HYDRO 2016 Paper 2A2

RapidCast: Analysis of spatio-temporal variability in high resolution speed of sound measurements

Mark Gray, Timothy Scott, Iain Slade, Jim Gardiner


Acoustic waves are the primary medium utilised by hydrographers for multibeam bathymetric surveys (MBES). Variation of speed of sound (SoS) through water causes these waves to bend and refract, which if not properly accounted for, can lead to misrepresentation of depth and features.

Traditional, static measurements of SoS are costly due to time "offline". The Teledyne Oceanscience RapidCast system with Valeport SwiftSVP reduces these losses and enables temporal and spatial quantification of SoS variance, with the potential to increase statistical confidence levels in swathe data with commercial applicability.

An MBES was undertaken in an 12 km2 area in Plymouth Sound with large SoS variances. Simultaneously, the RapidCast system collected sound velocity profiles (SVPs) at 100 m intervals. Survey of the site was repeated 6 times over half a tidal cycle. The resulting data has enabled graphical representation displaying SoS change spatially and temporally and the inclusion of high density SVPs into MBES data. This data provides the potential to display and quantify spatial and temporal SoS changes in a new, comprehensive manner. Geo-referenced 3D models of SoS variability will be created and analysed as a function of temporal change throughout the tidal cycle. We will test the hypothesis that when compared to reduced frequency SVP incorporation into MBES data, high density SVPs will increase the percentage of cells satisfying the 95% confidence level in total vertical uncertainty to achieve special order classification.

The RapidCast system allows the surveyor to minimise losses incurred with offline SoS measurements. Findings from statistical analysis of the MBES survey may highlight potential advantages to commercial activities. The high density of data the system can collect means detailed representation of temporal and spatial SoS variance has become possible.

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