Feasibility of Normal Incidence Techniques for use with REA Buoy

摘要

Literature relating to normal incidence techniques for determining geo-acoustic properties of sediment layers in the ocean is reviewed. The techniques outlined in the literature are assessed for use with a Rapid Environmental Assessment (REA) buoy currently under design at DRDC Atlantic. While there is extensive literature on normal incidence techniques for frequencies above 10 kHz and limited literature on experiments at lower frequencies in all cases there are critical differences in geometry of these previous experiments when compared to that of the REA buoy. One technique integrates the bottom and bottom-surface returns to determine roughness and reflection coefficient of the ocean-sediment interface. The frequency range for which this technique has been used (>10 kHz; 1-10 kHz) is not compatible with the current source of the REA buoy. A second technique uses waveform inversion to determine acoustic impedance (or other property) as a function of travel time. Geo-acoustic properties are found using either an iterative layer stripping technique or by minimizing the difference between perturbed models and the measured signal via least squares or simulated annealing. While the waveform inversion techniques have the most promise, the signal must be sampled at least 4 times the Nyquist criteria (or resampled using Fourier techniques if possible). A method for determining the attenuation of sub-surface layers is also reviewed, and a modified version is presented for use with the REA buoy.