Wave profile measurement by wavelet transform

A new technique for measuring wave profiles by wavelet transform using the Mexican Hat wavelet as the mother wavelet is introduced. This technique has the potential to provide low cost, high resolution field measurements of wave profiles in the laboratory. The experiments to capture the video image of the wave profile were carried out in a wave flume. Then, the Mexican Hat wavelet was adopted to trace out the exact profiles of the waves from the captured video images. The series of tests on numerical data and video images show promise as means of detecting two-dimensional profiles of waves.     

Measurements of short water waves using stereo matched image sequences

Image analysis techniques are used for retrieving water surface elevation fields spatially and temporally from CCD-images and CCD-image-sequences. The technique proposed herein utilizes binocular stereogrammetry to recover topographic information from a sequence of synchronous, overlapping video images. The method used differs from the traditional stereo-photogrammetric analysis of a single stereo-pair because the use of video allows for a continuous sequence of stereo-images to be digitally sampled and analyzed. For data acquisition two synchronized progressive-scan cameras were used.A partially supervised 3-D stereo system (called WASS, Wave Acquisition Stereo System) is shown here. It is used to reconstruct the 3-D shape of water surface waves, acquired at frame rate, with small computational time needed. The stereo method is presented, including the derivation of a relationship relating the geometry of the stereo rig and the expected errors. Finally, the 3-D calculated scattered points give the complete spatio-temporal distribution of the water surface elevations. The measurable length-scales depend on the pixel resolution, the triangulation accuracy, and the acquisition frame rate. Limitations in the stereo measurements are also discussed.Two experiments to test and to demonstrate the system took place: one on the Venice lagoon, north of the city of Venice in September 2004 and the second on the coast of California at San Diego in December 2005. For the second experiment, qualitative and quantitative intercomparisons of the stereo-matching and in-situ sensor measurements are presented. All the measurements of water surface waves indicate that the proposed approach is both accurate and applicable for measuring water surface waves. Moreover, shape estimates are accurately and extremely dense both in space and time, and the remote location of the instruments alleviates some difficulties associated with in situ instrumentation.     

海面微波辐射特性测量与分析

在对海面微波辐射特性现场测量和数据处理的基础上,结合对海面微波辐射理论的初步分析,研究了各种环境参数以及辐射计参数与海洋微波波谱特性的相关关系,为海洋微波遥感资料的分析和解译提供了基础数据.测量结果表明了辐射计参数和环境要素对海面微波辐射特性的影响,而这些影响因素在分析、解译遥感资料时是必需考虑的因素.     

Comparative analysis of the North Atlantic surface circulation reproduced by three different methods

Calculation results are presented for long-term mean annual surface currents in the North Atlantic based on direct drifter measurements and numerical experiments with the ocean general circulation model using both climatic arrays of hydrological data World Ocean Atlas 2009 and Argo profiling data. The calculations show that the technique suggested for model calculations of oceanographic characteristics of the World Ocean with the use of Argo data significantly improves the climatic fields of the temperature and salinity even on a coarse grid. The comparison of the model calculation results with drifter data showed that the temperature and salinity fields found from Argo data with the use of data variational interpolation on a regular grid allow the calculation of realistic currents and can be successfully used as initial conditions in hydrodynamic models of the ocean dynamics.     

The acoustic transient recording buoy (ATRB): system descriptionand initial measurement results

An acoustic transient recording buoy (ATRB) developed to provide improved dynamic range and recording capacity in a reconfigurable manner is described. This digital system can acquire and record up to 16 h of broadband wide dynamic range (≈80 dB) acoustic data from eight hydrophones. A unique feature is the use of two inexpensive video cassette recorders to obtain up to 10 Gb of data storage capacity. The system is self-contained and capable of unattended bottom-moored operation. An experiment designed and conducted using a single ship and this system to obtain simultaneous measurements of sea surface forward scatter, propagation loss, and sea floor interaction is reported. Data obtained demonstrate the utility of this system for ocean acoustic experiments. Explosive charge source levels using direct path measurements agreed with previous measurements. Surface reflected data exhibited a frequency dependence attributed to sea surface swell and roughness     

