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Since the late 1960s, various groups have investigated the influence of marine surface films on mechanisms dominating energy and mass transfer across the ocean/atmosphere interface. However, a compendium summarizing the state-of-the-art research in this field is still missing. The book fills this gap and transfers the accumulated knowledge to the scientific community. After a brief historical chapter basic chemical insights are presented, followed by theoretical and experimental approaches carried out in laboratory facilities. Air-sea interaction experiments are then described and finally, remote sensing applications with sea slicks and crude oil spills are presented.



General Section


Oil on troubled waters — a historical survey

Without Abstract
Heinrich Hühnerfuss

Carlo Marangoni and the Laboratory of Physics at the High School “Liceo Classico Dante” in Firenze (Italy)

Without Abstract
Giuseppe Loglio

Tribute to Erik John Bock

Without Abstract
Nelson Frew

Chemical Characteristics of Sea Slicks and Oil Spills


Basic physicochemical principles of monomolecular sea slicks and crude oil spills

Without Abstract
Heinrich Hühnerfuss

New chemical insights into the structure and morphology of sea slicks and their geophysical interpretations

The different morphology of palmitic acid methyl ester (PME) slicks spread from n-hexane or ethanol was studied by ‘Brewster Angle Microscopy’ [BAM]. In the latter case, already at large areas per molecule a foam-like structure is being formed on the water surface, i.e., a two-dimensional network which appears to be comparable with the morphological structure of biogenic sea slicks. In line with this assumption, both the magnitudes of the radar backscatter damping ratios and the characteristics of the damping ratio/wave number curves were comparable for the PME slick spread from ethanol and for the biogenic slicks, while in the presence of the PME slick spread from n-hexane lower damping ratios were determined. In the first instance, we were able to simulate the water wave damping characteristics of the biogenic sea slicks very well. Furthermore, the relaxation of alkanoic acid esters, which are often being found in biogenic sea slicks, was investigated by ‘Infrared Reflection-Absorption Spectroscopy’ [IRRAS]. It turned out that the ester group is continuously hydrated and dehydrated during compression and dilation on an undulating water wave field. It can be safely assumed that the strong water wave damping induced by these chemical compounds is centrally related to this phenomenon.
Heinrich Hühnerfuss, Frank Hoffmann, Johannes Simon-Kutscher, Werner Alpers, Martin Gade

Sea slicks: variability in chemical composition and surface elasticity

Surface-active organic matter collected from the air-sea interface is examined using mass spectrometry. The mass spectra reveal significant spatio-temporal variability in the chemical composition of sea slick materials. This compositional variability leads to differences in observed surface pressure-elasticity relationships.
Nelson M. Frew, Robert K. Nelson, Carl G. Johnson

Correlation studies of mass spectral patterns and elasticity of sea-slick materials

Variations in the chemical composition of surfactants from natural sea slicks are compared to variations in surface elasticity using mass spectrometry, Langmuir film balance measurements, and multivariate statistical techniques. It is shown that the information on chemical class and molecular structure contained in the mass spectra is strongly correlated with measured static elasticity and can be used to estimate film elasticity at a given surface pressure.
Nelson M. Frew, Robert K. Nelson, Carl G. Johnson

On surface renewal and sea slicks

Interaction between a current and surface-active material is considered. Some simple cases where the substrate motion is steady and 2D is analysed using standard boundary layer theory. Questions like: how is the transversal dimension of a slick related to the film pressure and the substrate convergence? and how strong substrate motion does it take to break up a surface film?, are addressed. It is pointed out that the answers depend on whether the film can be considered stagnant, or develops selforganised motion. It is further pointed out how small scale thermal convection at the ocean surface is easily suppressed by a slick.
Kristian B. Dysthe

Chemical Characteristics


Physicochemical effects of the marine microlayer on air-sea gas transport

4 Concluding remarks
Much progress has been made in understanding the role of surface films in air-sea gas transfer. Advances in both laboratory and field techniques have provided improved measurements and needed insight into this complex and important process. Current efforts focused toward better understanding the key physical mechanisms involved in interfacial gas transfer and the role of surfactants, along with innovative and interdisciplinary field and laboratory measurements, will help to further our present knowledge.
Sean P. McKenna, Erik John Bock

Static and dynamic surface tension of marine water: onshore or platform-based measurements by the oscillating bubble tensiometer

