Wave parameters and functions in wavelet analysis

A preliminary study of wave parameters and functions in wavelet analysis is conducted in this paper. The Morlet wavelet transform is used to calculate the time–frequency wavelet energy density function, its volume (i.e. the total energy), its frequency-integral (i.e. the wavelet smoothed instantaneous wave energy history), its time-integral (i.e. the wavelet spectral density function), and two non-dimensional wave indices (NIF, NIT). The processing of the measured wave data obtained from the Chi-Gu coastal observation tower during the period August 2000 to July 2001 indicates that the inter-comparison of wavelet smoothed instantaneous wave energy history and smoothed instantaneous wave energy history (SIWEH) as defined by Funke and Mansard (Proc 17th Int Conf on Ocean Eng, 1980) can reveal the noise structure of the wave signal. The wave data with index NIF greater than 2 is always accompanied with noise, therefore NIF can be used as one of the data quality criteria. The index NIT is linearly correlated with the significant wave period and with the significant wave height, therefore NIT can be used to study the wave growth and decaying phenomena.     

Separation of obliquely incident and reflected irregular waves by the Morlet wavelet transform

An existing 2D time-domain method for separating irregular incident and reflected waves by wavelet transform [Ma et al., 2010. A new method for separation of 2D incident and reflected waves by the Morlet wavelet transform. Coastal Eng., 57(6):597–603] is extended to account for obliquely incident irregular waves propagating over sloping bottoms. The linear shoaling and refraction coefficients are adopted to determine the amplitude and phase changes of waves. The optimal central frequency of the Morlet wavelet is determined by the minimum Shannon wavelet entropy. Numerical tests show that the present method can accurately separate waves over horizontal depths. For waves at sloping bottoms, however, the separation errors increase as bottom slope increases and are significant for waves with incident angle larger than π/3.     

A new method for separation of 2D incident and reflected waves by the Morlet wavelet transform

A new method based on the Morlet wavelet transform for separating a 2D wave field into incident and reflected waves is proposed in this paper. The principle of this method, first, is derived for constant depths. Then, using the linear shoaling theory, the method is extended to an arbitrary sloping bathymetry. Owing to the time-frequency characteristic of wavelet transform, the present method can separate waves in the real time domain and is not confined by the stationary assumption of waves. The efficiency and accuracy of this method are demonstrated using numerical simulated data.     

小波变换压制噪声在单道地震资料处理中的应用

海洋区域地质调查中,单道地震勘探以其高分辨率、方便快捷等优点成为揭示浅地层地质构造的重要手段之一.结合前人研究成果并出于保护信号的完整性,对低频部分的噪声采用带通滤波进行滤除,而对高频信号中的随机噪声采用小波阈值去噪方法进行压制处理.在小波阈值去噪过程中,通过比较选用最佳小波基函数sym5对地震剖面进行三层小波分解.结果表明:两种阈值去噪方法在带通滤波的基础上进一步提升了剖面的质量,使得反射层面变得连续可追踪.软阈值去噪滤掉了剖面上的较多信息,去噪效果较为彻底,硬阈值去噪则保留了较多的细节信息.     

用小波变换研究赤道西太平洋温度细结构

使用小波变换分析了赤道西太平洋暖池区的CTD资料的温度序列,表明垂向温度细结构具有自相似特征,有可能使用分数维模型来描述.计算了温度序列的奇异性指数及其概率分布,发现此分布具有近似普适的形式.非零概率的负指数意味着温度剖面可能存在奇异点.对经小波变换后的序列的周期性统计表明周期△和尺度α遵从幂律标度△～αβ,β≈1.这提示着细结构背后可能存在简单的二分串级机制.小波变换的振幅反映了温度脉动的强弱.据此可以确定脉动能量的垂向分布和能谱密度,和传统方法的比较表明小波能谱较为光滑,有利于精确测定谱斜率.     

Study of fine structure of temperature in the equatorial westernPacific Ocean with wavelet transform

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小波变换在HT-7Tokamak磁流体动力学振荡动态频谱分析中的应用

将Gauss复小波变换方法成功地应用于HT-7Tokamak磁流体动力学振荡动态频谱分析中.研究结果表明,这种方法具有较好的时间分辨率和空间分辨率,比较适用做动态频谱分析.对典型放电数据的分析结果表明, m=1模的振荡频率与等离子体压强梯度有着密切的关系.     