Space–time measurements of oceanic sea states

Stereo video techniques are effective for estimating the space–time wave dynamics over an area of the ocean. Indeed, a stereo camera view allows retrieval of both spatial and temporal data whose statistical content is richer than that of time series data retrieved from point wave probes. We present an application of the Wave Acquisition Stereo System (WASS) for the analysis of offshore video measurements of gravity waves in the Northern Adriatic Sea and near the southern seashore of the Crimean peninsula, in the Black Sea. We use classical epipolar techniques to reconstruct the sea surface from the stereo pairs sequentially in time, viz. a sequence of spatial snapshots. We also present a variational approach that exploits the entire data image set providing a global space–time imaging of the sea surface, viz. simultaneous reconstruction of several spatial snapshots of the surface in order to guarantee continuity of the sea surface both in space and time. Analysis of the WASS measurements show that the sea surface can be accurately estimated in space and time together, yielding associated directional spectra and wave statistics at a point in time that agrees well with probabilistic models. In particular, WASS stereo imaging is able to capture typical features of the wave surface, especially the crest-to-trough asymmetry due to second order nonlinearities, and the observed shape of large waves are fairly described by theoretical models based on the theory of quasi-determinism (Boccotti, 2000). Further, we investigate space–time extremes of the observed stationary sea states, viz. the largest surface wave heights expected over a given area during the sea state duration. The WASS analysis provides the first experimental proof that a space–time extreme is generally larger than that observed in time via point measurements, in agreement with the predictions based on stochastic theories for global maxima of Gaussian fields.     

Three- dimensional vector modeling and restoration of flat finite wave tank radiometric measurements

In this paper, a three-dimensional Fourier transform inversion method describing the interaction between water surface emitted radiation from a flat finite wave tank and antenna radiation characteristics is reported. The transform technique represents the scanning of the antenna mathematically as a correlation. Computation time is reduced by using the efficient and economical fast Fourier transform algorithm. To verify the inversion method, computations have been made and compared with known data and other available results. The technique has been used to restore data of the finite wave tank system and other available antenna temperature measurements made at the Cape Cod Canal. The restored brightness temperatures serve as better representations of the emitted radiation than the measured antenna temperatures.     

Acoustic backscattering by deepwater fish measured in situ from a manned submersible

An outstanding problem in fisheries acoustics is the depth dependence of scattering characteristics of swimbladder-bearing fish, and the effects of pressure on the target strength of physoclistous fish remain unresolved. In situ echoes from deepwater snappers were obtained with a sonar transducer mounted on a manned submersible next to a low-light video camera, permitting simultaneous echo recording and identification of species, fish size and orientation. The sonar system, consisting of a transducer, single board computer, hard disk, and analog-to-digital converter, used a 80 μs, broadband signal (bandwidth 35 kHz, center frequency 120 kHz). The observed relationship between fish length and in situ target strength shows no difference from the relationship measured at the surface. No differences in the species-specific temporal echo characteristics were observed between surface and in situ measures. This indicates that the size and shape of the snappers’ swimbladders are maintained both at the surface and at depths of up to 250 m. Information obtained through controlled backscatter measurements of tethered, anesthetized fish at the surface can be applied to free-swimming fish at depth. This is the first published account of the use of a manned submersible to measure in situ scattering from identified, individual animals with known orientations. The distinct advantage of this technique compared with other in situ techniques is the ability to observe the target fish, obtaining accurate species, size, and orientation information.     

Radar backscatter from mechanically generated transient breakingwaves. I. Angle of incidence dependence and high resolution surfacemorphology

This paper describes the results of an experimental investigation of the microwave backscatter from several laboratory generated transient breaking waves. The breaking waves were generated mechanically in a 35 m×0.7 m×1.14 m deep wave tank, utilizing chirped wave packets spanning the frequency range 0.8-2.0 Hz. Backscatter measurements, were taken by a X/K-band (10.525 GHz, 24.125 GHz) continuous wave Doppler radar at 30°, 45°, and 60° angles of incidence, and at azimuth angles of 0° and 180° relative to the direction of wave propagation. Surface profiles were measured with a high-speed video camera and laser sheet technique. Specular facets were detected by imaging the surface from the perspective of the radar. The maximum radar backscatter occurred in the upwave direction prior to wave breaking, was nearly polarization independent and corresponded to the detection of specular facets on the steepened wave face. This peak radar backscatter was predicted through a finite conductivity corrected physical optics technique over the measured surface wave profiles. Post break backscatter was predicted using a roughness corrected physical optics technique and the small perturbation method, which was found to predict the returns for vertical polarization, but to under predict the horizontal returns     

Effect of various surface-height-distribution properties on acoustic backscattering statistics

A problem of interest to underwater acousticians is understanding the relationship between ocean-bottom characteristics and acoustic backscattering statistics. This experimental work focused on examining surface roughness characteristics that cause backscattering strength statistics to deviate from the Rayleigh distribution. Several different scattering surfaces with known height distributions were designed for this study. The surfaces were modeled using a technique that allowed for different height-distribution functions and correlation lengths to be prescribed. Isotropic and anisotropic surfaces were fabricated having both Gaussian and non-Gaussian surface-height distributions. Many independent backscattering measurements were made for different aspects of each surface using a computer-controlled transducer-positioning system. Acoustic backscattering statistics were non-Rayleigh for the anisotropic surfaces when combining measurements from different aspects. Mean scattering strength was found to be dependent on both the surface-height distribution and correlation length. In addition, backscattering strength showed a dependence on the surface-height power distribution.     

