Validation of BS methodology for geo-acoustic sediment classification in Lampaul Canyon, Bay of Biscay.
Supervisors: Marco Terzariol, Ridha Fezzani (IFREMER; Brest)
Sustainable management exploitation and management of offshore marine resources requires the knowledge and understanding of seabed sediments. Remote sensing techniques such as the use of acoustics has significantly improved mapping and classification of marine sediments at a low cost. High frequency Multibeam echosounder backscatter strength, which was of particular interest in this study, have become relevant in the assessment of geoacoustic properties of surface seabed sediment at low effective stress conditions.

Recent advances have enabled to analytically derive backscatter strength measurements for different sediments structure However, this approach has not been applied within a natural context. This research adopts this novel approach to validate high frequency backscatter at a natural setting along a seabed-water interface with low effective stress conditions.

Lampaul canyon, was chosen as the natural study site, due to the availability of backscatter strength data and sediment samples. In addition, the canyon is of ecological importance as it hosts one of the highest marine biodiversity and the rarest cold-water coral reefs. As a result, the knowledge of sediment types and classification will assist in the management of its biodiversity.

The sediment structure void ratio at low effective stress was chosen as a relevant property applied to different sediment types. The seabed backscatter strength, on the other hand, was obtained by operating frequencies of 300 kHz and analysed using a semi-empirical approach, referred to as the Generalized Seabed Acoustic Model (GSAB), which analyses the angular regime of backscatter strength of sediment. The study area exhibited several types of sediments from fine-dominated to coarse-dominated structures. The acoustic backscatter strength measurements on the other hand, also showed a substantial range of sediments types. The novel approach revealed a promising relationship between high-frequency backscatter strength and sediment void ratio at low effective stress, highlighting the influence of a sediment’s loose structure at the seabed-water interface.

Keywords: Backscatter strength, geotech, void Ratio, GSAB, low effective stress, geoacoustic.