Illustrated below are a couple of examples of raw data collected by Ocean-i and stored on the flash card. The stern line shows the data from the upward looking sensor (ambient light) whilst the red and green lines show the data from the port and starboard sensors looking at the spectrum reflected from the water. It can be seen that there is often quite a difference between the data collected from the port and starboard sensors. This is due to one looking at foam or sunlight rather than the spectrum from the water. For this reason, Ocean-i looked at the ratio of 570:600nm for all of the upwelling data during the hour and sent back the sample with the highest ratio.
The four graphs below show examples of data that was transmitted back by Ocean-i. These are the best sample for each hour that were selected. They therefore only have data from the downwards looking sensor with the best data (blue line) and the corresponding ambient light data (green line). The number of the downwards looking sensor chosen and it's integration time are shown at the bottom of the graph. This data has also been windowed into 24 channels.
By looking at the different shapes of the spectra, an idea can be gained of the water and light conditions.
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This shows a typical ‘open ocean’ spectra in which the reflected light from the water (blue line) has a much higher blue content than the incident light. This is due to the strong absorption of red light and weak absorption of blue light by pure seawater (i.e. in the absence of phytoplankton, suspended sediment etc). |
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This data was collected near the start in the coastal waters of the Solent. Here it can be seen that the reflected light contains a much higher green content due to the presence of phytoplankton. The phytoplankton absorb blue and red light and reflect green. Looking at ocean colour spectra plays an important role in determining at the presence, quantity and type of phytoplankton. Suspended sediment also has an affect on coastal colour spectra. |
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It is assumed that the
downwards looking sensor was looking at white water in this example as the Lup spectrum
closely follows the line of the incident light. This implies that we are looking solely at
sky light reflected from white water at the sea surface and limited useful data can be
drawn from this image. |
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This data was collected first thing in the morning. (Ocean-i does not collect data when it’s dark – it determines dark by looking at the signal to noise ratio of the incident light) It can be seen that there is a large component of red light in the incident light spectra due to the rising sun. This is mirrored to some extent in the reflected light spectra, though it can be seen that a lot of the red light is absorbed and there is still more reflection of blue light by the water. |
The data shown is unprocessed; the transmitted data is subsequently turned into ‘real’ units and a calibration coefficient applied. The Edn and Lup data can then be combined to give reflectance – the spectrum of light reflected from the sea once the incident light has been taken into account. GPS information is also sent back allowing the data to be correlated with position.
This graph shows a couple of relative reflectance spectra gathered from Djuice during leg
4 on the approach to Rio.
The open ocean waters offshore show clear, blue waters but as they came into harbour not only did phytoplankton go up to high levels (seen by the rapid absorption of the blue light) but the "yellow substance" or dissolved organic matter also reached very high levels.
The reflectance data collected while the yachts are racing, along with corresponding satellite data can be viewed on the Volvo Ocean Adventure website.
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