Local and far-field surface elevation around a vertical cylinder in unidirectional steep wave groups

The problem of diffraction of a unidirectional incident wave group by a bottom-seated cylinder is considered. We assume the amplitude of the incoming wave to be small in comparison with other linear scales of the problem, and develop the corresponding second-order perturbation theory. We use the Fourier transform to treat time variation and separate spatial variables when solving the non-homogeneous second-order problem. The resulting set of non-homogeneous Bessel equations is solved numerically.Solutions for various types of incoming wave spectrum are obtained including the Gaussian spectrum and the Pierson–Moskowitz spectrum. To validate the method, problems with gradually decreasing bandwidth of Gaussian spectrum are solved and it is shown that the corresponding solution approaches that for the monochromatic case. The Pierson–Moskowitz spectrum with a set of realistic physical parameters is used as an example of extreme wave interaction with an offshore structure. The corresponding first- and second-order solutions are obtained and the effect of non-linearity on the solution is discussed with the emphasis on the growth of maximum free-surface elevation on the cylinder’s surface and generation of high frequency free radiated waves.     

Combined tidal velocity and duration asymmetries as a determinant of water transport and residual flow in Paranaguá Bay estuary

Net transports of water, salt and suspended particulate matter (SPM) for a cross-section in front of the Paranaguá Harbour (Paranaguá Bay, Brazil) are presented for eight distinct tidal cycles. Data include measurements over single spring and neap tidal cycles, during both wet and dry seasons. The main drive forces of circulation and SPM dynamics are identified. Advective transport dominated under moderate to high vertical salinity stratification and weak currents, while mixing processes dominated under well-mixed conditions generated by high currents and low freshwater input. Under partially mixed conditions, both advective and mixing processes were important. The tide-induced residual circulation dictated the magnitude and direction of residual currents and net transports of water and salt, but not of SPM transport. The SPM dynamics was intrinsically related to cyclical processes of erosion, resuspension and deposition driven by tidal currents. The turbulent mixing intensity conditioned the vertical mixing of SPM. Resuspension and vertical mixing were conspicuous in spring cycles, while the horizontal advection preponderated in the neaps. Lags between maximum currents and SPM peak concentrations occurred, with more pronounced hysteresis during ebb periods.     

The effect of resuspension on the fate of total mercury and methyl mercury in a shallow estuarine ecosystem: a mesocosm study

Sediments are the major repository of mercury in estuaries and could be a significant source of Hg to the overlying water column via release from the solid phase during resuspension. There is, however, little information on the effect of resuspension on Hg partitioning and release to the water column. The objective of this study was to determine the effect of resuspension on the cycling of THg and MeHg between the water column and the sediment. Tidal resuspension was simulated using the MEERC STORM facility. The facility can mimic both realistic bottom shear stress and water column turbulence simultaneously. There were three replicates of each resuspension (R) and no resuspension (NR) mesocosms. Two 4-week experiments were conducted in July and October of 2001: experiment 1 without clams and experiment 2 with clams. Both experiments showed that resuspension of muddy sediment introduced significantly higher particulate THg to the water column as TSS increased. The results suggest that THg was mostly bound to sediment particles with very little release during the resuspension events. In contrast, particulate MeHg was significantly lower in the R tanks where sediment particles with poor MeHg were dominant in the water column during the resuspension events. Dissolved THg and MeHg did not change in concert with changes in particulate load, suggesting that the dynamics between dissolved and particulate phases for both THg and MeHg cannot be explained by an equilibrium partitioning.     

经验模态与小波分解在光学遥感内波参数提取中的应用

内波遥感参数提取是利用遥感影像研究海洋内波的重要手段,通过提取内波的基本参数可以对海洋内波的生成与传播机制进行进一步的研究。提出了利用经验模态分解、小波分解与高阶多项式拟合从光学影像中提取内波半波宽度的方法。经验模态分解与小波分解对内波剖面数据进行尺度分解,根据归一化方差最大来提取内波分量;多项式拟合基于内波剖面的亮暗条纹变化完全由内波调制的假设,对数据进行拟合,并根据一阶导数来提取半波宽度。用南海北部东沙岛附近2004年7月10日的中-巴资源卫星(China-Brazil Earth Resources Satellite,CBERS)影像对方法进行了验证。结果表明,3种方法能较好地提取所需参数,获取的内波半波宽度具有较好的一致性;上述方法在处理非平稳及非线性遥感数据上,具有非常明显的优势。基于一维非线性内波理论,通过提取的内波半波宽度,辅以水深和混合层深度数据,反演了内波的振幅。     

