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

Abstract

The establishment of multibeam echosounders (MBES), as a mainstream tool in ocean mapping, has facilitated integrative approaches towards nautical charting, benthic habitat mapping, and seafloor geotechnical surveys. The combined acoustic response of the seabed and the subsurface can vary with MBES operating frequency. At worst, this can make for difficulties in merging the results from different mapping systems or mapping campaigns. However, at best, having observations of the same seafloor at different acoustic wavelengths allows for increased discriminatory power in seabed classification and characterization efforts. Here, we present the results from trials of a multispectral multibeam system (R2Sonic 2026 MBES, manufactured by R2Sonic, LLC, Austin, TX, USA) in the Bedford Basin, Nova Scotia. In this system, the frequency can be modified on a ping-by-ping basis, which can provide multi-spectral acoustic measurements with a single pass of the survey platform. The surveys were conducted at three operating frequencies (100, 200, and 400 kHz), and the resulting backscatter mosaics revealed differences in parts of the survey area between the frequencies. Ground validation surveys using a combination of underwater video transects and benthic grab and core sampling confirmed that these differences were due to coarse, dredge spoil material underlying a surface cover of mud. These innovations offer tremendous potential for application in the area of seafloor geological and benthic habitat mapping.

Mapping the Seabed and Shallow Subsurface with Multi-Frequency Multibeam Echosounders

“…A thorough analysis of two multi-frequency MBES datasets demonstrated significant variation in the measured bathymetries, BS levels, time series signals, and ARCs for fine sediments for the frequency range between 90 and 450 kHz. Similar variation in multispectral BS patterns for fine sediments between 100 and 400 kHz were also observed by Brown et al [20] and Gaida et al [18]. They argued that the lower frequency signals were acoustically sensitive to a buried layer of coarse dredge spoils whereas the higher frequencies are only sensitive to the surficial mud layer.…”

“…Except for the spatially limited rheometric profiling in the Port of Rotterdam, the ground-truthing of the subsurface was limited in this study. Ideally, a vibrocoring device, such as used by Brown et al [20], and a more extensive rheometric profiling would have been used. While the coring device provides a physical sample from the subsurface layer, the rheometric profiling takes an in situ measurement of the undisturbed sediment and can be used to accurately quantify the depth of a buried layer.…”

“…New developments in marine sonar technology allow the MBES to collect spatially and temporally co-located BS data at multiple frequencies (i.e., 90 to 450 kHz) using a single system. The first research studies, employing such a MBES, have also indicated a potential to increase the discrimination between different seabed environments using multi-frequency BS data [18][19][20][21]. In these studies different BS patterns at various frequencies (100, 200, and 400 kHz), indicating the benefits of multiple frequencies, were mainly observed for areas consisting of fine sediments (mud).…”

Where’s the Carbon: Exploring the Spatial Heterogeneity of Sedimentary Carbon in Mid-Latitude Fjords

“…Scottish and Irish fjords largely lack these data, a pattern observed in coastal systems globally. Other approaches to map the seabed and sediment type utilize multibeam and backscatter geophysical techniques (Serpetti et al, 2012;Lark et al, 2015;Audsley et al, 2016;Brown et al, 2019) where changes in acoustic responses are used to characterize seabed type. These methods provide an understanding of the spatial distribution of sediment types but have largely not been applied to fjord systems.…”

On the Information Advantage of Sidescan Sonar Three-Frequency Colour over Greyscale Imagery

“…Three-frequency multi-beam acoustic images are shown in Hughes Clarke and Muggah [3] and Hughes Clarke [4], and three-frequency sidescan sonar acoustic images were generated by Tamsett and McIlvenny [5], Tamsett, McIlvenny, and Watts [6], and McIlvenny et al [7]. There have recently been other investigations incorporating multi-frequency sonar, some including colour imagery [8][9][10][11].…”