CAN Bog Breathing be Measured by Synthetic Aperture Radar Interferometry

Abstract

Accounting for relatively large seasonal and short term peatland surface vertical displacements with Synthetic Aperture Radar Interferometry (InSAR) poses a problem of possible propagation of ambiguity errors. Notwithstanding, the absence of continuous high temporal resolution peatland surface levelling measurements for validation has been something characteristic. Based on the ground levelling from a raised bog, we demonstrate the Sentinel-1 distributed scatterer (DS) time-series InSAR technique underestimates real surface displacements and hereby we question the accuracy of the approach over peatlands. When the relative surface change from 6-day interferograms is used instead of accounting for the absolute change, the estimation accuracy improves (Spearman's rho 0.82, p-value < 0.002) because 6-day in situ surface changes are usually small and do not need InSAR phase unwrapping. Despite a serious unwrapping problem in peatlands, DS time series contain useful signal and differential InSAR (DInSAR) might have potential for assessment of short term peatland surface displacements in favourable conditions.

High Accuracy Estimation and Validation of InSAR-derived Surface Displacements at Temperate Raised Peatlands

“…Unwrapping is challenging for peatlands, because significant spatial discontinuities within InSAR observations can occur on and around peatlands (see Figure S1 in Supplementary Materials) depending on factors such as the nature of the peatland vegetation and the processing approach. Phase ambiguities in space and time also can be caused by large amplitude of displacement during dry periods (Marshall et al, 2022;Tampuu et al, 2022), and unwrapping is further hindered by low InSAR coherence -i.e., phase measurement quality -of lands around the peat soils during these periods (Hrysiewicz et al, 2023).…”

“…To estimate multi-annual surface displacement rates correctly, "long-term" interferograms spanning a 1-year period are needed to minimise the potential biases of InSAR phase arising from the interferogram network (Ansari et al, 2021). Although raised peatlands in good condition can yield 1-year interferograms with moderate to high InSAR coherence (see Figure S2 in Supplementary Materials) (Hrysiewicz et al, 2023), interferometric pairs such with such a long temporal baseline can be strongly affected by phase ambiguity (Marshall et al, 2022;Tampuu et al, 2022). To mitigate the ambiguities, a network of "short-term" interferometric pairs of high InSAR coherence can be used instead.…”

“…In addition, Tampuu et al (2022), which also undertook a ground validation of InSAR data on boreal peatlands on shorter period (July to October 2016). They show the same difficulties to obtain actual peat surface displacements on a raised bog in Estonia with similar in-situ technique than .…”

“…Most past efforts to measure peatland surface motions in-situ have focussed on the use of subsidence poles or precise levelling surveys. These have measured the elevations of ground on the peatland either directly (e.g., Howie & Hebda, 2018;Tampuu, 2022;Tampuu et al, 2022) or indirectly (e.g., Bradley et al, 2022;Marshall et al, 2022). These survey methods offer the advantage of giving cmscale to mm-scale precision but have the disadvantage of being highly discontinuous in time (measurements typically monthly to yearly) and cost-intensive in terms of equipment, fieldwork, and staff time.…”

“…Application of InSAR to measuring peatland surface motion has been undertaken for both temperate (e.g., Alshammari et al, 2020;Alshammari et al, 2018;Fiaschi et al, 2019) and tropical peatlands (Hoyt et al, 2020;Zhou, 2013;Zhou et al, 2016). However, recent ground validation efforts show that existing approaches to InSAR computations on peatlands apparently fail to capture with accuracy both the longer-term trend of peat surface motion and the shorter-term variations in surface motion superimposed on that trend (Marshall et al, 2022;Tampuu, 2022;Tampuu et al, 2022). Consequently, improved application of InSAR to mapping peatland surface motion requires refinement of the processing techniques, but such refinement must go hand-in-hand with validation through an in-situ technique of compatible precision and temporal resolution.…”