Highly nonlinear internal solitary waves over the continental shelf of the northwestern South China Sea

Large amplitude internal solitary waves (ISWs) often exhibit highly nonlinear effects and may contribute significantly to mixing and energy transporting in the ocean. We observed highly nonlinear ISWs over the continental shelf of the northwestern South China Sea (19°35′N, 112°E) in May 2005 during the Wenchang Internal Wave Experiment using in-situ time series data from an array of temperature and salinity sensors, and an acoustic Doppler current profiler (ADCP). We summarized the characteristics of the ISWs and compared them with those of existing internal wave theories. Particular attention has been paid to characterizing solitons in terms of the relationship between shape and amplitude-width. Comparison between theoretical prediction and observation results shows that the high nonlinearity of these waves is better represented by the second-order extended Korteweg-de Vries (KdV) theory than the first-order KdV model. These results indicate that the northwestern South China Sea (SCS) is rich in highly nonlinear ISWs that are an indispensable part of the energy budget of the internal waves in the northern South China Sea.     

Geoacoustic inversion for sediments in the South China Sea based on a hybrid inversion scheme

We performed a long range acoustic propagation experiment in the South China Sea (SCS) in November 2004. The environment of the experiment was with an isothermal sound speed profile, where influence of water volume fluctuation was small, meaning that bottom parameters can be well estimated from acoustic signals. We inverted the acoustic parameters of sediment by using a hybrid inversion scheme that combines the matched field processing inversion with Hamilton sediment empirical relationship and transmission loss data. The numerical results show excellent agreement with the experiment data, indicating validity of the inverted parameters.     

Hydroacoustic estimates of fish biomass and spatial distributions in shallow lakes

We conducted acoustical surveys with a horizontal beam transducer to detect fish and with a vertical beam transducer to detect depth and macrophytes in two typical shallow lakes along the middle and lower reaches of the Changjiang(Yangtze) River in November 2013. Both lakes are subject to active fish management with annual stocking and removal of large fish. The purpose of the study was to compare hydroacoustic horizontal beam estimates with fish landings. The preliminary results show that the fish distribution patterns dif fered in the two lakes and were af fected by water depth and macrophyte coverage. The hydroacoustically estimated fish biomass matched the commercial catch very well in Niushan Lake, but it was two times higher in Kuilei Lake. However, acoustic estimates included all fish, whereas the catch included only fish 45 cm(smaller ones were released). We were unable to determine the proper regression between acoustic target strength and fish length for the dominant fish species in the two lakes.     

High-frequency acoustic backscattering characteristics for acoustic detection of the red tide species Akashiwo sanguinea and Alexandrium affine

Harmful algal blooms(HABs),caused by the overgrowth of certain phytoplankton species,have negative effects on marine environments and coastal fisheries.In addition to cell-counting methods using phytoplankton nets,a hydroacoustic technique based on acoustic backscattering has been proposed for the detection of phytoplankton blooms.However,little is known of the acoustic properties of HAB species.In this study,as essential data to support this technique,we measured the acoustic properties of two HAB species,Akashiwo sanguinea and Alexandrium affine,which occur in the South Sea off the coast of Korea.Due to the small size of the target,we used ultrasound for the measurements.Experiments were conducted under laboratory and field conditions.In the laboratory experiment,the acoustic signal received from each species was directly proportional to the cell abundance.We derived a relationship between the cell abundance and acoustic signal received for each species.The measured signals were compared to predictions of a fluid sphere scattering model.When A.sanguinea blooms appeared at an abundance greater than 3 500 cells/mL,the acoustic signals varied with cell abundance,showing a good correlation.These results confirm that acoustic measurements can be used to detect HAB species.     

����������������Ժ����������ͷ���� ������

????о???????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????з??????????Ч?????????й???????з??????????Ч??????????????????????y??????????????????????????????????????????????????????????????????????????? ??????????????     

