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Remote Sensing of Oil Spills for Marine Life and Environmental Preservation

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A special issue of Remote Sensing (ISSN 2072-4292). This special issue belongs to the section "Ocean Remote Sensing".

Deadline for manuscript submissions: closed (31 December 2020) | Viewed by 16675

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Special Issue Editors

Dr. Stanislav Ermakov

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Guest Editor

Institute of Applied Physics, Russian Academy of Sciences
Interests: geophysical hydrodynamics; ocean remote sensing; microwave radar

Dr. José C.B. da Silva

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Guest Editor

Department of Geoscience, Environment and Spatial Planning (DGAOT), Faculty of Science, University of Porto, Rua do Campo Alegre, 687, 4169-007 Porto, Portugal
Interests: satellite oceanography; ocean remote sensing; submesoscale dynamics; internal waves; river plumes; ocean color
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Special Issue Information

Dear Colleagues,

Pollution of the sea surface is a serious threat for marine life and for the ecological state of the open ocean, coastal zones, and inland waters. The remote sensing of marine films, both oil spills and biogenic pollutions, aiming to identify the films and to quantify their characteristics is a very important problem in the context of marine environment safety. The problem is actively discussed in the literature, but is still far from its comprehensive solution.

Films on the sea surface result in enhanced attenuation of short wind waves and the formation of slicks (areas of flattened sea surface). Marine film slicks can be manifested in the signals of microwave and optical systems due to various physical mechanisms. Film slicks in optical images under natural illumination conditions appear as dark or bright areas depending on the sky brightness distribution and the geometry of observations. Microwave radar signatures of slicks are basically dark and determined mostly by the effect of suppression of short wind waves due to the film. The radar backscatter contrast of film slicks depends on radar wavelength, incidence angle, and polarization. Films can also be detected in IR or UV images because of their effect on water temperature, fluorescence spectra, etc. Slick signatures in different electromagnetic wavelength ranges depend dramatically on the physical/chemical characteristics of films, including film thickness, and on meteorological conditions.

Note that the remote sensing and characterization of marine film slicks is an intricate problem not only because of the complicated mechanisms of film signature formation, but also because of the existence of “look-alikes”. The latter are the areas of variable sea surface roughness which often look like slicks but are related to different oceanic/atmospheric processes in the absence of a film. Investigations of the problem of film slicks can improve our understanding of this phenomenon, which will lead to the development of new approaches/methods extending our capabilities of film detection and characterization with remote sensing techniques.

This Special Issue is focused on various aspects of the problem of film slick remote sensing, and the articles may address, but are not limited to, the following topics:

Microwave and optical, active and passive, remote sensing of oil spills on the sea surface, particularly, satellite methods of film characterization;
Remote sensing of natural marine films, relation between characteristics of marine films and biological processes in the upper ocean;
Theoretical aspects of the problem of film slicks;
Formation of marine film slicks and slick evolution due to small-scale and submesoscale processes such as cyclonic submesoscale vortices, internal waves, etc.;
Remote sensing of look-alikes of different origin; discrimination between oil and biogenic films and look-alikes.

Dr. Stanislav Ermakov
Dr. José C.B. da Silva
Guest Editor

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Keywords

oil spills
biogenic films
microwave radar
optical remote sensing
marine slicks
small-scale and submesoscale processes
slick look-alikes

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Research

16 pages, 3127 KiB

Open AccessArticle

The Role of Micro Breaking of Small-Scale Wind Waves in Radar Backscattering from Sea Surface

by Irina A. Sergievskaya, Stanislav A. Ermakov, Aleksey V. Ermoshkin, Ivan A. Kapustin, Olga V. Shomina and Alexander V. Kupaev

Remote Sens. 2020, 12(24), 4159; https://doi.org/10.3390/rs12244159 - 19 Dec 2020

Cited by 12 | Viewed by 2731

Abstract

The study of the microwave scattering mechanisms of the sea surface is extremely important for the development of radar sensing methods. Some time ago, Bragg (resonance) scattering of electromagnetic waves from the sea surface was proposed as the main mechanism of radar backscattering at moderate incidence angles of microwaves. However, it has been recently confirmed that Bragg scattering is often unable to correctly explain observational data and that some other physical mechanisms should be taken into consideration. The newly introduced additional scattering mechanism was characterized as non-polarized, or non-Bragg scattering, from quasi-specular facets appearing due to breaking wave crests, the latter usually occurring in moderate and strong winds. In this paper, it was determined experimentally that such non-polarized radar backscattering appeared not only for rough sea conditions in which wave crests strongly break and “white caps” occur, but also at very low wind velocities close to their threshold values for the wave generation process. The experiments were performed using two polarized Doppler radars. The experiments demonstrated that a polarization ratio, which characterizes relative contributions of non-polarized and Bragg components to the total backscatter, changed slightly with wind velocity and wind direction. Detailed analysis of radar Doppler shifts revealed two types of scatterers responsible for the non-polarized component. One type of scatterer, moving with the velocities of decimeter-scale wind waves, determined radar backscattering at low winds. We identified these scatterers as “microbreakers” and related them to nonlinear features in the profile of decimeter-scale waves, like bulges, toes and parasitic capillary ripples. The scatterers of the second type were associated with strong breaking, moved with the phase velocities of meter-scale breaking waves and appeared at moderate winds additionally to the “microbreakers”. Along with strong breakers, the impact of microbreaking in non-polarized backscattering at moderate winds remained significant; specifically the microbreakers were found to be responsible for about half of the non-polarized component of the radar return. The presence of surfactant films on the sea surface led to a significant suppression of the small-scale non-Bragg scattering and practically did not change the non-Bragg scatterer speed. This effect was explained by the fact that the nonlinear structures associated with dm-scale waves were strongly reduced in the presence of a film due to the cascade mechanism, even if the reduction of the amplitude of dm waves was weak. At the same time, the velocities of non-Bragg scatterers remained practically the same as in non-slick areas since the phase velocity of dm waves was not affected by the film. Full article

