Sea Bottom Sound Velocity Estimation Using High Resolution Multiswath Multibeam Bathymetric Echosounder

Sound velocity refraction induced errors in conventional multibeam bathymetric are usually corrected by measuring the sound velocity profile using a dedicated profiler sensor (SVP). We propose to use a new multibeam system geometry that is able, in real time, to estimate the sound velocity at the sea bottom and to correct bathymetry measurements from the refraction artefacts. To compensate sound refraction errors, data driven techniques have already been studied many years ago [1]. All these methods estimate a mean velocity value, independent of arrival angle and depth, that minimizes the bathymetry errors on overlapped survey lines. Using conventional multibeam echosounder geometry, these overlapped measurements are acquired at different time so the methods is sensitive to any changes in the environment: sound speed variability in space and time, tide, calibration error, high variation topography, navigation absolute precision. We propose a multiswath multibeam echosounder. This approach has first been evaluated in the 2000's for deep sea bathymetry (HYDROSWEEP[2]). Recently, a new compact high resolution multiswath-multibeam echosounder for mid to shallow water applications have been developed (SEAPIX [3]). The multiswath system is able to insonify the seabed in the along track and across track direction. Sound speed refraction corrections are automatically obtained by using the overlapped along track and across track bathymetry profile in a short time interval reducing sensitivity to environment variability. Using the multiswath , the mean harmonic celerity profile is obtained at each soundings depth. This profile can be inverted to recover an estimate of the true sound velocity at the soundings depths. A map of the sound velocity at the sea bottom may then be produced. The true grazing angle is estimated which improve backscatter imaging measurements.

[1] Snellen, M., Siemes, K. and D. G. Simons (2009), A model-based method for reducing the sound speed induced errors in multi-beam echo-sounder bathymetric measurements, OCEANS 2009 -EUROPE, pp. 1-7.

[2] ATLAS HYDROSWEEP DS-2, Hydrographic Deep Sea Multibeam Echosounder, Product Description, May 2003

[3] F. Mosca & al., Scientific potential of a new 3D multibeam Echosounder in fisheries and ecosystem research, Fisheries Research 178 pp. 130-141, 2016.