Ocean Bottom Seismometer (OBS) deployed in the seafloor can record information of ocean ambient noise, and typhoon can generate elastic waves propagating through the seafloor directly or indirectly. These factors cause great changes on OBS recording data to some extent. The OBS data during the process of typhoon was analyzed, and the methods of optimum filter were used in order to separate signals. After those work, we found that wind wave and ground swell were well recorded by short-period OBSs for the first time, and a new mode which typhoon affected seafloor ambient noise was put forward. We get three preliminary conclusions from above analysis: (1) Wind wave and ground swell caused by typhoon have their own distinctive modes to affect seafloor ambient noise;(2)The range and extent of seafloor ambient noise are obviously different which have been strengthened by two above waves;(3)Short-period OBSs can clearly record the information of ground swell, whose dominating period is 6—8 seconds and its energy is generally steady (we call it “8-second phenomenon); These results will have great significance for the future research on marine seismology and other aspects of oceanography.
YANG Xiao-qiu1,2,SHI Xiao-bin1,XU He-hua1,XU Xing3,LI Guan-bao4,GUO Xing-wei5,LUO Xian-hu3
The authors assimilate cruise data in the northern South China Sea using the Princeton Ocean Model (POM). The results show that the model assimilation improves the simulation effectively by reducing model errors, namely making the model results closer to the observations. The assimilation results, however, are not quite ideal in the regions where cruise observations are absent, indicating the method needs to be improved. This assimilation method once improved can provide re-analysis dataset for studying the South China Sea.
The noise from dynamic positioning system of a vessel interferes with the data of ultra short base line (USBL) underwater positioning system in hydrothermal sulfide field investigation, while in situ decision-making and continuing research demand much better underwater positioning data. It is therefore necessary to eliminate abnormal positioning data rapidly and effectively for the existing Posidonia 6000 USBL. This study takes the USBL procedure data (x, y, z) as elimination objects, sets up (x, y, z) elimination models according to field water depth and the block angle, and uses reasonable data structure and algorithm to realize interactive elimination according to maps between (x, y, z) and time series. With the models and interactive elimination, abnormal USBL positioning data can be eliminated effectively, and much better underwater positioning data can be provided for in situ decision-making of hydrothermal sulfide investigation and continuing research.
OPeNDAP stands for “Open-source Project for a Network Data Access Protocol.” The OPeNDAP protocol provides a discipline-neutral means of requesting and providing data across the World Wide Web. The goal is to allow all end users to access immediately whatever data they require in a form they can use, all while using applications they already possess and are familiar with. This article introduces the available OPeNDAP servers, and elaborates on the design and building of the OPeNDAP-based Sharing Platform of Physical Oceanographic Data of the South China Sea, which uses the GDS (GrADS Data Server), Dapper and THREDDS (Thematic Realtime Environmental Distributed Data Services) data servers to realize OPeNDAP services of wind, current, wave, Argo, and tide data. It also presents the OPeNDAP Services of the Sharing Platform. With the help of OPeNDAP, the service system achieves the objective of seamless access to physical oceanographic data of the South China Sea in a highly heterogeneously distributed environment over the network.
Taiwan Island is at the joint of Eurasian Continent and Pacific Plate, and is always threatened by typhoons and northeasterly winds, which may cause enormous loss of human life and property every year. Therefore it is necessary to de-velop a coastal sea-state monitoring system. The COMC (Coastal Ocean Monitoring Center, National Cheng Kung University) built the Taiwan coastal sea-state monitoring system, which is modern and self-sufficient, consisting of moored buoy, pile station, tide station, coastal weather station, and radar monitoring station. To assure the data quality, Data Quality Check Pro-cedure and Standard Operation Procedure were developed by the COMC. For data analysis, some new methods are introduced to make more detailed analysis, such as EMD (empirical mode decomposition) method that is used in the analysis of storm surge water level, wavelet transform that is used to discuss the near-shore wave characteristics from X-band radar images, and data assimilation technique that is applied in wave nowcast operation. The coastal sea-state monitoring system has a great potential in providing ocean information to serve the society.
In this study, we conducted barotropic detiding of the summertime shipboard ADCP (SADCP) dataset in the southern Taiwan Strait (TWS) from June to September during 2004-2013, employing the widely used spatiotemporal fitting by the least squares method (STF-LSM). The results show that the mean flow derived from STF-LSM and that derived from averaged vectors by dataset gridding both flowed northeastward. Moreover, these two mean flows had similar spatial distribution patterns of strong and weak flows, but their maximum velocities were slightly different: 0.46 and 0.34 m·s-1 for the former and the latter, respectively. Besides, the strong currents (about 0.3 m·s-1) were characterized by the narrow mainstream confined around the depth of 30 m off the sea at Dongshan and Longhai as well as the Taiwan Bank (TWB) with drastic topographic change, whereas the weak currents (less than 0.1 m·s-1) were mainly distributed in the central TWB. Estimation of the volume transport based on the above mentioned mean flows revealed similar results, up to (0.71±0.24) ×106 m3·s-1. The strong M2 tidal currents derived from SADCP were near the TWB and extended to the offshore of Dongshan, with a maximum M2 current amplitude as large as ~0.64 m·s-1, which was located in the TWB. The cotidal chart for M2 currents demonstrates the characteristics of progressive wave, which was delayed successively from the southwest to the northeast along the direction of the tidal wave as the wave propagated (about 2 h). Besides, there was a boundary line along northwestward to southeastward direction with ellipticity equal to zero at the offshore of Dongshan. The M2 currents rotated counterclockwise in the southern region of this boundary line and clockwise in the northern region.