(This article belongs to the Special Issue Remote Sensing of Oil Spills for Marine Life and Environmental Preservation)

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22 pages, 11872 KiB

Open AccessArticle

Suppression of Wind Ripples and Microwave Backscattering Due to Turbulence Generated by Breaking Surface Waves

by Stanislav A. Ermakov, Vladimir A. Dobrokhotov, Irina A. Sergievskaya and Ivan A. Kapustin

Remote Sens. 2020, 12(21), 3618; https://doi.org/10.3390/rs12213618 - 5 Nov 2020

Cited by 12 | Viewed by 3154

Abstract

The role of wave breaking in microwave backscattering from the sea surface is a problem of great importance for the development of theories and methods on ocean remote sensing, in particular for oil spill remote sensing. Recently it has been shown that microwave radar return is determined by both Bragg and non-Bragg (non-polarized) scattering mechanisms and some evidence has been given that the latter is associated with wave breaking, in particular, with strong breaking such as spilling or plunging. However, our understanding of mechanisms of the action of strong wave breaking on small-scale wind waves (ripples) and thus on the radar return is still insufficient. In this paper an effect of suppression of radar backscattering after strong wave breaking has been revealed experimentally and has been attributed to the wind ripple suppression due to turbulence generated by strong wave breaking. The experiments were carried out in a wind wave tank where a frequency modulated wave train of intense meter-decimeter-scale surface waves was generated by a mechanical wave maker. The wave train was compressed according to the gravity wave dispersion relation (“dispersive focusing”) into a short-wave packet at a given distance from the wave maker. Strong wave breaking with wave crest overturning (spilling) occurred for one or two highest waves in the packet. Short decimeter-centimeter-scale wind waves were generated at gentle winds, simultaneously with the long breaking waves. A Ka-band scatterometer was used to study microwave backscattering from the surface waves in the tank. The scatterometer looking at the area of wave breaking was mounted over the tank at a height of about 1 m above the mean water level, the incidence angle of the microwave radiation was about 50 degrees. It has been obtained that the radar return in the presence of short wind waves is characterized by the radar Doppler spectrum with a peak roughly centered in the vicinity of Bragg wave frequencies. The radar return was strongly enhanced in a wide frequency range of the radar Doppler spectrum when a packet of long breaking waves arrived at the area irradiated by the radar. After the passage of breaking waves, the radar return strongly dropped and then slowly recovered to the initial level. Measurements of velocities in the upper water layer have confirmed that the attenuation of radar backscattering after wave breaking is due to suppression of short wind waves by turbulence generated in the breaking zone. A physical analysis of the effect has been presented.
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(This article belongs to the Special Issue Remote Sensing of Oil Spills for Marine Life and Environmental Preservation)

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19 pages, 2817 KiB

Open AccessArticle

Internal Wave Dark-Band Signatures in ALOS-PALSAR Imagery Revealed by the Standard Deviation of the Co-Polarized Phase Difference

by Carina R. de Macedo and José C. B. da Silva

Remote Sens. 2020, 12(15), 2372; https://doi.org/10.3390/rs12152372 - 23 Jul 2020

Cited by 5 | Viewed by 2833

Abstract

Analysis of synthetic aperture radar (SAR) images in L-band of short-period internal waves (IWs), and classification of their radar signatures is presented by means of a polarimetric data set from ALOS-PALSAR mission. We choose the polarimetric feature named standard deviation(std) of the co-polarized phase difference (CPD) to identify fundamental differences in SAR signatures of internal waves, and divided them into three different classes, according to their backscattered modulation depths and morphology as well as the std CPD, namely: double-signed, single-negative, and single-positive signatures, for IW normalized image transects that display, respectively, signatures in the form of bright/dark, dark, and bright bands that correspond to positive/negative, negative, or positive variations of radar backscatter. These radar power types of signatures have a counterpart in the std CPD normalized transects, and in this paper we discuss those correlations and decorrelations. We focus in the single-negative type of signature, that is dark bands on gray background, and show that the std CPD is greatly enhanced over the troughs and rear slopes of those IWs. It is suggested that such behavior is consistent with the presence of surface slicks owing to enhanced surfactant concentration. Furthermore, those single-negative SAR signatures appear at locations where and when biological productivity is enhanced. It is found that the modulation depths associated to the std CPD is higher than the one associated to the HH-polarized radar backscatter for single-negative signatures propagating in the range direction, while the reverse occurs for the other types of signatures. Full article

