Seafloor change detection using multibeam echosounder backscatter: case study on the Belgian part of the North Sea

To characterize seafloor substrate type, seabed mapping and particularly multibeam echosounding are increasingly used. Yet, the utilisation of repetitive MBES-borne backscatter surveys to monitor the environmental status of the seafloor remains limited. Often methodological frameworks are missing, and should comprise of a suite of change detection procedures, similarly to those developed in the terrestrial sciences. In this study, pre-, ensemble and post-classification approaches were tested on an eight km2 study site within a Habitat Directive Area in the Belgian part of the North Sea. In this area, gravel beds with epifaunal assemblages were observed. Flourishing of the fauna is constrained by overtopping with sand or increased turbidity levels, which could result from anthropogenic activities. Monitoring of the gravel to sand ratio was hence put forward as an indicator of good environmental status. Seven acoustic surveys were undertaken from 2004 to 2015. The methods allowed quantifying temporal trends and patterns of change of the main substrate classes identified in the study area; namely fine to medium homogenous sand, medium sand with bioclastic detritus and medium to coarse sand with gravel. Results indicated that by considering the entire study area and the entire time series, the gravel to sand ratio fluctuated, but was overall stable. Nonetheless, when only the biodiversity hotspots were considered, net losses and a gradual trend, indicative of potential smothering, was captured by ensemble and post-classification approaches respectively. Additionally, a two-dimensional morphological analysis, based on the bathymetric data, suggested a loss of profile complexity from 2004 to 2015. Causal relationships with natural and anthropogenic stressors are yet to be established. The methodologies presented and discussed are repeatable and can be applied to broad-scale geographical extents given that broad-scale time series datasets become available

Insights into the short-term tidal variability of multibeam backscatter from field experiments on different seafloor types

Three experiments were conducted in the Belgian part of the North Sea to investigate short-term variation in seafloor backscatter strength (BS) obtained with multibeam echosounders (MBES). Measurements were acquired on predominantly gravelly (offshore) and sandy and muddy (nearshore) areas. Kongsberg EM3002 and EM2040 dual MBES were used to carry out repeated 300-kHz backscatter measurements over tidal cycles (~13 h). Measurements were analysed in complement to an array of ground-truth variables on sediment and current nature and dynamics. Seafloor and water-column sampling was used, as well as benthic landers equipped with different oceanographic sensors. Both angular response (AR) and mosaicked BS were derived. Results point at the high stability of the seafloor BS in the gravelly area (2 dB and 4 dB at 45° respectively. The high-frequency backscatter sensitivity and short-term variability are interpreted and discussed in the light of the available ground-truth data for the three experiments. The envelopes of variability differed considerably between areas and were driven either by external sources (not related to the seafloor sediment), or by intrinsic seafloor properties (typically for dynamic nearshore areas) or by a combination of both. More specifically, within the gravelly areas with a clear water mass, seafloor BS measurements where unambiguous and related directly to the water-sediment interface. Within the sandy nearshore area, the BS was shown to be strongly affected by roughness polarization processes, particularly due to along- and cross-shore current dynamics, which were responsible for the geometric reorganization of the morpho-sedimentary features. In the muddy nearshore area, the BS fluctuation was jointly driven by high-concentrated mud suspension dynamics, together with surficial substrate changes, as well as by water turbidity, increasing the transmission losses. Altogether, this shows that end-users and surveyors need to consider the complexity of the environment since its dynamics may have severe repercussions on the interpretation of BS maps and change-detection applications. Furthermore, the experimental observations revealed the sensitivity of high-frequency BS values to an array of specific configurations of the natural water-sediment interface which are of interest for monitoring applications elsewhere. This encourages the routine acquisition of different and concurrent environmental data together with MBES survey data. In view of promising advances in MBES absolute calibration allowing more straightforward data comparison, further investigations of the drivers of BS variability and sensitivity are required

High resolution multibeam and hydrodynamic datasets of tidal channels and inlets of the Venice Lagoon

Tidal channels are crucial for the functioning of wetlands, though their morphological properties, which are relevant for seafloor habitats and flow, have been understudied so far. Here, we release a dataset composed of Digital Terrain Models (DTMs) extracted from a total of 2,500 linear kilometres of high-resolution multibeam echosounder (MBES) data collected in 2013 covering the entire network of tidal channels and inlets of the Venice Lagoon, Italy. The dataset comprises also the backscatter (BS) data, which reflect the acoustic properties of the seafloor, and the tidal current fields simulated by means of a high-resolution three-dimensional unstructured hydrodynamic model. The DTMs and the current fields help define how morphological and benthic properties of tidal channels are affected by the action of currents. These data are of potential broad interest not only to geomorphologists, oceanographers and ecologists studying the morphology, hydrodynamics, sediment transport and benthic habitats of tidal environments, but also to coastal engineers and stakeholders for cost-effective monitoring and sustainable management of this peculiar shallow coastal system

