A wavelet-based algorithm to estimate ocean wave group parameters from radar images

In recent years, new remote sensing techniques have been developed to measure two-dimensional (2-D) sea surface elevation fields. The availability of these data has led to the necessity to extend the classical analysis methods for one-dimensional (1-D) buoy time series to two dimensions. This paper is concerned with the derivation of group parameters from 2-D sea surface elevation fields using a wavelet-based technique. Wave grouping is known to be an important factor in ship and offshore safety, as it plays a role in dangerous resonance phenomenons and the generation of extreme waves. Synthetic aperture radar (SAR) data are used for the analysis. The wavelet technique is introduced using synthetic ocean surfaces and simulated SAR data. It is shown that the group structure of the ocean wave field can be recovered from the SAR image if the nonlinear imaging effects are moderate. The method is applied to a global dataset of European Remote Sensing satellite (ERS-2) wave mode data. Different group parameters including the area covered by the largest group and the number of groups in a given area are calculated for over 33 000 SAR images. Global maps of the parameters are presented. For comparison, classical 1-D grouping parameters are calculated from colocated wave model data showing good overall agreement with the wavelet-derived parameters. ERS-2 image mode data are used to study wave fields in coastal areas. Waves approaching the island of Sylt in the North Sea are investigated, showing the potential of the wavelet technique to analyze the spatial wave dynamics associated with the bottom topography. Observations concerning changes of wavelength and group parameters are compared to linear wave theory.     

基于全天空流星雷达测量动量通量的新方法

全天空流星雷达是新一代的流星雷达, 可以接收流星余迹反射的无线电波, 目前主要应用于探测流星烧蚀区域的大气风场进而研究该区域的大气动力学.文中利用全天空流星雷达, 基于Hocking提出的测量重力波动量通量的新方法来研究昆明地区的大气环境参数—重力波的动量通量.由于采样时间间隔的不同, 该方法仍存在争议.本文主要利用昆明站工作频率为37.5 MHz的全天空流星雷达测得的流星数据来验证采样时间对重力波动量通量取值的影响, 同时利用工作频率分别为37.5 MHz、53.1 MHz的全天空流星雷达在2014年9月份观测的数据对该方法进行了对比分析, 结果表明:采样时间间隔的选取对重力波动量通量的取值有很大影响; 在合适的采样时间间隔内利用全天空流星雷达测量重力波的动量通量的新方法是可行的.     

Retrieval of gravity-capillary spectrum parameters by means of microwave radiometric techniques

The paper presents some results obtained during a series of microwave remote sensing experiments carried out on the Black Sea coast in 1999-2002. The measurements were made from a pier at the South Department of the Shirshov Institute of Oceanology with microwave radiometers operating at wavelengths of 1.5, 0.8, 0.5, and 0.3 cm. The surface wave spectrum parameters were estimated from angular radiometric measurements using a novel approach. Microwave brightness contrasts at Ka-band (37 GHz) measured over wide range of incidence angles were used for a curvature spectrum of gravity-capillary waves and a mean-square slope of gravity waves retrieval. The evolution of spectrum parameters under variable wind (ranged from 0.5-7 m/s) was investigated. The delay of brightness variation relative to wind speed as large as 1-2 h was found. The performed spectral analysis permitted to relate the observed brightness delay to short gravity-capillary surface waves, whereas longer gravity waves followed the wind practically without any delay. It is concluded that the proposed technique of gravity-capillary wave spectrum retrieval provides a useful tool for investigation of air-sea interaction and surface wave dynamics.     

ERS-1 SAR images of atmospheric gravity waves

ERS-1 synthetic aperture radar (SAR) images of atmospheric gravity waves over the ocean are discussed. Several case studies are presented in detail. It is shown that the well-organized long wavelength (1 to 10 km) wave phenomena which often are seen in SAR images over the ocean may be atmospheric gravity waves. The waves appear in the SAR images because they modulate the surface wind speed which in turn modulates the surface roughness and the radar cross section. The wavelength may be measured directly from the SAR image, and the mean wind speed and wind speed modulation near the ocean surface may be estimated from the observed radar cross section modulation using a wind retrieval model. The atmospheric gravity waves usually were generated by the approach or passing of a meteorological front. Atmospheric soundings and a two-layer model for the lower troposphere indicate that, in general, the observed atmospheric wave phenomena could have been supported by accompanying temperature inversion layers and wind shears     

