A method of digital spectral analysis of hydroacoustic signals

A sequence of operations for the spectral and correlation estimation of digitized random signals is suggested using a fast Fourier transform. A set of algorithms and their possible modifications, which can be used in developing the spectral estimation programmes aimed at operative derivation of processing data is represented in compressed information form.Translated by Mikhail M. Trufanov.     

Acoustic plane-wave scattering from a rough interface over an inhomogeneous transition fluid layer

Acoustic scattering of a plane wave incident upon a rough surface over a transition fluid layer within which both the density and sound speed vary with depth is considered. A theory based upon a boundary perturbation method has been applied to a typical seabed environment to study the power spectral density representing the energy distribution of the scattered field over the space. The effects of frequency and roughness properties, including the roughness height, spatial correlation, and power spectrum, on the power spectral density have been investigated. The results demonstrate that the power spectral density of the scattered field depends upon all the aforementioned parameters, particularly the correlation length and the power spectrum of the rough surface, a conclusion in distinct contrast to the results for the coherent field obtained in an earlier study. It was found that the constituents of the rough surface such as the correlation length and wavenumber spectrum dominate the angular distribution of the scattered energy. These results indicate that it is crucial to employ a suitable topological model in the study of rough seabed scattering.     

Gravity-wave-induced pressure fluctuations in the deep ocean

We establish a mathematically consistent theory of the pseudo-sound pressure fluctuation in the deep ocean induced by nonlinearly interacting random plane waves on the surface. In the process, a new set of the second-order perturbation equations is derived and power-correlation coefficients between random plane waves are introduced. A phenomenological model is adopted for wind pressure which excites the surface waves consisting of wind-driven sea and swell. By solving the first-order- and the second-order-perturbation equations with this wind pressure as the excitation, we obtain an expression for the pressure fluctuation and its power spectral density in the gravity-wave regime. It is concluded that only the swell part of the surface waves generates the pressure fluctuation and the spectral density is modified by the power correlation coefficient.     

Wave mechanism of the seasonal variability of the largescale current field in the northern Tropical Atlantic

A spatial-temporal analysis of the density field in the large-scale hydrological observation areas in the Tropical Atlantic is carried out. The spectral maximum corresponding to the first baroclinic mode of the planetary Rossby wave is discriminated and studied. It is shown that the seasonal transformation of the large-scale circulation of the current field is connected with the propagation of this wave. A simple quasi-geostrophic model is suggested which describes the seasonal variability of the North Equatorial Countercurrent. The results obtained by this model are compared with the hydrological survey data.Translated by Mikhail M. Trufanov.     

风浪波高间相关性和在波群中波高累积概率

通过分析实验室风浪资料,研究风浪波高间的相关性以及波群中波高累积概率问题,发现风浪波高间相关性虽然主要发生在相邻波之间,但在隔1个波和隔2个波的波高间仍存在一定的相关性。谱宽度对波高间的相关性产生影响,但在相邻波、隔1个波和隔2个波情形下,谱宽度对波高间的相关性的影响方式不同。在相邻波情形下,谱宽度主要影响较大波高间的相关性,对各种高度波高间的总体相关性影响很小。而在隔1个波和隔2个波情形下,谱宽度对各种高度波高间的总体相关性有明显影响。根据实验结果提出含有波高相关因子的波群中多个波波高累积概率分布。     

Acoustic plane-wave scattering from a rough surface over a random fluid medium

Acoustic plane-wave scattering from a rough surface overlying a fluid half-space with a sound-speed distribution subject to a small random variation is considered. Under the assumption that the surface roughness and medium randomness are statistically independent, the scattered field may be derived by first solving for the mean field in the medium, and then incorporating with boundary-perturbation method to obtain the total mean field and the power spectral density of the scattered field. The employed algorithm is compatible to the analysis available in the existing literature so that the formulations are conveniently integrated. The results for the power spectral density have shown that the effects of medium inhomogeneities on the rough surface scattering are limited in a spectral regime where the scattered components have shallow grazing angles. The distribution of the power spectral density over the space is primarily governed by the power spectrum and correlation lengths of the rough surface.     

