Feature ArticleTests and Upgrades for the East China Sea Seafloor Observatory
By Huiping Xu
School of Ocean and Earth Sciences
The environmental baseline data for the East China Sea are inadequate, so more research is needed to understand the physical and biogeochemical processes that will help develop environmental prediction models, and spot and survey algal blooms.
The observatory is located in an eel fishing area where the average water depth is 15 meters. The area is affected by an irregular semidiurnal tide, which is not far from the deep waterway. The sediment is medium to fine sand, and the water is highly turbid because of close proximity to one of the Yangtze River's transportation channels comprising mud and sand.
The observatory consists of a submarine fiber-optic composite power cable more than 1 kilometer long and a special junction box, which provides power and communication signals to different instruments. Power is continuously supplied by the solar panels and batteries on top of the platform, which sits above the water. There are several power interfaces for the sensors: One is for 10-to-48-voltage DC input, and four are for 12-to-24-voltage DC output, which can supply 10 watts each.
The main functions of the junction box are power conversion and supply, data collection, coding, and remotely controlling and managing all equipment. The junction box is installed in a trawl preventer and has three kinds of waterproof plugs. It connects to a CTD sensor, turbidity sensor and an acoustic Doppler current profiler (ADCP).
Instrumentation Testing and Initial Operations
All oceanographic instrumentation designed for the long-term seafloor observatory must pass a series of tests: laboratory, indoor pool, lake water and, finally, a shallow-water experiment in the ocean. There are several lab, indoor and lake testing sites in China, but none are shallow-water test beds. The East China Sea Seafloor Observatory can be modified to serve this function; for instance, by modifying the junction box to handle any kind of sensor and upgrading the renewable power supply system.
In-situ data are continuously transmitted through the junction box and fiber-optic cables to the platform, then wirelessly sent to a receiver located in the State Key Laboratory of Marine Geology at Tongji University in Shanghai.
For the first several months of the observatory's operations, the sampling rate of the ADCP and CTD was once per minute, and the spatial resolution was 25 centimeters. From October 2009, the frequency of ADCP and CTD data transmission shifted to 15-second intervals. The information received at Tongji University includes near-bottom temperature, conductivity, pressure, turbidity, current values and directions (in discrete layers of 25 centimeters per layer from 80 centimeters above the seafloor to the sea surface), and status parameters of all equipment.
Preliminary observation results show that the current field and fine suspended sediment transport of the East China Sea are complex and show considerable seasonal variation, affected by the influence of Changjiang's diluted water, Taiwan's warm current and the Yellow Sea coastal current. To continue this article please click here.
Huiping Xu, a professor at Tongji University, has bachelor's and master's degrees in applied geophysics and a Ph.D. in geoexploration and information technology. His main research interests are seafloor observation, data processing in exploration geophysics, GIS and remote sensing.
Yang Yu, a Ph.D. student of the School of Ocean and Earth Sciences at Tongji University, received a bachelor's degree in geoexploration and information technology in 2010. He is researching the architecture of the remote controlling system in the East China Sea Seafloor Observatory.
Rufu Qin received his Ph.D. in geoexploration and information technology from Jilin University in 2006. He is now a lecturer in the School of Ocean and Earth Sciences at Tongji University. His research interests include GIS, data visualization and the seafloor observation system.