Comb-Push Ultrasound Shear Elastography (CUSE): A Novel Method for Two-Dimensional Shear Elasticity Imaging of Soft Tissues

Fast and accurate tissue elasticity imaging is essential in studying dynamic tissue mechanical properties. Various ultrasound shear elasticity imaging techniques have been developed in the last two decades. However, to reconstruct a full field-of-view 2-D shear elasticity map, multiple data acquisitions are typically required. In this paper, a novel shear elasticity imaging technique, comb-push ultrasound shear elastography (CUSE), is introduced in which only one rapid data acquisition (less than 35 ms) is needed to reconstruct a full field-of-view 2-D shear wave speed map (40 × 38 mm). Multiple unfocused ultrasound beams arranged in a comb pattern (comb-push) are used to generate shear waves. A directional filter is then applied upon the shear wave field to extract the left-to-right (LR) and right-to-left (RL) propagating shear waves. Local shear wave speed is recovered using a time-of-flight method based on both LR and RL waves. Finally, a 2-D shear wave speed map is reconstructed by combining the LR and RL speed maps. Smooth and accurate shear wave speed maps are reconstructed using the proposed CUSE method in two calibrated homogeneous phantoms with different moduli. Inclusion phantom experiments demonstrate that CUSE is capable of providing good contrast (contrast-to-noise ratio ≥ 25 dB) between the inclusion and background without artifacts and is insensitive to inclusion positions. Safety measurements demonstrate that all regulated parameters of the ultrasound output level used in CUSE sequence are well below the FDA limits for diagnostic ultrasound.     

Point-source suppression for atmospheric wave extraction from airglow imaging measurements

Spectroscopic images of atmospheric chemiluminescent airglow contain fundamental diagnostic information about the structure and dynamics of the upper atmosphere. However, westward movement of stars causes non-negligible contamination in the two-dimensional horizontal wavenumber spectra extracted from all-sky airglow images. The conventional star-removing scheme is not effective in attenuating the star artifacts in the spectra of emission intensities of gravity wave perturbation, hindering the subsequent calculation of the angular spectra and the momentum fluxes of the profiles. To address this problem, a gradient-based edge detection algorithm for locating and attenuating the undesired star signal is developed. Comparatively, this algorithm is more effective than the conventional star-removing methods. The algorithm is validated in the context of experimentally obtained spectra.     

基于圆柱抛物方程的城市环境电波预测模型

提出了一种三维实体重建技术与柱坐标抛物方程结合的城市环境电波传播预测方法.从数字地图中提取城市建筑物高程矢量数据,建立立体光刻文件的拓扑数据结构,将其转化为抛物方程模型对应的扇形栅格数据,实现城市建筑群的三维几何模型重构.同时结合聚类算法并利用城市影像信息对地表环境进行分类,对不同类型的地表媒质赋予不同电磁参数,实现真...     

Bathymetry Estimation From Single-Frame Images of Nearshore Waves

Existing methods for determining bathymetry from remotely sensed images of nearshore waves exploit only information on the magnitude of wavenumber $(k = 2pi/L)$, ignoring spatial changes in wave direction $theta$ that can provide information about bathymetry gradients. These methods also require wave period information, so they can only be used when time series imagery is available. We present an algorithm where changes in direction of refracting waves are used to determine underlying bathymetry gradients based on the irrotationality of wavenumber condition. Depth dependences are explicitly introduced through the linear dispersion relationship. The final form of the model is independent of wave period so that all necessary input measurements can be derived from a single aerial snapshot taken from a plane, unmanned aerial vehicle, or satellite. Three different methods were tested for extracting wavenumber and angle from images, i.e., two based on spatial gradients of wave phase and one based on integrated travel times between sample locations (a tomographic approach). Synthetic testing using monochromatic and bichromatic waves, with and without noise, showed that while all three methods work well under ideal wave conditions, gradient methods were overly sensitive to data imperfections. The tomographic approach yielded robust wave measurements and provided confidence limits to objectively identify unusable areas. Further tests of this method using monochromatic waves on three synthetic bathymetries of increasing complexity showed a mean bathymetry bias of 0.01 m and a mean rms error of 0.17 m. While not always applicable, the model provides an alternative form of bathymetry estimation when celerity information is not available.      

