Improving Time-Lapse OBN Seismic through Accurate Node Depths and Subsidence Measurements

Time-lapse seismic remains the most widely applied tool for offshore reservoir monitoring. Its value, however, depends critically on acquisition repeatability and the robustness of processing workflows. In deep-water ocean-bottom node (OBN) surveys, uncertainty in receiver depth has long been a limiting factor, affecting statics, reducing 4D repeatability, and complicating the interpretation of depth-shift and compaction signals. At the same time, independent measurements of seabed subsidence are of increasing interest for calibrating geomechanical models and managing infrastructure integrity but are typically sparse or operationally demanding. We describe how principles originally developed for gravity-based subsidence surveys have been integrated into OBN operations to address both challenges. Initial work demonstrated that stabilised pressure-sensor systems embedded into node deployment workflows can deliver relative node depths with centimetre-level accuracy (Hatchell et al., 2019). Building on this foundation, the method was later applied across successive OBN campaigns in the same producing fields, enabling direct measurement of seabed subsidence with an uncertainty of approximately 3 cm over several years (Dutta et al., 2023). We report how accurate time-lapse node-depth constraints provide an independent subsidence monitoring capability with broad spatial coverage, while also strengthening 4D seismic results by improving statics and decoupling depth-related effects from water-velocity variability, node timing errors and other acquisition-related sources of non-repeatability. This demonstrates how modest operational additions to seismic surveys can enhance reservoir monitoring outcomes.