11th Panhellenic Symposium on Oceanography and Fisheries, Mytilene, Lesvos island, Greece
2015
An Underwater Optical Instrument Platform for the measurement of the optical properties of sea water 1,
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Spyridakis N. Psarra S. ,Karageorgis A. and Banks A.C.
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Hellenic Centre for Marine Research, Institute of Oceanography, Crete, Greece,
[email protected],
[email protected],
[email protected],
[email protected] 2 European Commission, DG Joint Research Centre(JRC), Institute of Environment and Sustainability (IES), Via Enrico Fermi 2749, Ispra (VA) 21027, Italy
Abstract The optical properties of seawater are of great importance for a number of studies, e.g. primary production, pollution etc. The first underwater - multiple sensor platform - optics measurements by HCMR were conducted during the "Inner Saronikos Gulf" project, between June 2010 and March 2012. Initially, irradiance and radiance measurements (45 deg. from vertical) were collected above the sea surface, using two TriOS hyperspectral radiometers installed on deck of the R/V Aegaeo. As it was soon realized that the potential of such radiometric measurements, especially the underwater ones, would be of great significance for future studies, the need of upgrading the existing equipment and adding new instruments and underwater sensors became obvious. Thus, the appropriate sensors were chosen and the supporting electronics (power supply, power control, data management, data storage) was designed and developed locally while specialised software had to be developed for the management and the analysis of the data collected by the various sensors. Keywords: irradiance, radiance, back scatter, attenuation, absorption, optical sensors, LabVIEW
1. Introduction Published optics measurements from the Hellenic seas are very scarce if not inexistent. The first spectral PAR profiles from the Hellenic seas were published by Ignatiades (1998)1, from the oligotrophic Cretan sea. Other publications focus on the association of optical variability and phytoplankton2,3. Collecting data about the optical characteristics of seawater, in the Aegean Sea had to be implemented by the combination of various sensors, instruments and software, all in one system, operating simultaneously below and above sea water. Data collected from sensors below sea water were the actual data to be studied, while data collected from sensors mounted on the deck of the vessels supporting this effort were the reference data. A basic problem faced from the very beginning was the synchronisation of all instruments and sensors, while most of them did not allow any intervention. Furthermore, no software existed for the in situ evaluation of the data or the basic handling and analysis. 2. Materials and Methods The instruments and sensors were divided into two major categories, those installed above water, operating as reference or support instruments and those installed onto an Underwater Optics Frame, which operate autonomously, but synchronised with the surface instruments. The surface optical instrumentation includes the instruments and sensors listed in Table 14,5. The submersible optical instrumentation includes the instruments and sensors listed in Table 24,5. Table 1. Surface Optical Instruments and Sensors #
Instrument
Manufacturer
Model
Sampling Rate
Specifications
1
Irradiance
TriOS
SAM 83D4
1 per 3 seconds
256 Channels of Spectrum.
2
Weather Station
Airmar
PB200
1 per second
Also includes vessel heading, pitch and roll and GPS.
3
PAR
Onset Computers
S-LIA-M003
1 per minute
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Table 2. Submersible Optical Instruments and Sensors #
Instrument
Manufacturer
Model
Sampling Rate
Specifications
1
Irradiance
TriOS
SAMIP 506C
1 per 3 seconds
256 Channels of Spectrum.
2
Radiance
TriOS
SAM 82E4
1 per 3 seconds
256 Channels of Spectrum.
3
Absorption - Attenuation
Wet Labs
AC-S
4 per second
84 Wavelengths.
4
Back Scatter
Wet Labs
Eco-BB
1 - 4 per second
3 Wavelengths.
5
Heading & Inclination
Airmar
H2183
15 per second
Pitch, Roll and Heading (Magnetic).
