Hyperspectral remote sensing for shallow waters. I. A semianalytical model

For analytical or semianalytical retrieval of shallow-water bathymetry and/or optical properties of the water column from remote sensing, the contribution to the remotely sensed signal from the water column has to be separated from that of the bottom. The mathematical separation involves three diffuse attenuation coefficients: one for the downwelling irradiance (K(d)), one for the upwelling radiance of the water column (K(u)(C)), and one for the upwelling radiance from bottom reflection (K(u)(B)). Because of the differences in photon origination and path lengths, these three coefficients in general are not equal, although their equality has been assumed in many previous studies. By use of the Hydrolight radiative-transfer numerical model with a particle phase function typical of coastal waters, the remote-sensing reflectance above (R(rs)) and below (r(rs)) the surface is calculated for various combinations of optical properties, bottom albedos, bottom depths, and solar zenith angles. A semianalytical (SA) model for r(rs) of shallow waters is then developed, in which the diffuse attenuation coefficients are explicitly expressed as functions of in-water absorption (a) and backscattering (b(b)). For remote-sensing inversion, parameters connecting R(rs) and r(rs) are also derived. It is found that r(rs) values determined by the SA model agree well with the exact values computed by Hydrolight (~3% error), even for Hydrolight r(rs) values calculated with different particle phase functions. The Hydrolight calculations included b(b)/a values as high as 1.5 to simulate high-turbidity situations that are occasionally found in coastal regions.     

Inversion of irradiance and remote sensing reflectance in shallow water between 400 and 800 nm for calculations of water and bottom properties

What we believe to be a new inversion procedure for multi- and hyperspectral data in shallow water, represented by the subsurface irradiance and remote sensing reflectance spectra, was developed based on analytical equations by using the method of nonlinear curve fitting. The iteration starts using an automatic determination of the initial values of the fit parameters: concentration of phytoplankton and suspended matter, absorption of gelbstoff, bottom depth, and the fractions of up to six bottom types. Initial values of the bottom depth and suspended matter concentration are estimated analytically. Phytoplankton concentration and gelbstoff absorption are initially calculated by the method of nested intervals. A sensitivity analysis was made to estimate the accuracy of the entire inversion procedure including model error, error propagation, and influence of instrument characteristics such as noise, and radiometric and spectral resolution. The entire inversion technique is included in a public-domain software (WASI) to provide a fast and user-friendly tool of forward and inverse modeling.     

基于神经网络方法的高光谱遥感浅海水深反演

利用人工神经网络方法进行了高光谱遥感反演浅海水深的初步研究.在产生模拟数据时,为保证模拟数据的合理性,引入了根据水体和海底特性来划分光学浅水和光学深水的方法,并初步研究了利用光谱徽分技术进行光学浅水和光学深水区分的有效性.在人工神经网络建模过程中,采用主成分分析的方法对网络的输入数据进行预处理,显著提高了网络的学习速度.建立的人工神经网络模型和基于非线性最优化方法的反演算法与实测数据的反演结果相比较,人工神经网络模型的反演精度明显高于非线性最优化反演算法.     

Trapped shallow water waves for continuous bottom topographies

Summary Trapped shallow water waves are studied for gently sloping, continuous bottom topographies. Asymptotic solutions, based on the smallness of the bottom slope are obtained for some two-dimensional and axisymmetric depth functions. Eigenfrequency spectra and corresponding trapped wave patterns are computed.

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Gefangene Seichtwasser-Wellen bei stetigen Bodentopographien

Zusammenfassung Untersucht werden gefangene Seichtwasser-Wellen für sanft geneigte stetige Bodentopographien. Asymptotische Lösungen, basierend auf der Kleinheit der Bodenneigung, werden für einige zweidimensionale und achsensymmetrische Tiefenfunktionen erhalten. Eigenfrequenzspektren und die zugehörigen Wellenbilder werden berechnet.