Determination of the methane content in the atmosphere using correlation radiometer

In this work the procedure of replacing the inhomogeneous atmosphere with homogeneous layers is discussed. The absorption and emission functions of a layer in a selected spectral region are the same as they are for the inhomogeneous layer. The dependence of the effective brightness temperature of a layer on its thickness is shown in the methane absorption band 1220–1260 cm^–1. Negative values of luminosity spectral density of a homogenous atmospheric layer with a width of over 3200 m indicate an increase in the weakening role of the layer for the outgoing radiation in the considered atmospheric model. The application of the method of gas optical filter correlation for a measurement of the methane content in the near-ground atmospheric layer from an aerospace platform is considered.     

The relationship between the amplitudinal characteristics of the high-frequency spectral components of wind-generated waves and the wind velocity over the sea

The relationship between the RMS amplitudes of the wind wave spectral components and the wind speed has been studied at ten frequencies in the band of 0.65–23 Hz. To measure the parameters of the high-frequenci waves, a resistance elevation wave gauge was operated, which was deployed in the Black See on an oceanographic platform near Katsively. The correlation between the wave amplitudes and the wind velocity at high frequencies of 5–23 Hz, corresponding to gravitation-capillary ripples, was found to reach a value of 0.8. At lower frequencies of 0.65–4.3 Hz, corresponding to short gravity waves, it dropped to 0.5–0.7. The response of spectral components to the wind speed variations in the gravity-capillary range is higher than in the range of short gravity waves. The results obtained differ from Phillips' idea about a saturated range for the frequency form of the spectrum of high-frequency gravity waves, since a linear dependence of the spectral amplitudes on the wind speed is established at a wind of force 1–8.Translated by Mikhail M. Trufanov.     

A Statistical Analysis of the Detection Performance of a Broadband Splitbeam Passive Sonar

An operational passive sonar is required to detect signals from sources, which are subject to spatial and temporal coherence losses via modifications by the ocean environment. Furthermore, these signals are to be detected in the presence of frequency-dependent correlated noise fields. For a system which employs splitbeam cross-correlation processing, the spatial and spectral properties of the signal and noise are of significant import. Therefore, the exact probability density and cumulative distribution functions of the N-sampled correlator outputs of a splitbeam broadband passive sonar are derived for the case of Gaussian inputs which are described by arbitrary cross-spectral density matrices. The validity of approximating the exact probability density function (pdf) as a Gaussian distribution is investigated. The effect of signal coherence loss and noise correlation on the detection performance is considered and the associated processing loss is expressed as a degradation factor within the detection threshold equation     

A criterion for the automatic identification of multimodal sea wave spectra

A simple and efficient procedure to automatically distinguish between uni-modal and multi-modal scalar spectra, which is consistent with the random nature of the spectral density estimations, is proposed. The suggested methodology is based on the logarithmic transformation of the spectral estimations. Examples that demonstrate the procedure are presented.     

改进的P—M谱

本文提出如下改进的无因次P—M谱 S(ω)=Aω~(-5)exp(-Bω~(-4-(?))并给出确定参数A、B、e的方法.改进P—M谱满足下述条件:谱峰通过给定点,谱面积等于1.     

单水滴致噪声平均能量密度谱的神经网络模型

单水滴击水致水下噪声平均能量密度谱是Medwin雨致噪声理论中的基本参量。基于人工神经网络算法,建立了利用一定粒度分布的水滴群及其击水后的噪声谱反演单水滴致水下噪声能量密度谱的模型。通过分析三种训练模式,得到反演值与真实值的对比及误差指标MSE的起伏性和收敛速度,验证了利用人工神经网络算法研究雨致噪声的有效性。     

Use of the kurtosis statistic in the frequency domain as an aid in detecting random signals

