Measuring and modeling time-dependent changes in seabed scatter caused by near-bottom hydrodynamics and biologic processes

Acoustic backscatter systems (ABS) can sense time-dependent changes in seabed surface roughness, internal structure, and composition caused by near-bottom hydrodynamics and biologic processes. However, a better understanding of how these processes affect the acoustic backscatter is required before data from ABS can be effectively used as a remote sensing tool. A Simrad EK80 Wideband Acoustic Transceiver (WBAT) was deployed on the seafloor at two sites off the coast of New Hampshire, and long-term backscatter measurements were acquired at 38, 70, and 200 kHz. The first site was located at the mouth of the Piscataqua River, near New Castle Island (NC), and the second at Star Island (SI) within the Isle of Shoals. The NC site exhibited strong tidal currents and mixed sediment with burrowing infauna, while the SI site was more susceptible to storm events and contained a coarser sediment with large quantities of shell fragments. Measurements of ping-to-ping decorrelation of scattered acoustic pulses were used to quantify timescales involved with near-bottom currents and storm events, while finite-element techniques were employed to study the backscatter from idealized burrows located at the water-sediment interface. An overview of these techniques and results will be discussed. [Work supported by ONR.]