Oskar Glowacki

Underwater acoustic signatures of glacier calving

Climate-driven ice-water interactions in the contact zone between marine-terminating glaciers and the ocean surface show a dynamic and complex nature. Tidewater glaciers lose volume through the poorly understood process of calving. A detailed description of the mechanisms controlling the course of calving is essential for the reliable estimation and prediction of mass loss from glaciers. Here we present the potential of hydroacoustic methods to investigate different modes of ice detachments. High-frequency underwater ambient noise recordings are combined with synchronized, high-resolution, time-lapse photography of the Hans Glacier cliff in Hornsund Fjord, Spitsbergen, to identify three types of calving events: typical subaerial, sliding subaerial, and submarine. A quantitative analysis of the data reveals a robust correlation between ice impact energy and acoustic emission at frequencies below 200 Hz for subaerial calving. We suggest that relatively inexpensive acoustic methods can be successfully used to provide quantitative descriptions of the various calving types.

Directionality of the ambient noise field in an Arctic, glacial bay

The directionality of ambient noise in an Arctic tidewater glacier bay was measured using two horizontally spaced, broadband hydrophones. Segments of noise were divided into two frequency bands and analyzed for arrival angle. These data show that different classes of source radiate noise in distinct spectral bands and are spatially diverse. A previously unidentified source, the interaction of surface gravity waves with underside of ice ledges at the periphery of icebergs, is described. The generation of noise by ice-wave interaction suggests that surface waves should be measured if ambient noise is to be used to monitor ice dynamics in glacial fjords.

The impact of glacier meltwater on the underwater noise field in a glacial bay

Ambient noise oceanography is proving to be an efficient and effective tool for the study of ice-ocean interactions in the bays of marine-terminating glaciers. However, obtaining quantitative estimates of ice melting or calving processes from ambient noise requires an understanding of how sound propagation through the bay attenuates and filters the noise spectrum. Measurements of the vertical structure in sound speed in the vicinity of the Hans Glacier in Hornsund Fjord, Spitsbergen, made with O(130) CTD casts between May and November 2015, reveal high-gradient, upward-refracting sound speed profiles created by cold, fresh meltwater during summer months. Simultaneous recordings of underwater ambient noise made at depths of 1, 10 and 20 meters in combination with propagation model calculations using the model Bellhop illustrate the dominant role these surface ducts play in shaping the underwater soundscape. The surface ducts lead to a higher intensity and greater variability of acoustic energy in the near-surface layer covered by glacially-modified waters relative to deeper waters, indicating deeper zones as most appropriate for inter-seasonal acoustic monitoring of the glacial melt. Surface waveguides in Hornsund are relatively shallow and trap sound above O(1 kHz). Deeper waveguides observed elsewhere will also trap low-frequency sounds, such as those generated by calving events for example. Finally, the ambient noise field in Hornsund is shown to be strongly dependent on the distribution of ice throughout the bay, stressing the importance of performing complementary environmental measurements when interpreting the results of acoustic surveys. This article is protected by copyright. All rights reserved.

Application of Passive Hydroacoustics in the Studies of Sea-Ice, Icebergs and Glaciers: Issues, Approaches and Future Needs

Arctic and Southern Oceans are extremely noisy places. Various geophysical and biological processes generate underwater sounds at different frequencies. Using spectral, wavelet and statistical analysis, it becomes possible to distinguish almost all individual phenomena. This allows the assessment of, among other things, the rainfall intensity, various characteristics of wind-generated waves, abundance of marine organisms and shipping traffic. These issues are now relatively well-understood. What is more, for such studies hydroacoustic methods have been widely used for many years and provided satisfactory results. In the last decades, however, more and more attention is paid to sea-ice processes and properties, calving events and drifting icebergs. Melting ice and retreating tidewater glaciers are also sources of underwater ambient noise. This becomes more and more noticeable due to the observed climate shifts. Dynamic nature of these phenomena and harsh conditions encountered during field measurements still limit the progress in this area of research. In spite of all, recent preliminary studies show the possibility of using passive acoustic methods for both analyzing calving events in the Arctic fjords and investigating the behavior of icebergs. It became possible, for instance, to identify and describe various stages of calving processes: large rumbles, ice fractures, impacts on the water and iceberg oscillations. On the other hand, ambient noise related with freshwater outflows and sound propagation in the vicinity of glaciers are still unstudied. Moreover, underwater sounds associated with sea-ice processes occurring in small basins are also poorly understood, as well as their directivity and relationships with meteorological and oceanographic conditions. These topics require further investigation, which will enable the development of appropriate classification algorithms. For this purpose, new field experiments and methods of data analysis as well as state-of-the-art measuring devices are needed. A review of existing research articles concerning underwater cryogenic sounds is presented here, supplemented by a summary of the main gaps and suggested future needs. All papers are sorted thematically and chronologically, showing the historical development of hydroacoustic methods and approaches in this area.