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With 5 Figures     

Deriving inherent optical properties from water color: a multiband quasi-analytical algorithm for optically deep waters

For open ocean and coastal waters, a multiband quasi-analytical algorithm is developed to retrieve absorption and backscattering coefficients, as well as absorption coefficients of phytoplankton pigments and gelbstoff. This algorithm is based on remote-sensing reflectance models derived from the radiative transfer equation, and values of total absorption and backscattering coefficients are analytically calculated from values of remote-sensing reflectance. In the calculation of total absorption coefficient, no spectral models for pigment and gelbstoff absorption coefficients are used. Actually those absorption coefficients are spectrally decomposed from the derived total absorption coefficient in a separate calculation. The algorithm is easy to understand and simple to implement. It can be applied to data from past and current satellite sensors, as well as to data from hyperspectral sensors. There are only limited empirical relationships involved in the algorithm, and they are for less important properties, which implies that the concept and details of the algorithm could be applied to many data for oceanic observations. The algorithm is applied to simulated data and field data, both non-case1, to test its performance, and the results are quite promising. More independent tests with field-measured data are desired to validate and improve this algorithm.     

Optical remote sensing of chlorophyll a in case 2 waters by use of an adaptive two-band algorithm with optimal error properties

Two-band algorithms that use the ratio of reflectances at 672 and 704 nm have already proved successful for chlorophyll a retrieval in a range of coastal and inland waters. An analysis of the effect of reflectance measurement errors on such algorithms is made. It provides important indications of the range of validity of these algorithms and motivates the development of an entirely new type of adaptive two-band algorithm for hyperspectral data, whereby the higher wavelength is chosen for each input spectrum individually. When one selects the wavelength at which reflectance is equal to the reflectance at the red chlorophyll a absorption peak, chlorophyll a retrieval becomes entirely insensitive to spectrally flat reflectance errors, which are typical of imperfect atmospheric correction, and is totally uncoupled from the retrieval or an estimation of backscatter. This new algorithm has been tested for Dutch inland and Belgian coastal waters.     

Uniqueness in remote sensing of the inherent optical properties of ocean water

We examine the problem of uniqueness in the relationship between the remote-sensing reflectance (Rrs) and the inherent optical properties (IOPs) of ocean water. The results point to the fact that diffuse reflectance of plane irradiance from ocean water is inherently ambiguous. Furthermore, in the 400 < lambda < 750 nm region of the spectrum, Rrs(lambda) also suffers from ambiguity caused by the similarity in wavelength dependence of the coefficients of absorption by particulate matter and of absorption by colored dissolved organic matter. The absorption coefficients have overlapping exponential responses, which lead to the fact that more than one combination of IOPs can produce nearly the same Rrs spectrum. This ambiguity in absorption parameters demands that we identify the regions of the Rrs spectrum where we can isolate the effects that are due only to scattering by particulates and to absorption by pure water. The results indicate that the spectral shape of the absorption coefficient of phytoplankton, a(ph)(lambda), cannot be derived from a multiparameter fit to Rrs(lambda). However, the magnitude and the spectral dependence of the absorption coefficient can be estimated from the difference between the measured Rrs(lambda) and the best fit to Rrs(lambda) in terms of IOPs that exclude a(ph)(lambda).     

Accuracy of remote sensing of water temperature by Raman spectroscopy

We present an experimental study of the Raman spectrum of pure and synthetic seawater with respect to its salinity and temperature dependence. Experiments made in the laboratory with both cw and pulsed excitation yield information on the limits and applicability of the technique in actual experiments in the field. We have also performed an experimental analysis to determine the presence of stimulated Raman scattering and its influence on the temperature dependence of the spectrum.     

The generalized Riemann problem method for the shallow water equations with bottom topography

This paper extends the generalized Riemann problem method (GRP) to the system of shallow water equations with bottom topography. The main contribution is that the generalized Riemann problem method (J. Comput. Phys. 1984; 55 (1):1–32) is used to evaluate the midpoint values of solutions at each cell interface so that the bottom topography effect is included in numerical fluxes, and at the same step the source term is discretized with an interface method in which only mid‐point values are plugged in. This scheme is well balanced between the flux gradient and bottom topography when incorporating the surface gradient method (SGM) (J. Comput. Phys. 2001; 168 (1):1–25) into data reconstruction step, and it is also suitable for both steady and unsteady flow simulations. We illustrate the accuracy of this scheme by several 1‐D and 2‐D numerical experiments. Copyright © 2005 John Wiley & Sons, Ltd.     

