Ocean Medium Research Articles

Generalized Ince–Gaussian beams propagation through oceanic medium

Generalized Ince–Gaussian beams propagation through oceanic medium

Bit error rate of M-pulse position modulated laser beams for vertical links operating in weak oceanic turbulence

Abstract The on-axis scintillation index of laser beams is investigated by employing the Rytov method in weakly turbulent oceanic medium for up/downlink coupling of laser communication between any underwater vehicles or divers. For vertical link, the formulation of the on-axis scintillation index of laser beams is derived analytically and evaluated for plane, collimated Gaussian and spherical beams in specific medium, including the Atlantic Ocean at mid and low latitudes associating temperature and salinity changes at low latitude, at mid latitude-summer and at mid latitude-winter. Using the scintillation index, bit error rate (BER) performances of M-pulse position modulation (M-PPM) are investigated for these types of laser beam. The variations of the scintillation index against the uplink/downlink propagation distances, source size and zenith angle are examined, and BER variations versus the Kolmogorov microscale and the symbol orders, and results are compared. It is noted that the behavior of the scintillation index that depends on the relative strength of temperature and salinity fluctuations which changes in depth, is different for uplink/downlink, and for each latitude due to its distinct characteristics. Source size that minimizes the scintillation index values is in the range of about 0.1 cm-0.2 cm for all latitudes.

Lagrangian modelling of reactive contaminant transport in the multi-component marine medium

The particle-tracking method for transporting radionuclides in multicomponent ocean medium (water and multifractional suspended and deposited sediments) is considered using a probabilistic approach for simulating interaction processes between several states of radioactivity. The state transformations as a result of reactions of the first order were described using the master equation for the probability of the particle being in the given state. Transition probabilities between all possible states can be obtained from the numerical solution of the matrix master equation that is derived in the paper. In the first approximation, the Euler method was used to obtain a solution for the next time step. This approach can be applied to any linear system of equations describing phase transitions with any number of states, but it requires small values of the transition probabilities to ensure only a single-phase change during one time step. The paper also focuses on deriving the Lagrangian interface conditions between the water column and bottom deposition. To apply the probabilistic approach, the boundary conditions were considered as the reaction terms in a thin near-bottom interface layer in which boundary conditions were converted into source terms. For this layer, the corresponding master equation was derived to obtain transitional probabilities for particle states. The developed approaches were tested on numerical and analytical solutions of two test cases. It was found that the optimal thickness of the interface layer must be larger than the maximum vertical displacement of the particle during the one-time step, but it must be small enough to approximate the condition of uniform distribution of concentration in this layer.

Iron reduction as a viable metabolic pathway in Enceladus’ ocean

AbstractRecent studies postulated the viability of a suite of metabolic pathways in Enceladus’ ocean motivated by the detection of H2 and CO2 in the plumes – evidence for available free energy for methanogenesis driven by hydrothermal activity at the moon's seafloor. However, these have not yet been explored in detail. Here, a range of experiments were performed to investigate whether microbial iron reduction could be a viable metabolic pathway in the ocean by iron-reducing bacteria such as Geobacter sulfurreducens. This study has three main outcomes: (i) the successful reduction of a number of crystalline Fe(III)-bearing minerals predicted to be present at Enceladus was shown to take place to differing extents using acetate as an electron donor; (ii) substantial bacterial growth in a simulated Enceladus ocean medium was demonstrated using acetate and H2(g) separately as electron donors; (iii) microbial iron reduction of ferrihydrite was shown to partially occur at pH 9, the currently accepted value for Enceladus’ ocean, whilst being severely hindered at the ambient ocean temperature of 0°. This study proposes the possibilities for biogeochemical iron cycling in Enceladus’ ocean, suggesting that a strain of iron-reducing bacteria can effectively function under Enceladus-like conditions.

Effect of the complex air–sea interface on a hybrid atmosphere-underwater optical wireless communications system

