Abstract
Abstract. Regions of anomalously low backscatter in C-band synthetic aperture radar (SAR) imagery of lake ice of Lake Neyto in northwestern Siberia have been suggested to be caused by emissions of gas (methane from hydrocarbon reservoirs) through the lake’s sediments. However, to assess this connection, only analyses of data from boreholes in the vicinity of Lake Neyto and visual comparisons to medium-resolution optical imagery have been provided due to a lack of in situ observations of the lake ice itself. These observations are impeded due to accessibility and safety issues. Geospatial analyses and innovative combinations of satellite data sources are therefore proposed to advance our understanding of this phenomenon. In this study, we assess the nature of the backscatter anomalies in Sentinel-1 C-band SAR images in combination with very high resolution (VHR) WorldView-2 optical imagery. We present methods to automatically map backscatter anomaly regions from the C-band SAR data (40 m pixel spacing) and holes in lake ice from the VHR data (0.5 m pixel spacing) and examine their spatial relationships. The reliability of the SAR method is evaluated through comparison between different acquisition modes. The results show that the majority of mapped holes (71 %) in the VHR data are clearly related to anomalies in SAR imagery acquired a few days earlier, and similarities to SAR imagery acquired more than a month before are evident, supporting the hypothesis that anomalies may be related to gas emissions. Further, a significant expansion of backscatter anomaly regions in spring is documented and quantified in all analysed years 2015 to 2019. Our study suggests that the backscatter anomalies might be caused by lake ice subsidence and consequent flooding through the holes over the ice top leading to wetting and/or slushing of the snow around the holes, which might also explain outcomes of polarimetric analyses of auxiliary L-band Advanced Land Observing Satellite (ALOS) Phased Array type L-band Synthetic Aperture Radar-2 (PALSAR-2) data. C-band SAR data are considered to be valuable for the identification of lakes showing similar phenomena across larger areas in the Arctic in future studies.
Highlights
Lakes and ponds are common features of the Arctic continuous permafrost zone and play an important role in the carbon cycle (e.g. Walter Anthony et al, 2012; Wik et al, 2016)
We investigated and quantified anomalies of Cband radar backscatter in synthetic aperture radar (SAR) images of lake ice of Lake Neyto in Yamal, Russia, and assessed their potential relation to gas emissions
This relation was suggested before using visual comparisons between Sentinel-1 data and mediumresolution optical data, but here we provided a quantification of relations between features in SAR and very high resolution (VHR) imagery, examined the spatio-temporal dynamics of backscatter anomaly regions, and assessed potential scattering and formation mechanisms in greater detail
Summary
Lakes and ponds are common features of the Arctic continuous permafrost zone and play an important role in the carbon cycle (e.g. Walter Anthony et al, 2012; Wik et al, 2016). Methane (CH4) is a powerful greenhouse gas, and the global trend of its atmospheric concentration has shown significant changes over the last few decades. CH4 produced by microorganisms in the sediments of Arctic lakes can escape to the atmosphere through upward bubbling (ebullition) in the water column and contributes significantly to the total global methane emissions Geologic methane accumulated in sub-surface hydrocarbon reservoirs, previously sealed by permafrost or glaciers that act as a cryosphere cap, can seep into the atmosphere through lake sediments and the water column in the case of open taliks under big lakes and rivers in the continuous permafrost zone or in regions of glacial retreat (Walter Anthony et al, 2012)
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