The functionality and performances of an oscillating bubble tensiometer designed for the measurement of static and dynamic surface tension of marine water samples are illustrated. The instrumentation completely works in automatic mode. Semi-continuous operation can also be easily implemented. By virtue of its geometrical configuration, the apparatus shows a relative insensitivity to external disturbances, in comparison to other measurement techniques, and consequently it can be employed even under severe environmental conditions, such as those encountered onshore or on platform-based locations. Experimental observations of the dynamic surface tension response to harmonic surface area changes are reported, for samples of sea water and for a dilute aqueous solution of a standard surfactant. The observed results are considered useful for a physical characterisation of marine water in respect to interfacial ocean-atmosphere phenomena and processes. Furthermore, the oscillating-air-bubble tensiometer mimics the real behaviour of the bubbles (ubiquitously present in the near surface of the oceans) and appears to be a suitable instrument for the assessment of the presence of natural and man-made adsorption-film-forming organic materials at the air/water interface.
Giuseppe Loglio, Boris Noskov, Piero Pandolfini, Reinhard Miller

Multiple scattering of surface waves by two-dimensional colloid systems

Propagation of capillary waves along the surface of water covered by a homogeneous insoluble film has been a subject of numerous experimental and theoretical studies. However, it has been shown only recently that real surface films contain two-dimensional aggregates, which influence the characteristics of surface waves. The problem of multiple scattering of surface waves by two dimensional viscoelastic particles is considered briefly below. The results can be compared with the experimental data for condensed films with two-dimensional bubbles of gaseous phase.
Boris A. Noskov, Giuseppe Loglio

Laboratory study of the damping of parametric ripples due to surfactant films

Laboratory studies of damping of capillary-gravity waves (CGW) due to organic films were carried out in a wide range of surfactant concentrations using a method of parametrically excited surface waves (Faraday ripples) at wave frequencies from about 15 Hz to 30 Hz. The threshold acceleration for CGW excitation and CGW wavelengths were measured and the wave damping and the surface tension coefficients were retrieved for a clean water surface and for water covered with organic films. It is obtained that the damping coefficient for ordinary surface-active substances (e.g., oleyl alcohol and oleic acid) exhibits a maximum at small surfactant concentrations, comparable with the concentrations needed for monomolecular coverage and remains practically constant at large concentrations. The damping coefficient for polymer films (polyoxyalkylene glycol - “Emkarox”) shows two maxima, one of which is a narrow peak at small concentrations similar to the case of ordinary surfactants, and a “plateau”-like maximum at large concentrations. The dynamic film elasticities are retrieved from the measured damping coefficient using a theory of wave damping for purely elastic films. The dynamic elasticities are shown to differ strongly from the static elasticities, especially at large surfactant concentrations.
Stanislav A. Ermakov, Sergei V. Kijashko

Wave tank study of phase velocities and damping of gravity-capillary wind waves in the presence of surface films

The wave number-frequency spectra of gravity-capillary waves were measured using two optical spectrum analysers and an artificial gradient illuminator. It was found that phase velocities of centimetre-millimetre (cm-mm) scale waves on clean water do not obey the dispersion relation of free surface waves, but they increase with fetch, while frequencies of dominant decimetre (dm)-scale wind waves decrease with fetch. This observation implies that the wind wave spectrum contains nonlinear cm-mm-scale harmonics bound to the dominant waves and propagating with the phase velocities of the dominant waves. The relation between bound and free waves can be estimated from measurements of phase velocity. Wind ripple damping was found to be maximum at wavelengths around 5–7 mm. The latter effect agrees with results of our field experiments using artificial slicks and can be explained qualitatively by a nonlinear “cascade” damping mechanism, when the damping of dm-scale dominant waves leads to strong damping of their cm-mm-scale nonlinear harmonics including damping of “parasitic” capillary ripples.
Stanislav A. Ermakov, Irina A. Sergievskaya, Emma M. Zuikova, Vladimir Yu. Goldblat, Yury B. Shchegolkov

Laboratory measurements of artificial rain impinging on slick-free and slick-covered water surfaces