南海北部海域油气资源调查技术及其应用研究——震源子波检测及模拟分析

在野外采集中考察气枪的稳定性以及室内进行确定性子波反褶积处理,都要求能够获得气枪阵列的实时远场子波。从震源子波检测方法出发,阐述了近场子波采集、幅值转换及记录流程,并列举出实测近场子波时需要考虑解决的问题;接着介绍了震源远场子波的获取方法和主要途径;震源近场子波及远场子波的实测方案,检测震源子波所需的仪器设备和采集技术方法。文中分析了气枪子波的气泡干扰问题,虚反射与后续气泡震荡间的关系以及吸收衰减对子波的影响,并根据实测远场子波,对齐氏和范氏两种气体模型进行了模拟效果对比,结果表明:采用范氏气体模型所模拟的子波与实际子波更为吻合,模拟出的气枪的子波波形、振幅大小、气泡周期上,与业界通用的Nucleus软件的模拟结果吻合较好,在后续的使用中将继续优化。     

Transfer-function modelling between environmental variation and mesozooplankton in the Baltic Sea

Time series of freshwater runoff, seawater salinity, temperature and oxygen were used in transfer functions (TF) to model changes of mesozooplankton taxa in the Baltic Sea from the 1960’s to the 1990’s. The models were then compared with long term zooplankton monitoring data from the same period. The TF models for all taxa over the whole Baltic proper and at different depth layers showed statistically significant estimates in t-tests. TF models were further compared using parsimony as a criterion. We present models showing 1) r² > 0.4, 2) the smallest residual standard error with the combination of exploratory variables, 3) the lowest number of parameters and 4) the highest proportional decrease in error term when the TF model residual standard error was compared with those of the univariate ARIMA model of the same response variable. Most often (7 taxa out of a total of 8), zooplankton taxa were dependent on freshwater runoff and/or seawater salinity. Cladocerans and estuarine copepods were more conveniently modelled through the inclusion of seawater temperature and oxygen data as independent variables. Our modelling, however, explains neither the overall increase in zooplankton abundance nor a simultaneous decrease found in the neritic copepod, Temora longicornis. Therefore, biotic controlling agents (e.g. nutrients, primary production and planktivore diets) are suggested as independent variables for further TF modelling. TF modelling enabled us to put the controlling factors in a time frame. It was then possible, despite the inherent multiple correlation among parameters studied to deduce a chain-of-events from the environmental controls and biotic feedback mechanisms to changes in zooplankton species. We suggest that the documented long-term changes in zooplankton could have been driven by climatic regulation only. The control by climate could be mediated to zooplankton through marine chemical and physical factors, as well as biotic factors if all of these were responding to the same external control, such as changes in the freshwater runoff. Increased runoff would explain both the increasing eutrophication, causing the overall increase of zooplankton, and the changes in selective predation, contributing to decline of Temora.     

Vertical distribution of the macrobenthic community and its relationships to trace metals and natural sediment characteristics in the lower Douro estuary, Portugal

Vertical distribution (0–15 cm) of the macrobenthic community and its relationships to natural sediment characteristics and trace metal contents and bioavailability were studied at five locations in the lower Douro estuary, Portugal. An analysis of vertical metal distribution, for the interpretation of anthropogenic impact on the estuarine sediments, was also investigated. Sediment characterisation included organic matter, grain size, metals (Al, Fe, Cu, Pb, Cr, Ni, Cd, Zn and Mn), acid volatile sulphide (AVS) and simultaneously extracted metals (SEM). The macrobenthic community had low diversity (14 species), was dominated by small size opportunists and seemed to be controlled mainly by natural factors such as grain size distribution, Al and Fe contents and sediment depth. The vertically heterogeneous distribution of macrobenthic community appears to affect redox status of the sediments and consequently metal bioavailability. Despite anthropogenic contamination in terms of Zn, Cu, Pb, Cr and Ni having already been detected in the north bank, the analysis of vertical distribution was essential for the identification of current anthropogenic contamination in terms of Zn, Pb and Cd in the south bank.     