A role for remote sensing information in storm water planning and management

The Environmental Protection Agencys (EPAs) Phase II Storm Water regulations offer challenges and opportunities to the nations smaller communities, those with populations of less than 100,000. The requirement to address storm water issues from the quality perspective provides the impetus to consider the issues of community growth on a watershed basis, and over the entire timeframe of watershed development. At the same time, remote sensing (RS)-based information can now be collected with a resolution that is useful for addressing urban issues. This paper discusses particular EPA requirements within the Phase II program and identifies RS applications that can inform decisions and activities that meet the requirements. Planning, on-going operations, and enforcement activities can all be supported through the use of RS image data. In addition, community leaders have the ability to formulate rational policies and to defend the basis for those policies based on RS imagery. The paper concludes with an assessment of the potential for RS imagery and tools to support site specific and watershed-based planning and ongoing management. Specific actions are required on the part of both government agency users/decision makers and the research community to take full benefit of the available information.     

Airborne and satellite remote sensing of the mid-infrared water vapour continuum

Remote sensing of the atmosphere from space plays an increasingly important role in weather forecasting. Exploiting observations from the latest generation of weather satellites relies on an accurate knowledge of fundamental spectroscopy, including the water vapour continuum absorption. Field campaigns involving the Facility for Airborne Atmospheric Measurements research aircraft have collected a comprehensive dataset, comprising remotely sensed infrared radiance observations collocated with accurate measurements of the temperature and humidity structure of the atmosphere. These field measurements have been used to validate the strength of the infrared water vapour continuum in comparison with the latest laboratory measurements. The recent substantial changes to self-continuum coefficients in the widely used MT_CKD (Mlawer-Tobin-Clough-Kneizys-Davies) model between 2400 and 3200?cm(-1) are shown to be appropriate and in agreement with field measurements. Results for the foreign continuum in the 1300-2000?cm(-1) band suggest a weak temperature dependence that is not currently included in atmospheric models. A one-dimensional variational retrieval experiment is performed that shows a small positive benefit from using new laboratory-derived continuum coefficients for humidity retrievals.     

Effect of suspended particulate and dissolved organic matter on remote sensing of coastal and riverine waters

We use remote sensing reflectance (RSR) together with the inherent optical properties of suspended particulates to determine the backscattering ratio b(b)/b for coastal waters. We examine the wavelength dependence of b(b)(lambda) and f(lambda)/Q(lambda) and establish the conditions when C(lambda) in RSR(lambda) approximately or = C(lambda)b(b)(lambda)/a(lambda) can be treated as a constant. We found that for case 2 waters, RSR was insensitive to the natural fluctuations in particle-size distributions. The cross-sectional area of the suspended particulate per unit volume, x(g), showed an excellent correlation with the volume scattering coefficient.     

A stereoscopic technique for seafloor statistical characterization by acoustic remote sensing

The authors present a new method for performing statistical characterization of underwater surfaces by acoustic remote sensing. A wideband technique is outlined that, by exploiting stereoscopic observation, estimates the roughness height of underwater surfaces however oriented with respect to the instrument. The surface is ensonified by a wideband signal and simultaneously viewed from two or more slightly different directions of view. The corresponding echoes are compared in the frequency domain, and a roughness estimate is obtained. Once the instrument centerband frequency has been fixed, the scale factor in the estimation can be adjusted by varying the differences between the view directions. This permits a wide estimation interval, ranging from a small fraction to many times the wavelength corresponding to the centerband frequency. Preliminary experiments have been carried out in a water tank, and the estimated roughness values are in agreement with the theory.     

用激光烧结法制备的SnO2薄膜的气敏性质

用溶胶-凝胶法制备SnO2前驱液,然后用提拉法分别在单晶Si和Al2O3基片表面制备出SnO2前驱膜,再用脉冲Nd:YAG激光烧结前驱膜使其转变为晶体SnO2薄膜.用XRD分析了单晶Si表面的SnO2薄膜,研究激光功率对SnO2薄膜相组成的影响.TEM观察表明,激光烧结后的薄膜SnO2颗粒均匀,直径约为10 nm.用激光烧结法制备的SnO2薄膜对浓度为1.80×10-4丙酮的最高灵敏度为30～40,明显高于用传统烧结法制备的SnO2薄膜的灵敏度.激光烧结能降低薄膜具有最高灵敏度的工作温度.     

