Wu Hui-Xia

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  • Thesis
    Ambient noise measurements in the 200-300 Hz band during the Greenland Sea tomography experiment
    (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 1992-06) Wu, Hui-Xia
    A six mooring acoustic tomography array was jointly deployed by Scripps Institution of Oceanography and Woods Hole Oceanographic Institution in 1988, and a year long time series of ambient noise in the 200-300 Hz was collected by those moorings. Large scale meteorological environmental information, particularly wind, was provided during that same year by the British Meteorological Office. Time series of ice type and ice concentration were provided by Special Sensor Microwave Imager and Advanced Very High Resolution Radiometer satellite. Using those data sets, we were able to look at the characteristics of the ambient noise and to correlate the noise against significant environmental variables such as wind , ice concentration, ice edge position, etc. The largest noise levels are generally seen during the winter months and are associated with periods of moderate to strong wind speed. The lowest noise levels are confined to summer; however, there is also an extremely quiet period in mid-winter, coincident with heavy ice cover. During the ice-free periods, the ambient noise is higher than the Wenz ambient noise for open water. The regressions between the noise and wind speed show that the noise is wind dependent, with slightly lower slopes than the Wenz curve. Under the heavy pack ice cover conditions, noise levels are much lower than during the ice-free periods, even lower in fact than Wenz noise for open water when the wind speeds pass 11 m/s. The ambient noise is almost wind independent during this period. The overall noise levels are highest during the ice edge advance/ retreat period; the noise is also wind dependent , with the regression slopes higher than that for ice-free period, but still lower than that of the Wenz curves. . Noise and wind fields correlate well in fall and during the ice edge advance/ retreat periods, but are less correlated under the heavy ice cover and during low wind speed periods. The spatial cross correlations of the noise fields show quite high levels, up to 0.9 in fall and during the ice edge advance/ retreat period, but there is less correlation during other periods. The MIZ noise levels are dependent on the distance between the receiver and the ice edge and also the ice concentration. The noise peaks at the ice edge and diminishes faster going under the ice than into the open water. The measured noise levels near the ice edges are about 4 to 7 dB higher than open water, and about 7 to 10 dB higher than levels far into the ice field. In the MIZ, on-ice-wind results in higher noise than off-ice-wind. Ambient noise increases as on-ice-wind increases, but increases much slower or even decreases as off-ice-wind increases.