Laboratory measurements with artificial rain were carried out in the wind-wave tank of the University of Hamburg, in order to gain better understanding of the radar backscattering from a slick-free and slick-covered water surface, particularly when it is agitated by strong rain. We used a coherent 9.8 GHz (X band) scatterometer at different polarisations and at an incidence angle of 28 degrees, a resistance-type wire gauge, and a two-dimensional laser slope gauge. A water surface area of 2.3 m2 was agitated by strong artificial rain with a rain rate of 160 mm h−1 and rain drops of 2.9 mm diameter. The wind-speed range used in the present investigation was between 2 and 10 m s−1. A monomolecular surface film was produced by deploying oleyl alcohol on the water surface. The results of the analyses of the measured radar Doppler spectra and wave amplitude and slope spectra are presented. We show that while the wind-induced surface roughness is strongly reduced in the presence of the slick, at a high rain rate (of 160 mm h−1) the surface slick less strongly affects the rain-induced increase of the surface roughness (i.e., the generation of crowns, cavities, stalks, ring waves, and secondary drops).
Nicole Braun, Martin Gade, Philipp A. Lange

Imaging surfactant concentration distribution at the air/water interface

Part 1: Surfactant concentration gradient on a laminar channel flow
The surface specific spectroscopic probes of reflected second harmonic generation (SHG) and reflected sum frequency generation (SFG) have been successfully employed to measure surfactant monolayer concentration gradients on the ocean surface. Reflected SHG was adapted for areal measurements of monolayer concentrations using an intensified pulse gated CCD camera for detection and a dichromatic image splitter to allow for signal normalisation and scaling. As a preliminary test prior to field use, the imaging probe was used to measure the surfactant concentration gradients for an insoluble monolayer on a channel flow downstream of the Reynolds ridge. The resulting concentration measurements had a temporal resolution of 3 nanoseconds and spatial resolution of 0.21 millimetres or less per pixel.
Gerald M. Korenowski, Elizabeth A. van Wagenen, Amir Hirsa

Imaging surfactant concentration distributions at the air/water interface

Part 2: Insoluble monolayer concentrations on standing capillary waves
Surfactant concentrations at the peaks and troughs of a standing capillary wave field were measured using a reflected second harmonic generation (SHG) imaging technique. The results revealed behaviour that is consistent with a compaction of the insoluble monolayer at the crests of the capillary waves and a dilation of the monolayer in the troughs. Each measurement was obtained using a single 3 nanosecond laser pulse, providing essentially instantaneous measurements of surfactant concentrations.
Gerald M. Korenowski, John R. Saylor, Elizabeth A. van Wagenen, Joseph S. Kelley, Mark E. Anderson, Elizabeth J. Edwards

Modelling and Air-Sea Interactions


Variability of the wave number spectra of short surface waves in the ocean and their modulation due to internal waves and natural slicks

Spatial measurements of capillary-gravity (CG) waves in the ocean are collected using a scanning slope sensor mounted on a free-drifting, wave-following buoy. The spectral analysis of spatial measurements yields wave number resolution directly. The range of wavelengths resolved from the optical sensor is from 0.4 to 6.2 cm. This paper investigates the variability of the spectral densities of these capillary-gravity waves under natural field conditions. For mild to medium wind speeds, the variation factor (to be further defined in the text) is typically between 1.5 and 2.5 in the short gravity wave regime, and between 2 and 4 in the capillary wave regime. The quantitative enhancement of the roughness contrast by natural slicks and surface current strains induced by internal waves is also presented.
Paul A. Hwang

On the imaging of biogenic and anthropogenic surface films on the sea by radar sensors

Radar signatures of sea surface films of different origin are investigated, which have been acquired by airborne and spaceborne multi-frequency/multi-polarisation microwave sensors during the Spaceborne Imaging Radar-C/X-Band Synthetic Aperture Radar (SIR-C/X-SAR), missions in 1994, as well as by the ERS SAR in 1996–98. During SIR-C/XSAR, controlled surface film experiments were performed in the German Bight by deploying various quasi-biogenic substances and mineral oil on the sea surface, in order to study the radar signatures caused by surface films of different visco-elastic properties. In general, our results show that multi-frequency capabilities, rather than multi-polarisation capabilities, are needed for a radar-based system for the discrimination of marine surface films. We show that, under high wind conditions (> 10 m s−1), a discrimination between the different kinds of surface films is very difficult, whereas at low to moderate wind speeds (≤ 5 m s−1) a discrimination seems to be possible. This finding is supported analytically by means of a new model for the wave number-dependent radar contrast at high wind speeds (> 10 m s−1) and statistically through the analyses of more than 700 ERS SAR images. In addition, results of laboratory experiments are presented that were carried out in the wind-wave tank of the University of Hamburg. At certain wind-speed ranges a different damping of bound and freely propagating surface waves by monomolecular surface films is observed, which may explain the high radar contrasts measured by the microwave sensors.
Martin Gade

