Model-oriented ocean tomography using higher frequency, bottom-mounted hydrophones

A tomographic scheme is presented that ingests ocean acoustic measurements into an ocean model using data from bottom-mounted hydrophones. The short distances between source-receiver pairs (1-10 km) means arrival times at frequencies of 8-11 kHz are readily detectable and often distinguishable. The influence of ocean surface motion causes considerable variability in acoustic travel times. Techniques are presented for measuring travel times and removing the variability due to surface waves. An assimilation technique is investigated that uses differences in measured and modeled acoustic travel times to impose corrections on the oceanographic model. Equations relating travel time differences to oceanographic variables are derived, and techniques are presented for estimating the acoustic and ocean model error covariance matrices. One test case using a single source-receiver pair shows that the tomographic information can have an impact on constraining the solution of the ocean circulation model but can also introduce biases in the predictions. A second test case utilizes knowledge of a bias in a model-predicted variable to limit grid cells that are impacted by the tomographic data. In this case, using the tomographic data results in significant improvements in the model predictions without introducing any biases.     

An acoustic shadowgraph trial to detect convection in the Arctic

Abstract

The EU Framework 5 project CONVECTION aims to understand convection processes in the Greenland Sea. By studying water motion close to the surface we hope to determine how convection is linked to atmospheric conditions and local surface features.

The usual methods of studying such processes in the ocean are by taking multiple soundings of conductivity, temperature and pressure or towing a large chain measuring temperature and salinity through a cross-section of ocean. These have the disadvantage of yielding information only while the research vessel is in the area. We have employed an alternative acoustic method that can provide data for long periods using semi-permanent moorings.

The acoustic shadowgraph method relies on the fact that when an acoustic signal propagates through a region containing convective irregularities the temperature variations along the path cause the signal amplitude to fluctuate. Unlike tomography, the shadowgraph does not require travel times to be measured and so the equipment can be much cheaper.

This paper describes the experimental apparatus, its testing and deployment on Vesteris Bank in the Greenland Sea in October 2001 and its recovery in April 2002. It also gives an overview of some of the acoustic intensity results and shows how they can be interpreted to yield estimates of sub-surface convection velocities.     

Statistics of travel time and intensity of two first arrivals of short pulses backscattered by a rough 3D surface

The time history of a pulse backscattered by a rough surface contains information about the position of the surface and the properties of the scatterers. This information is utilized successfully in a number of remote sensing techniques ranging from echo sounding of the ocean bottom to medical ultrasonics and satellite altimetry. In the current paper, statistical properties of backscattered waves are considered in a geometrical optics approximation. The probing pulse duration is assumed to be sufficiently short so that signals backscattered in the vicinity of individual specular points on a rough surface do not overlap in time. Theoretical results previously obtained for a 2D problem [I. M. Fuks and O. A. Godin, 2005, Waves in Random and Complex Media, 14, 539-562; M. I. Charnotskii and I. M. Fuks, 2005, Waves in Random and Complex Media, 15, 451-467] are extended to wave scattering by 3D rough surfaces by following a mathematical approach developed in stochastic geometry. Predictions of an asymptotic theory are verified against results of Monte-Carlo simulations. Probability density functions of travel times and intensities of the first and second arrivals of the backscattered wave are quantified in terms of statistical moments of roughness assuming normal distribution of elevations. It is found that, as in the 2D case, the travel time and the intensity are strongly correlated; on average, the earlier a signal arrives, the smaller is its intensity.     

Smoothing of the macrorelief on the surface of a solid of arbitrary stoichiometric composition under irradiation

A general system of equations is formulated describing the evolution of the macrorelief on the surface of a solid of arbitrary stoichiometric composition under irradiation. Irradiation is treated only as an additional source of point defects (vacancies and interstitial atoms). A general formula for the smoothing of the macrorelief with time is obtained with consideration of the real conditions of the problem. A two-component substance is considered as a special case: new forms of the dependence of the surface smoothing rate on the roughness parameters appear, which are not observed for a one-component material. The surface and bulk diffusion coefficients of the material, as well as the constants characterizing the interaction of radiation with the substance, can be found by experimentally investigating the surface smoothing rate as a function of the roughness. Fiz. Tverd. Tela (St. Petersburg) 39, 746–751 (April 1997)     

Comment on ‘X-ray scattering from a randomly rough surface’

Abstract

Recently, in this journal, Leskova and Maradudin published a new method for calculating x-ray scattering from a rough surface. In this comment their results will be compared with those obtained by other methods, especially the distorted-wave Born approximation. It is concluded that their results are useful in the limit of small transverse correlation length of the surface roughness. For intermediate values of the correlation length the results of both approximations are equivalent, being valid for small root-mean-square roughness and/or large incident perpendicular wavevector. In the case of large correlation length, neither approximation is correct. It is noted that the new method includes cross-polarization effects.     