A New Video and Digital Camera System for Studies of the Dynamics of Microtopographic Features on Tidal Flats

A newly developed video and camera system for tidal flat microtopographic studies is presented. It consists of a SONY handy cam placed in an underwater housing mounted on a frame about 70?cm above the sediment surface. A rectangular surface area of 30?×40?cm is imaged by the camera. The camera records video sequences and/or digital images at predetermined time lapses. The total number of images is about 540, and a similar number of 10-second long video sequences can be recorded. The camera is programmed with a PC before deployment, and the total deployment time depends on the time lapse between recordings. The camera is connected to an external power supply (12?volt), and a halogen projector pointing towards the sediment surface ensures that the system is operable on a 24-hours scale. The system has been tested in the Danish Wadden Sea. It has proved to be a very useful tool in studies of topographic effects of erosion and deposition sequences, and for studies of benthic organisms-sediment interactions. The test site was further equipped with sensors for water and seabed measurements, which proved to be indispensable regarding the interpretation of recorded image time-series.     

A New Video and Digital Camera System for Studies of the Dynamics of Microtopographic Features on Tidal Flats

A newly developed video and camera system for tidal flat microtopographic studies is presented. It consists of a SONY handy cam placed in an underwater housing mounted on a frame about 70 cm above the sediment surface. A rectangular surface area of 30 ×40 cm is imaged by the camera. The camera records video sequences and/or digital images at predetermined time lapses. The total number of images is about 540, and a similar number of 10-second long video sequences can be recorded. The camera is programmed with a PC before deployment, and the total deployment time depends on the time lapse between recordings. The camera is connected to an external power supply (12 volt), and a halogen projector pointing towards the sediment surface ensures that the system is operable on a 24-hours scale. The system has been tested in the Danish Wadden Sea. It has proved to be a very useful tool in studies of topographic effects of erosion and deposition sequences, and for studies of benthic organisms-sediment interactions. The test site was further equipped with sensors for water and seabed measurements, which proved to be indispensable regarding the interpretation of recorded image time-series.     

Integrated System for Robust 6-DOF Positioning Utilizing New Closed-Form Visual Motion Estimation Methods in Planar Terrains

Estimating the relative positions and (or) trajectory of a camera from video images is a fundamental problem in motion vision. Of special relevance is the closed-form solution for planar scenes, for processing fly-over imagery from airborne and underwater robotics platforms, automated airplane landing utilizing runway landmarks, photomosaicing, etc. However, the method's robustness can break down in certain scenarios, e.g., due to inherent translation-rotation ambiguity of visual motion with short baselines and narrow field of view. The robustness can be improved by devising methods that compute a smaller set of motion parameters, utilizing other sensors to measure the remaining components. This paper addressed key issues in six degrees of freedom positioning from fly-over imagery by integrating vision with rotational angle sensors. First, we propose and utilize robust closed-form solutions for estimating the motion and orientation of a planar surface from the image flow variations up to first order, given measurements of pitch and roll motions. We also describe a calibration technique to enable the integration of angle sensor and visual measurements. Next, an error analysis enables us to evaluate the impact of inaccurate pitch and roll measurements on the estimates from the new closed-form solutions. Finally, the performance of our new methods and the integrated positioning system are evaluated in various experiments with synthetic and real data     

Estimation of winds over the sea from land-based measurements

Simultaneous measurements of wind velocities at two different sites, one over the sea and the other over land, can differ substantially and therefore cannot be interchanged. In situations where the wind data at an offshore site are missing while simultaneous measurements from a land-based station exist, a linear mean-square estimation (LMSE) technique can be used to estimate the missing data. This technique relies on past wind data gathered simultaneously at the two locations, and it generates from the associated correlation a set of four transfer functions capable of predicting one data set from the other. In the present case, the LMSE technique is outlined briefly, and is then applied to construct seasonal transfer functions between a land-based station and two coastal/offshore sites in Kuwait. Comparisons between the actually observed wind characteristics and those predicted by the LMSE technique are favorable, and thus tend to confirm the applicability of the technique under appropriate conditions.     

Interannual and seasonal variations in ozone in different atmospheric layers over St. Petersburg based on observational data and numerical modeling

This paper analyzes atmospheric ozone variability at different altitudes over St. Petersburg for the period 2009–2014 on the basis of surface observations at the Peterhof station, satellite measurements with an SBUV instrument, and numerical simulations. Simulation data on temperature, wind velocity, humidity, and surface pressure are taken from the MERRA reanalysis database. Based on ozone measurements, numerical modeling, and reanalysis data, characteristics of ozone seasonal and interannual changes are identified; the role of photochemical and dynamic factors in ozone variations is estimated.     

