Detection and measurement performance of imaging sonar with application to miter gate monitoring

Navigational locks and dams are critical to U.S. inland navigation waterways, but unscheduled outages caused by miter gate damage can disrupt transportation and lead to significant economic losses. Monitoring these structures for damage is challenging because submerged components cannot be readily visually inspected or instrumented with contact sensors such as strain gages. Imaging sonar provides a potential non-contact alternative capable of operating in turbid underwater environments; however, its ability to detect and measure structural displacements has not been rigorously established for structural health monitoring (SHM) applications. This study evaluates whether imaging sonar can 1) detect structural displacement and 2) accurately measure its magnitude. Controlled laboratory experiments displaced a steel plate while an imaging sonar recorded the motion. From this imagery, optical flow methods quantified displacements. Statistical hypothesis tests were used to determine whether these sonar measurements were significantly correlated with true plate motion (detection) and whether sonar measurements agreed with reference measurements (measurement accuracy). Results demonstrate that imaging sonar reliably detects sub-millimeter displacement under favorable viewing geometries, with sensitivity diminishing as sonar-to-plate distance increases. To assess field relevance, the sonar measurement performance was compared with displacement changes predicted by a finite element model of a miter gate at The Dalles Lock and Dam. Model results indicate that expected displacement changes from most induced damage cases exceeded the measurement accuracy, supporting the feasibility of imaging sonar as a practical, non-contact sensing technology for SHM of submerged infrastructure.