Wireless optical communication (WOC) through the atmosphere–ocean system is fundamentally complex because of the diversity, concentration, types and vertical thickness of the constituents and poses a major challenge from the optical characteristics of bubbles at the air–sea interface and water constituents (phytoplankton, suspended sediments, and dissolved organic matter), and stratification (medium inhomogeneity). The present study aims to analyse the effects of atmospheric aerosols (their intensity, diversity, and vertical thickness), waves/bubbles at the air–sea interface, seawater constituents (phytoplankton, suspended sediments, and dissolved organic matter), and medium inhomogeneity along the path between a laser transmitter in the airborne platform and a receiver in the underwater platform using Monte Carlo technique. For the first time, our analysis showed that for a homogeneous atmosphere–ocean channel (7 km in the atmosphere column and 42 m in the ocean medium), the estimated normalized received power reduced to 17 dB and the time delay spread increased to 1.01 ns (considering a receiver of 7” and an aperture FOV of 180°) when compared to those values for a stratified atmosphere–ocean channel. The bubbles (both clean and coated) relative to the particulates in open oceanic water under high wind speeds have a significant impact on the spatio-temporal spread of the laser beam. There is also a well pronounced effect of coated bubbles on the received signal power in open ocean water. In contrast, the particulates have a higher impact on the received signal power than the clean/coated bubbles in turbid coastal water. These results suggest that the normalized received power when calculated for the cases of dense bubble populations under high wind speeds (at a wind speed 18 ms−1) reduced to 8–9 dB and increased to 0.0025 scintillation index. The coated bubbles at the air–water interface reduced the normalized received power by 1.41 dB compared to the clean bubbles. Further, our analysis indicated that the expansion of beam width and the reduction of beam divergence angle can be utilized to mitigate the performance degradation (power loss) caused by bubbles at the air–sea interface. These results and analyses will be helpful for a system designer to accurately define/decide the system parameters and build a functional communication system for the inhomogeneous atmosphere–ocean system.

Ocean Media: Digital South China Sea and Gilles Deleuze's Desert Islands

Ocean Media: Digital South China Sea and Gilles Deleuze's Desert Islands

Ocean Media: Digital South China Sea and Gilles Deleuze's Desert Islands

Ocean Media: Digital South China Sea and Gilles Deleuze's Desert Islands

Inversion of Volume Scattering Function for Estimation of Bubble Size Distribution in Ocean Waters

Angular distribution of the intensity of light scattered by a spherical air bubble in the oceanic medium is distinctive near the critical scattering angle region, when light interacts with a bubble with a refractive index less than its surrounding medium. This study is aimed at presenting theoretical and experimental results for the volume scattering function (VSF) of the bubbles generated under oceanic and laboratory conditions. The effect of bubbles on the VSFs was investigated using Mie calculations for the bubble sizes ranging from 5 to 200 μm and their size distribution slopes ranging from 3.6 to 4.6. Findings revealed that the critical angle scattering patterns (50°-80°) produced by bubbles become well-pronounced, enhanced and dependent on the minimum and maximum bubble sizes and their size distribution slopes (rmin, rmax and α). A relationship of the volume scattering phase function (β(θ, λ)) between an inverse power-law and a normally-distributed bubble population was established based on experimental data. Both experimental results and Mie simulations showed that the angular position of the first critical dark fringe (θ1) shifts in the direction of the critical angle as the minimum and mean values of the bubble size increase in accordance with the power law and normally-distributed bubble populations. The new inversion method predicted the light scattering patterns produced by bubbles near the critical angle region in close agreement with results from Mie theory simulations. The inversion method for estimating the bubble size distribution based on the measured VSF data have significant implications for underwater optical communication, imaging and ocean colour remote sensing studies.

ACSIS Atlantic Ocean medium resolution SST dataset: Reconstructed 5‐day, ½‐degree, Atlantic Ocean SST (1950‐2014)

AbstractA new dataset, the ACSIS Atlantic Ocean medium resolution SST dataset, is presented. This new dataset spans the period 1950–2014 at a 5‐day, ½‐degree resolution and covers the Atlantic Ocean. The dataset is based on in situ sea surface temperature (SST) observations from the International Comprehensive Ocean‐Atmosphere Data Set interpolated using Kriging to infill gaps and is available from the Centre for Environmental Data Analysis (CEDA) archive. Compared to existing datasets, the resolution is increased by a factor of 4 spatially and 6 temporally.

Adaptive Algorithms for Interferometric Processing

A numerical experiment on approving adaptive sound-source localization algorithms making it possible to solve the inverse problem in the absence of data on the hydroacoustic characteristics of ocean medium has been performed. These results may be useful for identifying low-noise sources in water areas where acoustic calibration is impossible and there is no a priori information about the transfer function. The conditions for the few- and multimode regimes of sound propagation are discussed.

Adaptive optics corrections of scintillations of Hermite–Gaussian modes in an oceanic medium

Adaptive optics correction of the scintillation index is found when Hermite-Gaussian laser beams are used in oceanic turbulence. Adaptive optics filter functions are used to find how the tilt, focus, astigmatism, coma, and total correction will behave under high order mode excitation. Reduction of the oceanic scintillation under various oceanic turbulence and system parameters is examined under different high order modes. Also, the effects of the source size, wavelength, and link length on the total adaptive optics correction of Hermite-Gaussian modes in an oceanic medium are investigated for different modes.