A two-scale model from the point of view of the small-slope approximation

Abstract

General results for the scattering cross section following from the small-slope approximation (SSA) are applied to the case of two-scale surface roughness which can be represented as a superposition of small-scale and large-scale components. The purpose of the paper is to obtain analytically tractable results with obvious physical meaning which can be used for estimates prior to undertaking extensive numerical calculations according to exact unambiguous expressions of the SSA. The general case of vector (electromagnetic) or scalar (sound) waves is considered. The statistics of small-scale roughness is not assumed to be Gaussian (in any sense) or space-homogeneous, and the possible dependence of the statistics of small-scale roughness on a large-scale undulating surface is taken into account. As a result, a modified local spectrum of small-scale components of roughness enters into corresponding expressions for the scattering cross section. It is demonstrated that under appropriate conditions, the resulting formulae for the scattering cross section reduce to the conventional two-scale model.     

高偏压金属─绝缘体─金属结的光发射

我们制作了一种可在6-11V偏压范围内均匀发射可见光的新型金属-绝缘体-金属结型发光器件,其内层结构是Al-Al₂O₃-MgF₂-An(Cu),其承受偏压、单位面积发光功率及相应的外量子效应高过迄今已知的M-O-M遂道结型发光器件.本文首次报导并论证了这一由Schottky热电子所激发的光发射及其物理图象:Schottky热电子在AO(Cu)-真空界面激发表面等离极化激元(SPP);Au(Cu)-真空界面的SPP通过表面粗糙度与外光子耦合.这一图象与该器件的电流-电压(I-V)、电流-温度(I-T)关系及其发射光谱的主要特征一致.     

Mechanisms for the SAR Imaging of the Ocean

We perform a comparative analysis of two mechanisms for the formation of a synthetic aperture radar (SAR) image of the ocean, which are related to orbital velocities on the surface. It is shown that the mechanism of fluctuations of the effective density of scatterers, which is commonly accepted at present as the physical basis for the SAR diagnostics of roughness, actually works only in the case of a sufficiently flat swell. In the presence of wind roughness, the dominant role is played by another mechanism, namely, fluctuations of the number of surface elements whose images are randomly shifted and superimposed in the image plane due to orbital velocities. In the case of a developed wind roughness propagated in the azimuthal direction, the power contributions of two above-mentioned mechanisms to the SAR imaging of the ocean differ by about two orders of magnitude.     

Propagation effects of a fractal rough ocean surface on the vertical electric field generated by lightning return strokes

Based on an improved two-dimension (2D) fractal model of rough ocean surface, the propagation effects of the rough ocean surface on the vertical electric fields generated by lightning return strokes are analyzed. The results show that the rough ocean surface has much effect on the electric field derivatives, but has no or little effect on the field peaks. The frequency above 10 MHz is attenuated significantly by the rough ocean surface, and the rapid attenuation of frequency above 10 MHz in the experimentally obtained spectrum may be taken into account the errors introduced by the roughness of the ocean surface.     

Enhanced backscattering and transmission of light from random surfaces on semi-infinite substrates and thin films

Abstract

The enhanced backscattering of light from a random surface is manifested by a well defined peak in the retro-reflection direction in the angular distribution of the intensity of the incoherent component of the light scattered from such a surface. In this paper we present several new theoretical and experimental results bearing on the conditions under which enhanced backscattering occurs, and the way in which this phenomenon depends on the nature of the random surface roughness, both in the case that the random surface bounds a semi-infinite scattering medium and in the case that it bounds a film, either free-standing or on a reflecting substrate. In addition, we present new results on the transmission of light through thin metallic films bounded by random surfaces, which display the phenomenon of enhanced transmission, namely a well defined peak in the antispecular direction in the angular distribution of the intensity of the incoherent component of the light transmitted through such films.     

The Brewster effect for a one-dimensional random surface

Abstract

By the use of a unitarity and reciprocity conserving theory for the scattering of p-polarized light from a one-dimensional random dielectric surface the we determine the shift in the Brewster angle toward smaller angles of incidence caused by the surface roughness, and the (non-zero) value of the reflectivity at its minimum. Both quantities may be useful in the characterization of the roughness of such surfaces.     

