Toward a mechanistic understanding of coherent seabed decorrelation of a wave-dominated sandy seafloor

Temporal changes in biological and hydrodynamic activity near the seafloor have been shown to significantly influence seabed scattering strength and decorrelation rates. In particular, coherent decorrelation of the backscattered acoustic field is highly sensitive to small changes in the amplitude and phase of seafloor backscatter. Because of this sensitivity, measurements of coherence have the potential to sense how near-bottom biology and hydrodynamics alter the seabed. To this end, an instrumented lander was deployed at a sandy site off Star Island, NH, in 20 m of water. Calibrated acoustic echosounders pointed toward the seafloor at a nominal grazing angle of 20 deg transmitted acoustic 38, 70, 200 kHz tones every hour and received complex seafloor backscattered returns for five consecutive months. Temperature, salinity, and surface wave statistics were also measured throughout the deployment. The magnitude of the complex Pearson’s correlation coefficient between subsequent backscattered returns for different seafloor patches were estimated. A statistical analysis over space and time was performed to quantify the percentiles of seabed decorrelation over the entire deployment, during storm events, and during quiescent periods. The impact of wave-induced currents at the seabed on coherent decorrelation rates will be discussed. [Work supported by ONR Awards N000142412559 and N000141912732.]