Measurement accuracy of bathymetric sonars as a function of signal/noise ratio

Multibeam echosounders and bathymetric side-scan sonars are nowadays widely used for seafloor topographic mapping. Depending on the measurement angle sector and on the sonar array structure, various methods are usable to estimate time-angle pairs needed to estimate sounding point positions. Distinctions are to be made between methods based on signal amplitude or phase, on one hand, and measurements at given time or angle, on the other hand; this leads to define four main methods, covering most of bathymetry systems available today (maximum amplitude instant, phase difference direction, zero-phase difference instant, maximum amplitude direction). The principles of these four approaches are described, and their respective measurement accuracy is evaluated as a function of the signal-to-noise ratio value. The formulae developed here account for the array geometry and beamforming weighting. The performances of two current methods for oblique incidences are compared and commented. The particular case of measurements close to the vertical is discussed. The performances of elementary measurements of phase, time and angle for fluctuating signals are presented in an appendix.Les sondeurs multifaisceaux et les sonars latéraux à fonction bathymétrique sont largement utilisés pour les relevés topographiques des fonds marins. En fonction du secteur angulaire mesuré, et de la structure des antennes du système sonar, différentes méthodes peuvent être employées pour l'estimation des couples temps-angle nécessaires à l'estimation de position des points de sonde. Les distinctions entre méthodes basées sur l'amplitude ou sur la phase des signaux d'une part, et entre mesure à instant donné ou à angle fixé d'autre part, permettent de définir quatre grandes approches, qui couvrent l'essentiel des systèmes bathymétriques disponibles aujourd'hui. Ces quatre méthodes classiques sont décrites dans leurs principes, et leurs précisions de mesure respectives sont évaluées en fonction du rapport signal sur bruit. Les formules données tiennent compte de la géométrie des antennes et de leur pondération. Les performances de deux méthodes utilisées en incidence oblique sont comparées et commentées. Le cas particulier des mesures aux incidences proches de la verticale est discuté. Les performances des mesures élémentaires de phase, de temps et d'angle pour des signaux fluctuants sont présentées en Annexe

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

Modelling of Transonic Shallow Cavity Flows, and Store Release Simulations from Weapon Bays

This paper aims to obtain more insight in the physics of cavit y flows. CFD results are analysed using a superposition of refle cted acoustic waves driven by the cavity flow showing clearly the r elative amplitude, and the time modulation of cavity tones. The tona l, and a part of the broadband noise, is found to be driven by the turb u- lent length scales, disabling the phase superposition of th e smaller wavelength. This can be seen as a way to complement the estab- lished Rossiter formula. In addition Scale-Adaptive Simul ations of store release from weapon bays using overset grids are prese nted. A six-degree-of-freedom model is coupled with the HMB3 flow so lver, and store release simulations are performed for a finned stor e inside an idealised bay. It is found that the trajectories of stores released from the cavities are affected by the mean flow field, the standin g waves, and the dynamics of the shear layer formed along the ca vity opening

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

Sonar detection and monitoring of sunken heavy fuel oil on the seafloor

Abstract The oil products transported at sea that have the potential to become suspended in the water column and sink after weathering or mixing with sediment are quite numerous: asphalt, carbon black oil, bunker C, fuel oil n°5 and 6. In many incidents and accidents part of the spilled heavy oil product has actually sunk and has been difficult to track because of the lack of means of detection. To compensate this gap, a comprehensive sonar experiment has been performed through the use of a large seawater tank on the bottom of which several patches of three different heavy fuel oils have been laid on top of a sandy layer. In this facility several kinds of sonar have been tested as for their response according to their frequency, resolution and type (side scan sonar, multibeam/panoramic sonar, 3D acoustic camera). The results proved to be very valuable according to the acoustic specificity of the products involved. Indeed the acoustic properties are similar to those of the seawater when considering density and sound speed; however the attenuation is much more important and its variation has been measured from 100 kHz to 500 kHz preliminary to the testing. The results of the sonar experiment that have been obtained do confirm the capability of current sonars to detect heavy oil patches over sand seafloors, and should make it possible to select the most adequate survey strategy according to the oil patches dimensions and thickness and to the environmental conditions

Scientific rationale for Uranus and Neptune <i>in situ</i> explorations

The ice giants Uranus and Neptune are the least understood class of planets in our solar system but the most frequently observed type of exoplanets. Presumed to have a small rocky core, a deep interior comprising ∼70% heavy elements surrounded by a more dilute outer envelope of H2 and He, Uranus and Neptune are fundamentally different from the better-explored gas giants Jupiter and Saturn. Because of the lack of dedicated exploration missions, our knowledge of the composition and atmospheric processes of these distant worlds is primarily derived from remote sensing from Earth-based observatories and space telescopes. As a result, Uranus's and Neptune's physical and atmospheric properties remain poorly constrained and their roles in the evolution of the Solar System not well understood. Exploration of an ice giant system is therefore a high-priority science objective as these systems (including the magnetosphere, satellites, rings, atmosphere, and interior) challenge our understanding of planetary formation and evolution. Here we describe the main scientific goals to be addressed by a future in situ exploration of an ice giant. An atmospheric entry probe targeting the 10-bar level, about 5 scale heights beneath the tropopause, would yield insight into two broad themes: i) the formation history of the ice giants and, in a broader extent, that of the Solar System, and ii) the processes at play in planetary atmospheres. The probe would descend under parachute to measure composition, structure, and dynamics, with data returned to Earth using a Carrier Relay Spacecraft as a relay station. In addition, possible mission concepts and partnerships are presented, and a strawman ice-giant probe payload is described. An ice-giant atmospheric probe could represent a significant ESA contribution to a future NASA ice-giant flagship mission