Jaroslaw Tegowski

Effects of intermittent entrainment of air bubbles by breaking wind waves on ocean reflectance and underwater light field

Light-scattering properties of air bubbles suspended in water and observational evidence of bubble entrainment by breaking wind waves indicate that bubble clouds may influence ocean reflectance and in-water light field characteristics within the surface layer. We estimate potential changes in remote sensing reflectance and in-water light field associated with a bubble entrainment event observed at a wind speed of 10 m s−1. Our approach combines acoustic measurements of bubble concentration as a function of time and depth and radiative transfer simulations of the light field within and leaving a water body. We show that the remote sensing reflectance can increase significantly (more than twofold) due to bubble entrainment, and these large variations occur over time periods on the order of minutes or less. The bubble clouds have a spectral effect on ocean reflectance such that the water patch containing bubbles will appear greener or more yellowish than the surrounding waters with no bubbles. These results are relevant to measurements of remote sensing reflectance made from just above the water surface with downlooking radiance meters whose spot size at the surface is of O(0.1–1) m. The light field characteristics within the bubble layer are also significantly affected, with most pronounced effects seen in the profiles of upwelling irradiance and upwelling radiance. Therefore the bubble entrainment can be a source of error in the estimation of reflectance from extrapolation of underwater measurements at a depth up to and across the surface. While our radiative transfer simulations were made using a model with a plane-parallel geometry, future efforts to examine the effects of spatial distribution of bubble clouds and their properties on ocean reflectance from spatially integrating [O(10–1000) m] satellite measurements will have to be based on a three-dimensional approach.

Statistical analysis of acoustic echoes from underwater meadows in the eutrophic Puck Bay (southern Baltic Sea)

In order to monitor the recovery of vegetation from pollution and the success of re-seeding efforts, acoustic echoes from the sea floor, covered and uncovered by underwater vegetation, were collected in Puck Bay (southern Baltic sea) using a 208 kHz Biosonics DT 4200 scientific echo sounder. The echo envelopes were examined and several of their parameters were recommended for further analysis. The possibility of using these parameters to distinguish between a bare sea floor and underwater meadows was tested. The parameters may be helpful in the identification of the species composition of the meadows and in accurate biomass assessment.

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.

Estimation of macrophytes using single‐beam and multibeam echosounding for environmental monitoring of arctic fjords (Kongsfjord, West Svalbard Island)

This paper presents results of a study on the spatial distribution and biomass of macrophytobentos in a fjord of Arctic Svalbard. Kongsfjord represents a periglacial environment with intense morphodynamic processes and rapidly progressing changes in the biotic environment, making it one of the most promising areas to research climate impact on ecosystems. The main objective was to provide an acoustic tool for the evaluation of benthic habitats. The 2007 field survey included systematic, co‐registered, single‐beam and multibeam echosounder measurements. Acoustic observations were verified by biological samplings and observations for the classification algorithm development and verification. Analyses of acoustic signals scattered on bottoms covered by algae indicate the good quality of the data recorded, providing a map of phytobenthos distribution and biomass estimation in Kongsfjord. The algorithms designed and tested for processing single‐ and multibeam data allow extracting the morphological forms of th...

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.

Multibeam Bathymetry and Slope Stability of Isvika Bay, Murchisonfjorden, Nordaustlandet

A research expedition to the polar region of Murchisonfjorden (Nordaustlandet, Svalbard) on the research vessel Horyzont II took place in August 2009. This paper presents results from an extensive bathymetric measurement campaign of the Isvika Bay, southern part of Murchisonfjorden. The aim of this campaign was to select optimal sites for sediment sampling. A detailed analysis of the bathymetric features is performed with a special emphasis on the slope stability conditions. A simple method for identifying areas of sediment redeposition is proposed. The results confirm that the Isvika Bay has two distinct basins separated by a ridge. Both basins display flat central regions surrounded by steep slopes with gullies. In addition, results of analysed Quaternary geological data have indicated that the area was affected by ice repeated glacial activity in the past and that there is no morphological form typical for glacier erosion and sedimentation.

