Graduation of Nick van de Voort

Inversion Methods of the Sound Velocity Profile for Improving the Accuracy of the USBL Positioning System

• Professor of graduation: Prof. dr. J.D. Pietrzak

• Supervisors:  Prof. dr. ir. M. Snellen (TU Delft), Ir. L.M. Keyzer (TU Delft), Dr. ir. G.J. de Boer (Van Oord), Ir. B.J. van den Berg (Van Oord)

   

Remotely Operated Vehicles (ROVs) are positioned relative to an underwater acoustic positioning system: Ultra-Short BaseLine (USBL). The accuracy is affected by refraction artifacts, caused by the variations of the sound velocity in the water column. Accurate positioning would require continuous measuring of the sound velocity profile (SVP) which is unpractical and preferably to be minimized. It requires alternative or inversion methods to obtain SVP information. In this thesis, two inversion methods are introduced: HISOM (Hull In Situ Ocean Model) and OMBES (Overlapping MultiBeam EchoSounder). 

HISOM analysis whether the SVP can be approximated via other sources: 1) a constant profile based on the in situ surface (ISS) sound velocity, or 2) a profile derived from ocean model data in combination with the ISS sound velocity. These simplifications inevitably introduce a refraction error but for fallpipe ROVs operating at small incident angles (e.g. θ<6°) this error may be within acceptable margins (e.g. threshold of 0.2 m). The applicability is assessed by estimating the horizontal refraction error with a ray-tracing technique using daily-mean SVPs, derived from freely-available ocean model data. A spatiotemporal quantification for the North Sea yielded maps of sea areas where SVP measurements are necessary, and areas where constant surface SVPs suffice. The latter are the shallow parts of the North Sea (<80 m), where the error is always smaller than 0.2 m. For deeper locations, the gradients of the SVP cannot be neglected. Then, the model-based SVPs can be used with or without the use of ISS sound velocity data. Comparing a collection of observed SVPs in the North Sea revealed that these daily-mean model-based SVPs are accurate enough for the positioning of fallpipe ROVs at least up to 370 m depth.

OMBES uses the synchronous overlap in depth measurements between two dual-head multibeam echosounders (MBES). In previous studies, the overlap is obtained by sailing two adjacent tracks with one MBES on a ship. Here, we propose to use two MBESs deployed on the same ship, thereby reducing the uncertainty of the ship’s motions that affect the quality of the depth measurements. The mean sound velocity can be inverted by mathematically minimizing the depth differences in the overlap. This technique completely minimizes the refraction error with frequent updates of the inverted mean sound velocity on the flight. Subsequently, the mean sound velocity can be used to locate ROVs that operate close to the seafloor in shallow water (<80 m), even for large incident angles (e.g. θ~65°). Simulations showed that the best performance of the inversion technique is established when maximizing the distance between the multibeam heads, and by inward-tilting one or two heads. Practically, it means the deployment of MBESs on either side of the ship, rather than one pair of MBESs at mid-ship.   
With increasing access to reliable ocean data, HISOM methods can potentially be run on ocean forecast model data to assess where and when refraction artifacts become dominant for fallpipe ROV positioning. This automatic assessment tool supports the SVP measuring strategy in subsea rock installation projects. OMBES can potentially be used as monitoring tool by comparing near real-time updates of the inverted mean sound velocity with the measured SVP. This data-driven decision tool can assist when to take an additional SVP. OMBES also improves the accuracy of multibeam bathymetric surveys by automatic collection of SVPs.