Matched-field inversion of sound speed profile in shallow water using a parallel genetic algorithm

A sound speed profile plays an important role in shallow water sound propagation. Concurrent with in-situ measurements, many inversion methods, such as matched-field inversion, have been put forward to invert the sound speed profile from acoustic signals. However, the time cost of matched-field inversion may be very high in replica field calculations. We studied the feasibility and robustness of an acoustic tomography scheme with matched-field processing in shallow water, and described the sound speed profile by empirical orthogonal functions. We analyzed the acoustic signals from a vertical line array in ASIAEX2001 in the East China Sea to invert sound speed profiles with estimated empirical orthogonal functions and a parallel genetic algorithm to speed up the inversion. The results show that the inverted sound speed profiles are in good agreement with conductivity-temperature-depth measurements. Moreover, a posteriori probability analysis is carried out to verify the inversion results.     

Measurement of single-fish target strength in the South China Sea

We measured the target strength (TS) of three commercial fish species: whitespotted spinefoot (Siganus canaliculatus), black porgy (Acanthopagrus schlegelii), and creek red bream (Lutjanus argentimaculatus), in the South China Sea. The TS of caged or tethered fish (n=76 total) was measured using a Simrad EY60 portable scientific echosounder at 120 kHz. We evaluated the relationship between TS and total length (TL, cm) for the three species. This is the first attempt to use split-beam acoustics to measure single-fish TS in the South China Sea by Chinese researchers. Our results will improve the accuracy and precision of acoustic abundance estimates of commercially important species and further the development of underwater acoustic survey techniques in fisheries in the South China Sea.     

利用GPS TEC探测2015年尼泊尔地震激发的电离层扰动

利用尼泊尔周边区域6个IGS GPS跟踪站数据,对2015-04-25尼泊尔地震激发的电离层扰动进行分析。结果表明,地震发生后,尼泊尔上空电离层总电子含量（total electron content,TEC）存在不同频率的电离层扰动,扰动中心频率约为3 mHz和4 mHz。前者发生在震中附近1 000 km左右的范围,后者范围达2 000 km,推断其分别为地震破裂产生的声重波信号以及瑞利波向上传播到电离层所激发的异常扰动信号。联合两个频率扰动信号,根据观测的扰动位置以及经验扰动速度反演震中位置,结果与实际情况符合较好。     

�Ŵ��㷨�ڷ����Իع�ģ�ͽ����е�Ӧ��

????????????????????????????????????????????????????????????????????????????????????????????t?????????????????????????????飬?????????????????????????????????????????????????????????3????????????????     

Comparison of nonlinear and linear PCA on surface wind, surface height, and SST in the South China Sea

We compared nonlinear principal component analysis (NLPCA) with linear principal component analysis (LPCA) with the data of sea surface wind anomalies (SWA), surface height anomalies (SSHA), and sea surface temperature anomalies (SSTA), taken in the South China Sea (SCS) between 1993 and 2003. The SCS monthly data for SWA, SSHA and SSTA (i.e., the anomalies with climatological seasonal cycle removed) were pre-filtered by LPCA, with only three leading modes retained. The first three modes of SWA, SSHA, and SSTA of LPCA explained 86%, 71%, and 94% of the total variance in the original data, respectively. Thus, the three associated time coefficient functions (TCFs) were used as the input data for NLPCA network. The NLPCA was made based on feed-forward neural network models. Compared with classical linear PCA, the first NLPCA mode could explain more variance than linear PCA for the above data. The nonlinearity of SWA and SSHA were stronger in most areas of the SCS. The first mode of the NLPCA on the SWA and SSHA accounted for 67.26% of the variance versus 54.7%, and 60.24% versus 50.43%, respectively for the first LPCA mode. Conversely, the nonlinear SSTA, localized in the northern SCS and southern continental shelf region, resulted in little improvement in the explanation of the variance for the first NLPCA.     

Impact of nonlinear processes on formation of the Kuroshio large meander path in a barotropic inflow-outflow model

Based on a barotropic inflow-outflow model,we examine the formation of the Kuroshio large meander(LM) using conditional nonlinear optimal perturbation(CNOP) method.Both linear and nonlinear evolutions of such perturbations obtained by this method are investigated.The results show that the nonlinear evolution can result in the Kuroshio transition from a straight to LM path,whereas the linear evolution cannot.This implies that nonlinearity plays an important role in the formation of the Kuroshio LM path.The nonlinearity exists as advection in the evolution equations of the perturbation derived from the barotropic inflow-outflow model,namely the nonlinear advection of the perturbation by the perturbation(NAPP).By examining the role of this nonlinearity,we find that the NAPP tends to move the cyclonic eddy induced by the CNOP-type perturbation westward.Together with the beta effect,this offsets part of the eastward advection caused by the interaction between the perturbation and the background flow.Hence,the eastward movement of the cyclonic eddy is significantly weakened,effectively causing the eddy to develop.The sufficient evolution of this cyclonic eddy leads to the formation of the Kuroshio LM.     

