default Using a vertical line array and ambient noise to obtain measurements of seafloor reflection loss

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Using a vertical line array and ambient noise to obtain measurements of seafloor reflection loss. Victor Young. SR-410. March 2005.

Bottom reflection properties can be obtained from ambient noise directionality. The results obtained at six sites in 2002 with NURC’S 62m VLA are summarised. At four of the sites the VLA was moored, in order to study a single seafloor environment, while at the other two sites the VLA was allowed to drift, in order to study geographical changes in the seafloor environment. Several variants of the measurement and processing techniques are investigated here. Firstly, rather than using only the uniformly spaced central section of a nested vertical array (VLA), one can expand the useful size of the array to about three quarters (rather than one half) of the full array length by padding out the array’s correlation matrix. The padding makes the apparently good assumption that the noise crossspectral-density matrix is Toeplitz. Thus the initially sparse matrix for the outer sections of the array (with wider hydrophone separations) can be filled out with values from elsewhere in the true matrix. This provides better angular resolution which is beneficial for the method. Secondly, the possibility of synthesising a VLA with a pair of hydrophones is investigated. The benefit of a synthetic aperture would be the lower cost of the equipment in an operational context. For the process to work (without extremely long integration times) the noise source spatial distribution needs to be stationary.