Re-recorded from presentation at GeoHab 2024 conference (May 6-10, 2024, Arendal Norway).
Full Title: "Don’t let the mud fool you - Ambiguous multibeam seafloor bathymetry and backscatter measurements using a broad frequency range"
Full list of authors: J. Schneider von Deimling [1], J. Hoffmann [2,3], P. Pocwiardowski [4], P. Held [1]
1. Institute of Geosciences, Kiel University, Germany
2. Alfred-Wegener-Institute, Helmholtz Centre for Polar and Marine Research, Germany
3. Marine Geology and Seafloor Surveying, University of Malta, Malta
4. NORBIT Sonar Systems, Santa Barbara, California, USA
Abstract:
Acoustic echosounder systems have become the standard tool for determining seafloor depth. Paradoxically, there is no clear definition of where the seafloor begins acoustically, it is usually taken as the first prominent echo. In soft sediments, the acoustic determination of seafloor depth can become ambiguous, with consequences for both bathymetry and backscatter. Reasons for this behaviour are geo-acoustic properties of muddy sediments with low density contrast to seawater, low attenuation and sometimes reduced sound velocity, rendering them a slow reflector. Here we present examples from muddy seabeds in shallow water between 0 and 100 m, recorded over a wide multibeam echosounder systems (MBES) frequency range between 12 and 400 kHz. Some of the areas we observed are charged with gas. Some of them were found to have acoustic velocities lower than the overlying seawater, and others contain suspended sediments and mobile fluid mud layered on top of a stationary seafloor. The latter setting is critical for hydrographic authorities concerned with safe navigation, where the question arises as to which acoustic depth seabeds are navigable for cargo ships and where further dredging is required.
We have collected MBES data over the last decades with different systems (KONGSBERG, ELAC, NORBIT) including 12, 50, 80, 95, 180, 300 and 400 kHz frequencies. Along with the multibeam surveys, synchronized parametric sub-bottom profiles were acquired to determine the first acoustic 'seafloor' echoes and to identify differences between the two depths. The largest of these occurred in mud when surveyed with 12 kHz MBES, resulting in a depth difference of 12 m below the so-called nautical depth. This data set virtually draws a palaeo bathymetric map of the subsurface. In the same sediment, 50 kHz MBES showed penetration and false bottom detection down to 2 m, forming strange looking ring shapes on the seabed. Even at 300 kHz, a very pronounced penetration of 30 cm could be observed, forming eyed pockmarks in the resulting backscatter map.
The vast majority of the seafloor is covered by sediments. In the deep sea, and particularly in coastal inlets and harbours, soft mud with low acoustic attenuation is among the most common sediments and penetration effects are ubiquitous. Although the contribution of volume scatterers from within the sediments is a well-described phenomenon, we suggest more data is needed to gain a better understanding of which geogenic or biogenic settings give rise to which pattern. This could lead to a better understanding and interpretation of multispectral backscatter imaging.
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