Quantitative characterisation of seafloor substrate and bedforms using advanced processing of multibeam backscatter—Application to Cook Strait, New Zealand

Abstract

A comprehensive EM300 multibeam echo-sounder dataset acquired from Cook Strait, New Zealand, is used to develop a regional-scale objective characterisation of the seafloor. Sediment samples and high-resolution seismic data are used for ground-truthing. SonarScope ® software is used to process the data, including signal corrections from sensor bias, specular reflection compensation and speckle noise filtering aiming at attenuating the effects of recording equipment, seafloor topography, and water column. The processing is completed by correlating a quantitative description (the Generic Seafloor Acoustic Backscatter-GSAB model) with the backscatter data. The calibrated Backscattering Strength (BS) is used to provide information on the physical characteristics of the seafloor. The imagery obtained from the BS statistical compensation is used for qualitative interpretation only; it helps characterizing sediment facies variations as well as geological and topographic features such as sediment waves and erosional bedforms, otherwise not recognised with the same level of detail using conventional surveying. The physical BS angular response is a good indicator of the sediment grain size and provides a first-order interpretation of the substrate composition. BS angular response for eight reference areas in the Narrows Basin are selected and parameterised using the GSAB model, and BS angular profiles for gravelly, sandy, and muddy seafloors are used as references for inferring the grain size in the reference areas. We propose to use the calibrated BS at 45° incidence angle (BS 45) and the Specular-To-Oblique Contrast (STOC) as main global descriptors of the seafloor type. These two parameters enable global backscatter studies by opposition to compensated imagery whose intensity is not comparable from one zone to the other. The results obtained highlight the interest of BS measurements for seafloor remote sensing in a context of habitat-mapping applications.

Modern morpho-sedimentological patterns in a tide-dominated estuary system: the Bay of Brest (west Britanny, France)

“…The side-scan sonar signatures result from backscatter variation (imagery) that depends on the seabed material and its macro-morphology (Augustin & Lurton, 2005;Augustin et al, 1996;Lamarche, Lurton, Verdier, & Augustin, 2011;Le Chenadec, Boucher, & Lurton, 2007;Lurton, 2003). Their identification relies on the interpretation of shades of grey and apparent textures in the mosaic.…”

Control of the repeatability of high frequency multibeam echosounder backscatter by using natural reference areas

“…In order to detect a possible drift of backscatter without an absolute calibration curve, the backscatter data is used without any prior angular compensation. The angular sector that best discriminates the different types of sediment is the oblique-incidence sector often forming a plateau within approximately ± [30°, 50°] (APL 1994; Lamarche et al 2011). Here, we consider that variations in the nature of the seabed will be reflected by a significant variation in the average level of backscatter measured in this particular angular sector.…”

Seafloor multibeam backscatter calibration experiment: comparing 45°-tilted 38-kHz split-beam echosounder and 30-kHz multibeam data

“…The selected areas are flat, homogeneous in terms of sediment character and without significant seafloor backscatter features. The sediment homogeneity was confirmed by sample collection and corroborated by the homogeneous backscatter facies (Le Gonidec et al 2003;Lamarche et al 2011;Lucieer and Lamarche 2011). The sites are located close to the home port of RV Tangaroa, which optimizes and facilitates regular passages over the patch-test areas.…”

“…It is now well accepted that such data provide qualitative information on substrate and benthic habitat (e.g. Brown and Blondel 2009;Lamarche et al 2011;McGonigle and Collier 2014). Backscatter data are commonly acquired concomitantly with bathymetric data, at least with multibeam echosounders (MBES) which are presently the most widely used systems for acquisition of such data (Mayer 2006).…”