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CTD
Sea Bird
CTD-19
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CTD, PAR
The surface irradiance sensor was controlled by a computer installed in the Dry Laboratory of the vessel, running the manufacturer dedicated software. The software supplied with the weather station proved to be unsuitable for this particular use, therefore new software had to be developed. This was done in the LabVIEW software development environment. The meteorological data, as well as the vessel heading and movement data were visualised and stored in real time. The PAR sensor was autonomous and was supported by a data logger. At the end of each scientific cruise, the PAR data were downloaded from the data logger, using any computer available and the manufacturer software. All instruments and computers used were synchronised, at least once per day (in the morning), to the GPS time stamp, but it was decided that all systems will run in local (winter or summer) time, not UTC, with the exception of the Weather Station (PB200), which contains the GPS device used. During the first cruises, a USB camera modified to be waterproof was also installed vertically on the deck of the vessel, to collect one image of the sky, at the beginning of the deployment of the Underwater Optics Frame. Since this required an extra computer to run, the camera was soon removed. The necessary images were collected using any means available at the time of the Underwater Optics Frame deployment, like still cameras, mobile telephones or tablets. Later, the image taken was downloaded to the Main Operation Computer. The SkyImager piece of software was run and the cloud coverage of the sky was instantly calculated. A sample of this software is shown in Figure 1.
Fig. 1 : SkyImager software. In this sample, the sky is calculated to be covered by cloud in 38.3%, according to the parameters entered.
Fig. 2 : PB200 software, running the meteorological data and the vessel motion data, of March 23, 2013.
At the end of the cruise, when all data were collected and transferred to the Main Operation Computer, the software PB200, also developed in LabVIEW environment was run and the meteorological data and the vessel motion data were evaluated and merged in custom format. In Figure 2, the screen of this piece of software is shown. In order for the submersible instruments and sensors to be deployed safely, as well for the power supply and the data management and storage necessary for such an operation, a special frame was designed and constructed of INOX metal. In order for the metal part to reflect the least possible, the 2
11th Panhellenic Symposium on Oceanography and Fisheries, Mytilene, Lesvos island, Greece
2015
metal parts of the frame were covered with black heat shrink tube, while the frame was assembled and welded. This frame, with the original instrument configuration is shown in Figure 3. There, three (3) underwater hyperspectral radiometers (two irradiance and one radiance TriOS sensors) were designed to be installed. Thus, the first irradiance sensor was installed vertically, but looking upwards, the second irradiance sensor was also installed vertically, but looking downwards; finally the radiance sensor was also installed vertical looking downwards. The final configuration of the sensors is shown in Figure 4. The downwards looking irradiance sensor was removed to be the reference sensor for above water incident irradiance, nevertheless the supporting electronics are capable of hosting this sensor when needed. A Seabird CTD is attached next to the Underwater Optics Frame, running autonomously, to collect CTD data, as well as PAR. The supporting power supply and electronics were housed in a modified old Li-COR underwater housing. The original electronics were removed and special receptacles were manufactured, in order for all necessary electronics and a big rechargeable battery to fit. The TriOS hyperspectral photometers are controlled by a mini-PC, running the manufacturer software. This software triggers the two sensors once every 4 to 6 seconds, approximately and the collected data are being saved in engineering units, in separate files for each sensor and sampling. This method ensures that in a case of power break down or computer crash, only the measurements being collected at the instant of the break down will be lost. Otherwise, if the data were saved in the system data base (software default option), the risk of corrupting the whole data base exists, losing any data stored up to the instant of the break down. The mini-PC computer was synchronised to the GPS clock, running in local (winter or summer) time. The WET Labs AC-S is also connected to the Underwater Electronics Housing, for power supply as well as data transmission. The AC-S instrument operates in the simplest mode possible. When powered up, a