Abstract
Multi-frequency multibeam backscatter (BS) has indicated, in particular for fine sediments, the potential for increasing the discrimination between different seabed environments. Fine sediments are expected to have a varying signal penetration within the frequency range of modern multibeam echosounders (MBESs). Therefore, it is unknown to what extent the multispectral MBES data represent the surface of the seabed or different parts of the subsurface. Here, the effect of signal penetration on the measured multi-frequency BS and bathymetry is investigated. To this end, two multi-frequency datasets (90 to 450 kHz) were acquired with an R2Sonic 2026 MBES, supported by ground-truthing, in the Vlietland Lake and Port of Rotterdam (The Netherlands). In addition, a model to simulate the MBES bathymetric measurements in a layered medium is developed. The measured bathymetry difference between the lowest (90 kHz) and highest frequency (450 kHz) in areas with muddy sediments reaches values up to 60 cm dependent on the location and incident angle. In spatial correspondence with the variation in the depth difference, the BS level at the lowest frequency varies by up to 15 dB for the muddy sediments while the BS at the highest frequency shows only small variations. A comparison of the acoustic results with the ground-truthing, geological setting and model indicates that the measured bathymetry and BS at the different frequencies correspond to different parts of the seabed. However, the low-frequency BS cannot be directly related to a subsurface layer because of a significant sound attenuation in the upper layer. The simulation of the MBES bottom detection indicates that the bathymetry measured at the highest and lowest frequency can be used to determine the thickness of thin layers (∼20 cm). However, with an increasing layer thickness, the bottom detection becomes more sensitive to the incident angle and small variations in the sediment properties. Consequently, an accurate determination of the layer thickness is hampered. Based on this study, it is highly recommended to analyze multi-frequency BS in combination with the inter-frequency bathymetry difference to ensure a correct interpretation and classification of multi-frequency BS data.
Highlights
Multibeam echosounders (MBESs) are the most efficient and widely used sonar technology for seabed mapping
This study investigates the effect of frequency-dependent signal penetration on multi-frequency backscatter (BS) and bathymetry data
Two multi-frequency datasets were acquired with the multispectral mode of an R2Sonic 2026 multibeam echosounder (MBES) at two different study sites
Summary
Multibeam echosounders (MBESs) are the most efficient and widely used sonar technology for seabed mapping. Beamforming in the across-track direction enables measurements of the signal travel time to the seabed for a large number of beam angles. As such, it provides detailed and extensive information about the seabed bathymetry. 2020, 12, 52 of the acoustic backscatter (BS) strength can be retrieved [1] This is a measure of the sound that is scattered or reflected from the water-sediment interface (interface scattering) and from the sediment volume (volume scattering) back towards the transmitter [2,3]. While higher frequencies are more sensitive to the seabed roughness, lower frequency signals are more affected by volume heterogeneities due to lower sound attenuation, resulting in an increased signal penetration, in the sediment
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