Abstract
The accuracy of multibeam echosounder bathymetric measurement depends on the accuracy of the data of the sound speed layers within the water column. This is necessary for the correct modeling of ray bending. It is assumed that the sound speed layers are horizontal and static, according to the sound speed profile traditionally used in the depth calculation. In fact, the boundaries between varying water masses can be curved and oscillate. It is difficult to assess the parameters of these movements based on the sparse sampling of sound velocity profiles (SVP) collected through a survey; thus, alternative or augmented methods are needed to obtain information about water mass stratification for the time of a particular ping or a series of pings. The process of water column data collection and analysis is presented in this paper. The proposed method updates the sound speed profile by the automated detection of varying water mass boundaries, giving the option to adjust the SVP for each beam separately. This can increase the overall accuracy of a bathymetric survey and provide additional oceanographic data about the study area.
Highlights
One of the essential components of every bathymetric survey is the measurement of the sound speed value, which is necessary to calculate the depths based on the two-way travel time of an acoustic signal sent from the echosounder and reflected back from seabed features
To include the impact of internal waves in hydrographic measurements, the current boundaries between water masses with different properties should be determined at the time of measurement, knowing that boundaries change both spatially and over time
It was proposed that this boundary recognition could be conducted by the analysis of water column data, treating the samples of all the beams as an image that is analyzable by using image processing techniques
Summary
One of the essential components of every bathymetric survey is the measurement of the sound speed value, which is necessary to calculate the depths based on the two-way travel time of an acoustic signal sent from the echosounder and reflected back from seabed features. In the case of a multibeam echosounder (MBES), is the mean value of the sound speed of great importance, and the data about water column stratification, which are used to estimate the refraction of outer beams This requires the measurement of the sound velocity profile (SVP), using SVPs (sound velocity profilers—direct measurement) or CTD (conductivity, temperature, depth—indirect measurement) devices, which are lowered down from the sea surface to the seabed and measure the sound speed during downward and upward travel. There is an option to use an MVP (moving vessel profiler), which does not need a vessel to stop, but requires dedicated, high-cost equipment on-board to automatically release and recover the MVP Both options of sound speed measurements are based on a discrete sampling of water stratification in time and space (measurement along a straight or close-to-straight line from the surface to seabed). The measured values indicate the parameters of the stratified water mass, and the boundaries between those masses are assumed to be straight and horizontal
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