Long-term fish and macro-crustacean community variation in a Mediterranean lagoon

The fish and macro-crustacean community of the Vaccarès lagoon (Rhône River delta, South France) was sampled monthly from 1993 to 2002. The lagoon salinity shifted from 15 in 1993 to 5 in 1994–1997 and went back to 15 in 1999–2002. Connections with the sea also varied during the study period with larger openings in 1996–1997. During the study period, the community changed to revert in 2002 to a state similar to 1993. These changes consisted of a sequence of increased and decreased patterns of freshwater species and some marine species. Typical lagoon species tended to resist to salinity changes. Freshwater species colonised the lagoon when the salinity was low. Marine species may have varied both in relation to connections with the sea and to indirect effects of freshwater outflow. This study shows that community changes following environmental variations can be delayed in time, and emphasises the need for long-term studies.     

Nutrient, sulfur and carbon dynamics in a hypersaline lagoon

We measured benthic and water column fluxes in a hypersaline coastal system (Baffin Bay, Texas) in 1996–1997, a period of decreasing salinity (increased freshwater input) and turbidity. Salinity decreased from a mean of 60 to 32 practical salinity units (psu) and turbidity decreased from a mean of 78 to 25 NTU over the study period. Associated with hydrological changes, there were important changes in nutrient fluxes and metabolism. There was a shift of total respiration from the water column to the sediments and an increased amount of the benthic metabolism (2–67%) was attributed to sulfate reduction in this system when salinity was lowest, perhaps a consequence of increased benthic light levels and photosynthetic production of labile carbon in the sediments. The sediments were a large sink for both N and P. Sediment particulate C:N (9.8) and C:P (119) ratios were lower than those in the water column. However, ammonium:phosphate fluxes increased coincident with increased sulfate reduction rates and porewater sulfide concentrations. Efficient N-retention mediated through dissimilative nitrate reduction to ammonium, and high rates of N-fixation in shallow, hypersaline systems may facilitate transitions from N-limitation to P-limitation. During the most hypersaline period, seston exhibited some of the most extreme nutrient ratios ever reported for a marine ecosystem (C:N 10–37 and C:P 200–1200) and suggest that plankton are likely to be P-limited or are very well adapted to low P availability. When salinity and N:P and C:P ratios were highest, the plankton was dominated by a brown tide alga (Aureoumbra lagunensis), supporting evidence that this organism is adapted to low P, long residence time systems.     

Contrasts in sedimentation flux below the southern California Current in late 1996 and during the El Niño event of 1997–1998

The vertical flux of particulate matter at 330 m depth in San Lázaro Basin off southern Baja California ranged from 63 to 587 mg m⁻² d⁻¹ between August and November 1996. Organic carbon contents were between 5.6 and 14.8%, yielding flux rates of 9–40 mgC m⁻² d⁻¹. In December 1997 and January 1998, at the height of the strong El Niño event, the respective fluxes (47–202 mg m⁻² d⁻¹ and 3–8 mgC m⁻² d⁻¹) were comparable. The February–June 1998 records, however, revealed sharply reduced mass (1–6 mg m⁻² d⁻¹) and organic carbon (0.2–0.8 mgC m⁻² d⁻¹) fluxes. The organics collected in 1996 were predominantly autochthonous (δ¹³C=−22‰; C/N=8). The variations in δ¹⁵N (8.3–11.0‰) suggest an alternation of new and regenerated production, possibly associated with fluctuations in the intensity of deep mixing during that autumn. The relatively high organic matter fluxes in December 1997 appear to be associated with regenerated production. The average composition from February to June 1998 (δ¹³C=−23.6‰; ¹⁵N=11.7‰; C/N=10.5) indicates degraded material of marine origin. The maximum δ¹⁵N value found (14‰) suggests that deeper, denitrified waters were brought to the surface and possibly advected laterally. Regime changes in the waters of the basin occur at 6–10 week intervals, evidenced by concurrent shifts in most of the measured parameters, including fecal pellet types and metal chemistry. The marine snow-dominated detritus collected showed a shift from a mixed diatom-rich-radiolarian-coccolith assemblage in late 1996 to a coccolith-dominated assemblage, including the contents of fecal pellets, during the 1997–1998 El-Niño period. T–S profiles, plankton analysis and chlorophyll contents of the upper water column indicated that the strong phytoplankton bloom, normally associated with seasonal upwelling along the Pacific coast of Baja, did not occur during the spring of 1998. The persistence of oligotrophic conditions during the 1997–1998 El Niño event favored the dominance of nanoplankton and reduced the vertical flux of particles.