(This article belongs to the Special Issue Remote Sensing of Oil Spills for Marine Life and Environmental Preservation)

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24 pages, 3890 KiB

Open AccessArticle

Classification of Oil Slicks and Look-Alike Slicks: A Linear Discriminant Analysis of Microwave, Infrared, and Optical Satellite Measurements

by Gustavo de Araújo Carvalho, Peter J. Minnett, Nelson F. F. Ebecken and Luiz Landau

Remote Sens. 2020, 12(13), 2078; https://doi.org/10.3390/rs12132078 - 28 Jun 2020

Cited by 9 | Viewed by 2895

Abstract

We classify low-backscatter regions observed in Synthetic Aperture Radar (SAR) measurements of the surface of the ocean as either oil slicks or look-alike slicks (radar false targets). Our proposed classification algorithm is based on Linear Discriminant Analyses (LDAs) of RADARSAT-1 measurements (402 scenes off the southeast coast of Brazil from July 2001 to June 2003) and Meteorological-Oceanographic (MetOc) data from other earth observation sensors: Advanced Very High Resolution Radiometer (AVHRR), Sea-Viewing Wide Field-of-View Sensor (SeaWiFS), Moderate Resolution Imaging Spectroradiometer (MODIS), and Quick Scatterometer (QuikSCAT). Oil slicks are sea-surface expressions of exploration and production oil, ship- and orphan-spills. False targets are associated with environmental phenomena, such as biogenic films, algal blooms, upwelling, low wind, or rain cells. Both categories have been interpreted by domain-experts: mineral oil (n = 350; 45.5%) and petroleum free (n = 419; 54.5%). We explore nine size variables (area, perimeter, etc.) and three types of MetOc information (sea surface temperature, chlorophyll-a, and wind speed) that describe the 769 samples analyzed. Seven attribute–domain combinations are tested with three non-linear transformations (none, cube root, log₁₀), with and without MetOc, adding to 39 attribute subdivisions. Classification accuracies are independent of data transformation and improve when selected size attributes are combined with MetOc, leading to overall accuracies of ~80% and sound levels of sensitivity (~90%), specificity (~80%), positive (~80%) and negative (~90%) predictive values. The effectiveness of this data-driven attempt supports further commercial or academic implementation of our LDA algorithm. Full article

(This article belongs to the Special Issue Remote Sensing of Oil Spills for Marine Life and Environmental Preservation)

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26 pages, 4374 KiB

Open AccessArticle

Mineral Oil Slicks Identification Using Dual Co-polarized Radarsat-2 and TerraSAR-X SAR Imagery

by Dmitry Ivonin, Camilla Brekke, Stine Skrunes, Andrei Ivanov and Nataliya Kozhelupova

Remote Sens. 2020, 12(7), 1061; https://doi.org/10.3390/rs12071061 - 25 Mar 2020

Cited by 11 | Viewed by 4013

Abstract

This study is devoted to a generalization of C-band Radarsat-2 and X-band TerraSAR-X synthetic aperture radar (SAR) data in the form of a diagram serving to easily identify mineral oil slicks (crude oil and emulsions) and separate them from the other oil slicks. The diagram is based on the multi-polarization parameter called Resonant to Non-resonant signal Damping (RND) introduced by Ivonin et al. in 2016, which is related to the ratio between damping within the slick of the short waves and wave breakings. SAR images acquired in the North Sea during oil-on-water exercises in 2011–2012 containing three types of oil spills (crude oil, emulsion, and plant oil) were used. The analysis was performed under moderate sea conditions (wind speeds of 2–6 m/s and sea wave heights of less than 2 m), the incidence angles of 27°–49°, and the signal-to-noise ratio (SNR) of −3 to 11 dB within slicks. On the diagram plane, created by the RND parameter and the Bragg wave number, the mineral oil samples form a well-outlined zone, called a mineral oil zone. For C-band data, the plant oil samples were clearly distinguished from the mineral oils in the diagram. Determination of the confidence level for the detection of mineral oils versus plant oil was proposed using the mineral oil zone boundaries. The mineral oil data with SNR within slicks better than 2 dB lay within this zone with a confidence level better than 65%. The plant oil data with the same SNR lay outside this zone with a confidence level of better than 80%. For mineral oil with SNR of −3 dB, the confidence level is 55%. Full article

(This article belongs to the Special Issue Remote Sensing of Oil Spills for Marine Life and Environmental Preservation)

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