Real-time Monitoring of Uncertainty due to Refraction in Multibeam Echo Sounding

A software toolkit has been developed to objectively monitor uncertainty due to refraction in multibeam echosounding, specifically mapping systems that employ underway sound speed profiling hardware. The toolkit relies on the use of a raytrace simulator which mimics the sounding geometry of any given echosounder, specifically array type, angular sector, draft, and availability of a surface sound speed probe. The simulator works by objectively comparing a pair of consecutively collected sound speed profiles and reporting sounding uncertainty across the entire potential sounding space. Realtime visualizations of the uncertainty as a function of time and space allow the operator to tune the sound speed profile collection regime to maintain a desired sounding uncertainty while at the same time minimizing the number of casts collected

Modeling the Effect of Oceanic Internal Waves on the Accuracy of Multibeam Echosounders

When ray bending corrections are applied to multibeam echosounder (MBES) data, it is assumed that the varying layers of sound speed lie along horizontally stratified planes. In many areas internal waves occur at the interface where the water’s density changes abruptly (a pycnocline), this density gradient is often associated with a strong gradient in sound speed (a velocline). The internal wave introduces uncertainty into the echo soundings through two mechanisms: (1) tilting of the velocline, and (2) vertical oscillation of the velocline’s depth. A model has been constructed in order to examine how these effects degrade the accuracy of MBES measurements. The model numerically simulates the 3D ray paths of MBES soundings for a synthetic flat seafloor, as though the soundings have been collected through a user-defined internal wave. Along with sound speed information, the ray paths are used to estimate travel times which are then utilized as inputs for a conventional 2D ray trace. The discrepancy between the 3D and 2D ray traced solutions serve as an estimate of uncertainty. The same software can be extended to model the expected anomalies associated with tidal fronts and other phenomena that result in significant tilting or oscillation of the velocline. A case study was undertaken using observed internal wave parameters on the Scotian Shelf. The case study examines how survey design parameters such as line spacing, direction of survey lines, and water column sampling density can influence the uncertainty introduced by internal waves. In particular, an examination is undertaken in which 2D ray tracing models are augmented with MBES water column imaging of the velocline. The investigation shows that internal waves have the potential to cause vertical uncertainties exceeding IHO standards and that the uncertainty can potentially be mitigated through appropriate survey design. Results from the case study also indicate that acoustic tracking of the velocline has the potential to counteract the effects of internal waves through augmentation of 2D ray tracing models. This technique is promising, however, much more research and field testing is required to ascertain the practicality, reliability and repeatability of such an approach

Modelling Uncertainty Caused by Internal Waves on the Accuracy of MBES

A 3D ray tracing model has been developed to estimate the effects of internal waves upon the accuracy of multibeam echosounders (MBES). A case study examines the variability in these effects as a function of survey line direction and also considers the case of improving 2D ray tracing models with wave parameters derived from MBES water column imagery. Results indicate that, under certain circumstances, the effects of internal waves can prove to be a significant source of uncertainty that detracts from the ability to efficiently map the seafloor with wide swath angles

Water-Column Variability Assessment for Underway Profilers to Improve Efficiency and Accuracy of Multibeam Surveys

With the advent of underway profilers, sampling the water-column to obtain sound speed corrections is no longer a detriment to hydrographic survey efficiency. Instead, the challenge has become deciding how many casts are necessary to maintain a desired level of multibeam sounding accuracy, while not needlessly overworking the profiler. Ray tracing uncertainty analysis can determine in hindsight whether a particular sampling interval is adequate or not. Based on this methodology, an algorithm was developed to generate recommended sampling intervals based on successively acquired sound speed profiles, allowing the MVP to run in a “cruise-control” mode where the sampling interval is altered in response to changing oceanographic conditions. In collaboration with Rolls Royce, the algorithm was implemented in Python and loosely couples with the MVP controller software such that the recommended sampling interval can be adjusted without operator intervention. Integration of the software with the MVP controller was successfully tested aboard the NOAA Ship Ferdinand R. Hassler in September of 2012. Initial results from field trials and from analysis of existing data sets are presented

Split-beam echosounder observations of natural methane seep variability in the northern Gulf of Mexico

A method for positioning and characterizing plumes of bubbles from marine gas seeps using an 18 kHz scientific split-beam echo sounder (SBES) was developed and applied to acoustic observations of plumes of presumed methane gas bubbles originating at approximately 1400 m depth in the northern Gulf of Mexico. A total of 161 plume observations from 27 repeat surveys were grouped by proximity into 35 clusters of gas vent positions on the seafloor. Profiles of acoustic target strength per vertical meter of plume height were calculated with compensation for both the SBES beam pattern and the geometry of plume ensonification. These profiles were used as indicators of the relative fluxes and fates of gas bubbles acoustically observable at 18 kHz and showed significant variability between repeat observations at time intervals of 1 h–7.5 months. Active gas venting was observed during approximately one third of the survey passes at each cluster. While gas flux is not estimated directly in this study owing to lack of bubble size distribution data, repeat surveys at active seep sites showed variations in acoustic response that suggest relative changes in gas flux of up to 1 order of magnitude over time scales of hours. The minimum depths of acoustic plume observations at 18 kHz averaged 875 m and frequently coincided with increased amplitudes of acoustic returns in layers of biological scatterers, suggesting acoustic masking of the gas bubble plumes in these layers. Minimum plume depth estimates were limited by the SBES field of view in only five instances

Rethinking the Patch Test for Phase Measuring Bathymetric Sonars

While conducting hydrographic survey operations in the Florida Keys, NOAA Ship Thomas Jefferson served as a test platform for the initial operational implementation of an L-3 Klein HydroChart 5000 Swath Bathymetry Sonar System1 , a hull-mounted phase measuring bathymetric sonar (PMBS). During the project it became apparent that the traditional patch test typically utilized for multibeam echosounder (MBES) systems was poorly suited to the HydroChart – and perhaps other PMBS systems as well. These systems have several inherent characteristics that make it difficult to isolate and subsequently solve for biases under the traditional patch test paradigm: presence of a nadir gap, wide swaths (typically greater than 6 times water depth), and relatively poor object-detection capability in the outer swath. After “rethinking” the patch test to account for these characteristics, the authors propose a new patch test paradigm that is better suited to the HydroChart and other PMBS systems

Estimation of Sounding Uncertainty from Measurements of Water Mass Variability

Analysis techniques are introduced that allow for estimation of potential sounding uncertainty due to water mass variability from reconnaissance campaigns in which oceanographic parameters are measured at a high temporal and spatial resolution. The analysis techniques do not require sounding data, thus analyses can be tailored to match any survey system; this allows for pre-analysis campaigns to optimize survey instrumentation and sound speed profiling rates such that a desired survey specification can be maintained. Additionally, the output of the analysis methods can potentially provide a higher fidelity estimation of sounding uncertainty due to water mass variability than uncertainty models in common use