Posterior capsular opacification and intraocular lens surface micro-roughness characteristics: an atomic force microscopy study

ObjectiveSurface roughness parameters of various intraocular lenses (IOLs) biomaterials using atomic force microscopy (AFM) are compared. Variation, if any, in the micro-roughness properties of different IOLs made up of the same biomaterial is also explored. Retrospective analysis of posterior capsular opacification (PCO) incidence has been followed up for a period of four years post IOL implantation to evaluate the correlation of PCO formation with surface roughness of IOLs.DesignExperimental materials study.Materials and participantsSurface characteristics of 20 different IOL models were assessed using AFM. These IOL models were made up of PMMA or HEMA or acrylic hydrophobic or acrylic hydrophilic or silicone. Retrospective analysis of PCO incidence in 3629 eyes of 2656 patients implanted with the same IOL models was performed.MethodsTopological characteristics of 20 different IOLs made up of 5 different biomaterials including (i) PMMA, (ii) HEMA, (iii) acrylic hydrophobic, (iv) acrylic hydrophilic and (v) silicone were evaluated using AFM in the tapping mode. Images were acquired with a resolution of 256 × 256 data points per scan at a scan rate of 0.5 Hz per line and a scan size of 10 × 10 μm. Rate of PCO formation in 3629 eyes of 2656 patients implanted with the five different IOL biomaterials was retrospectively analyzed.ResultsAFM images of IOL optic surfaces showed a collection of pores, grooves, ridges and surface irregularities. Surface roughness parameters of the IOL optics were significantly different on comparing lenses of different materials. Acrylic hydrophobic IOLs had minimum surface roughness while acrylic hydrophilic IOLs showed the highest surface roughness. Different IOL models of the same biomaterial showed varied topological roughness characteristics. Retrospective analyses of PCO formation rate after IOL implantation was carried out, which revealed that rate of PCO incidence, was directly proportional to the increase in surface micro-roughness of IOLs.ConclusionsAFM is a powerful technique for the topological characterization of IOLs. Acrylic hydrophobic IOLs showed minimum surface roughness properties as well as minimum PCO incidence over a period of four years post implantation. It is, therefore, tempting to consider acrylic hydrophobic IOLs over other IOL biomaterials as the ideal biocompatible material for lowering PCO incidence. These results suggest an urgent need for manufacturers to optimize the various steps involved in the fabrication of IOLs.     

Fractional order vs. exponential fitting in UTE MR imaging of the patellar tendon

PurposeQuantification of the T₂¹ relaxation time constant is relevant in various magnetic resonance imaging applications. Mono- or bi-exponential models are typically used to determine these parameters. However, in case of complex, heterogeneous tissues these models could lead to inaccurate results. We compared a model, provided by the fractional-order extension of the Bloch equation with the conventional models.MethodsAxial 3D ultra-short echo time (UTE) scans were acquired using a 3.0 T MRI and a 16-channel surface coil. After image registration, voxel-wise T₂¹ was quantified with mono-exponential, bi-exponential and fractional-order fitting. We evaluated all three models repeatability and the bias of their derived parameters by fitting at various noise levels. To investigate the effect of the SNR for the different models, a Monte-Carlo experiment with 1000 repeats was performed for different noise levels for one subject. For a cross-sectional investigation, we used the mean fitted values of the ROIs in five volunteers.ResultsComparing the mono-exponential and the fractional order T₂¹ maps, the fractional order fitting method yielded enhanced contrast and an improved delineation of the different tissues. In the case of the bi-exponential method, the long T₂¹ component map demonstrated the anatomy clearly with high contrast. Simulations showed a nonzero bias of the parameters for all three mathematical models. ROI based fitting showed that the T₂¹ values were different depending on the applied method, and they differed most for the patellar tendon in all subjects.ConclusionsIn high SNR cases, the fractional order and bi-exponential models are both performing well with low bias. However, in all observed cases, one of the bi-exponential components has high standard deviation in T₂¹. The bi-exponential model is suitable for T₂¹ mapping, but we recommend using the fractional order model for cases of low SNR.     

The effects of surface topography on the accumulation of biofouling

Experiments in the laboratory and the natural marine environment examined the changes in the rate of the accumulation of bacterial biofouling for different surface roughness values. The range of surface roughness values examined was that suitable for use in optical sensors. The fouling build-up was found to be greater at around 10 nm rms roughness value than at either 5 or 15 nm. For most materials tested, the fouling build-up at 10 nm was not equalled for roughness values below 25 nm. Different experimental exposure regimes were adopted: a purely static exposure, static exposure followed by dynamic removal of the bacterial layer and fully dynamic exposure where the bacterial suspension flowed across the surface of the samples. The results suggest that the effect of surface roughness on bacterial adhesion is almost instantaneous. The static fouling experiments were repeated with a range of materials and the results were the same, indicating that the response is physical in nature. As the surface imperfections are far smaller than the size of bacteria, it is suggested that the results are due to a similarity between the size and shape of the imperfections and the size and configuration of the adhesive molecules produced by marine bacteria.     

