UBC Theses and Dissertations
A two-component Arctic ambient noise model Greening, Michael Victor
Short term Arctic ambient noise spectra over the frequency band 2 - 200 Hz are presented along with a two component noise model capable of reproducing these spectra. The model is based on the measured source spectrum and the spatial, temporal and source level distributions of both active pressure ridging and thermal ice cracking. Modeled ambient noise levels are determined by summing the input energy of the distributions of ice cracking and pressure ridging events and removing the propagation loss. Measurements were obtained on a 22-element vertical array along with a 7-element horizontal array deployed beneath the Arctic pack ice in 420 meters of water. Over 900 thermal ice-cracking events were detected in approximately 2 hours of data col lected over several days during April 1988. The source directivity for events beyond 40 wavelengths range was found to be accurately represented by a dipole with an approximate 3 dB increase above the dipole directivity pattern near 60° - 65° caused by the leaked longitudinal plate wave. A technique for measuring the bottom reflectivity function by correcting the bottom reflection of a thermal ice crack for the measured directivity is presented. The spatial distribution of thermal ice-cracking events is consistent with a uniform distribution. Source levels were measured from 110 to 180 dB //μPa²/ Hz at 1 m with the distribution of all events approximating a linearly decreasing function on a log-dB scale of the number of events versus source level. Near the end of the data collection period, measurements from a nearby active pressure ridge were obtained. Evidence is presented that the infrasonic peak observed near 10 Hz in Arctic ambient noise spectra may result from a frequency dependent propagation loss acting on the source spectrum of pressure ridging. Both modeled and measured ambient noise spectra show that ice cracking may dominate the spring-time ambient noise to frequencies as low 40 Hz. Below 40 Hz, the ambient noise is dominated by a single or few active pressure ridges at ranges of tens of kilometers. Above 40 Hz, the ambient noise is dominated by a large distribution of thermal ice-cracking events with over 50% of the total noise level produced by events within 6 km range and over 80% produced by events within 30 km range.
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