Anomalous frequency behavior in low-frequency sea-surface backscatter

Surface backscattering strengths have been measured between 70 and 1500 Hz using broadband SUS charges as sources in the Critical Sea Test 7 at-sea test. While the results of the measurements for frequencies of 1 kHz and below have been presented previously, the extension of the results up to 1.5 kHz have shown some unexpected frequency behavior in the scattering strengths as a function of wind speed. For wind speeds of 15 m/s and higher, scattering strengths at a fixed grazing angle are found to peak in the 600- to 1000-Hz region and then decline with increasing frequency, while for lower wind speeds the scattering strengths increase roughly monotonically with increasing frequency. Possible explanations for this behavior will be discussed. [Work supported by ONR.]