Ambient Noise from Turbidity Currents in Howe Sound
Date
2017-08-31T14:39:47Z
Authors
Hatcher, Matthew Gordon
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Abstract
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.
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Keywords
Turbidity Current, Sediment Generated Noise (SGN)