基于瑞利多普勒激光雷达数据的重力波测量与分析

对于平流层的高空间分辨率的重力波连续测量和研究非常稀少。近几年中国科学技术大学的瑞利多普勒激光雷达观测了大量的重力波事件,这得益于激光雷达系统在重力波研究中的优越特性。文中对激光雷达系统作出了一个简单的介绍,并且展示了位于中国酒泉（39.741N,98.495E）,垂直高度范围在15~60 km,自2015年10月7日起持续了两个月的重力波夜间观测结果。在对一些重力波事件分析过程中,对中尺度的水平风速进行了二维波谱分析之后,发现重力波波动非常明显,这些波动的波长范围主要集中在3~6 km,而周期大约为10 h。这些观测结果都肯定了瑞利多普勒激光雷达在重力波观测应用方面的优势。     

周期线性变化光子晶体用于色散补偿的研究

文章研究了周期线性变化的一维光子晶体的色散补偿特性.根据周期性介质中电磁场在界面处的传递规律,利用传输矩阵方法分析了不同频率的光波在周期线性变化的一维光子晶体中的反射特性,结果表明这种光子晶体对于不同频率的光波成分具有不同的延时,利用这种延时特性,可以制作用于密集波分复用(DWDM)系统的色散补偿器件.     

Modeling the slow wave shapes of spreading depression in a rat cortex: a methodology for seeking physiological parameters

Spreading depression (SD) consists of a transient significant suppression of the spontaneous neural electrical activity that spreads slowly across regions of the gray matter in a wave form. Nowadays, this phenomenon is being studied by means of mathematical and computational models to reproduce the main characteristics of SD. Given the high number of parameters and their unknown ranges of variation, the setting of parameters for current SD models is usually a hard task that must be addressed in order to make such models reproduce real data. In this paper, we present a 1-D model which is able to reproduce the most important characteristics of SD waves observed in laboratory experiments: the slow extracellular potential shift and extracellular ionic concentration variations regarding speed, shape, and amplitude. Such a reproduction is possible due to a methodology that we introduced to set the parameters of the SD models. The methodology allows the impact of each parameter on the results produced by the model and the range of parameters for which the model displays plausible behavior to be determined. The methodology also helps to identify features that the model cannot produce and it gives insights about what parts of the model should be modified to improve its capacities through the identification of parameters involved with each behavior.     

Errors in bathymetric retrievals using linear dispersion in 3-D FFTanalysis of marine radar ocean wave imagery

The phenomenon of ocean wave-shoaling, and the associated reduction of ocean wave phase speed with decreased water depth, provides useful information for inferring water depth D (bathymetry) in coastal environments. One strategy for relating D to phase speed C and wave-vector K of long wavelength ocean waves involves using the one-dimensional (1-D) linear (gravity wave) dispersion relationship C ²=g*tanh(KD)/K. In principle, this approach has limitations because the approach is based on a WKB approximation, so it cannot be applied when D varies appreciably over the wavelength of a shoaling wave. Also, the approach is restricted to waves that have small wave height. The author uses a set of marine radar image sequences and applies this linear approximation, using a 3-D FFT analysis of 88 sets of image sequences spaced half an hour apart. The author inverts the dispersion relation to solve for D. Depths between 3.6 and 5.8 m were tested, for root mean square (RMS) wave heights offshore between 8 and 3 m. The author shows that for low to moderate wave heights, the approach does generally retrieve the correct depth in water depths of 5 m and greater for moderate wave RMS heights. However, an increase in the RMS wave height from 1 m to 3.5 m produced a much poorer depth estimate, proving the need for an application of a nonlinear wave model to the problem. The errors also increase with shallower depths as expected, as the error dependence on depth and wave height is determined     

