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

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.

Complex Eyed Pockmarks and Submarine Groundwater Discharge Revealed by Acoustic Data and Sediment Cores in Eckernförde Bay, SW Baltic Sea

“…We showcase that even at 400 kHz frequency, backscattering strength is significantly increased by subsurface volume scattering from shallow gas. Our study also highlights the potential for shallow gas to confuse bathymetric depth and backscatter interpretations, in addition to studies dealing with lower frequencies (e.g., Gaida et al, 2019; Schneider von Deimling et al, 2013). In particular, we show that high‐resolution hydroacoustic surveys followed by targeted coring can be used to identify specific locations of SGD and provide new insight into how SGD influences seafloor geochemistry and morphology.…”

Hydroacoustic Mapping of Geogenic Hard Substrates: Challenges and Review of German Approaches

“…Advantages of MBES backscatter investigation is the additional depth information which can be used to detect stones directly in bathymetric data, while a corresponding disadvantage of MBES remains the depth-dependent size of the acoustic footprint which may not be constant across an investigation area. Increasingly common multi-frequency surveys [86][87][88][89][90], in which the sonar cycles through different frequencies, provide different resolutions for imagery derived from each frequency. While the use of multiple frequencies for habitat mapping is undoubted, the capacity of multi-frequency maps for object detection will be reduced due to increased ping intervals per frequency.…”

Assessing the Performance of the Multi-Beam Echo-Sounder Bathymetric Uncertainty Prediction Model

“…As an example, Eleftherakis et al [15,16] combined the so-called depth residuals (related to the depths' standard deviation) with backscatter strength measurements to increase the discrimination performance of the sediment classification methods and to solve the ambiguity in the relationship between backscatter value and median grain size [17]. Recently, Gaida et al [18] used the bathymetry uncertainty to assess the statistical significance of the changes in the measured depth differences between the various frequencies with the incident angle. Lacking information with regard to the uncertainties inherent to the MBES can lead to misinterpretation.…”