Insights into the Short-Term Tidal Variability of Multibeam Backscatter from Field Experiments on Different Seafloor Types

Abstract

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 (<0.5 dB variability at 45° incidence) and significant variability in the sandy and muddy areas with envelopes of variability >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.

Impact of Sparse Benthic Life on Seafloor Roughness and High-Frequency Acoustic Scatter

“…Hence, by comparing the observed backscatter variations with the seafloor interface-roughness parameter (RMS roughness, slope and intercept) no significant relationships were found (Table 1). In fact, the observed lander backscatter difference (1.1 dB) may be within a natural time-varying fluctuation in sand environments [45,46] and is less than 4 dB for small patches with increased abundance of benthic life in the investigation area based on a 200 kHz ship-based multibeam echo sounder survey reported by reference [19]. The lander data-model comparison showed no correlation.…”

Picking Up the Pieces—Harmonising and Collating Seabed Substrate Data for European Maritime Areas

“…At present, data are compiled regardless of their vintage; hence, datasets for different time periods are often combined to create a single map. In near-coastal areas with high sediment dynamics, seabed surface monitoring typically shows highly variable substrates through time, with seasonal, event-or activity-related and unexplained patterns [55]. The present phase of EMODnet has focused on the static mapping of available data, but future efforts should account for this temporal behaviour.…”

Assessing the Repeatability of Automated Seafloor Classification Algorithms, with Application in Marine Protected Area Monitoring

“…Yet, these novel automated methods have scarcely been applied to the assessment of temporal changes in deep-water habitats [14][15][16][17], with little evidence of any attempt to conducted a systematic study to assess the suitability and repeatability of different methods in temporal change detection. To validate the use of automated methods in the monitoring of MPAs, it is necessary not only to assess their accuracy, but also to establish that they are repeatable, such that users can be confident that any differences in the resultant benthic habitat maps are the result of true changes to seafloor habitats, and not simply artefacts of the method.…”

“…This was acknowledged by Rattray et al [14], who identified the unknown repeatability of the classification process as a difficult to control source of error in the classification step when researching benthic habitat change. It has been acknowledged that current acoustic technologies provide limited repeatability depending on the instrument, equipment settings, processing algorithms, operational ranges, environmental conditions, and survey methods [17,20]. Nevertheless, there are few if any studies that have attempted to compare the repeatability of different classification algorithms.…”