Expected Technological advancements
The preliminary design for CeltNet involved a series of nodes linked via sea-floor cables (Figure 5). However, for financial and technical reasons it may be more appropriate for nodes in deeper waters to operate as standalone nodes with communication links and near real-time data transfer. Other ESONET sites, like MoMAR and the Arctic may also require standalone and re-locatable solutions, e.g. to monitor the effects of receding ice. Data telemetry for non-cabled systems is therefore an important topic for the design of the ESONET NoE observatory network. MODOO will demonstrate the functioning of a relocatable system with underwater acoustic telemetry as well as surface buoy telemetry of scientific and engineering data.
Figure 5: Cabled observatory CeltNet (planned) and location of the PAP/MODOO deployment site
Infrastructure
The MODOO
MODOO consists of two observatory components (Figure 6): a steel/plastic wire mooring and a benthic lander. The mooring is equipped with a bi-directional telemetry buoy which connects via the mooring wire to inductively linked instruments. One of this “instruments” is a so called “Data Collection and Dissemination” (DCD) node. This node acoustically links the lander data with the surface buoy via the mooring wire. The lander also has a DCD node where lander instruments are connected to. The central components: DCD nodes, mooring, telemetry buoy, and lander will be described now.
Figure 6: Schematic of MODOO communication components, steel wire mooring (left) with inductive coupled sensors; Lander (right) with directly connected sensors. There is an acoustic communication between the two “Data Collection and Dissemination” (DCD) nodes.
Data Collection and Dissemination (DCD) node
The central hardware which links the observatories components into one system is constructed around acoustic telemetry modems. These devices, the “Data Collection and Dissemination” (DCD) nodes will not 'only' be used for the communication between them, they will also control sensors, store data, apply a precise time stamp to all data, compress/uncompress data for efficient transmission, and check the quality of transmitted data. Given the central importance of this device for the project we decided to use a fully commercial product. First market investigations indicated that some companies have devices available that fit our requirements.
The MODOO partners have formulated a list of basic requirements on functionality and endurance of the DCD nodes:
- Endurance: up to 16 month deployment
- Housing either glass sphere or cylinder (6000m rated), housing hosts also required energy source
- Acoustic transducer implemented, directivity 35°/-3dB
- Mooring DCD node shall have an inductive modem coupler to link the DCD node to the mooring wire
- Mooring DCD node communicates (bi-directional) with surface telemetry buoy via mooring wire
- Mooring DCD node communicates (bi-directional) with Lander DCD node
- Mooring DCD node shall send data along mooring wire by request from surface buoy (once a day)
- Lander DCD node takes up to 6 external devices via connectors (SEACON AW)
- Lander DCD node ports are programmable, RS 232 and RS 485 interfaces
- Lander DCD node established communication for data transfer with mooring DCD node
- Lander DCD node logs and saves data from connected devices (with an appropriate time stamp)
- Lander DCD node control connected instruments (event control) via mooring DCD node/ surface telemetry link
The PAP mooring
The MODOO DM will make use of an existing mooring which is located at the PAP site as parts of the EuroSITES project. At the PAP site there two sort of deep sea moorings deployed: deep water sediment trap moorings, and since 2002 a full water depth mooring multidisciplinary mooring with surface telemetry. MODOO will connect to the latter (see figure 7) using an inductive coupled DCD node. The PAP mooring has a total length of about 6200m while the deep sea part is mainly Polyester/Polystell robe, the upper 1300m are made of Norselay® steel wire.
Figure 7: Drawing of the EuroSITES full water depth PAP mooring with surface telemetry buoy as it will be deployed in summer 2009. The MODOO DCD node will be added to this PAP mooring and is connected via an inductive link. For clarity the mooring line is drawn in three pieces (points A & B are connected)
Surface telemetry system
The data logging and telemetry system (Figure 8) on the PAP mooring was originally developed at NERC-NOCS for the RAPID program but has also been used for the PAP mooring (first time in 2007). The system is based around a Persistor CF2 computer board linked to an Iridium 9522 modem, a Seabird inductive modem, a GPS receiver, a compass/pitch/roll unit and internal voltage/current/temperature monitors. The buoy contains a solar charging controller and highintensity LED recovery lamps that can be switched on remotely. The computer’s clock is regularly compared to the GPS clock and is adjusted to stay within +/- 1 second accuracy. All data received from inductively coupled sensors (in MODOO one will be a DCD node) is time stamped with this clock, as are all the other data logged in the system.
Figure 8: Surface telemetry buoy with Iridium 9522A, Seabird SIM, Trimble Lassen GPS, pitch/roll/compass, solar panels for battery recharge.
The Iridium modem is programmed to send regular position and engineering data reports via SBD email messages, while scientific data is transferred using the dial-up mechanism. Commands can be sent to the unit via email messages according to a predefined command set for sensor control from shore (event tripper). The system has successfully communicated with MicroCAT sensors at depths of up to 4960m.
The Benthic Boundary lander (BoBo)
The BoBo lander (Figure 9) was developed as stand-alone long-term monitoring system (van Weering et al., 2000) that in its MODOO configuration will carry a Seabird 16 CT probe (3m above the bottom) with one Seapoint optical backscatter sensors (1m above the bottom) and a new combined OBS-Fluorometer sensor (Wetlabs; 2m above the bottom) connected to it. A 1200kHz downward looking ADCP is fixed in the frame 2m above the sediment surface to measure bottom currents in high resolution (5cm vertical bin size). At the same time the backscatter values of the four acoustic beams give additional 3D information about the amount of re-suspended material when correlated with the OBS sensors. In addition a Technicap PPS 4/3 sediment trap with a rotating carousel of 12 bottles (250 ml) is mounted in the frame with the aperture (0.05m2), 4 m above the bottom. A pan/tilt camera and three lights can be mounted in the frame at 2 m above the seabed. The camera can be programmed to record video images in six different settings and/or positions (these images are only accessible after retrieval). A power-unit supplies both, CT and ADCP with power and combines the RS232 connections of both sensors in one underwater plug to establish the link to the DCD node. The lander itself is a steel construction with three legs, flotation and two releasers needed for later retrieval (compare Figure 9). The two guest sensor packages for Deep Sea Marine Life ad Geohazard will be fixed to the frame at the best suitable position.
 Figure 9: BoBo lander post-deployment
The BoBo lander will be deployed either as free fall lander (released at the sea surface) or, to get a defined and more accurate positioning, it will be lowered on a wire and dropped approx. 50m above the bottom by using a acoustic releaser. Retrieval of the lander works by releasing weights via one of the two acoustic releasers. Due to the incorporate flotation the lander will surface and can be picked up by the ship. If needed, deployment and retrieval can be done more than once during the project. The system was already repeatedly used for long-term monitoring of up to 1 year.
System geometry
The Lander will be placed in 5500m deep wate and in a maximal horizontal distance of approximately 3000m from the mooring anchor to ensure save ship operations. We will mount the mooring DCD node at a depth where ambient noise can be expected to be low (away form the surface) but which still guarantees good acoustic transmission. The sound speed profiles (Figure 10) indicate a deep SOFAR channel at around 1500m but with seasonal movement by some hundred meters. Using the sound speed profiles and performing a simple ray trace analysis of the sound paths (based on a 30° transducer opening) indicates that for this geometry some directional orientation might be required (Figure 10).
Figure 10: (left) Sound speed profiles PAP site. (right) Ray traces for instruments at 1000m (upper) and at 4900m depth (lower) as for DCD nodes. Colour indicates sound speed variability
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