Monitoring delta subsidence with Interferometric Synthetic Aperture Radar (InSAR)

Abstract:

Can subsidence in river deltas be monitored in near real-time at the spatial and temporal resolution needed for informing critical management decisions? Interferometric Synthetic Radar Aperture (InSAR) is a satellite-based technique that can map ground deformation with millimeter-scale vertical resolution over thousands of square kilometers. InSAR has enormous potential to shed light on the dynamics of actively subsiding deltas, but the technique is not commonly applied outside of major cities due to the difficulty of performing InSAR in wet, vegetated settings. Given these limitations, how can InSAR best serve the global effort to monitor sinking deltas?

Here, an overview of InSAR processing is provided that addresses delta-specific challenges, including frequent cloud-cover in tropical areas; noisy signals in wetlands and flooded fields; dense forests that interact unpredictably with different radar wavelengths; flat landscapes that hinder image stacking algorithms; rapid urban development that can render Digital Elevation Models (DEMs) inaccurate; and a lack of in situ GPS (Global Positioning System) receivers for InSAR calibration. InSAR has unique value for monitoring subsidence in deltas, and some natural and anthropogenic drivers of subsidence can be resolved by InSAR.

High-resolution InSAR measurements from the Ganges-Brahmaputra Delta (GBD) are then presented and validated against GPS data. Surface motion is shown to reflect subsurface stratigraphy, and sediment compaction is shown to be the most important factor in this delta on short (non-tectonic) timescales. Average compaction rates throughout the eastern delta range from 0 to > 18 mm/y, varying by more than an order of magnitude depending on the ages and grain sizes of surface and subsurface sediment layers. Fastest subsidence is observed in Holocene organic-rich mud, and slowest subsidence is observed along the Meghna River and in areas with surface or subsurface sand deposits. Although groundwater extraction rates are high in Dhaka, much of the city is built on a consolidated Pleistocene terrace that appears to compact little even under sustained drawdown. This has minimized the impact of anthropogenic fluid extraction in the Dhaka region.