Slick radar image modelling with an extended “VIERS-1” wave spectrum

The VIERS-1 model (Janssen et al. 1998) was originally devised to describe ERS scatterometer measurements over the sea on a physical basis. This model is extended to include the saturation range, and it is used to derive equilibrium wave spectra and formulations for the relaxation toward that equilibrium, with and without slicks present. It was attempted to derive actual relaxation rates from the model, but this was only successful on part of the spectral range. The extended VIERS model is combined with a radar imaging model originally designed for radar bathymetry, to yield quantitative computations of radar contrast along transects through slicks. The resulting model shows an apparent size increase of the slick and a smoothing of its edges, due to the finite relaxation time of the slick-dampened, backscattering surface roughness. The present work offers no validation by observed images, but the trend in frequency and polarisation dependence of the radar contrast is supported by published measurements.
Harm Greidanus

Thermal imagery of surface renewal phenomena

Processes within the sea surface microlayer have been relatively little studied considering their major role in air-sea interaction, especially gas exchange. This state of affairs can be explained by the inaccessibility of this very thin layer to most methods. One of the few suitable tools is radiometry which can probe a surface layer of ∼1–1000 micrometres according to the wavelength chosen. In particular, thermal imagery can be used to investigate turbulence impinging on the sea surface - “surface renewal patterns”. Field measurements with thermal imaging cameras show evidence of fairly large-scale (∼ 1 metre) organised turbulent structures. Thermal imagery is also a powerful tool in the study of breaking waves. Thermal images of breaking waves are characterised by a “hot” high-emissivity crest ahead of a warm patch. Interpretation of field observations remains difficult. Greater control on variables is possible in the laboratory. Laboratory experiments show that in dynamically weak situations (e.g., free convection), though the influence of eddies is readily apparent, these eddies do not appear to bring bulk water to the absolute surface. Even quite weak bubble plumes are highly effective in renewing the surface, suggesting that they will influence the thermal signature of the sea surface following breaking waves, and may be an effective agent of air-sea gas exchange. Surface-active materials both influence the nature of surface renewal and the measurement process (through alteration of emissivity).
David K. Woolf, Nicholas Ward

Infrared imaging: a novel tool to investigate the influence of surface slicks on air-sea gas transfer

The influence of surface films on air-sea gas exchange at low and moderate wind speeds is investigated. Observations were made in the small Heidelberg circular wind-wave facility and in coastal and offshore waters south of Cape Cod, New England. The passive controlled flux technique was used to investigate the micro turbulence very near the water surface, which controls the rate of transfer of momentum, heat, and mass across the air-sea interface. The analysis of infrared image sequences allows the estimation of the net heat flux at the water surface, the skin-bulk temperature difference across the thermal sublayer and thus the heat transfer velocity. Using Schmidt number scaling, estimates of the gas transfer velocity are obtained. Experimental evidence shows that increased surface film concentrations suppress near surface turbulence and thus decrease the gas exchange compared to a slick-free ocean interface. If a surfactant is present, turbulent mixing is dampened and direct renewal of the surface is inhibited. A surface slick changes the hydrodynamic boundary conditions in that the length scales of near surface turbulence controlling air sea gas exchange are modified. The micro-scale temperature fluctuations at the water surface indicate that at low wind speeds the transport process is dominated by large-scale turbulence, whereas at higher wind speeds the smallest observed scales dominate the transport.
Uwe Schimpf, Nelson M. Frew, Bernd Jähne

Remote Sensing Applications


Detection of oil spills by airborne sensors

Aerial surveillance of marine oil spills has become a multinational effort, especially for the deterrence of potential polluters and support of oil spill clean-up crews. For many years, the main effort has been directed towards developing sensors with enhanced spill monitoring capabilities. These sensors can be divided into different classes, namely basic sensors and advanced sensors including thermal imagers with active laser illumination. Recently, more attention has been paid to the automated derivation of high-level information from airborne remotely sensed multispectral oil spill data. In this paper, we present an algorithmic framework for automated analysis and fusion of multi-spectral oil spill data acquired by a near range sensor suite. It is shown how infrared/ultraviolet, microwave radiometer and imaging laser fluorosensor data can be converted into high-level information using single-sensor data processing and multi-sensor data fusion. It is also explained how this knowledge can, in combination with external information processing, improve the usability of the maritime surveillance system.
Oliver Zielinski, Theo Hengstermann, Nils Robbe

