HYPERSPECTRAL IMAGING FEATURE
Mathias Bochow, GFZ Helmholtz Centre, Potsdam, is working on the Trace project to track marine plastic
and field data and, although it is too early to share any definite results, he said the scientists are already finding that material thickness and water depth have profound and sometimes unexpected effects on the hyperspectral signature of plastics. Throughout the project the Spots team will make use of a range of hyperspectral imaging technology, including a Bayspec OCI-F imager, a Spectral Evolution SR- 3501 spectrophotometer, and a Specim IQ snapshot hyperspectral camera. ‘This equipment is only suitable for use in
controlled environments,’ de Vries explained. ‘This means that we collect data of plastic in a controlled environment, to learn how we can best extrapolate it to applied remote sensing in the future.’ He added: ‘At this stage, it is challenging
to apply multispectral and hyperspectral sensors in the field for applied remote sensing for dispersed marine litter. The main reasons for this are the relatively high cost of the equipment and the limited spatial resolution, as well as the long sensor
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integration times, compared with a dynamic environment such as flowing rivers or ocean waves.’
Polarised potential Elsewhere, the Ocean Plastics Polarisation Properties (OP3) project is focused on obtaining fully characterised polarisation signatures of plastic marine litter in relation to other natural seawater constituents. This is through a combination of laboratory experimentation, in situ measurements and existing polarisation data from satellite missions, such as Parasol. As Tristan Harmel, research scientist
at Géosciences Environnement Toulouse, explained, remote sensing of plastic marine litter can be challenging because of the very high water absorption in the NIR-SWIR range, and the signal being masked during atmospheric correction. Harmel said that complementary optical
information is needed to detect plastics in the oceans. The polarisation state of light leaving water has been shown
to be a significant tool to disentangle complex marine signals to retrieve water constituents. To sense the two important elements of polarisation and directionality, Harmel explained that specific setups are needed in the laboratory, as well as in the field and from satellite platforms. For the latter, he said the historical French mission, Parasol, which contained the Polder instrument, was of particular interest because Polder sensors provided polarisation at three spectral bands (blue, red and near infrared) for up to 16 different viewing directions. Moving forward, he said the next
generation of satellite polarimeters will also provide a much more refined dataset encompassing the visible, NIR and SWIR bands. These include the 3MI instrument, to be launched on the Eumetsat Metop- SG series of satellites, which will provide multispectral (410-2,130nm), multi-polarised (-60°, 0°, and +60°) and multi-angular images of outgoing radiance at the top of the atmosphere. In addition, the SpexOne
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Mathias Bochow/GFZ
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