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Acoustic methods are routinely used to provide broad scale information on the geographical distribution of benthic marine habitats and sedimentary environments. Although single-frequency multibeam echosounder surveys have dominated seabed characterisation for decades, multifrequency approaches are now gaining favour in order to capture different frequency responses from the same seabed type. The aim of this study is to develop a robust modelling framework for testing the potential application and value of multifrequency (30, 95, and 300 kHz) multibeam backscatter responses to characterize sediments’ grain size in an area with strong geomorphological gradients and benthic ecological variability. We fit a generalized linear model on a multibeam backscatter and its derivatives to examine the explanatory power of single-frequency and multifrequency models with respect to the mean sediment grain size obtained from the grab samples. A strong and statistically significant (p < 0.05) correlation between the mean backscatter and the absolute values of the mean sediment grain size for the data was noted. The root mean squared error (RMSE) values identified the 30 kHz model as the best performing model responsible for explaining the most variation (84.3%) of the mean grain size at a statistically significant output (p < 0.05) with an adjusted r2 = 0.82. Overall, the single low-frequency sources showed a marginal gain on the multifrequency model, with the 30 kHz model driving the significance of this multifrequency model, and the inclusion of the higher frequencies diminished the level of agreement. We recommend further detailed and sufficient ground-truth data to better predict sediment properties and to discriminate benthic habitats to enhance the reliability of multifrequency backscatter data for the monitoring and management of marine protected areas.

Accurate seabed sediment classification is essential for mapping marine geological features, assessing benthic habitat, and planning coastal infrastructure. This study investigated the utility of multifrequency multibeam echosounder (MBES) backscatter data for improving seabed sediment classification compared to traditional single-frequency approaches. MBES data were acquired at three frequencies (170, 300, and 450 kHz), and post-processed to produce frequency-specific backscatter mosaics and a composite red–green–blue image. Classification was performed using unsupervised clustering methods, including K-means and isodata clustering, with input vectors composed of normalized backscatter intensities from the three frequencies. The integrated multifrequency approach successfully identified three distinct sediment classes, which were validated using grab samples analyzed for grain size, water content, total organic carbon, and slope. These classes exhibited strong correspondence with underlying geomorphological features and local hydrodynamic regimes, confirming the influence of topography and tidal currents on sediment distribution. Lower-frequency data (170 kHz) were more sensitive to subsurface variability, while higher-frequency data (450 kHz) captured surface texture differences more effectively. The combined use of all three frequencies improved classification performance, particularly in transitional sediment zones where single-frequency methods proved ambiguous. The methodology proved robust across varying water depths, sediment types, and complex seabed terrains, aligning with recent advances in MBES-based sediment mapping and supporting its general applicability for other dynamic coastal systems. These results demonstrate that the use of multifrequency MBES backscatter data enhances the resolution and reliability of sediment classification results, providing a robust framework for high-resolution seabed mapping in dynamic coastal environments.

A SeaBat T50 calibration that combines measurements in a test tank with data from numerical models is presented. The calibration is assessed with data obtained from a series of tests conducted over a sandy seabed outside the harbor of Santa Barbara, California (April 2016). The tests include different tone-burst durations, sound pressure levels, and receive gains in order to verify that the estimated seabed backscattering strength (Sb) is invariant to sonar settings. Finally, Sb-estimates obtained in the frequency range from 190 kHz in steps of 10 kHz up to 400 kHz, and for grazing angles from 20∘ up to 90∘ in bins of width 5∘, are presented. The results are compared with results found in the literature.