Seafloor motion from offshore man-made structures using satellite radar images – A case study in the Adriatic Sea

Abstract

Space geodetic techniques have achieved centimeter to even millimeter precision in measuring earth surface deformation. However, a large data gap remains in the offshore area. Offshore man-made structures (e.g., oil/gas platforms) anchored to the ocean bottom provide an opportunity to study seafloor motion in some areas. Although satellite InSAR (Interferometric Synthetic Aperture Radar) has been widely used to study earth surface deformation, its application to offshore regions is extremely limited. Continuous GNSS (Global Navigation Satellite System) observations at several tens of offshore platforms in the Adriatic Sea have recently been released. Measuring the same platforms with InSAR provides a great opportunity to assess the feasibility of applying this technique to study seafloor motion on a regional scale using offshore structures. We processed a six-year-long time series of SAR images from the Sentinel-1A satellite using the Permanent Scatterer InSAR (PS-InSAR) method. We assessed the feasibility of phase unwrapping using synthetic data with different velocity fields and noise levels. Correct phase unwrapping could be achieved in the Adriatic Sea and two other large offshore oil/gas fields: the Gulf of Mexico and the North Sea. Different calibration strategies were applied, and we suggest that the InSAR results could be calibrated with limited and even no GNSS stations. Our InSAR results show good agreement with the GNSS measurements and the InSAR observations from the European Ground Motion Service. In addition, our InSAR results provide deformation measurements at about twenty offshore structures where GNSS stations are not present. Most of the offshore structures have a subsidence rate of no more than 5 mm/year, while a few of them reach about 10 mm/year. Our work demonstrates that it is feasible to apply the InSAR technique to measure displacement of discrete offshore man-made structures (fixed to the ocean bottom) on a regional scale but still on a case-by-case basis. Pre-acquired information including geological settings, existing geodetic observations, and human activity records (e.g., hydrocarbon production) are useful information to assess the feasibility and to validate the InSAR results.