Small-slope expansion for electromagnetic-wave diffraction on a rough surface

Abstract

The diffraction and absorption of the plane electromagnetic wave on a rough surface is considered to find the scattering and emissivity of the surface. For this purpose a system of integral equations for unknown surface fields is derived from Green's formula for the Helmholtz equation. The small-slope approach is used to find a solution, i.e. the solution is determined from an expansion over the roughness spectrum that, in the limit of the large-scale roughness, turns out to be the expansion over the slope spectrum.     

Specular propagation over rough surfaces: numerical assessment of Uscinski and Stanek's mean Green's function technique

The purpose of this paper is to numerically evaluate the effectiveness and accuracy of Uscinski and Stanek's mean Green's function technique for computing the mean field of a wave scattered by a rough surface. We present here a direct comparison of this technique with a rigorous numerical method, the forward scattering integral equation method, and another analytical method, the first-order smoothing approximation. Furthermore, we compare the roughness generated equivalent admittance using the three methods. Numerical computations reveal that the scattered field calculated by this technique is not accurate particularly for the equivalent admittance at low grazing angles, even though the mean surface current density is recovered when the wave has traversed several correlation lengths on the surface.     

Anisotropy of Two-Dimensional Electron and Hole Mobilities in (0 0 1) GaAs/Al x Ga 1− x As Heterostructures Under Uniaxial Stress

Transport properties of two-dimensional electron and hole gas at (0 0 1) GaAs/Al x Ga 1 m x As heterointerface in [1 1 0] and [1 m 1 0] directions have been investigated for the first time under in plane uniaxial compression up to 5 kbar. Resistivity, Shubnikov-de Haas oscillations and Hall effect were measured and carrier densities and mobilities were determined. The in-plane uniaxial compression significantly modifies the energy spectrum of p-GaAs/Al x Ga 1 m x As that leads to strong anisotropy of hole mobility under compression. In the case of the n-type heterostructure uniaxial compression causes only change of the carrier concentration and the corresponding small change of the initial mobility anisotropy, determined by anisotropic surface roughness scattering.     

Dynamics of an electron beam from a high-current picosecond accelerator

The dynamics of a high-current (10²–10⁴ A) electron beam with energies of 10⁵–10⁶ eV and picosecond duration (10⁻¹⁰ s) at the output of the accelerator tube is investigated. The slowing of electrons by the residual positive charge on the surface of the tube is found to have a significant influence in the case of short pulse durations. The distance of the electron beam from the surface of the tube in vacuum is estimated on the basis of a one-dimensional model. It is shown that the electron radiation can travel to a distance of several centimeters from the surface at current densities below 20 A/mm², whereas at high current densities the beam is trapped near the surface. Zh. Tekh. Fiz. 69, 111–115 (May 1999)     

Adhesion studies of latex film surfaces on the meso- and nanoscale

In this paper the effects of surface roughness and annealing temperature (T) of latex coating films on adhesion are discussed for the different stages of the film formation process. The surface free energy of latex films was assessed in terms of practical work of adhesion (W) (or adherence) using a custom-built adhesion-testing device (ATD), atomic force microscopy (AFM), and contact angle measurements. For preannealed latex films surface roughness averages (R_a) were determined from AFM height images and were related to the values of W obtained from ATD measurements at room temperature. The results obtained using these tests exhibiting surface behavior on different length scales indicate a dependence of the measured adhesion on surface roughness and temperature, as well as on the length scale of the measurements.First preannealed samples were studied, which were obtained by heat treatment above the respective glass transition temperatures (T_g). Increasing the temperature of preannealing resulted in a decrease of the adherence observed in ATD experiments at room temperature. However, on the nanoscale, using AFM, no significant variation of the adherence was observed. This observation can be explained by roughness arguments. Preannealing decreases roughness which results in lower adherence values measured by ATD while for essentially single asperity AFM experiments roughness has an insignificant effect. Specimens were also annealed over a constant period of time (90 min) at different temperatures. At the end of the heat treatment, adhesion was measured at the treatment temperature by ATD. The amplified effect of temperature observed in this case on adherence is attributed to the combination of roughness decrease and increasing test temperature. In a third set of experiments completely annealed samples were studied by ATD as well as by AFM as a function of temperature. With increasing T values ATD showed a decrease in adherence, which is attributed to a decreasing surface free energy of the annealed films at elevated T values. AFM, on the other hand, showed an opposite trend which is assigned to increasing penetration of the tip into the tip/wetting polymer samples versus increasing temperature. Finally, annealing isotherms as a function of time were investigated by ATD in situ at different temperatures. This last set of experiments allowed us to optimize annealing time and temperature to achieve complete curing.