Performance analysis of electromagnetic (EM) wave in sea water medium

In this work, the performance analysis of Electromagnetic (EM) waves in sea water medium was described. It is very challenge-able situation to communicate the EM waves in ocean or sea because of high path loss and low signal-to-noise ratio, high bit error rate. However, in this examination on employment of EM waves in sea water medium, some theoretical observations through mathematically and simulations tools were performed to meet practical specifications for analyzing the feasibility and applicability of EM waves in this high loss environment of sea water medium. In addition, total path loss (TL), signal-to-noise ratio (SNR), bit error rate (BER) and Data rate channel capacity (DRC) parameters were also examined to understand applicability of EM wave at frequency range 4 GHz in sea water medium. In this work, transmission power is also a most important factor which helps to vary different required parameters. It was analyzed that transmission power also affects the propagation of EM wave in sea water medium. In addition, therefore, the impact of EM waves is less on marine life of ocean water medium as compared to acoustic waves and optical waves. Our results show that the 4 GHz frequency band has been analyzed for its suitability for small size sensor developments. After a detailed investigation a model for the performance analysis of the channel medium in terms of TL, SNR, BER and DRC was presented. Our research has shown that efficient communication between sensor nodes operating in the 4 GHz frequency band is theoretically possible for distances up to 2.6 m in underwater environment with total path loss 37.4761 db at 4 GHz band and system loss 70 db. The Values of SNR, BER, DRC are expected to be 0.0069, 0.4534, 0.3940 × 108 bits/s for distances up to 2.6 m in underwater environment with transmission power PT = 26 db m at 4 GHz frequency.

Anti submarine Warfare Oceanography

Anti-submarine warfare (ASW) Oceanography has become an important and independent discipline, especially after World War II as the threat perceptions from hostile underwater platforms has been on a continuously rising trajectory worldwide. There is no doubt that till date, acoustic energy remains the only viable and most effective means to detect underwater objects. However, sound waves are highly susceptible to refraction, reflection, scattering, reverberation and absorption in the ocean medium. Among these, refraction is controlled by the 3D sound speed structure in the specific ocean region and reflection loss by the sea state and ocean bottom characteristics. Water quality parameters such as sediment concentration and suspended biological masses influence attenuation, scattering, reverberation and absorption. Oceanographic variability emerges from diurnal heating, seasonal changes, river discharges, etc. and are the most important environmental factors deciding the 3D sound speed structure in any given area in the ocean.

BER OF ANNULAR BEAMS IN WEAK OCEANIC TURBULENCE

Based on Rytov method, on-axis scintillation index of laser communication link in a weak oceanic medium is formulated for collimated annular beam. Employing these obtained scintillation values, average bit error rate ( ) is evaluated where the intensity has log-normal distribution. Scintillation indices of collimated annular beams are found for fixed primary source size , varying annular beam thickness, propagation distance , source size , the rate of dissipation of the mean squared temperature , non-dimensional parameter representing the relative strength of temperature and salinity fluctuation . versus the source size and the average signal to noise found for the collimated annular beams are exhibited for various rate of dissipation of turbulent kinetic energy per unit mass of fluid and source sizes . At the stated link lengths, as secondary source size of annular beam equals to zero, that is, for Gaussian beam, will offer more advantages.

Decoherence effects in 3D fluctuating environments: Numerical and experiment study

This paper is devoted to the study of the effects of ocean fluctuations on acoustic propagation. The development of an ultrasonic testbench allowing to reproduce, under laboratory conditions, the influence of 3D fluctuations on received acoustic data is presented. The experimental protocol consists in transmitting, in a water tank, a high-frequency wavetrain throughout an acoustic slab presenting a plane input face and a randomly rough output face. The various regimes of saturation and unsaturation classically used in the literature are explored by tuning the statistics of the so-called RAFAL (RAndom Faced Acoustic Lens). Comparisons to a “corresponding” oceanic medium are obtained via a scaling procedure. In parallel, numerical tools were developed to provide meaningful comparison with the acquired data. Both based on a split-step Fourier algorithm, a 3D PE simulation of the tank experiment and a 3D PE simulation of real scale acoustic propagation programs are presented. Features of acoustic fields pertu...

A Theoretical Study of a Vector Radiative Transfer Equation for Atmosphere and Ocean Medium

Radiative transfer plays a fundamental role in several disciplines of sciences and engineering. For certain applications in turbid media, such as atmosphere or ocean, due to the polarization effect of light, the vector radiative transfer equation (VRTE) rather than the scalar radiative transfer equation must be adopted. Although there have been numerous discussions on the application and numerical simulations of the VRTE, a rigorous theoretical analysis of the equation is still needed. This letter provides a well-posedness analysis of the VRTE. Under some assumption on the combined size of the single-scattering albedo and the scattering phase matrix of the medium, existence and uniqueness of a solution are proved for the VRTE supplemented with a boundary condition. The solution is shown to be Lipschitz continuous dependent on the source function and the boundary condition function.