Power spectral density estimation is often employed as a method for signal detection. For signals which occur randomly, a frequency domain kurtosis estimate supplements the power spectral density estimate and, in some cases, can be employed to detect their presence. This has been verified from experiments with real data of randomly occurring signals. In order to better understand the detection of randomly occurring signals, sinusoidal and narrow-band Gaussian signals are considered, which when modeled to represent a fading or multipath environment, are received as non-Ganssian in terms of a frequency domain kurtosis estimate. Several fading and muitipath propagation probability density distributions of practical interest are considered, including Rayleigh and log-normal. The model is generalized to handle transient and frequency modulated signals by taking into account the probability of the signal being in a specific frequency range over the total data interval. It is shown that this model produces kurtosis values consistent with real data measurements. The ability of the power spectral density estimate and the frequency domain kurtosis estimate to detect randomly occurring signals, generated from the model, is compared using the deflection criterion. It is shown, for the cases considered, that over a large range of conditions, the power spectral density estimate is a better statistic based on the deflection criterion. However, there is a small range of conditions over which it appears that the frequency domain kurtosis estimate has an advantage. The real data that initiated this analytical investigation are also presented.     

The generation of digital random time histories

“A method to generate digital random time histories is described. A random number sequence is shaped to give the desired spectral density curve. This finite set of numbers is then Inverse Fast Fourier Transformed (IFFT). The result is a pseudo random time history which has given spectral characteristics. An application of this technique is described.”     

基于EOF分析对南海西北海域水体光谱特性的研究

This study presents an analysis of the spectral characteristics of remote sensing reflectance(R_(rs)) in northwestern South China Sea based on the in situ optical and water quality data for August 2018.R_(rs)was initially divided into four classes,classes A to D,using the max-classification algorithm,and the spectral properties of whole R_(rs) were characterized using the empirical orthogonal function(EOF) analysis.Subsequently,the dominant factors in each EOF mode were determined.The results indicated that more than 95% of the variances of R_(rs) are partly driven by the back-scattering characteristics of the suspended matter.The initial two EOF modes were well correlated with the total suspended matter and back-scattering coefficient.Furthermore,the first EOF modes of the four classes of R_(rs)(A-D R_(rs)-EOF_1) significantly contributed to the total variances of each R_(rs) class.In addition,the correlation coefficients between the amplitude factors of class A-D R_(rs)-EOF_1 and the variances of the relevant water quality and optical parameters were better than those of the unclassified ones.The spectral shape of class AR_(rs)-EOF_1 was governed by the absorption characteristic of chlorophyll a and colored dissolved organic matter(CDOM).The spectral shape of class B R_(rs)-EOF_1 was governed by the absorption characteristic of CDOM since it exhibited a high correlation with the absorption coefficient of CDOM(a_g(λ)),whereas the spectral shape of class C R_(rs)-EOF_1 was governed by the back-scattering characteristics but not affected by the suspended matter.The spectral shape of class D R_(rs)-EOF_1 exhibited a relatively good correlation with all the water quality parameters,which played a significant role in deciding its spectral shape.     

Water-temperature effect on the spectral density of wind gravity waves and on sea-surface roughness

The results of hourly measurements of sea roughness and hydrometeorological parameters, which were automatically taken from special buoys over a long period of time, were used. These buoys were located in the open regions of both the Atlantic and Pacific oceans in different climatic zones; the mean water-surface temperature around the buoys varies from 1–3°C to 26–28°C. In addition to measurement results, the tables contain data on the spectral density of sea roughness for a wide range of frequencies. An analysis of these data, which was made for a short-wave region of the wind-wave spectrum, for the first time revealed a noticeable watertemperature dependence of the spectral density of wind waves within the frequency range 0.30–0.40 Hz, which corresponds to wave lengths of 9-4 m. The presence of such dependence is explained by a rapid temperature change in kinematic sea-water viscosity. Earlier, we indicated the temperature dependence of only very short spectral components that relate to a centimetric wavelength range. The statistical significance of the watertemperature effect on the spectral density of waves of the indicated frequency is supported by the results of a variance analysis. Temperature variations in the parameter of sea-surface roughness, which is determined, first of all, by the energy of the spectral shortwave region, are estimated. Altimetry is the basic method which is used in remotely determining the velocity of near-water wind. This method allows one to obtain records of deviations of the sea surface from the geoid surface and to calculate (on the basis of these records) the spectral density of wave components of almost any frequency. It is known that the wave-spectrum components in the region of low frequencies are almost always affected by ripple. Consequently, the energy of these components is determined not only by wind forcing, and only the components in the range of frequencies exceeding approximately 0.3 Hz are purely windy. Therefore, using the results of sea-surface altimetry in order to determine the velocity of near-water wind, one should use the spectral densities of wave components in this frequency region. The water-temperature dependence of the spectral density of short wind waves is manifested only in a certain frequency interval, which supports this recommendation.     

