Geophysical Assessment of Land Subsidence Risk Using InSAR and Geotechnical Data in Lagos Island, South West Nigeria

Lagos Island, a densely built urban area on reclaimed and natural low-lying land in Lagos State, is experiencing apparent land subsidence processes associated with groundwater withdrawal and the compaction of unconsolidated fill materials, yet the magnitude, spatial distribution, and driving mechanisms of subsidence have not been quantified through a rigorous geophysical and geodetic investigation. This study uses Sentinel-1 SAR imagery processed by the Persistent Scatterer Interferometric SAR technique in combination with geotechnical borehole data and groundwater monitoring records to quantify land surface displacement across Lagos Island over a 2019 to 2024 observation period. A total of 234 Sentinel-1 ascending and descending pass images were processed using the StaMPS software chain to produce displacement velocity maps at sub-centimetre per year precision. Mean annual subsidence rates across Lagos Island range from less than 1 millimetre per year in areas of natural rock outcrops to between 12 and 28 millimetres per year in areas of reclaimed land and thick soft sediment fill, with localised hotspots reaching 40 millimetres per year beneath large commercial structures. Correlation of PS-InSAR displacement with groundwater head decline rates at monitored piezometers confirms that 60 to 70 percent of the observed subsidence variance is explained by aquifer head depletion. Geotechnical compression parameters from oedometer tests on borehole samples provide a hydrogeomechanical explanation for the observed deformation magnitudes. The study recommends groundwater recharge augmentation and regulatory control of abstraction as primary mitigation measures. Keywords: InSAR, land subsidence, Lagos Island, groundwater, PS-InSAR