Parametrical and Textural Analysis of Sidescan Sonar Images of the Seafloor

This paper presents a comparative analysis of a new method of sidescan sonar imagery segmentation helpful in the estimation and spatial distribution of macrophytobentos in Arctic conditions. In our research, we designed three methods, two based on a parametric analysis of sidescan sonar echo signals and the third on a textural analysis relying on mathematical morphology. Acoustic observations were verified by video recordings and biological samplings. A single beam echosounder was also used. The Spitsbergen fjords represent a periglacial environment with great diversity of morphodynamic processes and sensitivity for global warming changes, so it is one of the most promising areas to study the influence of climate change on an ecosystem. Sidescan sonar is a very effective and economic tool for mapping marine vegetation on the seafloor, but interpretation of data still causes many problems especially in the specific conditions of the Arctic fjords (underwater rocks, postglacial sediments, steep slopes). Proposed segmentation and classification algorithms enable estimation of the bottom area covered by algae and, together with results of biological sampling and single beam echosounder measurements, estimation of algae biovolume and biomass.

Nonlinear acoustical methods in the detection of gassy sediments

The applieation oj nonlinear backscattering oj aeoustieal signals in deteetion ojgas bubbles in subsurjaee layer oj sediment oj the Gulf oj Gdansk is presented. Gas bubbles concentration was estimated assuming that nonlinear scattering in soft sediments is similar as in water. Summary, difJerence and double harmonics generated only by gas bubbles were reeorded and used jor bubble density estimation. Comparisons oj the eoneentrations reeeived from difJerent nonlinear components show generally agreement in cafeulated bubble density distributions, although values oj densities acquired from almost identieal volumes in a single transmission differ.

Statistical and Spectral Features of Corrugated Seafloor Shaped by the Hans Glacier in Svalbard

High-resolution images of the seabed obtained with the use of hydroacoustic measurements allow a detailed identification of inaccessible seabed areas such as the Hans Glacier foreland in the Hornsund Fjord on Spitsbergen. Analyses presented in the paper were carried out on a Digital Elevation Model (DEM) of the bay’s seafloor exposed in the process of deglaciation, obtained from bathymetric data recorded by a multibeam echosounder. The main objective of this study was to show the relevance of the autocorrelation length parameter used to describe the roughness of the bottom surface based on the example of seafloor postglacial forms in the Hans Glacier foreland. The resulting parameter reflects the scale of the terrain roughness, which varies between geomorphologic forms. Maps of the autocorrelation length were derived from successive tiles of the data, overlapping by 90%. Based on this, the two-dimensional Fourier transform (2D FFT) was successively conducted, and the power spectral density and autocorrelation were calculated following the Wiener–Khinchin theorem. The thus obtained parameter describes the scale of the glacial bay seafloor roughness, which was assigned to the geomorphological features observed.

Spectral and statistical analyses of ambient noise

Calving of glaciers generates sound audible underwater, useful in studying glacier processes and ice floes dynamics. Measurements of underwater ambient noise were carried out in two completely different Spitsbergen fjords - Hornsund, surrounded by melting glaciers, and Murchison, devoid of glaciers but full of floating ice floes, at frequencies from 20 Hz to 24 kHz using an omnidirectional hydrophone deployed at 18 meters deep. Calm weather during the experiments enabled measurements without noise coming from wind, rain or breaking waves. Statistical and spectral analyses of its variations were used to identify calving events and phenomena associated with the dynamics of ice floes. We hypothesise that, at low frequencies, the probability density distribution of the noise significantly differs from the normal distribution and gives clues about the number and diversity of contributing sources. Goodness-of-fit tests were performed across all frequency bands, and their results showed central-limit behavior in a high frequency range (>2.5 kHz). For low frequencies (below 1 kHz), the histograms of instantaneous noise spectrum level averaged over 1-minute intervals were compared to the theoretical Gaussian distribution, whose 0.9th quantile was determined as a threshold value in order to distinguish high energy spectral components of non-Gaussian origin.