From meters of subsidence to millimeters of slow slip: monitoring deformation and associated uncertainties from InSAR

Over the past two decades, InSAR evolved from the occasional processing of single interferograms over arid terrains to monitoring continuous time series of SAR acquisitions at the continental scale. Challenges, including atmospheric phase screen mitigation, automatic careful SAR image co-registration, ionospheric phase screen corrections, or discontinuous acquisition planning, were met through various technical and methodological advances by many research groups globally. The resulting methodologies now allow us to image a vast range of processes, from sudden large earthquakes to continuous subsidence involving metric to millimetric displacements. In addition to the ability to process datasets over continental scales, we can now measure natural signals of a few millimeters over distances lower than a kilometer.

In recent years, we proposed technical solutions to issues that were seriously impeding our ability to measure small, millimeter-scale displacements over natural terrains. First, I will discuss the early development of tropospheric corrections using numerical weather models and highlight some of the most recent tools and methods stemming from there. Second, I will illustrate our approach to tackle the issue of continuously incoming SAR acquisitions, which we addressed by developing a data assimilation-based method involving a Kalman filter. This tool allows the rapid update of pre-existing time series of deformation as new SAR images are available while carefully propagating forward some of the uncertainties associated with time series analysis. Third, I will show how we handle the automatic denoising of InSAR time series using a fully convolutional neural network, allowing us to detect sub-millimeter tectonic fault slip with no prior knowledge of the faults. Fourth, I will present some recent developments about the effect of fading signals and time-dependent coherence evolution over temperate regions, depending on land covers.

All these developments allowed us to image surface deformation processes, including several continuously creeping faults globally, transient tectonic slow slip events, intriguing post-seismic deformation signals, and strong subsidence patterns.