Investigation of Land Subsidence in Lagos Using Differential Interferometric Synthetic Aperture Radar

Lagos, as Nigeria's commercial capital built partly on reclaimed coastal and lagoonal sediments, is highly susceptible to land subsidence driven by groundwater extraction, sediment compaction, and construction loading. This study investigates land subsidence patterns across selected districts of Lagos State using Differential Interferometric Synthetic Aperture Radar analysis of Sentinel-1 C-band SAR images acquired between 2021 and 2024. A stack of interferometric pairs is processed using the Small Baseline Subset algorithm to derive time-series displacement maps at millimetric resolution. Results are validated against repeated spirit levelling observations conducted at benchmark points in Victoria Island, Eti-Osa, and Ikorodu. Subsidence rates ranging from three to eighteen millimetres per year are detected in reclaimed land areas of Victoria Island, the Lekki peninsula, and parts of Apapa, with the highest rates coinciding with locations of intensive groundwater pumping and high-rise construction. Comparatively stable zones are observed over pre-existing laterite rock outcrops. The study highlights the critical structural risk to foundations, drainage systems, and coastal flood defences posed by ongoing subsidence. Outputs are communicated to the Lagos State Ministry of Physical Planning and Urban Development and the Lagos State Water Corporation to inform groundwater management policies and building code revisions. Keywords: land subsidence, InSAR, Sentinel-1, Lagos, groundwater.