Enhancing Methane Source Attribution Using InSAR: A Study of Open-Pit Coal Mines and Landfills

Abstract

Methane is the most impactful greenhouse gas in the short term, and its mitigation can help reduce future warming, especially in the waste and coal mining sectors, which contribute to 20% of anthropogenic emissions. Emission monitoring enables mitigation and is possible through the daily global detections provided by the TROPOMI instrument. Low-resolution TROPOMI acquisitions can be used to guide high-resolution targeted observations from other satellites, allowing for site-level plume localisation. Further attribution to specific underlying activities or sub-areas at landfills and openpit coal mines is difficult due to their topography. Interferometric synthetic aperture radar (InSAR) can support this effort by using satellite-based SAR sensors to measure millimetre-accurate surface deformations. The small-baseline subset (SBAS) and hybrid phase-linking (PL) methods enable multi-year deformation monitoring over unstable areas by leveraging many acquisitions with short revisit times provided by the Sentinel-1 (S-1) C-band SAR constellation. However, their use over methane emission sites has been limited. This study combined measurements from high-resolution methane satellites with InSAR-derived surface deformations to enhance the use of InSAR methods and understanding of methane plume development at landfills and open-pit coal mines. S-1 workflows of varying complexity were compared alongside an Advanced Land Observing Satellite-2 (ALOS-2) based workflow, included for its longer L-band SAR wavelength. All S-1 workflows showed error values less than 16 mm annually when compared against GNSS-measured ground movement, with the most complex hybrid PL method showing the most precise results. Surface deformations were calculated over several super-emitters, identifying that standard SBAS methods often masked active dumping signals with subsidence trends, whereas the hybrid PL method uniquely captured rapid positive deformation spikes. In contrast, ALOS-2 measurements indicated a greater ability to capture the scale of deformations due to its wavelength but were restricted by spatial resolution and coverage. While correlations between deformation magnitude and emission rate were insignificant in landfills, positive surface deformations in coal mines showed a correlation with emission rates. Furthermore, spatial proximity analysis revealed that up to 97% of landfill plumes originated within 100 m of high-deformation clusters. These findings highlight the potential of current methods, particularly the hybrid PL method, for improved methane detection targeting over complex landfills and open-pit coal mine emitters. They are capable of identifying highly deforming areas corresponding to emissionproducing activity, such as dumping or mining, but cannot capture their full extent. To better measure deformation at these sites, next-generation L-band satellites with improved resolutions and revisit times should be used in combination with processing techniques that utilise ground-level data.