Acoustic Propagation in Tidewater Glacial Fjords: A Dynamic and Extreme Underwater Environment
Abstract
The description of this effort can be located in R2 Sub-Activity Ocean Sciences in PE 0601153N. Tidewater glacial environments serve as an effective ground zero in the relationship between Earths ice and its oceans, thus it is necessary to develop novel methods to more effectively characterize these environments. A decade of accelerated research on ice-ocean interactions has identified two main processes (calving and submarine melting) that contribute to loss of ice at glacier termini, but neither has been parameterized in models and few in-situ observations are available. These processes cannot be completely observed by satellites, and regular calving events make near-terminus measurements challenging. Despite successful implementation within a variety of terrestrial and underwater environments, remote underwater acoustic monitoring remains underutilized in tidewater glacial fjords. Existing studies have revealed the potential of acoustic sensing in glacial fjords to reveal time-varying processes that occur there. Integrated noise in this environment comes from a myriad of physical processes at the ice-ocean boundary, including calving events, crevasse propagation, and the release of pressurized bubbles escaping due to melting of the submarine terminus face and icebergs at the fjord surface resulting in the loudest measured natural oceanic environment (120 dB re 1 Pa) [2]. Unfortunately, to date, acoustic surveys have been ad hoc, thus not adequately accounting for the effects of sound propagation in the ice-ocean environment and precluding accurate determination of how these noise contributors can change over time.Akin to understanding a speech from a distant speaker in a noisy, reverberant hall, the ability to accurately attribute noise to any specific process within a glacial fjord depends largely on understanding the effects of sound propagation within the environment. Sensors must be placed at a sufficient distance from the terminus to remain operational, and the acoustic environment of marine-terminating glaciers is adequately complex that the meaning of sounds must account for the depth-dependence of the propagation of signals from the source to the receiver. Additionally, the noise field in the fjord changes over time as the submarine face evolves, icebergs melt, and water properties adjust to the freshwater flux emerging at the grounding line. Importantly, by characterizing the time-dependent propagation environment we believe we can understand how calving and submarine melt change over time, which would be a first-of-its-kind observation.Previous studies of underwater acoustic signals in ice-marginal areas have not produced an understanding of sound propagation in the fjord nor its variabil.This project seeks to increase the United States Navys tactical advantage in underwater warfare through understanding the acoustical characteristics and sound propagation of ambient noise processes in tidewater glacial fjords to improve current (and develop new) techniques to monitor ice-ocean dynamics. We will conduct a propagation experiment using a known sound source and receivers in a prearranged geometry in LeConte Bay near LeConte Glacier in Southeast Alaska in order to study acoustic propagation within the fjord. Conductivity, temperature, and depth (CTD) measurements combined with underwater cameras and time-lapse photography of the fjord taken throughout the experiment will provide physical characterization of the underwater environment.Using these data, we will develop a numerical model to aid in the prediction of acousticpropagation within this and other dynamic, ice-covered environments.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Jul 20, 2020
- Source ID
- N000142012654
Entities
People
- Preston S Wilson
Organizations
- Office of Naval Research
- United States Navy
- University of Texas at Austin