An Under-Ice Hyperspectral and RGB Imaging System to Capture Fine-Scale Biophysical Properties of Sea Ice

Abstract

Sea-ice biophysical properties are characterized by high spatio-temporal variability ranging from the meso-to the millimeter scale. Ice coring is a common yet coarse point sampling technique that struggles to capture such variability in a non-invasive manner. This hinders quantification and understanding of ice algae biomass patchiness and its complex interaction with some of its sea ice physical drivers. In response to these limitations, a novel under-ice sled system was designed to capture proxies of biomass together with 3D models of bottom topography of land-fast sea-ice. This system couples a pushbroom hyperspectral imaging (HI) sensor with a standard digital RGB camera and was trialed at Cape Evans, Antarctica. HI aims to quantify per-pixel chlorophyll-a content and other ice algae biological properties at the ice-water interface based on light transmitted through the ice. RGB imagery processed with digital photogrammetry aims to capture under-ice structure and topography. Results from a 20 m transect capturing a 0.61 m wide swath at sub-mm spatial resolution are presented. We outline the technical and logistical approach taken and provide recommendations for future deployments and developments of similar systems. A preliminary transect subsample was processed using both established and novel under-ice bio-optical indices (e.g., normalized difference indexes and the area normalized by the maximal band depth) and explorative analyses (e.g., principal component analyses) to establish proxies of algal biomass. This first deployment of HI and digital photogrammetry under-ice provides a proof-of-concept of a novel methodology capable of delivering non-invasive and highly resolved estimates of ice algal biomass in-situ, together with some of its environmental drivers. Nonetheless, various challenges and limitations remain before our method can be adopted across a range of sea-ice conditions. Our work concludes with suggested solutions to these challenges and proposes further method and system developments for future research.

Mapping the in situ microspatial distribution of ice algal biomass through hyperspectral imaging of sea-ice cores

“…In situ hyperspectral images were captured beneath the sea ice area from which the ice cores were sampled. We used an underwater HI and photogrammetric payload mounted on an under-ice sled fully described in a previous study 25 .…”

“…We used a tethered under-ice hyperspectral and RGB imaging system to capture 10–30 m transects with the same HI camera as the ice-core scanning set-up 25 . Spatial resolution and spectral dimensions were binned at-sensor yielding a native spatial resolution of 0.624 mm, and a spectral sampling interval of 3.5 nm.…”

“…Underwater Hyperspectral Imaging (HI) is one method that could deliver a methodological turning-point for quantitative mapping of fine-scale sea-ice biophysical conditions that is relevant to larger scale analyses 24 , 25 . HI can be used to quantify biogeochemical properties of a target in each spectrally-resolved pixel within an image 26 – 28 .…”

“…We have recently demonstrated how HI can qualitatively capture biomass variability at the sub-mm spatial resolution in both artificial laboratory ice 24 and in situ under Antarctic fast ice 25 . In this study, we advance the use of HI technology to map the fine-scale vertical and horizontal distribution of sea-ice algae through optical quantification of the photosynthetic pigment chlorophyll a (Chl a ).…”

Underwater Hyperspectral Imaging Technology and Its Applications for Detecting and Mapping the Seafloor: A Review

“…Cimoli et al [ 87 ] creatively developed an under-ice hyperspectral imager to observe the sea-ice at Cape Evans, Antarctica, from the particular “inverted” under-ice prospective. The hyperspectral imager was deployed in an under-ice sled system for scanning the bottom of sea-ice in push-broom mode.…”