Current Subsidence Rates of the Mississippi River Delta From Satellite Radar Interferometry: Onshore and Offshore

The Mississippi River Delta has long been recognized as an area experiencing substantial subsidence and land loss. Existing GNSS stations are quite sparse and there is no station at the Brid's Foot area and the delta front (offshore) where the Holocene sediment is the thickest. Six-year-long Sentinel-1 satellite data was used to estimate the subsidence rates using PS-InSAR method. Taking advantage of the many oil/gas platforms in the Gulf of Mexico, the subsidence rates at the platforms were estimated. These unique measurements filled the data gap in the offshore region which is important to understand the dominating factor for the subsidence on a regional scale. The maximum subsidence rate measured by InSAR is about 12 mm/year (converted to the vertical direction from InSAR line-of-sight direction) and is around the Bird's Foot area. An abrupt increase in subsidence rate was observed immediately downstream near the end of the artificial levee system. By excluding active platforms, the offshore subsidence rates due to natural processes were obtained. Both on-land and offshore InSAR subsidence rates showed a nearly linear relation with the Holocene sediment thickness. InSAR observations from targets with relatively deep foundations suggest that the background subsidence rate due to deformation of the Pleistocene and underlying strata is about 0–2.5 mm/year and shows a gradient approximately perpendicular to the coastline. This is in general less than half of the observed total subsidence rate. The compaction of Holocene sediment likely dominates the subsidence of the Mississippi River Delta area on the regional scale, although local deformation could be dominated by other processes.