Multi-Modal and Short-Range Transmission Loss in Ice-Covered, Near-Shore Arctic Waters

In the past century, extensive research has been done regarding the sound propagation in arctic ice sheets. The majority of this research has focused on low frequency propagation over long distances. One of the most commonly used excitation methods for air-ice-water layers has been explosives. However, environmental regulation has become more stringent, disallowing the use of almost all explosive excitation types. Due to changing climate conditions in these environments, new experimentation is warranted to determine sound propagation characteristics in, through, and under thin ice sheets, in shallow water, over short distances. In April, 2016 several experiments were conducted approximately 2 km off the coast of Barrow, Alaska on shore-fast, first year ice, approximately 1 m thick. To determine the propagation characteristics of various sound sources, Frequency Response Functions (FRFs) were measured between a source location and several receiver locations at various distances from 1 m to 1 km. The primary sources used for this experiment were, an underwater speaker with various tonal outputs, an instrumented impact-hammer on the ice, and a propane cannon that produced an acoustic blast wave in air. The transmission characteristics of the multipath propagation (air, ice, water) are investigated and reported

Ocean Surface Gravity Wave Excitation of Flexural Gravity and Extensional Lamb Waves in Ice Shelves

Flexure and extension of ice shelves in response to incident ocean surface gravity waves have been linked to iceberg calving, rift growth, and even disintegration of ice shelves. Most modeling studies utilize a plate bending model for the ice, focusing exclusively on flexural gravity waves. Ross Ice shelf seismic data shows not only flexural gravity waves, with dominantly vertical displacements, but also extensional Lamb waves, which propagate much faster with dominantly horizontal displacements. Our objective is to model the full-wave response of ice shelves, including ocean compressibility, ice elasticity, and gravity. Our model is a 2D vertical cross-section of the ice shelf and sub-shelf ocean cavity. We quantify the frequency-dependent excitation of flexural gravity and extensional Lamb waves and provide a quantitative theory for extensional Lamb wave generation by the horizontal force imparted by pressure changes on the vertical ice shelf edge exerted by gravity waves. Our model predicts a horizontal to vertical displacement ratio that increases with decreasing frequency, with ratio equal to unity at ~0.001 Hz. Furthermore, in the very long period band (<0.003 Hz), tilt from flexural gravity waves provides an order of magnitude larger contribution to seismometer horizontal components than horizontal displacements from extensional Lamb waves

ACOUSTIC LOCALIZATION TECHNIQUES FOR APPLICATION IN NEAR-SHORE ARCTIC ENVIRONMENTS

The Arctic environment has undergone significant change in recent years. Multi-year ice is no longer prevalent in the Arctic. Instead, Arctic ice melts during summer months and re-freezes each winter. First-year ice, in comparison to multi-year ice, is different in terms of its acoustic properties. Therefore, acoustic propagation models of the Arctic may no longer be valid. The open water in the Arctic for longer time periods during the year invites anthropogenic traffic such as civilian tourism, industrial shipping, natural resource exploration, and military exercises. It is important to understand sound propagation in the first-year ice environment, especially in near-shore and shallow-water regions, where anthropogenic sources may be prevalent. It is also important to understand how to detect, identify, and track the anthropogenic sources in these environments in the absence of large acoustic sensory arrays. The goals of this dissertation are twofold: 1) Provide experimental transmission loss (TL) data for the Arctic environment as it now exists, that it may be used to validate new propagation models, and 2) Develop improved understanding of acoustic vector sensor (AVS) performance in real-world applications such as the first-year Arctic environment. Underwater and atmospheric acoustic TL have been measured in the Arctic environment. Ray tracing and parabolic equation simulations have been used for comparison to the TL data. Generally good agreement is observed between the experimental data and simulations, with some discrepancies. These discrepancies may be eliminated in the future with the development of improved models. Experiments have been conducted with underwater pa and atmospheric pp AVS to track mechanical noise sources in real-world environments with various frequency content and signal to noise ratio (SNR). A moving standard deviation (MSD) processing routine has been developed for use with AVS. The MSD processing routine is shown to be superior to direct integration or averaging of intensity spectra for direction of arrival (DOA) estimation. DOA error has been shown to be dependent on ground-reflected paths for pp AVS with analytical models. Underwater AVS have been shown to be feasible to track on-ice sources and atmospheric AVS have been shown feasible to track ground vehicle sources

Diurnal seismicity cycle linked to subsurface melting on an ice shelf

ABSTRACTSeismograms acquired on the McMurdo Ice Shelf, Antarctica, during an Austral summer melt season (November 2016–January 2017) reveal a diurnal cycle of seismicity, consisting of hundreds of thousands of small ice quakes limited to a 6–12 hour period during the evening, in an area where there is substantial subsurface melting. This cycle is explained by thermally induced bending and fracture of a frozen surface superimposed on a subsurface slush/water layer that is supported by solar radiation penetration and absorption. A simple, one-dimensional model of heat transfer driven by observed surface air temperature and shortwave absorption reproduces the presence and absence (as daily weather dictated) of the observed diurnal seismicity cycle. Seismic event statistics comparing event occurrence with amplitude suggest that the events are generated in a fractured medium featuring relatively low stresses, as is consistent with a frozen surface superimposed on subsurface slush. Waveforms of the icequakes are consistent with hydroacoustic phases at frequency ${\bf \gt} \bf 75\,{\bf Hz}$ and flexural-gravity waves at frequency $\bf {\bf \lt}25\,{\bf Hz}$. Our results suggest that seismic observation may prove useful in monitoring subsurface melting in a manner that complements other ground-based methods as well as remote sensing.</jats:p