稀土Dy掺杂纳米SnO2薄膜的结构与气敏特性

采用真空蒸发法在玻璃衬底上制备稀土Dy掺杂Sn薄膜,对薄膜进行合适的氧化、热处理后获得Dy掺杂SnO2薄膜.用X射线衍射、场发射扫描电子显微镜、静态配气法对薄膜性能进行测试,研究不同掺Dy含量和热处理条件对SnO2薄膜的影响.结果显示,制备的SnO2薄膜呈金红石结构为n型;在相同热处理条件下,Dy掺杂可明显缩短薄膜氧化、热处理的时间;适当掺入稀土Dy可明显改善SnO2薄膜的结构、气敏特性.掺Dy 3at%后可大大提高SnO2薄膜对丙酮气体的灵敏度.     

基于PSO优选参数的SVR水质参数遥感反演模型

为进一步提高多光谱图像水质反演的精度,提出了一种基于PSO优选参数的SVR水质参数遥感反演模型。该模型利用高分辨率多光谱遥感SPOT-5数据和水质实地监测数据,采用CV估计模型推广误差,并使用PSO优选SVR模型参数,实现了模型参数的自动全局优选,在训练好的SVR模型基础之上对水质进行反演。以渭河陕西段为例进行实证研究,实验结果表明,所提出的水质反演模型较常规的线性回归模型有更高的反演精度,为内陆河流环境遥感监测提供了一种新方法。     

An optical fiber sensor for remote pH sensing and imaging

A fiber-optical probe for pH sensing and real-time imaging is successfully fabricated by connecting a polymer imaging fiber and a gradient index (GRIN) lens rod which was modified with a sensing film. By employing an improved metallographic microscope, an optical system is designed to cooperate with the probe. This novel technique has high-quality imaging capabilities for observing remote samples while measuring pH. The linear range of the probe is pH 1.2-3.5. This technique overcomes the difficulty that high-quality images cannot be obtained when directly using conventional imaging bundles for pH sensing and imaging. As preliminary applications, the corrosion behavior of an iron screw and the reaction process of rust were investigated in buffer solutions of pH 2.0 and 2.9, respectively. The experiment demonstrated that the pH values of the analytes' surface were higher than that of buffer solutions due to the chemical reaction. It provides great potential for applications in optical multifunctional detection, especially in chemical sensing and biosensing.     

L/S波段微波遥感海水盐度和温度的反演算法

对基于多元线性回归构造的非线性多项式形式的L/S双波段微波遥感海水盐度和温度反演算法进行了误差分析,并优化了多项式的系数.进一步提出了一种半解析的反演算法,该算法基于海面微波发射理论模型,构建包含盐度和温度两个反演目标的二元非线性方程组,利用数值解法直接求解该方程组,以反演盐度和温度.该反演算法较以往的算法具有更高的反演精度.     

Discrete-ordinated finite-element method for atmospheric radiative transfer and remote sensing

Advantages and disadvantages of modern discrete-ordinates finite-element methods for the solution of radiative transfer problems in meteorology, climatology, and remote sensing applications are evaluated. After the common basis of the formulation of radiative transfer problems in the fields of neutron transport and atmospheric optics is established, the essential features of the discrete-ordinates finite-element method are described including the limitations of the method and their remedies. Numerical results are presented for 1-D and 2-D atmospheric radiative transfer problems where integral as well as angular dependent quantities are compared with published results from other calculations and with measured data. These comparisons provide a verification of the discrete-ordinates results for a wide spectrum of cases with varying degrees of absorption, scattering, and anisotropic phase functions. Accuracy and computational speed are also discussed. Since practically all discrete-ordinates codes offer a builtin adjoint capability, the general concept of the adjoint method is described and illustrated by sample problems. Our general conclusion is that the strengths of the discrete-ordinates finite-element method outweight its weaknesses. We demonstrate that existing general-purpose discrete-ordinates codes can provide a powerful tool to analyze radiative transfer problems through the atmosphere, especially when 2-D geometries must be considered.