Over the last few centuries, mapping the ocean seabed has been a major challenge for marine geoscientists. Knowledge of seabed bathymetry and morphology has significantly impacted our understanding of our planet dynamics. The history and scientific trends of seabed mapping can be assessed by data mining prior studies. Here, we have mined the scientific literature using the keyword “seabed mapping” to investigate and provide the evolution of mapping methods and emphasize the main trends and challenges over the last 90 years. An increase in related scientific production was observed in the beginning of the 1970s, together with an increased interest in new mapping technologies. The last two decades have revealed major shift in ocean mapping. Besides the range of applications for seabed mapping, terms like habitat mapping and concepts of seabed classification and backscatter began to appear. This follows the trend of investments in research, science, and technology but is mainly related to national and international demands regarding defining that country’s exclusive economic zone, the interest in marine mineral and renewable energy resources, the need for spatial planning, and the scientific challenge of understanding climate variability. The future of seabed mapping brings high expectations, considering that this is one of the main research and development themes for the United Nations Decade of the Oceans. We may expect a new higher resolution ocean seafloor map that might be as influential as The Floor of the Oceans map.
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.
ABSTRACT. This research aimed to map distinct types of echo-character in order to recognize estuarine sedimentary processes along the Piraquê-Açu System, in Aracruz, Espírito Santo State, Brazil. Echo-character mapping was undertaken using high frequency sub-bottom profiling data. Four echo-character (EC1, EC2, EC3,EC4) patterns were recognized by combining sub-bottom and surficial sedimentological data. These EC patterns defined three distinct areas within the system that arecharacterized by the predominance of a specific echo-character and a seabed type: i) estuarine mouth, where sand beds with low penetration prevail; ii) a transitionzone, where sandy-muddy andmuddy-sandy beds are associated with a high penetration echo-character; and, iii) the rivers/estuary Piraquê-Ac¸u and Piraquê-Mirim withsandy-muddy and muddy beds with high penetration echo pattern. The interpretation showed that the association of EC and surficial sediment may represent modernsedimentary processes within the estuary. Three sedimentary processes were recognized: a) marine dominated processes showing sand transport by tidal currents;b) estuarine processes with fine sediment input, resuspension and deposition; and c) erosive estuarine processes or scour lags, exposing possibly relict deposits.Keywords: estuarine sedimentation, shallow geophysics, sub-bottom profiler. RESUMO. Esse estudo busca reconhecer os tipos de ecocaráter existentes para o melhor entendimento dos processos sedimentares do sistema estuarino do Piraquê-Açu e Piraquê-Mirim, localizado no município de Aracruz, Espírito Santo. O reconhecimento e o mapeamento dos tipos de ecocaráter foram realizados a partir deregistros de perfilador de subfundo de alta frequência. Através da associação dos dados geofísicos com dados sedimentológicos foram reconhecidos e definidos quatropadrões de ecocarácteres (EC1, EC2, EC3, EC4) associados a tipos de fundo distintos. A distribuição espacial dos tipos de eco permitiu a divisão do estuário em três áreas distintas que diferem pela predominância de diferentes tipos de ecocaráter e sedimento: i) A foz do sistema estuarino, interpretada como região de domiâancia marinha, onde predomina o fundo arenoso e a baixa penetração do sinal; ii) A zona de confluência, onde predomina sedimentos areno-lamosos a lamo-arenosos compenetração do sinal acústico; iii) Os braços Piraquê-Açu e Piraquê-Mirim de predomínio de fundo areno-lamoso a lamoso, com penetração do sinal. Estas áreas estão associadas a distintos processos sedimentares, sendo eles: a) processos marinhos com transporte de sedimentos arenosos por correntes de maré (foz do sistema até a confluência dos braços); b) processos estuarinos com aporte, resuspensão e deposição de sedimentos finos, típicos da região interna do sistema; e c) processosestuarinos erosivos ou de ravinamento, ocorrendo localmente nos braços.Palavras-chave: sedimentação estuarina, geofísica rasa, perfilador de subfundo
Improvements to acoustic seafloor mapping systems have motivated novel marine geological and benthic biological research. Multibeam echosounders (MBES) have become a mainstream tool for acoustic remote sensing of the seabed. Recently, “multispectral” MBES backscatter, which is acquired at multiple operating frequencies, has been developed to characterize the seabed in greater detail, yet methods for the use of these data are still being explored. Here, we evaluate the potential for seabed discrimination using multispectral backscatter data within a multi-method framework. We present a novel MBES dataset acquired using four operating frequencies (170, 280, 400, and 700 kHz) near the Doce River mouth, situated on the eastern Brazilian continental shelf. Image-based and angular range analysis methods were applied to characterize the multifrequency response of the seabed. The large amount of information resulting from these methods complicates a manual seabed segmentation solution. The data were therefore summarized using a combination of dimensionality reduction and density-based clustering, enabling hierarchical spatial classification of the seabed with sparse ground-truth. This approach provided an effective solution to synthesizing these data spatially to identify two distinct acoustic seabed classes, with four subclasses within one of the broader classes, which corresponded closely with seafloor sediment samples collected at the site. The multispectral backscatter data also provided information in likely, unknown, sub-surface substrate differences at this site. The study demonstrates that the adoption of a multi-method framework combining image-based and angular range analysis methods with multispectral MBES data can offer significant advantages for seafloor characterization and mapping.
