2:15 PM - 2:30 PM
[AOS17-03] Three-dimensional acoustical imaging of microstructure in the coastal ocean using a multibeam echosounder
Keywords:multibeam echosounder, oceanic microstructure, pycnocline, acoustical oceanography, underwater acoustics
A multibeam echosounder (MBES) has the capability of high-resolution underwater acoustic observation, taking advantage of multiple formations of highly-directive narrow beams. The MBES measures underwater acoustic scatterings to detect sea bottom. The recorded multibeam echo profiles, called water column data (WCD), can provide three-dimensional visuals of backscattering from pycnocline and velocline associated with oceanic microstructure. While multi-channel seismic reflection and single-beam echo-sounding have acoustically portrayed oceanographic structures in previous studies, two-dimensional transects provided from those techniques have a limitation in describing the continuity and variability of microstructures in the coastal ocean. Although only some examples of MBES-applied oceanographic observation exist, the method will significantly contribute to physical oceanography.
We conduct shipborne multibeam echo-sounding and record WCD in the northwestern Pacific, 25-30 km off the coast of Fukushima. The instrument for observation is a 256-beam echosounder with a Mills-Cross array configuration. Each beam has a 1-degree width in both along-track and across-track directions. The transmit pulse is 71-87 kHz continuous wave of 5 milliseconds duration. The analyzing process of WCD consists of two steps; constructing grayscale images of mid-water scattering corresponding to the reception of each emitted pulse and making cross-sections from the stack of those images by taking the median of scattering intensity. Since the WCD is three-dimensional, three types of cross-sectional views can be created: horizontal, vertical, and orthogonal to the direction of travel.
As a result, some of the along-track vertical cross-sections exhibit a periodic scattering pattern resembling Kelvin-Helmholtz (KH) billow at approximately 20 m above the sea bottom of 140-150 m depth. The corresponding scattering pattern also appears in a horizontal cross-section. The cross-track view portrays intertwined thin scattering layers progressing in time, resembling KH billow. The thickness of the scattering pattern is 10-20 m. In the vertical cross-sectional images, the horizontal distance between two adjacent troughs reads 60-90 m.
A vertical profile of temperature and salinity observed close to the echo-sounding area shows a mixed layer at the surface and small discontinuities in the lower part. At the boundaries of those discontinuities, steep pycnocline and velocline make significant contrasts in acoustic impedance, hence the spikes in acoustic reflectivity. The observed periodic pattern is considered to reflect oceanic microstructure since the pattern is unlikely to be an artifact, and the depth of the strong scattering is consistent with the vertical reflectivity profile.
The advantages of multibeam echo-sounding, where beam pointing angles and round-trip times are accurately known, also provide a quantitative perspective for mid-water physical structure. The WCD can indicate the geographic location of observed scattering with an accuracy of ~1 m in both range and angular directions. The observed KH-billow-like pattern most likely exhibits apparent elongation and shrinkage corresponding to the vessel velocity; therefore, the horizontal periodicity of the periodic structure itself can be estimated independent of the speed of the observation platform. The horizontal cross-sectional image provides implications for estimating the direction of movement of progressive waves. The analysis of WCD derived from MBES can be beneficial in identifying temporal and spatial properties of oceanic microstructure.
We conduct shipborne multibeam echo-sounding and record WCD in the northwestern Pacific, 25-30 km off the coast of Fukushima. The instrument for observation is a 256-beam echosounder with a Mills-Cross array configuration. Each beam has a 1-degree width in both along-track and across-track directions. The transmit pulse is 71-87 kHz continuous wave of 5 milliseconds duration. The analyzing process of WCD consists of two steps; constructing grayscale images of mid-water scattering corresponding to the reception of each emitted pulse and making cross-sections from the stack of those images by taking the median of scattering intensity. Since the WCD is three-dimensional, three types of cross-sectional views can be created: horizontal, vertical, and orthogonal to the direction of travel.
As a result, some of the along-track vertical cross-sections exhibit a periodic scattering pattern resembling Kelvin-Helmholtz (KH) billow at approximately 20 m above the sea bottom of 140-150 m depth. The corresponding scattering pattern also appears in a horizontal cross-section. The cross-track view portrays intertwined thin scattering layers progressing in time, resembling KH billow. The thickness of the scattering pattern is 10-20 m. In the vertical cross-sectional images, the horizontal distance between two adjacent troughs reads 60-90 m.
A vertical profile of temperature and salinity observed close to the echo-sounding area shows a mixed layer at the surface and small discontinuities in the lower part. At the boundaries of those discontinuities, steep pycnocline and velocline make significant contrasts in acoustic impedance, hence the spikes in acoustic reflectivity. The observed periodic pattern is considered to reflect oceanic microstructure since the pattern is unlikely to be an artifact, and the depth of the strong scattering is consistent with the vertical reflectivity profile.
The advantages of multibeam echo-sounding, where beam pointing angles and round-trip times are accurately known, also provide a quantitative perspective for mid-water physical structure. The WCD can indicate the geographic location of observed scattering with an accuracy of ~1 m in both range and angular directions. The observed KH-billow-like pattern most likely exhibits apparent elongation and shrinkage corresponding to the vessel velocity; therefore, the horizontal periodicity of the periodic structure itself can be estimated independent of the speed of the observation platform. The horizontal cross-sectional image provides implications for estimating the direction of movement of progressive waves. The analysis of WCD derived from MBES can be beneficial in identifying temporal and spatial properties of oceanic microstructure.