The determination of confidence limits associated with estimates of the spectral peak frequency

A Monte-Carlo simulation technique is used to evaluate the suitability of proposed techniques for the estimation of the spectral peak frequency. Due to the statistical variability of spectral estimates and the discrete frequency resolution of spectra, the calculated values of peak frequency are stochastic variables. The probability density functions for such estimates are functions of the frequency resolution and the number of degrees of freedom of the spectrum from which they are derived as well as the spectral peakedness. The mean values of all techniques are biased high, indicating the derived values of peak frequency are an overestimate of the true value. The probability density functions do not follow an obvious analytical form. Tabular values are, however, presented to enable the determination of confidence limits for estimates of the peak frequency.     

Acoustic wave scattering from a rough seabed over a sediment layer with generalized-exponential density and inverse-square sound speed profiles

This paper considers acoustic plane wave scattering from a rough seabed on a transition sediment layer overlying an elastic sea basement. The transition sediment layer is assumed to be fluid-like, with density and sound speed distributions behaving as generalized-exponential and inverse-square functions, respectively. This specific class of density and sound speed profiles deserves special attentions not only because it is geologically realistic, but also renders analytical solutions to the Helmholtz equation, making it particularly useful in the study of ocean and seabed acoustics. Based upon a boundary perturbation approach, the computational algorithm for the spatial spectrum in terms of the power spectral density of the scattered field has been developed and implemented. The results have shown that, while the coherent field mainly depends upon the gross structure of the seabed roughness, e.g., RMS roughness, the scattered field is significantly affected by the details of the roughness distributions specialized by the roughness power spectrum and the spatial correlation length of the rough surface. The dependence of the power spectral density of the scattered field on the various types of sediment stratifications, including the constant and the k²-linear sound speed distributions, is also included in the analysis.     

An In-Situ Study on the Influence of Ascidian Suspension Feeding on the Subtidal Nepheloid Layer in the Northern Adriatic Sea

Abstract. In order to evaluate the effect of epibenthic suspension-feeding organisms on nepheloid-layer processes, a novel flow-through 'Benthic Ecosystem Tunnel' was used on sublittoral marine sediment substrate. Our test organism was the ascidian Microcosmus sulcatus . Particle and chlorophyll a reduction were recorded. The correlation of seasonal fluctuations of chlorophyll a concentrations with chlorophyll a reduction by M. sulcatus indicates the potential for Microcosmus to control local phytoplankton input to the subtidal nepheloid layer. However, the relatively low density of Microcosmus at the study site, the nature of the sediment surface and the susceptibility of ascidians to high turbidity ( i.e. , high sedimentation rates of total particular matter (TPM) causing frequent periods of siphon closure and inactivity) makes it unlikely that M. sulcatus is an important element controlling the composition of the nepheloid layer on a large scale.     

Correlation of the distribution of normalized spectral radiances as an indicator of the features of water exchange in the Black Sea

A set of 128 images of the Black Sea obtained using the SeaWiFS color scanner data in 1998–2004 was processed to estimate the new capabilities of indication of the vertical inhomogeneity of the upgoing normalized radiance. The basin of the sea was divided into 430 frames with a size of 30 × 30 km. Correlation coefficients R between all the spectral radiances were calculated for all the frames of all the images. The correlation between the radiance distributions at 490 and 670 nm was most sensitive to the conditions of the radiance formation. This allowed us to find the situations in which the variations in the shortwave and long wave radiances in the frame appear opposite and are characterized by estimates R < ?0.7. It was shown that such distributions are indicators of the water exchange between the coastal and open sea areas. They are caused by a special optical stratification during the events of such exchange. A conclusion is made about the prospective of applying the estimates of the correlation of spectral radiances for diagnosis of the water exchange based on the current and archive materials of ocean color scanners.