Satellite monitoring of accidental and deliberate marine oil pollution

The present activities of the Joint Research Centre (JRC), a Directorate General of the European Commission, in the field of monitoring sea-based oil pollution are described. The methodology used to process and interpret a massive set of satellite images (i.e., mosaics over all European Seas) and the relevant legal framework are briefly introduced. Both the issues of accidental and deliberate oil discharges are addressed. The presentation then focuses on JRC experience acquired on the use of satellite images during the Prestige accident emergency in 2002. At that time, the JRC provided technical assistance on the interpretation of the satellite images to the EC Civil Protection Mechanism and interested EU Member States. In addition to this task, JRC is carrying out a systematic mapping of the illicit vessel discharges using mosaics of satellite images over all the European Seas. These maps and the associated statistics are repeated on an annual basis in order to assess its evolution. The results obtained for the Mediterranean and the Black Sea are of special interest due to the lack of a regular aerial surveillance as that in the North and in the Baltic Sea. This action helped to reveal for the first time the dimension of the oil pollution problem, thus stressing the need for more concerted international actions. Finally, a strategy to tackle this issue is presented. In this context, the need for additional research efforts involving the competent regional/national authorities and the importance of securing the role of space-borne sensors in support to aerial and naval means are outlined. The proposed strategy is based on the involvement of the end-users. Consequently, the JRC together with DG Environment has established a permanent Group of Experts on satellite monitoring of sea-based oil pollution.
Guido Ferraro, Dario Tarchi, Joaquim Fortuny, Alois Sieber

Long-term microwave radar monitoring of ocean slicks at low grazing angles

This paper proposes the use of a digital microwave marine radar system for the detection and monitoring of surfactant slicks at sea. The spatial, temporal and radiometric resolutions of this low grazing angle instrument are compared to those of satellite and airborne radars traditionally used for oil pollution monitoring. Experimental evidence is presented in two artificial surfactant releases in the coastal zone which were successfully detected and tracked with a shore-based marine radar. The information contained in a long-term time-series of NRCS images is explored and its synergistic use with traditional satellite- and air-borne radar data discussed.
Christine P Gommenginger, Simon R Boxall

Oil spills on ALMAZ-1 and ERS-1 SAR images: results from the DOSE-91 experiment

This paper presents the results of the oil spill observations using Almaz-1 and ERS-1 SAR images collected in the Norwegian Sea during the Dedicated Oil Spill Experiment (DOSE-91) in 1991. Three artificial spills were released from a vessel in August 1991 and data on sea and weather conditions near the test area were determined. The analysis of the acquired SAR images shows that the reduction of the backscatter from oil-covered sea surface attained values of up to 10–15 dB. For the first time an effect of an intensification of wind waves both within the area of the spill and at the windward edge of the oil spill, expressed as a magnification of the SAR image brightness, was detected. The increase in relative backscatter power was up to 2.0 dB. It is concluded that both Almaz-1 and ERS-1 SARs are valuable tools for oil spill detection and localisation, but that the detectability essentially depended on wind speed, sea state and age of spills.
Konstantin Litovchenko, Andrei Ivanov

Estimation of average surface currents from ERS SAR images of oil-tank cleaning spills

Synthetic Aperture Radar (SAR) images from ERS-1/2 have been used to study current-induced perturbations of spill bands in the Kuroshio east of Taiwan, stemming from ballast water of several ships that had been dumped after the cleaning of tanks during the movement of the respective ship parallel to Taiwan’s coast. Under weak winds, the variable surface currents normal to a ship’s track were the main cause of the displacement of spills relative to a straight line. The currents were associated with the interaction of the Kuroshio with the island of Lutao and ocean bottom variations as well as with vortex structures of different scales, which were observed near the Kuroshio boundary. The average current velocity was determined by dividing the magnitude of the spill displacement by its residence time, which in turn was inferred from the distance of the spill from a ship and the supposed ship velocity. The current shear zones manifested themselves on the SAR images as narrow light lines. Sharp shifts in a spill were observed at its crossing lines. The estimates of current shear, based on the magnitude of the shifts, agree with published data. The width of a spill band observed on May 20th, 1994, at different distances from a ship, was found from the sections normal to it. The change in width as a function of the residence time may be well approximated by a power function with an exponent of about 0.86.
Leonid Mitnik, Kun-Shan Chen, Chih-Tien Wang


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