Acoustic Sensing of Sediment Properties using a WWW-controlled Shallow Water Mooring

PI: Henrik Schmidt, MIT Sea Grant

Project Number:2000-RCM-5

Start Date:2000-03-01End Date:2002-02-28

Proposal Summary

Objectives: The main scientific objective is to establish the theoretical foundation for the development of small-scale acoustic sensing techniques for remotely sensing the geotechnical properties of the seabed, which can be implemented within the AOSN framework. The initial effort will focus on the spectral properties of the acoustic backscattering, in particular in regard to the extraction of information regarding gradients, and the design requirements in terms of the acoustic arrays .On the technology side, the proposed effort is aimed at interfacing a complex, high-data rate ocean sensing system with a state of the art distributed information system, Poseidon. This effort will establish a versatile framework for executing long and short term acoustic field experiments which, once the hardware is deployed, do not require any logistics support.

Methodology: The tower mooring recently developed at Woods Hole Oceanographic Institution under the Regional Sea Grant Project "Acoustic Monitoring System Augmentation to Coastal Observatories," is designed to accommodate a generic set of payloads, although the prototype mooring is primarily focusing on an acoustic receiving capability in the form of a linear hydrophone array. Thus, the electronics used is directly capable of controlling and driving an acoustic source, which will make the mooring into a self-contained instrumentation platform for carrying out acoustic backscattering measurements. This project investigates the potential for using such measurements as a new remote sensing capability for seabed properties, including important geotechnical parameters of significance to deep ocean seabed construction. The project initially focuses on the establishment of the computational infrastructure and the implementation of an acoustic source on the existing tower mooring, and a continuation of the acoustic modeling work.
If funding is identified for completing a second mooring a reciprocal tomographic experiment to be performed in a suitable location, either in Narragansett Bay or Massachusetts Bay. The present effort will continue to focus on using the two moorings for stand-alone backscattering experiments, although the theoretical and experimental analysis may suggest a benefit to using bi-static configurations for sediment characterization, in which case some mooring-to-mooring experiments will be carried out.

Rationale: Even though the technology now exists for reaching the deep ocean seabed, the remote sensing capabilities necessary for large scale surveys is limited, in particular in regard to characterizing the geotechnical properties of the seabed. Thus, existing side scan and multi beam sonar systems are extremely capable for bathymetric mapping, but provide little information regarding the bottom composition, even though some information may be extracted from the backscattering strength. Sub bottom profiling systems are suitable for implementation on small AUVs for mapping seabed layering but these systems do not provide much direct information regarding the bottom composition either. It seems impossible to design a remote sensing technique that directly measures the geotechnical properties such as shear strength and creep rate of sediments, but it may be possible to achieve robust estimates of these by remotely sensing a combination of seabed parameteres, such as for example, layering structure and depth-dependent visco-elastic moduli. This research project is aimed at taking the first steps towards the development of such an acoustic remote sensing approach for seabed geotechnical classification by investigating whether acoustic array processing techniques can be used to robustly determine seabed parameters which can be established as indicators of the structural strength of the seabed.



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