Underwater Bridge Inspection: Platforms, Sensors, and Data Processing for Condition Assessment—A Review

Underwater bridge inspection combines direct observation, acoustic imaging, optical measurement, and data interpretation to examine components that are difficult to access. This review aims to organize the principal inspection methods and explain how their operating principles and technical characteristics relate to bridge condition assessment. The qualitative synthesis covers deployment platforms, optical and acoustic equipment, image and point-cloud processing, representative applications, and assessment practice. Three themes emerge. First, platform accessibility and sensor viewing geometry jointly determine inspection coverage. Second, acoustic methods provide structural and riverbed information under low visibility, while optical, laser, and tactile methods contribute local surface detail. Third, processing methods connect observations to component identification, geometric description, and comparisons across inspections. The review distinguishes the roles of two-dimensional images, three-dimensional point clouds, and visual or contact observations, and relates these information types to scour, surface damage, joint anomalies, and deformation. Its contribution is an integrated account of the technical characteristics and complementary uses of inspection methods across the acquisition-to-assessment process. Future directions include coordinated platform operation, multisensor alignment, processing adapted to underwater data, and integration of repeated inspections into bridge maintenance.