The vertical directionality of melt noise from a glacier terminus

Measurements of the vertical directionality of the sound of bubbles released explosively from a melting glacier terminus are presented. These data are motivated by a desire to infer ice melt rates using the sound generated by bubbles, trapped in the glacier ice at the base of the fern layer and pressurized over time, as they are released by the melting terminus. The free energy available to generate noise is a function of the difference between the bubble internal gas pressure and hydrostatic pressure, both of which can vary with depth beneath the sea surface. Previous studies of bubble gas pressure in glacier ice suggest that the noise generated by bubble release should decrease with increasing depth below the surface. Measurements of noise directionality made with a compact, 4-element hydrophone array approximately 300 m in front of Hansbreen Glacier in Hornsund Fjord, Southwestern Svalbard in the summer of 2017 will be presented and discussed. This study suggests that only the top few 10’s of meters of...

Studying melting icebergs with ambient noise oceanography

Marine-terminating glaciers are retreating at an unprecedented pace, largely as a result of enhanced submarine melting. However, studying ice-ocean interactions is complicated due to both harsh conditions prevailing in glacial bays and lack of scientific methods enabling continuous measurements. Recent studies have shown that high underwater noise levels measured in the Arctic are related to glacier melt, but quantitative research requires proper separation of individual noise sources, including icebergs and glacier fronts. Therefore, we show results of field experiments carried out in 2013, 2015, and 2016 in Hornsund fjord, Svalbard, to present directionality and statistics of the noise produced by melting icebergs. Measurements of noise directionality were conducted with 3-hydrophone acoustic array. Calculated angles of arrivals for the noise at the frequency range of 1–10 kHz correspond well to locations of individual, grounded icebergs. The amplitude of sound emitted by these sources has a symmetric α...

A propagation experiment in an Arctic glacial fjord

A propagation experiment was undertaken in the meltwater-modified surface layer in the bay of a marine-terminating Arctic glacier. Broad-band, m-sequence transmissions were made with an International Transducer Corporation 1007 source deployed from a drifting boat tracked using gps, approximately 500 m in front of Hansbreen Glacier in Hornsund Fjord, Southwestern Svalbard in the summer of 2017. Signals were received with a short array of Hitech HTI-96 hydrophones tethered to an autonomous, anchored boat. The distance between the hydrophone array and source was measured every 2 minutes using a laser rangefinder. Propagation effects in this noisy and dynamic environment include waveguide propagation through the meltwater-modified surface layer and occlusion of transmissions by drifting icebergs. Details of the experiment and propagation effects noted during the transmission period will be presented and discussed. [Work funded by ONR, Grant No. N00014-17-1-2633, and by the Polish National Science Centre, Gra...

An acoustic study of sea ice behavior in a shallow, Arctic bay

Recent acceleration of sea ice decline observed in the Arctic Ocean draws attention to environmental factors driving this phenomenon. One of the main conclusions is a growing need for better understanding of sea ice drift, deformation, and fracturing. In response to that call, several ambient noise recordings were carried out in the coastal zone of Hornsund Fjord, Spitsbergen, in spring 2015 to study underwater acoustic signatures of sea ice behavior. The noise levels varied significantly with sea ice type and intensity of external forces. Low-frequency signatures were strongly related to tidal cycle, which manifested in much higher SPL values at low water. Compacted ice cover is periodically deformed and crushed, representing a significant contribution to the ambient noise field in the study site. Average noise levels at frequencies above 1 kHz are, in turn, considerably higher in front of marine-terminating glacier than in the neighboring, non-glacial bay. These differences, expanding with the rise of w...