Ocean wave dispersion surface measured with airborne IR imagingsystem

Image sequences of the ocean have been collected at long range and low grazing angle with an airborne infrared system. The images are geographically registered, and 3D frequency-wavenumber spectra are calculated and shown to have a strong 2D dispersion surface that is characteristic of wind waves and swell. Wave directions compare well with in situ measurements, and their speeds are consistent with the water depth. The authors conclude that temporal sequences of IR images acquired from long distance can provide information for extracting surface wave parameters     

The near-surface current velocity determined from image sequencesof the sea surface

A method to measure the ocean's near-surface current velocity vector based on the analysis of remote sea-surface image sequences was developed. The spatial and temporal records were transformed to the wavenumber-frequency domain, resulting in a three-dimensional (3-D) image power spectrum. In the spectrum, the signal energy of the waves is localized on a shell defined by the dispersion relation of surface waves. The sum of the sensor's velocity and the near-surface current profile deforms the dispersion shell due to the Doppler-frequency shift. An iterative least-squares fitting technique and an error-estimation model was implemented. To improve the method's accuracy, spectral wave energy found in higher harmonics of the dispersion shell and aliasing effects are taken into account. The most important nonlinear mechanism leading to higher harmonics is explained as resulting from wave shadowing due to the low grazing angles typical for ground- or ship-based radars. The improved method is examined analytically and is tested with Monte Carlo simulations. The variation of the shape of the measured or simulated 3-D image spectra, especially the peak wavenumber, the directional spread, and the main travel direction, controls the behavior and accuracy of the technique. A comparison of velocities acquired by nautical radar and independent Doppler log current measurements is presented. The technique's accuracy, its limits, and its adaptability are discussed. Additional improvements are proposed     

Constrained registration for motion compensation in atrial fibrillation ablation procedures

Fluoroscopic overlay images rendered from preoperative volumetric data can provide additional anatomical details to guide physicians during catheter ablation procedures for treatment of atrial fibrillation (AFib). As these overlay images are often compromised by cardiac and respiratory motion, motion compensation methods are needed to keep the overlay images in sync with the fluoroscopic images. So far, these approaches have either required simultaneous biplane imaging for 3-D motion compensation, or in case of monoplane X-ray imaging, provided only a limited 2-D functionality. To overcome the downsides of the previously suggested methods, we propose an approach that facilitates a full 3-D motion compensation even if only monoplane X-ray images are available. To this end, we use a training phase that employs a biplane sequence to establish a patient specific motion model. Afterwards, a constrained model-based 2-D/3-D registration method is used to track a circumferential mapping catheter. This device is commonly used for AFib catheter ablation procedures. Based on the experiments on real patient data, we found that our constrained monoplane 2-D/3-D registration outperformed the unconstrained counterpart and yielded an average 2-D tracking error of 0.6 mm and an average 3-D tracking error of 1.6 mm. The unconstrained 2-D/3-D registration technique yielded a similar 2-D performance, but the 3-D tracking error increased to 3.2 mm mostly due to wrongly estimated 3-D motion components in X-ray view direction. Compared to the conventional 2-D monoplane method, the proposed method provides a more seamless workflow by removing the need for catheter model re-initialization otherwise required when the C-arm view orientation changes. In addition, the proposed method can be straightforwardly combined with the previously introduced biplane motion compensation technique to obtain a good trade-off between accuracy and radiation dose reduction.     

Thermoacoustic Emission from Carbon Nanotubes Imaged by Atomic Force Microscopy

The thermoacoustic effect of isolated single‐wall carbon nanotubes aligned between electrodes is experimentally observed for the first time by imaging the emitted acoustic wave using an atomic force microscopy‐based technique specifically developed for the task. The capability of such a technique for single‐point thermoacoustic measurements is first verified on carbon nanotubes layers with two electrodes for injecting alternate electric current. The technique is then demonstrated to allow the acquisition, simultaneously with the topography, of images reflecting the pressure of the acoustic wave at fixed distance from the sample. Such a capability is used to collect images reflecting the amplitude of acoustic waves generated by isolated nanotubes and nanotube bundles by the thermoacoustic effect.     

