Ambient Noise from Turbidity Currents in Howe Sound
Hatcher, Matthew Gordon
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Turbidity currents are gravity currents that derive their density difference from sediment suspended within the fluid. In the marine environment they are responsible for sediment transport on large scales (e.g. it is thought that the bulk of terrigenous mobile sediments in the ocean were carried to abyssal depths by turbidity currents, through geologic time) and smaller scales, such as river deltas. Previous research has shown that sediment being transported in a fluid can produce sediment self-generated noise (SGN), arising from inter-particle collisions within the flow, or the associated bedload transport. Generally turbidity currents are difficult to measure in situ, due to their unpredictability in time and space; however, environments where sediment-laden rivers enter fjords, forming deltas, can be an exception. The spatial uncertainty is drastically reduced due to the topographical constraints, and the temporal uncertainty may also be reduced, depending on the trigger mechanism. During a 5 day period in June 2013, measurements were made of turbidity currents in Howe Sound, using both active and passive acoustic instrumentation. The primary goal of this thesis is to explore the use of passive acoustics for turbidity current detection and monitoring, and further—from the spectral characteristics of turbidity current noise—to establish the likely sound generation mechanism. The spectral shape of the measured turbidity current noise, and that predicted by the SGN mechanism are consistent, indicating turbidity current noise is generated by particle collisions. The secondary goal is to establish a relationship between the noise signal, and the dynamical properties of these sediment-laden flows. The relationship between sound pressure squared—normalized by turbidity current width—and head speed to the power of seven, is consistent with both the measurements and predictions. The predictions use the linear relationship between sound pressure and collision speed (for a single collision), and estimates of rates of collision occurrence, between moving particles, based on the kinetic theory of gases.