�������˲��ķ�����ǿ�Ⱥ͹������ܵĶ���׼��

????????????????????????????????????????????????????????????????????????????????????     

Application of hydroacoustics to investigate the distribution,diel movement,and abundance of fish on Zhubi Reef,Nansha Islands,South China Sea

A combination of traditional fish sampling methods(hand-line and gill net) and modern hydroacoustic techniques were used to study fish community structure,distribution,and diel movements of fish on Zhubi Reef to enhance understanding of the ecosystem.We collected 126 individuals from 29 species,20 genera,17 families,three orders,and two classes using traditional gear.Perciforms were the dominant group in terms of species richness and biomass.The acoustic data indicated that very small(target strength[TS],dB) -60 dB) and small(-60 dB≤TS-45 dB) fish contributed the most to abundance and species richness on the coral reef,and that the proportion of medium-sized(-45 dB≤TS-35 dB) and largesized(-35 dB≤TS) fish increased gradually as depth increased.The single-target detection method revealed two distinct size classes during the day in the 12-16 and 16-20-m layers.One group consisted of very smallsized fish(TS-60 dB) and the other consisted of medium and large-sized fish(TS-55 dB).The number of single-target detections was significantly higher during the night than during the day(P0.05).The singletarget TS frequency distribution during the day was significantly different than during the night at depths of4-8,8-12,12-16,and 16-20 m.Significant differences were observed among the 4-8,8-12,12-16,and16-20-m-depth layers during day and night.Diel vertical movement was evidenced as fish began to spread and move upward just before sunset and began to assemble and descend shortly(15 min) after sunrise.     

不同频率对埋入式压电陶瓷辐射声波的影响

不同频率激励埋入式压电陶瓷,辐射声波的声压、声强和指向性不同。为提高超声无损检测的精度,正确选用激励频率十分重要。通过ANSYS软件对埋入式压电陶瓷进行有限元分析,得到在20~100kHz频率范围内不同振动模态对应的固定频率,使用所得频率对压电陶瓷进行激励,分析不同频率激励压电陶瓷辐射超声波的影响,选定最佳固定频率。结果表明在20~100kHz频率范围内,共有14个振动模态,9个振动模态的振幅达到极值,声压随频率的增大逐渐增强,当频率达到79.666kHz时为最大值,而后逐渐衰减,且在67.301~84.895kHz频段内变化十分迅速。     

基于Prophet-RF模型的GNSS高程坐标时间序列预测分析

针对GNSS高程坐标时间序列非平稳性与非线性等特点,在深入分析Prophet模型与随机森林(ran-dom forest,RF)模型特性的基础上,构建了Prophet-RF组合预测模型,解决了Prophet模型对时间序列非线性部分预测能力较弱的缺陷,且该组合模型具有较强的鲁棒性.本文选用BJFS站高程方向的连续观测数据...     

基于经验模态分解方法的山西断裂带跨断层形变资料分析

利用经验模态分解方法重构并提取山西断裂带跨断层形变监测高频平稳信号和低频趋势信号。基于高频平稳信号分析地震前兆异常及其表现形式,结果表明,在山西断裂带及邻近地区中强地震前部分跨断层测段出现明显异常;强震前山西断裂带跨断层异常测点数量多、分布广、距离震中较远,中强地震前异常测点多集中在震中附近。基于低频趋势信号分析断层形变时空分布特征表明,山西断裂带断层以继承性正断运动为主,断层分时分段差异特征明显。     