水下机器人视频图像高效压缩方法研究

水下视频图像压缩一直是有限带宽水声信道实时传输海量视频数据的关键技术之一。本文首先介绍了目前水下机器人和水下视频图像压缩研究存在的主要问题,并综合分析了目前几种高效视频压缩方法的特点并探讨了进一步研究的方向。此外,根据水下视频的成像特点,提出了高效的全局与局部运动混合补偿方案和基于小波变换的预处理方法。初步实验结果表明：本文提出的预处理方法可以有效去除视频图像中存在的大量视觉冗余和空间冗余,提出的混合运动补偿方案可以获得很高的压缩编码效率;但必须进一步研究快速、有效的全局运动估计方法。     

Rain measurement on buoys using hydrophones

The impact of rain and spray on the ocean disturbs the sea surface and generates underwater ambient noise. The short scale roughness is influenced by impacting drops due to the momentum transfer. Radar and sonar signals are scattered by the short elements of the sea surface. Spray and rain impact change their characteristics, and consequently affect radar and sonar backscatter. In situ measurements of rain and spray impact are necessary to study their effects on the sea surface. Accurate sea measurements of rain momentum fluxes and drop size distributions are a complex problem, especially on buoys. A new measuring technique has been developed using hydrophones. Exposed to precipitation, these instruments are affected directly by the impact of rain. A drop falling on the hydrophone deforms its surface and is sensed by a piezoelectric transducer. The voltage output of the sensor is a rapidly decaying oscillation. The integral value of this signal is a measure of the drop momentum, and the drop size can be deduced. Laboratory studies of defined drops as well as field measurements of natural rain have shown that hydrophones can be used to determine drop momentums and drop size distributions. Based on simultaneous rain measurements by a Joss-Waldvogel Disdrometer and a hydrophone, an analytical function has been derived which relates drop size and hydrophone voltage output     

Mean current velocity measurement from a movable base

This paper discusses the possibility of measurement of the current velocity vector characteristics averaged over the time of continuous regular down-up probings from a moving carrier. The measurements can be carried out either from a moving vessel or from a helicopter, floating on or hovering above the sea surface. As an illustration for the suggested technique,in situ observations compiled by a free-component (±XYZ) VEGA-3 probe in the Guiana current region in March 1990 are provided. Comparison of the results with buoy data shows good agreement within the accuracy limits of the instruments employed.Translated by Vladimir A. Puchkin.     

Statistical Characteristics of the Moduli of Slopes of the Sea Surface

On the basis of data of measurements carried out on an oceanographic platform, we study the variability of statistical characteristics of the modulus of slopes of the sea surface under various hydrometeorological situations. We show that the law of distribution of the modulus of slopes of the sea surface depends on the velocity of the wind and note that the empirical distribution differs considerably from the approximation proposed by Longuet-Higgins for it by the results of analysis of a Gauss random moving surface. The possible reasons for the difference in model and empirical distributions are discussed.     

Remote sensing of breaking wave phase speeds with application to non-linear depth inversions

A number of existing models for surface wave phase speeds (linear and non-linear, breaking and non-breaking waves) are reviewed and tested against phase speed data from a large-scale laboratory experiment. The results of these tests are utilized in the context of assessing the potential improvement gained by incorporating wave non-linearity in phase speed based depth inversions. The analysis is focused on the surf zone, where depth inversion accuracies are known to degrade significantly. The collected data includes very high-resolution remote sensing video and surface elevation records from fixed, in-situ wave gages. Wave phase speeds are extracted from the remote sensing data using a feature tracking technique, and local wave amplitudes are determined from the wave gage records and used for comparisons to non-linear phase speed models and for non-linear depth inversions. A series of five different regular wave conditions with a range of non-linearity and dispersion characteristics are analyzed and results show that a composite dispersion relation, which includes both non-linearity and dispersion effects, best matches the observed phase speeds across the domain and hence, improves surf zone depth estimation via depth inversions. Incorporating non-linearity into the phase speed model reduces errors to O(10%), which is a level previously found for depth inversions with small amplitude waves in intermediate water depths using linear dispersion. Considering the controlled conditions and extensive ground truth, this appears to be a practical limit for phase speed-based depth inversions. Finally, a phase speed sensitivity analysis is performed that indicates that typical nearshore sand bars should be resolvable using phase speed depth inversions. However, increasing wave steepness degrades the sensitivity of this inversion method.