Data Descriptor: high resolution multibeam and hydrodynamic datasets of tidal channels and inlets of the Venice Lagoon

Tidal channels are crucial for the functioning of wetlands, though their morphological properties, which are relevant for seafloor habitats and flow, have been understudied so far. Here, we release a dataset composed of Digital Terrain Models (DTMs) extracted from a total of 2,500 linear kilometres of high-resolution multibeam echosounder (MBES) data collected in 2013 covering the entire network of tidal channels and inlets of the Venice Lagoon, Italy. The dataset comprises also the backscatter (BS) data, which reflect the acoustic properties of the seafloor, and the tidal current fields simulated by means of a high-resolution three-dimensional unstructured hydrodynamic model. The DTMs and the current fields help define how morphological and benthic properties of tidal channels are affected by the action of currents. These data are of potential broad interest not only to geomorphologists, oceanographers and ecologists studying the morphology, hydrodynamics, sediment transport and benthic habitats of tidal environments, but also to coastal engineers and stakeholders for cost-effective monitoring and sustainable management of this peculiar shallow coastal system

Classification of multibeam sonar image using the Weyl transform

In this paper we develop a novel classification method for multibeam sonar images based on the Weyl transform. The texture descriptor based on Weyl coefficients describes effectively the multiscale correlation features appearing in the sonar images. Our classification approach combines the Weyl coefficients with statistical features that are commonly used in the analysis of seabed sonar images and captures the morphological variation and geoacoustic characteristics of the seafloor. We employ a neural network as a classifier. The proposed combined feature extraction method demonstrates better performance than the commonly used statistical methods in this application

INDI 67. Developments of Indicators to improve monitoring of MSFD descriptors 6 and 7 (INDI67) : final report

Context To protect the marine environment more effectively, the European Union adopted the Marine Strategy Framework Directive (MSFD) in 2008, aiming to achieve the Good Environmental Status (GES) of the EU's marine waters by 2020 and to protect the resource base upon which marine-related economic and social activities depend. A major challenge in the implementation of the MSFD is to achieve the necessary scientific knowledge on the marine environment, its processes and the methods to monitor them. The focus of INDI67 is on the evaluation and development of indicators to monitor GES of descriptor 6 and 7. Seafloor integrity (descriptor 6) refers to the structure and functions of the benthic ecosystems. It relates to the comprises physical, chemical and biological properties as well as to spatial and temporal connectedness, avoiding artificial fragmentation of habitats or temporal sealing due to ephemeral sediment deposits or armouring. Hydrographic conditions (descriptor 7) imply that the nature and scale of any long-term changes to the prevailing hydrographical conditions resulting from anthropogenic activities (individual and cumulative), do not lead to significant negative impacts on the benthic and pelagic habitats, functioning or on hydro-geomorphological impacts on the seabed. Objectives The overall objective of INDI67 is to develop and evaluate tools and methods to support the monitoring of MSFD descriptors 6 and 7. The subject is the monitoring of seafloor integrity and hydrography using both modelling and measurements of hydro- and sediment dynamic processes and seabed characteristics. Three parameters have been selected as key indicator, i.e. turbidity, bottom shear stress and seabed/habitat type. These parameters are all related to sea floor dynamics and are strongly linked as changes in seafloor integrity and turbidity occur as a result of the combined force that waves and currents exert on the sea floor. Furthermore, they are witnesses of changes induced by human activities (dredging/disposal, aggregate extraction, constructions, fishery). Turbidity (used both in terms of suspended particulate matter(SPM) concentration and light availability) and bottom shear stress are currently measured and modelled. Bottom shear stress and seabed/habitat type are included in the Belgian MSFD monitoring programme, while turbidity is not yet included as GES indicator. Conclusions The major conclusion with respect to the measuring and modelling of the key indicators turbidity/SPMC, bottom shear stress and seabed/habitat type are: 1) Despite calibration to a reference solution and the use of ISO-normed optical turbidity sensors, model calibration may vary considerably in recorded turbidity for a same SPMC solution across different instruments resulting in instrument-specific turbidity-SPMC relation. Turbidity (or dB for acoustic sensors) should therefore not be used as it is not standardized and will diminish the comparability of the data. Instead, the optical and acoustic sensor output should be transformed into a mass concentration, a unit that is comparable in time and between regions. Monitoring in situ SPMC should follow common guidance and protocols to restrict their measurement uncertainties. The main challenge is now to evaluate model results uncertainty and improve the formulation of natural processes, such as flocculation and bottom shear stress, together with the effects of pressures in the models