submersible pump circulates the sea water through the absorption ("a") and attenuation ("c") tubes of the instrument. Any data collected are transmitted through a serial (RS232) port to any appropriate device connected at the other end. No date or time stamp data are included in the transmitted data. A very powerful serial data logger was used to collect the AC-S data. During the initial tests, a date and time stamp was applied to the collected data, by a special selection in the data logger, but since the AC-S data are in raw binary format, it proved practically impossible for the data to be resolved. Therefore, it was decided that the data should be stored in raw format, with no date - time addition. A Deck Log Book was kept, with the precise time of powering up and down all instruments, which was particularly important for the AC-S. Inside the Underwater Electronics Housing a gyro compass was installed, to monitor the behaviour of the Underwater Optics Frame. This device was also connected to a serial data logger, similar to the one used for the purposes of the AC-S. All instruments were powered up by a single 12 Volt - 21 Ah rechargeable battery, located at the bottom of the Underwater Electronics Housing. The location was chosen for the case of minor water leakage inside the Housing. In such a case, the battery would be flooded, before the water reaches the electronics. The battery would be sacrificed for the electronics to be kept dry, until action was taken. The WET Labs Eco-BB instrument was running autonomously, from batteries contained inside the instrument and logging the data in internal memory. The Seabird CTD attached later to the Underwater Optics Frame was also running autonomously. Upon retrieval of the Underwater Optics Frame, data had to be downloaded from the autonomous instruments (WET Labs Eco-BB and Seabird CTD) and the Underwater Electronics Housing had to be opened, to access the mini-PC and the two data loggers. This procedure is quite time-consuming, also applying great risk of imperfect assembly, under the pressure of the cruise time table. During tests, the Underwater Optics Frame was left running almost all day, with no interruption, proving that the power system used is capable of supporting a sufficiently long operation.
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Fig. 3 : Underwater Optics Frame. Original engineering design.
Fig. 4 : Underwater Optics Frame. Final configuration.
3. Results - Discussion The Underwater Optics Frame, in combination with the reference instruments installed on the deck of the research vessel constitute an an advanced optical sensor system, basically designed and developed by HCMR. The system is fully operational since April 2013,, with no modifications necessary from the original design, other than moving one TriOS hyperspectral radiometer sensor from the Underwater Optics Frame to the vessel deck. The upgrade of the System is already designed, with new underwater housings for the electronics and the power supply system and capable of being submerged to greater depths. New ew technology more powerful and less le massive battery packs have already been introduced into the plans and will eventually be implemented within near future projects involving studies of sea water optical properties. properties 4. Acknowledgements This research has been supported by the projects: "Policy-oriented "Policy oriented marine Environmental Research in the Southern European Seas" (PERSEUS PERSEUS, EC 7th FP) and "Technological and Oceanographic ceanographic Cooperation Network for the Study of mechanisms fertilising the North-East North Aegean Sea" (AegeanMarTech AegeanMarTech), co-financed by the European Union (European Social Fund - ESF) and Greek national funds ds through the Operational Program "Education and Lifelong Learning" of the National Strategic Reference Framework (NSRF) - Research Funding Program : THALES.
5. References Ignatiades L. 1998. The productive and optical status of the oligotrophic waters of the Southern Aegean Sea (Cretan Sea), Eastern Mediterranean. Mediterranean Journal of Plankton Research. Drakopoulos P., Petyhakis G., Valavanis V., Nittis K. and G. Triantafyllou, 2003. Optical variability associated with phytoplankton dynamics in the Cretan Sea during 2000 and 2001. In “Building the European Capacity in Operational Oceanography”, Elsevier Oceanography O Series No 69, Elsevier BV, pp. 554-561. 554 Drakopoulos P.G., Nittis K., Petihakis G., Kassis D, Pagonis P., Ballas D. and M. Ntoumas, 2011. Monitoring chlorophyll concentrations with Poseidon system's optical instruments. Proceedings of the Sixth International Conference on EuroGOOS, 4–6 October 2011, Sopot, Poland. www.wetlabs.com, Western Environmental Technologies. Technologies www.trios.de, TriOS Mess und Datentechnik GmbH. GmbH
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