Application of 2-D Hilbert transform in geophysical imaging withpotential field data

The relationship between two-dimensional (2-D) Hilbert transforms and three-dimensional (3-D) potential field data (gravity and magnetic) is reviewed and applied to quantitative interpretation of magnetic and gravity anomalies as well as their derivatives. The vertical and horizontal derivatives of potential field anomalies due to multiprism models were used to test the 2-D Hilbert transform algorithms and their application. The gravity and magnetic contour maps of the Sudbury area, Canada, were digitized and Hilbert-transformed to be correlated with satellite digital images and with the previously known geology. The results indicate that the 2-D Hilbert transform technique can, in a robust fashion, be utilized for the extrapolation of geological boundaries through overburdened areas and subsequently provide a mathematical tool for a step-by-step integration of potential field data and geological remote sensing     

SIR-B Observations of Ocean Waves in the NE Atlantic

Synthetic-aperture radar (SAR) imagery from SIR-B was obtained over a deep-water site in the NE Atlantic Ocean. Selected images were Fourier analyzed in order to compare SAR measurements of ocean waves with simultaneous data obtained from wave buoys deployed in the site. The wave heights were not large enough to be detected in the first pass over the site, but larger wave amplitudes were present in two subsequent passes that were almost orthogonal and separated by 6 h. The first of these passes showed a system of waves traveling close to the range direction, and secondary wave systems were present in some spectra. Azimuth-traveling waves were seen clearly on only one of the spectra from the second of these passes. All the spectra had the appearance expected from a linear imaging theory, and no significant discrepancies were found between the SAR and buoy measurements of the wavelengths and directions of the dominant wave systems. However, the difficulty in detecting azimuth-traveling waves here may be hard to explain in terms of existing theories, but this is not a firm conclusion because large variations (factors of about 4) in spectral intensity were observed across distances as small as 20 km. This variability means that it is very difficult to determine whether SIR-B was observing the true sea state. Procedures to obtain more accurate tests of sea-imaging theories are discussed.     

An augmented reality system for patient-specific guidance of cardiac catheter ablation procedures

We present a system to assist in the treatment of cardiac arrhythmias by catheter ablation. A patient-specific three-dimensional (3-D) anatomical model, constructed from magnetic resonance images, is merged with fluoroscopic images in an augmented reality environment that enables the transfer of electrocardiography (ECG) measurements and cardiac activation times onto the model. Accurate mapping is realized through the combination of: a new calibration technique, adapted to catheter guided treatments; a visual matching registration technique, allowing the electrophysiologist to align the model with contrast-enhanced images; and the use of virtual catheters, which enable the annotation of multiple ECG measurements on the model. These annotations can be visualized by color coding on the patient model. We provide an accuracy analysis of each of these components independently. Based on simulation and experiments, we determined a segmentation error of 0.6 mm, a calibration error in the order of 1 mm and a target registration error of 1.04 +/- 0.45 mm. The system provides a 3-D visualization of the cardiac activation pattern which may facilitate and improve diagnosis and treatment of the arrhytmia. Because of its low cost and similar advantages we believe our approach can compete with existing commercial solutions, which rely on dedicated hardware and costly catheters. We provide qualitative results of the first clinical use of the system in 11 ablation procedures.     

Novel BI-FDTD approach for the analysis of chiral cylinders and spheres

A versatile time-domain technique, known as bi-isotropic finite difference time domain (BI-FDTD), has recently been introduced for the numerical analysis of electromagnetic wave interactions with complex bi-isotropic media. However, to date only one-dimensional BI-FDTD schemes have been successfully implemented. This paper presents novel two-dimensional (2-D) and three-dimensional (3-D) dispersive BI-FDTD formulations for the first time. The update equations for these new 2-D and 3-D BI-FDTD approaches are developed and applied to the analysis of electromagnetic wave scattering by chiral cylinders and spheres in free space. The distinctive feature of this technique is the use of two independent sets of wavefields representing the left- and right-polarized waves in the chiral medium. This wavefield decomposition approach allows dispersive models for the chirality parameter as well as the permittivity and permeability of the medium to be readily incorporated into an FDTD scheme. The 2-D and 3-D BI-FDTD simulation results are compared with available analytical solutions for the scattering from a circular chiral cylinder and a chiral sphere respectively.