Joint retrieval of aerosol and water-leaving radiance from multispectral, multiangular and polarimetric measurements over ocean

Abstract. An optimization approach has been developed for simultaneous retrieval of aerosol properties and normalized water-leaving radiance (nLw) from multispectral, multiangular, and polarimetric observations over ocean. The main features of the method are (1) use of a simplified bio-optical model to estimate nLw, followed by an empirical refinement within a specified range to improve its accuracy; (2) improved algorithm convergence and stability by applying constraints on the spatial smoothness of aerosol loading and Chlorophyll a (Chl a) concentration across neighboring image patches and spectral constraints on aerosol optical properties and nLw across relevant bands; and (3) enhanced Jacobian calculation by modeling and storing the radiative transfer (RT) in aerosol/Rayleigh mixed layer, pure Rayleigh-scattering layers, and ocean medium separately, then coupling them to calculate the field at the sensor. This approach avoids unnecessary and time-consuming recalculations of RT in unperturbed layers in Jacobian evaluations. The Markov chain method is used to model RT in the aerosol/Rayleigh mixed layer and the doubling method is used for the uniform layers of the atmosphere–ocean system. Our optimization approach has been tested using radiance and polarization measurements acquired by the Airborne Multiangle SpectroPolarimetric Imager (AirMSPI) over the AERONET USC_SeaPRISM ocean site (6 February 2013) and near the AERONET La Jolla site (14 January 2013), which, respectively, reported relatively high and low aerosol loadings. Validation of the results is achieved through comparisons to AERONET aerosol and ocean color products. For comparison, the USC_SeaPRISM retrieval is also performed by use of the Generalized Retrieval of Aerosol and Surface Properties algorithm (Dubovik et al., 2011). Uncertainties of aerosol and nLw retrievals due to random and systematic instrument errors are analyzed by truth-in/truth-out tests with three Chl a concentrations, five aerosol loadings, three different types of aerosols, and nine combinations of solar incidence and viewing geometries.

Scintillation index in strong oceanic turbulence

Scintillation index of spherical wave in strongly turbulent oceanic medium is evaluated. In the evaluation, modified Rytov solution and our recent formulation that expresses the oceanic turbulence parameters by the atmospheric turbulence structure constant, are employed. Variations of the scintillation index in strong oceanic turbulence are examined versus the oceanic turbulence parameters such as the rate of dissipation of kinetic energy per unit mass of fluid, the rate of dissipation of mean-squared temperature, viscosity, wavelength, the link length, and the ratio of temperature to salinity contributions to the refractive index spectrum.

Seabed property inversion—Sequential and direct approaches

The goal of this work is to improve our knowledge of the ocean medium using physics of sound propagation and statistical signal processing. Two of the approaches we discuss are based on global and local optimization combined with sequential filtering. A third one falls under the category of direct (“exact”) methods. The first two approaches are based on the extraction and the association between paths/modes and detected arrivals and also produce posterior probability density functions, characterizing the uncertainty structure of modal/multipath arrivals. These are then propagated backwards through an acoustic model for posterior probability density function calculation of environmental parameters. A third method extends Stickler’s exact inverse approach, employing measurements of a reflection coefficient at low frequencies to obtain the sediment sound speed profile. We develop an approximation based on interpolation and a regularization approach. The former improves on results of a previous method for sol...

Expressing oceanic turbulence parameters by atmospheric turbulence structure constant

The parameters composing oceanic turbulence are the wavelength, link length, rate of dissipation of kinetic energy per unit mass of fluid, rate of dissipation of mean-squared temperature, Kolmogorov microscale, and the ratio of temperature to salinity contributions to the refractive index spectrum. The required physical entities such as the average intensity and the scintillation index in the oceanic medium are formulated by using the power spectrum of oceanic turbulence, which is described by oceanic turbulence parameters. On the other hand, there exists a rich archive of formulations and results for the above-mentioned physical entities in atmospheric turbulence, where the parameters describing the turbulence are the wavelength, the link length, and the structure constant. In this paper, by equating the spherical wave scintillation index solutions in the oceanic and atmospheric turbulences, we have expressed the oceanic turbulence parameters by an equivalent structure constant used in turbulent atmosphere. Such equivalent structure constant will help ease reaching solutions of similar entities in an oceanic turbulent medium by employing the corresponding existing solutions, which are valid in an atmospheric turbulent medium.