On attaching acoustic imaging instrumentation to the LEO-15 observatory for sediment transport and bottom boundary layer studies

Abstract

The Woods Hole Oceanographic Institution (WHOI) and Dalhousie University conducted a sediment transport study at the LEO-15 observatory maintained by Rutgers University and WHOI engineers off the central New Jersey coast. This study was designed to take advantage of the unique capabilities (power and telemetry) provided at the site. The various sensing systems were attached to a bottom-mounted frame and connected by divers to the underwater-mateable guest ports on the LEO-15 node. The node supplied power for the various acoustic and electronic systems and provided telemetry for control of the remote instrumentation and acquisition of several GBytes of data per day. A shore-based support station of several PCs and workstations networked together and connected to the Internet was set up at Rutgers Tuckerton Field Station to receive, display and store this data. Instrumentation was deployed in mid-November 1999, remained active until Christmas 1999, and was recovered in January 2000. The methodology of using the LEO-15 observatory was different from previous approaches used by the WHOI and Dalhousie sediment transport teams and had some subtle problems not obvious until the equipment was set up and utilizing the observatory. A second LEO-15 node, on shore at the Tuckerton field station, facilitated testing of each subsystem and the fully integrated system before offshore deployment. One set of problems involved the node's ground fault detection system that was designed to shut down all scientific instrumentation to protect the node when a current leak to seawater was detected on any power or telemetry line. WHOI connected all their sensing systems together to provide an optimum test of any system problem. Dalhousie had isolated their instrumentation (power and communications) so the ground fault circuit only tested the cables from the LEO-15 node to their electronics. The WHOI instrumentation developed a slowly increasing ground fault in one line that resulted in some unnecessary data loss. The underwater-mateable connectors required at the LEO-15 sometimes showed a low ground fault level leakage that disappeared when they were reconnected. Because or the constant power source, the electronics on the tripod remained powered up and active during the deployment, and individual sensors were powered and burst-sampled once or twice an hour, collecting /spl sim/4 GBytes of data per day that was telemetered to shore. WHOI's high-speed link operated successfully at /spl sim/400 kbits/sec and Dalhousie's at 2 Gb/s. Dalhousie experienced some problems with their remote system suddenly stopping that may be associated with changes made to interface it with the LEO-15 observatory. All data were archived at the Tuckerton facility as the Internet link to WHOI and Dalhousie could only provide 1/3 of the required speed to transmit the data collected daily. In spite of these problems, the systems worked much of the time and collected many gigabytes of data on the suspended sediment transport and structure in relationship to bedform roughness, and wave and current forcing. Real-time sensor functionality and data acquisition operations were monitored over the Internet. The goal of plugging instruments into the LEO-15-node and obtaining continuous long time-series remotely with no interruptions is not yet fully realized. Someone had to monitor the connection to keep systems functioning optimally. However, we learned much of the methodology to move toward full remote operations in the future which will improve future observatories, such as the new Martha's Vineyard Coastal Observatory.