On the propagation of acoustic-gravity waves under elastic ice sheets

The propagation of wave disturbances in water of varying depth bounded above by ice sheets is discussed, accounting for gravity, compressibility and elasticity effects. Considering the more realistic scenario of elastic ice sheets reveals a continuous spectrum of acoustic–gravity modes that propagate even below the cutoff frequency of the rigid surface solution where surface (gravity) waves cannot exist. The balance between gravitational forces and oscillations in the ice sheet defines a new dimensionless quantity Ka. When the ice sheet is relatively thin and the prescribed frequency is relatively low (Ka≪1), the free-surface bottom-pressure solution is retrieved in full. However, thicker ice sheets or propagation of relatively higher frequency modes (Ka≫1) alter the solution fundamentally, which is reflected in an amplified asymmetric signature and different characteristics of the eigenvalues, such that the bottom pressure is amplified when acoustic–gravity waves are transmitted to shallower waters. To analyse these scenarios, an analytical solution and a depth-integrated equation are derived for the cases of constant and varying depths, respectively. Together, these are capable of modelling realistic ocean geometries and an inhomogeneous distribution of ice sheets

NASA oceanic processes program: Status report, fiscal year 1980

Goals, philosophy, and objectives of NASA's Oceanic Processes Program are presented as well as detailed information on flight projects, sensor developments, future prospects, individual investigator tasks, and recent publications. A special feature is a group of brief descriptions prepared by leaders in the oceanographic community of how remote sensing might impact various areas of oceanography during the coming decade

Paleobathymetric Reconstruction, Modeled Ocean Circulation, and Sedimentation History in the Weddell Sea, Antarctica

The Weddell Sea basin is of particular significance for understanding climate processes, including the generation of ocean water masses and their influences on ocean circulation as well as the Antarctic ice sheets dynamics. The sedimentary record, preserved in the basin serves as an archive of the pre-glacial to glacial development, ocean circulation and tectonic evolution. This thesis focuses on understanding the sedimentation history and reconstructing paleo-water depths, using all available multichannel seismic lines and existing drilling sites, with the aim to apply the paleo-water depths to General Circulation Models (GCM) of the Weddell Sea basin. A series of sedimentary thicknesses grids (pre-glacial, transitional, full-glacial) and paleobathymetric grids produced in this work are essential contributions for numerical climate simulations and ocean circulations. These sedimentary thickness grids allow the comparison of sedimentary regimes of the pre-glacially dominated and glacially dominated stages of Weddell Sea history. The pre-glacial deposition with thicknesses of up to 5 km was controlled by the tectonic evolution and sea-floor spreading history interacting with terrigenous sediment supply. The transitional unit shows a relatively high sedimentation rate and has thicknesses of up to 3 km, which may be attributed to an early formation of the East Antarctic Ice Sheet having partly advanced to the coast or even inner shelf. The main deposition centre of the full-glacial unit lies in front of the Filchner-Ronne Ice Shelf and has sedimentation rates of up to 140-200 m/Myr, which infers that ice sheets grounded on the middle to outer shelf and that bottom-water currents strongly impacted the deep-sea sedimentation in the middle Miocene. The paleobathymetric grids at 15, 34 and 120 Ma are reconstructed by using a backstripping technique and applied to constrain paleoclimate models. Coupled GCM runs are forced by global warm climatic boundary conditions of the Mid-Miocene and the new Weddell Sea paleobathymetry data. The GCM model results suggest that deep water formation and ocean circulation are especially sensitive to the paleobathymetric configuration of the Weddell Sea which is mainly characterized by a more southerly shelf break than at present or in previous paleobathymetric reconstructions for the Miocene. The southwards shifted shelf break of the Weddell Sea results in dramatic changes in simulated mixed layer depth and bottom water formation. Intensification of this bottom water plays a significant role in sediment distribution and the geomorphology of the Weddell Sea margin, e.g. through the build-up of a number of large sediment drifts. In addition to the paleobathymetric study of the Weddell Sea, I carried out two seismic interpretation studies in the southeast Weddell Sea and along the Dronning Maud Land margin. Large deposition centers, the Crary Trough Mouth Fan and prominent sediment ridges, are interpreted as glacial deposits of the southeastern Weddell Sea. Two giant, sinuous, NE-SW-oriented sediment ridges are interpreted as turbidity-contourites, due to the complicated down-slope/along-slope processes occurring across their margins. The large catchment area, abundant sediment supply, fluctuating sea level and ice sheet dynamics are the major contributions for the sedimentation. The remarkable increase in mass-transport deposits during the Late Miocene and Middle to Late Pliocene is related to the build-up of pore overpressure during rapid sediment accumulation as well as changing sea level and may be triggered by glacio-isostatic paleoearthquakes. Based on seismic reflection data and well data acquired on the continental margin offshore Dronning Maud Land, the sedimentation processes are investigated. My investigations reveal that the Jutul-Penck Graben system on the Dronning Maud Land plays a significant role in erosion, transport, and deposition of sedimentary material. I further found seismic chimney structures in this region for the first time and attribute their formation to volcanic processes

Abstracts of manuscripts submitted in 1993 for publication

This volume contains the abstracts of manuscripts submitted for publication during calendar year 1993 by the staff and students of the Woods Hole Oceanographic Institution. We identify the journal of those manuscripts which are in press or have been published. The volume is intended to be informative, but not a bibliography. The abstracts are listed by title in the Table of Contents and ar grouped into one of our five departents, Marine Policy Center, Coastal Research Center, or the student category. An author index is presented in the back to facilitate locating specific papers