Advances in high-resolution seafloor characterization in support of high-frequency underwater acoustics studies: techniques and examples

Abstract

Studies designed to examine high-frequency seafloor scattering or penetration mechanisms have been limited by the difficulty of accurately characterizing seabed properties at millimetre scales. Both the two-dimensional (2D) interface controlling roughness scattering and the three-dimensional (3D) internal structure of the top tens of centimetres of seafloor sediments controlling volume scattering require accurate measurement tools that are non-destructive and that have high spatial resolution. Subcritical penetration mechanisms have not been satisfactorily explained to date due in part to the lack of two-dimensional seafloor roughness measurements. Recently, tools for addressing these experimental shortfalls have been developed to support seafloor acoustics measurements, and information obtained from these systems is being used to address many open questions in high-frequency seafloor acoustics. This paper provides a review with examples of the development and use of several non-traditional technologies in seafloor acoustics studies including x-ray computed tomography and digital photogrammetry.

Multispectral Multibeam Echo Sounder Backscatter as a Tool for Improved Seafloor Characterization

“…The surface roughness of the sediment can affect the backscatter intensity that is based on the wavelength of the signal and the scale of roughness at the seafloor (e.g., surface roughness that is caused by grainsize through to bedforms) [22]. The sediment parameters at the site can influence backscatter based on local roughness, which is often unknown or unmeasured [31], which in turn can cause different backscatter responses at different frequencies [16]. Generally speaking, lower frequency signals will penetrate deeper into the substrate than higher frequency signals, which will attenuate over a shorter distance [22,32].…”

Measurements of high-frequency acoustic scattering from glacially eroded rock outcrops

“…Images were processed to obtain height fields using the OpenCV library (Kaehler and Bradski, 2015), and the intrinsic and extrinsic camera parameters were obtained using the camera calibration toolbox (Bouguet, 2008). Details on stereo photogrammetry can be found in Wong (1980) and an example of an underwater system can be found in Lyons and Pouliquen (2004).…”

Secondary dispersal of seagrass seeds in complex microtopographies

“…Characterizing the hierarchy of roughness elements of the micro-topography, remains an important difficulty. Experimental quantification of small-scale bottom features has been attempted with high-resolution sonar mapping systems, stereo photogrammetry and microtopographic laser scanners (Briggs, 1989;Lyons and Pouliquen, 2004;Du Preez and Tunnicliffe, 2012). Still, the lack of a robust approach to characterize in situ the seafloor micro-topography is repeatedly cited as a source of uncertainty in derived analysis (Lyons and Pouliquen, 2004).…”

“…The use of a power law form to characterize bottom roughness over a wide range of spatial frequencies, is a useful and popular method (Fox and Hayes, 1985;Lyons et al, 2002). Even though the seafloor is continually changing, the previous statistical description is assumed to be relatively stable (Lyons and Pouliquen, 2004).…”