Towards a Relationship Between Acoustic Backscatter and Soil Geotechnical Properties: insights from Bay of Morlaix, France

Offshore infrastructures are essential for energy production, telecommunication, and transportation sectors. High-frequency offshore acoustic backscatter is widely used as a proxy for seabed and soil classification. When properly interpreted, it can significantly reduce exploration costs by optimizing survey design, guiding targeted sampling strategies, and improving the detection and interpretation of natural seafloor features. In the context of offshore wind farms, acoustic backscatter data are particularly valuable for early-stage site screening, preliminary economic assessment, and the identification of potential geohazards prior to detailed geotechnical investigations. Although hydroacoustic surveys can cover large areas efficiently, their ability to estimate geotechnical properties is still limited.   Hence, this study aims to investigate the relationship between high-frequency acoustic backscatter and geotechnical parameters of marine sediments. The dedicated research cruise RHYSMA was conducted in the Bay of Morlaix to acquire both hydroacoustic and geotechnical data on 8^th of May 2025 during ten days. Eleven sites were selected to represent a wide range of sediment types with average water depth varying approximately 3m to 50m, ranging from muddy sediments to boulders, and exposed rock. Such diversity provides a natural laboratory to investigate the interaction between acoustic signals and sediment properties. Acoustic measurements were performed using SIMRAD EK80 single beam echosounders with frequencies ranging from 90 kHz to 440 kHz, with incidence angles varying from 0° to 70°. In total 10 cores were obtained; 840 kg of sediments and 9 hours of seafloor videos were recovered. Hydroacoustic data was calibrated and fitted using the Generic Seafloor Acoustic Backscatter (GSAB) model to estimate the seabed angular response and to assess the influence of angle and frequency on different sediment types. Typical backscatter ranges from -15 dB up to -4 dB in the exposed rock. Geotechnical results show particle sizes ranges from 4 microns up to 8 cm boulders, while dry densities range from 1 to 1.6 g/cc and specific gravity spans from 2.64 to 2.96 and fine content for sediment ranges from 1% percent to 56%. Preliminary results indicate an emerging relationship between acoustic backscatter and sediment porosity particularly for high frequency response at near-normal incidence angles (~0°). Overall, this study improves the understanding of acoustic backscatter behavior across contrasting sediment types and highlights the complexity of seabed acoustic responses.