A Method to Quantify Bedform Height and Asymmetry from a Low-Mounted Sidescan Sonar

Abstract

Rotary sidescan sonars are widely used to image the seabed given their high temporal and spatial resolution. This high resolution is necessary to resolve bedform dynamics and evolution; however, sidescan sonars do not directly measure bathymetry, limiting their utility. When sidescan sonars are mounted close to the seabed, bedforms may create acoustical ''shadows'' that render previous methods that invert the backscatter intensity to estimate bathymetry and are based on the assumption of a fully insonified seabed ineffective. This is especially true in coastal regions, where bedforms are common features whose large height relative to the water depth may significantly influence the surrounding flow. A method is described that utilizes sonar shadows to estimate bedform height and asymmetry. The method accounts for the periodic structure of bedform fields and the projection of the shadows onto adjacent bedforms. It is validated with bathymetric observations of wave-orbital ripples, with wavelengths ranging from 0.3 to 0.8 m, and tidally reversing megaripples, with wavelengths from 5 to 8 m. In both cases, bathymetric-measuring sonars were deployed in addition to a rotary sidescan sonar to provide a ground truth; however, the bathymetric sonars typically measure different and smaller areas than the rotary sidescan sonar. The shadow-based method and bathymetric-measuring sonar data produce estimates of bedform height that agree by 34.0% 6 27.2% for wave-orbital ripples and 16.6% 6 14.7% for megaripples. Errors for estimates of asymmetry are 1.9% 6 2.1% for wave-orbital ripples and 11.2% 6 9.6% for megaripples.

Direct Measurements of Mean Reynolds Stress and Ripple Roughness in the Presence of Energetic Forcing by Surface Waves

“…The ripple wavelength k was calculated as the mean distance between the zero downcrossings along the defined radius. For a sonar mounted near the seafloor, (12) are illustrated graphically including the distance from the sonar to the start of a ripple shadow (L), the length of the shadow (S) and the length of the illuminated seabed to the start of the next shadow (B); (b) Amplitude of the acoustic backscatter along the transect perpendicular to the inferred ripple crests, showing the zero crossings (circles) and the associated estimate of the ripple wavelength (k5B 1 S) ripple heights can be estimated from the length and location of the acoustic shadows cast from the bedforms (Jones & Traykovski, 2018). For uniform, symmetric ripples on a flat seabed, g can be estimated from:…”

“…The ripple wavelength k was calculated as the mean distance between the zero downcrossings along the defined radius. For a sonar mounted near the seafloor, Journal of Geophysical Research: Oceans 10.1002/2017JC013252 ripple heights can be estimated from the length and location of the acoustic shadows cast from the bedforms (Jones & Traykovski, 2018). For uniform, symmetric ripples on a flat seabed, g can be estimated from:…”

“…The ripple wavelength was calculated as the mean distance between the zero downcrossings along the defined radius. For a sonar mounted near the seafloor, ripple heights can be estimated from the length and location of the acoustic shadows cast from the bedforms (Jones & Traykovski, ). For uniform, symmetric ripples on a flat seabed, can be estimated from: where H i is the height of the sonar, L is the distance from the sonar to the start of a ripple shadow, S is the length of the shadow and B is the length of the illuminated seabed to the start of the next shadow (see Figure , for example).…”

Neural Shape-from-Shading for Survey-Scale Self-Consistent Bathymetry from Sidescan

“…While more complete than the simple shadow detection approach of [19], it faces similar challenges in reconstructing general types of seafloors. Recently, Jones & Traykovski [22] employed shadow geometry to reconstruct the surface surrounding a rotating sidescan mounted close to the seabed. They perform thorough evaluations of the efficiency of their method, with quantitative comparisons to multibeam bathymetry.…”

Measurements and dynamics of multiple scale bedforms in tidally energetic environments

“…This method accounts for the periodic structure of bedform fields and the projection of the shadows on adjacent bedforms. We This chapter was previously published as Jones and Traykovski (2018) (Irish et al, 1998;Hay and Wilson, 1994;Rubin et al, 1983) Although sidescan sonars provide an image of the seafloor, they are unable to obtain direct measurements of seafloor elevation. Recent developments in image processing have enabled backscatter intensity to be inverted to obtain seabed elevation maps using an image model based on surface roughness scattering strength (Coiras et al, 2007;Tang et al, 2009;Nishimura, 1997).…”