By regarding the moderate resolution imaging spectroradiometer (MODIS) remote sensing data as data sources and the southwest coastal waters of Hainan Island as the study area, the improved spilt-window algorithm is used to inverse the SST of the study area from 2005 to 2014. To verify the retrieval accuracy of the SST, we calculate the correlation coefficient between the inversion and measured results using the measured values of 12 observation points; the coefficient is 0.9. Based on the inversion results of SST, the data of four seasons in a year and a decade are analyzed to show the spatial and temporal variations of SST distribution, respectively. Results show that using the MODIS multi-channel improved spilt-window algorithm we can inverse Hainan Island offshore SST accurately. The spatial and temporal variations of SST distribution thus obtained can provide important reference values for marine fishery, change of marine environment and meteorological monitoring.
A two-dimensional ocean bottom seismometer (OBS) survey line (OBS2015-2) was carried out in 2015, which was an E-W trending and located in the front of the Manila Trench subduction zone. In this study, we take two OBS stations (OBS04 and OBS08) as examples to introduce the data processing flow for this short-period OBS type of data. The process includes data format transforming from original RAW data to SAC format and then to SEGY format; and it contains shot relocation and OBS position relocation. The preliminary results of data processing demonstrate that the OBS data are of high quality in this experiment. The seismic phases, such as PsP, Pg, PmP, etc. are very clear along the OBSs’ seismic record sections. They are further identified by the forward modeling using the Rayinvr software. Especially, the reflective seismic phases from the Moho interface are abundant and obvious in the incoming plate along the Manila Trench subduction zone, which will provide good constraints for the crustal bottom of the incoming plate.
It is of great significance to fully exploit the oceanic application potential of the GaoFen-1 data, which is the first domestic high-resolution satellite of Earth observation in China. In this study, a GaoFen-1 multi-spectral image is used to retrieve the island shallow water depth of Jinqing Island in Xisha sea area by adopting the dual-band linear model and log-ratio model, after serials of image processing including image geometric correction, atmospheric correction and sun-glint correction. The accuracy of the two models is evaluated by using actual water depth data. Meanwhile, possible influence factors of retrieval accuracy of island shallow water depth from satellite data are discussed. It is shown that the result from the dual-linear model is obviously better than that from the log-ratio model, and the dual-band linear model is more suitable for the retrieving shallow water depth of Jinqing Island. For water depth less than 20 m, the overall root mean square (RMS) error is 1.8 m with RMS error of 1.14 m within 5 m, which reaches the level of accuracy of water depth inversion by satellite remote sensing.
Using offshore airguns and onshore explosions as seismic sources, along with portable seismic stations, permanent seismic stations and Ocean Bottom Seismometer (OBS) as a synchronously receiving array, a onshore-offshore 3D seismic exploration experiment was carried out for the first time in the Zhujiangkou area of the northern South China Sea. This paper introduces in detail the seismic data acquisition, observation system, data preprocessing method, and phase identification. A variety of seismic phases, including Pg, PcP, PmP, Pn, and Sg, can be clearly identified in most data collected by the permanent, portable seismic stations and OBS. Jointly using airgun and explosion sources, and dense stations receiving seismic signals simultaneously both on land and at sea, it can highly improve the ray coverage and cross overlap beneath the onshore-offshore transitional belt, which lays a foundation for obtaining fine crustal structure beneath such onshore-offshore area.
This study was based on the raw eddy-covariance (EC) data measured in February 2016 from the observation tower on Yongxing Island. EddyPro software was used for data post-processing, and the influence on flux calculation was quantitatively analyzed before and after each processing step. Furthermore, flux data quality classification, footprint analysis and comparison with calculated results using easyflux_dl real time programme were conducted. The results showed that despiking, coordinate rotation, frequency response correction and density correction all influenced the flux calculation result to different degrees. The influences of coordinate rotation on momentum flux, latent heat flux and carbon dioxide flux were evident, which revealed increases of 11.28%, 7.36% and 18.53%, respectively. The density correction had a significant effect on the carbon dioxide flux calculation, which was reduced by 7.87% after density correction. Comprehensive quality evaluation results revealed that the ratios of high quality data (grades 1~3) of momentum flux, sensible heat flux, latent heat flux, and carbon dioxide flux were 90.29%, 90.22%, 88.46%, and 85.41%, respectively. Footprint analysis showed that 72.04% source area of the flux was scattered in the first interesting area (10~80°, the underlying surface is the ocean surface with an average water depth of more than 10 m). Comparison of flux calculation results between EddyPro and EasyFlux showed that the two flux time series were fundamentally consistent; the results obtained by EddyPro were smaller but more reliable and accurate due to more strict despiking adopted to get rid of outliers’ impacts.
Based on the Operational Sea Surface Temperature and Sea Ice Analysis (OSTIA) data in 2015, we analyzed the seasonal variation and genesis mechanisms of thermal fronts in Guangdong coastal zone. The Guangdong coastal thermal fronts are strong in winter and weak in summer. In the eastern Guangdong coastal zone, thermal fronts are seen all year round, while in the western Guangdong coastal zone, thermal fronts disappear in summer. In the Pearl River estuary zone, thermal fronts are associated with the Pearl River plumes, which extend east northward in summer and southward in autumn. We suggest wind speed play important roles in the genesis and disappearance of these thermal fronts. Using a simple model, we find that monsoon influences the front strength through air-sea turbulent heat fluxes. Under southwesterly (northeasterly) wind condition, warm and wet (cold and dry) air mass could decrease (increase) the latent heat release near the cold coastal zone, leading to a temperature increase in the cold zone, and finally the front disappears.