ABSTRACT. Understanding the behaviour of acoustic signals in marine sediments is of great importance for applied coastal studies. In areas with high suspended sediment concentration, the detection and delineation of these fluid mud layers are imperative for the determination of nautical depths (navigability) and dredging projects. Herein, we investigate the response of the acoustic signal according to the frequency and surface sediments characteristics (grain size and density). The dataset was collected along the Amazon inner shelf (North Brazil), including high-frequency acoustic data (3.5, 33 and 210 kHz), surface sediment physical properties, in situ bottom density and suspended particulate matter derived from optical backscatter sensors. Results show a cross-shelf transition from a sandy to a muddy bottom, followed by a decrease in sediment seabed density. Acoustic data also show this transition, illustrated by different levels of signal penetration. Statistical analysis relating the geophysical records and the physical properties of the sediments showed that density was a determining variable for the interpretation of surface acoustic reflection. The acoustic sources were able to detect the occurrence of fluid mud layers but the optical backscatter sensor had the best resolution in delineating the upper boundary of the fluid mud layer. The sub-bottom profiler (3.5 kHz) detected the fluid mud layers, whereas the echo displacement were not always related to the presence of fluid mud. Finally, the results pointed out that for coastal engineering projects and navigation purposes, the mapping of seabed density along with geophysical surveys is effective, and highlighted the issue about delineation of upper fluid mud boundary.Keywords: geoacoustic, fluid mud, Amazon River. RESUMO. Fundamental para estudos aplicados a regiões costeiras, a geoacústica submarina auxilia na compreensão do comportamento do sinal acústico no sedimento marinho. A detecção de depósitos lamosos em registros acústicos de alta resolução é de fácil distinção, aparecendo normalmente como pacotes sedimentares de baixa reflexão. Em áreas com alta concentração de material particulado em suspensão, a detecção e a delimitação dessas camadas de lama fluida são fundamentais para a determinação da profundidade náutica e para realização de projetos de dragagem. Esse estudo tem como principal objetivo investigar variações no sinal acústico de diferentes fontes, de acordo com a frequência e as características do sedimento superficial (granulometria e densidade). A base de dados analisada foi coletada na Plataforma Interna do Canal Norte do Rio Amazonas, e é composta por registros geofísicos de alta frequência (3,5, 33, 210 kHz), propriedades físicas do sedimento superficial, densimetria in situ e medidas de material particulado em suspensão. Os resultados sedimentológicos e geofísicos mostram uma transição de um fundo arenoso para lamoso na plataforma continental, acompanhada por uma diminuição na densidade do sedimento superficial do fundo marinho. A análise estatística apontou a densidade como variável determinante para interpretação da reflexão superficial do sinal acústico. As fontes acústicas foram capazes de detectar a ocorrência da lama de fluida, mas o sensor óptico obteve a melhor resolução para delinear o limite superior da camada de lama fluida. O perfilador de sub-fundo (3,5 kHz) registrou camadas transparentes que indicam a presença de estratos sedimentares pouco consolidados, enquanto o deslocamento do eco registrado pela ecobatimetria nem sempre foi relacionado unicamente com a presença de lama fluida. Finalmente, os resultados apontaram que para projetos de engenharia costeira e com fins de navegação, o mapeamento da densidade fundo do mar em conjunto com levantamentos geofísicos é eficaz, além de destacar essa questão da delineação do limite superior da lama fluida. Palavras-chave: geoacústica, lama fluida, Rio Amazonas.
ABSTRACT. Understanding the behaviour of acoustic signals in marine sediments is of great importance for applied coastal studies. In areas with high suspended sediment concentration... RESUMO. A geoacústica submarina auxilia no entendimento do comportamento do sinal acústico no sedimento marinho e é fundamental para estudos aplicados a regiões costeiras...
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