Distribution of the nonlinear random ocean wave period

Because of the intrinsic difficulty in determining distributions for wave periods, previous studies on wave period distribution models have not taken nonlinearity into account and have not performed well in terms of describing and statistically analyzing the probability density distribution of ocean waves. In this study, a statistical model of random waves is developed using Stokes wave theory of water wave dynamics. In addition, a new nonlinear probability distribution function for the wave period is presented with the parameters of spectral density width and nonlinear wave steepness, which is more reasonable as a physical mechanism. The magnitude of wave steepness determines the intensity of the nonlinear effect, while the spectral width only changes the energy distribution. The wave steepness is found to be an important parameter in terms of not only dynamics but also statistics. The value of wave steepness reflects the degree that the wave period distribution skews from the Cauchy distribution, and it also describes the variation in the distribution function, which resembles that of the wave surface elevation distribution and wave height distribution. We found that the distribution curves skew leftward and upward as the wave steepness increases. The wave period observations for the SZFII-1 buoy, made off the coast of Weihai (37°27.6′ N, 122°15.1′ E), China, are used to verify the new distribution. The coefficient of the correlation between the new distribution and the buoy data at different spectral widths (ν=0.3−0.5) is within the range of 0.968 6 to 0.991 7. In addition, the Longuet-Higgins (1975) and Sun (1988) distributions and the new distribution presented in this work are compared. The validations and comparisons indicate that the new nonlinear probability density distribution fits the buoy measurements better than the Longuet-Higgins and Sun distributions do. We believe that adoption of the new wave period distribution would improve traditional statistical wave theory.     

Monthly variation in the fat content of anchovy (Engraulis japonicus) in the Yellow Sea: implications for acoustic abundance estimation

Anchovy is a key species in the Yellow Sea ecosystem. An accurate estimate of anchovy abundance is vital for the management of the anchovy stock and measurement of the ecosystem response to changes in anchovy abundance. However, the acoustic fish abundance estimate may be biased by 30%?C40% if the fat-content induced target strength variation is not taken into account. We measured the monthly variation in the fat content of anchovy (Engraulis japonicus) in the Yellow Sea, and evaluated the potential effect of variation in fat content on the acoustic assessment of anchovy abundance. The fat content of anchovy varied seasonally, with two maxima and two minima in a year. The highest fat content (14.75%) was measured in the pre-spawning period in May, and the lowest fat content (2.48%) was measured during the post-spawning period in October. Fat content appeared to correlate with water content, but not body size. Assuming that the target strength is decreased by 0.2dB for every 1% increase in fat content, the seasonal difference in the target strength of anchovy may be as high as 2.45 dB. Given this, the acoustic abundance estimate may be biased by between 43% and 76%. Our results highlight the need for more information on the changes in fat content of fishes whose abundance is estimated by acoustic surveys.     

A study of oceanic reverberation modeling based on a nonlinear low-dimensional dynamic system

Acoustic reverberation signals generated by an experimental explosive source are analyzed by nonlinear dynamical methods. Three characteristic parameters, i.e., the correlation dimension, the largest Lyapunov exponent, and the Koimogorov en-tropy, are estimated in the reconstructed phase space. The results indicate that the reverberation signals are nonlinear. The Volterra adaptive prediction method is introduced to model the oceanic reverberation signals. The reverberation time series can be predicted in short term with small prediction errors. A preliminary conclusion can be reached that the nonlinear low-dimensional dynamic sys-tem model is more suitable for modeling oceanic reverberation than the classical random AR model.     

Relationship between in situ sound velocity and granular characteristics of seafloor sediments in the Qingdao offshore region

The sound velocity of seafloor sediments from shallow seas can provide important information for harbor design, and ocean and seacoast engineering projects. In this study, in situ measurements were used to obtain accurate sediment sound velocities at 45 stations offshore of Qingdao. The relationships between the sound velocity and granular properties of the seafloor sediments were analyzed. Sound velocity showed an increase with the sand content, sand-clay ratio, and sorting coefficient; and a nonlinear decreasing trend with increasing mean grain size and clay content. We plotted a sound velocity distribution map, which shows that the sound velocity was closely related to the geological environment. Previous empirical equations suggested by Hamilton, Anderson, and Liu were used to calculate the velocity with grain size. A comparison between the measured and calculated velocities indicates that the empirical equations have territorial limitations, and extensive data are essential to establish global empirical equations. Future work includes the calibration of the laboratory acoustic measurements with an in situ technique.