Abstract
There is a great demand to develop new acoustic techniques to efficiently map the seabed and automate the interpretation of acoustic, sedimentological, and imaging data sets, eliminating subjectivity. Here, we evaluate the potential, limitations and complementariety of distinct supervised and automatic classification techniques in the mapping of reefs by comparing these results with a reference map. The study was carried out in the Abrolhos Continental Shelf (Eastern Brazilian Continental Margin) using a multibeam echosounder and side scan sonar (SSS) dataset. Two automatic supervised techniques were applied. A reference map was derived by detailed manual interpretation carried out by three experts. The two supervised classification techniques were: benthic terrain modeler (BTM), a morphometric classification with focus on spatial analyses of the bathymetric grid derivatives, and object-based image analysis (OBIA), a segmentation applied to the backscatter data from the SSS mosaic. Both automatic techniques obtained similar values of reef coverage area, but overestimated the reef area when compared with the reference map. The agreement between BTM and OBIA results and the reference map was 69% and 67%, respectively. Disagreement was mainly due to quantity of reef (both methods over-estimated reef), while the disagreement in spatial allocation was relatively low, it indicates that both methods are reasonable representation of the spatial patterns of reef. Efficient mapping of reef in the wider area of the Abrolhos Continental Shelf will be best achieved by a further development of automatic methods tested against reference maps obained from representative areas of the seabed. By combining the results of the two automatic methods, it was possible to create an ensemble map, which achieved better agreement with the reference dataset.
Highlights
Terrestrial remote sensing has a comparatively long history since the 1970s and benefits from the availability of land surface imagery at various spatial and spectral resolutions, supplemented by ancillary data such as environmental data and digital terrain models
We have presented the results of two automated methods (BTM and object-based image analysis (OBIA)) for mapping reefs in a test site situated in the Abrolhos outer reef arc
The accuracy of these methods in detecting reef against a background of non-reef was estimated by comparing them with a reference map that was obtained by detailed manual interpretation of side scan sonar (SSS) backscatter and multibeam echosounders (MBES) bathymetry data
Summary
Terrestrial remote sensing has a comparatively long history since the 1970s and benefits from the availability of land surface imagery at various spatial and spectral resolutions, supplemented by ancillary data such as environmental data and digital terrain models. This enabled the development of sophisticated classification methods and algorithms that extract critical information for environmental management from remote sensing imagery [1,2]. The increasing interest and necessity for seabed habitat mapping in marine spatial planning initiatives has forced the industry of sonar makers, software developers and scientific research institutions to invest in new technology capable of producing seabed classification over regional areas [6]. Oil and gas exploration, offshore engineering, as well as shallow and deep sea mining are examples of industries that are requiring seabed classification to support environmental risk assessments and spatial planning [6,7,8]
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