Monitoring Highly Dynamic Land Surface Motion with Satellite Radar Interferometry

Abstract

Over the past three decades, synthetic aperture radar (SAR) interferometry (InSAR) has become one of the most important Earth observation technologies in the world, and its use has become common in applications such as topographic mapping, monitoring earthquakes and volcanoes, as well as the built environment. Despite these advances, many technical and scientific challenges remain unsolved in the field, which prevent its use across diverse regions and biomes. One such type of region are wetlands and peatlands, which are notoriously challenging to monitor remotely due to poor signal quality and rapidly changing conditions between SAR acquisitions. This problem is particularly relevant in the Netherlands, because a significant portion of the country is composed of drained peat and clay soils which lie below sea level. These “soft soils” exhibit highly dynamic temporal behaviour that is closely linked to the phreatic groundwater system. In addition, they also exhibit a slow, irreversible subsidence caused by compaction and oxidation, the latter of which is a greenhouse gas (GHG) emitting process. It is this slow, irreversible subsidence component which scientists, governments, farmers and other stakeholders are trying to better understand, and evaluate the risks it poses. Previous efforts in monitoring the cultivated soft soil regions of the Netherlands by InSAR have been hampered by two main problems, which in this work are referred to as “cycle slips” and “loss-of-lock”. The former refers to consistent errors made in ambiguity resolution due to signals which exhibit such highly dynamic behaviour that standard algorithms cannot correctly interpret the wrapped phase data. The latter term refers to a permanent and irreparable loss of coherence in an interferometric SAR data stack. It is common in peatland regions for coherence levels to rise and fall seasonally, and in general, no coherent interferometric combination exists between the coherent periods. This condition means that the interferometric time series is severed during these incoherent periods, and only intermittent, disconnected temporal subsets of data are useable...