects on Earth by means of propagated signals Remote sensing...
to quantitatively analyze the multibeam backscatter intensity data. After image segmentation and classification, acoustic imagery can be used to discriminate between areas with different morphological
Geomorphology
Geomorphology is the scientific study of landforms and the processes that shape them...
properties. A fundamental point to note when using any classification method is that, no classification map is 100% accurate and some attempt must always be made to assess the accuracy (e.g. confusion matrix
Confusion matrix
In the field of artificial intelligence, a confusion matrix is a specific table layout that allows visualization of the performance of an algorithm, typically a supervised learning one . Each column of the matrix represents the instances in a predicted class, while each row represents the...
).
The classification map needs to be subject to ground-verification in order to identify the class compositions and bottom type. The functionality of Geographic Information Systems (GIS) can be used to integrate data from different sources, for example, integrating with groundtruthing
Ground truth
Ground truth is a term used in cartography, meteorology, analysis of aerial photographs, satellite imagery and a range of other remote sensing techniques in which data are gathered at a distance. Ground truth refers to information that is collected "on location." In remote sensing, this is...
data. This includes 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 acoustically
Acoustics
Acoustics is the interdisciplinary science that deals with the study of all mechanical waves in gases, liquids, and solids including vibration, sound, ultrasound and infrasound. A scientist who works in the field of acoustics is an acoustician while someone working in the field of acoustics...
(or other method) derived information about the area, such as fish distribution and abundance or vegetation characteristics, to establish habitat groups based on the associations. This allows classification maps derived from multibeam data to help characterize the seabed and more effectively manage its use.
The use of optical sensors has been restricted to depths less than 40 m due to absorption of electromagnetic radiation
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
by water. Most recently, processing tools have been developed to classify data acquired using airborne bathymetric LiDAR
LIDAR
LIDAR is an optical remote sensing technology that can measure the distance to, or other properties of a target by illuminating the target with light, often using pulses from a laser...
systems, (QTC, 2008). Nevertheless, acoustics remain the preferred method of imaging the seafloor as data can be acquired over a much larger area (than in-situ sampling) from almost any depth.
Sub-surface seabed classification is commonly referred to as sub-bottom profiling and is generally used for geological
Geology
Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...
assessment of the sub-surface characteristics, down to hundreds of meters, as necessary in the exploration for oil deposits.
External links
- What is habitat mapping? (www.searchmesh.net)
- Lawrence & Bates, 2001, Acoustic ground discrimination systems (AGDS)
- Lockhart et. al., 2007
- Hughes-Clarke et.al., 1997, Ocean Mapping Group, UNB
- QTC, 2008
- Applied Acoustics- Special Issue:The Application of Underwater Acoustics for Seabed Habitat Mapping
- Acoustic seabed classification: current practice and future directions
- Sediment Classification Software (2009)
- Acoustic Seabed Classification Systems (2001)
- Bottom Classification
- Acoustic Seabed Classification – Applications in Fisheries Science and Ecosystem Studies
- Seabed classification
Resources - seabed surface:
- Acoustic Seabed Classification Bibliography
- Quester Tangent (QTC) Marine
- Mapping European Seabed Habitats (MESH) Project
- RoxAnn Seabed Classification System
- ICES Study Group on Acoustic Seabed Classification
- Case Studies and References for Lakes, Rivers & Marine
- BioSonics VBT Seabed Classification Software
- Quester Tangent QTC IMPACT
- ECHOplus seabed discrimination
- GEOHAB - Marine Geological and Biological Habitat Mapping - Conferences