Abstract: In the survey of major modern urban projects and underground drainage network, the measurement of large sewage caissons is an important but difficult task. The traditional method of frogmen going down to the well to explore has been gradually replaced by high-precision sonar detection systems due to its high risk factor and large measurement errors. In order to achieve highprecision and highefficiency measurement of large sewage caissons, this paper developes a multi-beam sonar caisson measurement system, and compares it with the dual-axis scanning sonar measurement system from multiple angles. The experimental results show that the multi-beam sonar measurement system is light in weight, small in size, high in precision, high in field measurement efficiency, and high in point cloud density. It can quickly obtain the three-dimensional shape of the well chamber and inlet and outlet of the water storage caisson. With the advancement of technology, the sonar measurement system will become an effective tool for measuring urban underground water storage spaces such as pipelines, caissons, and hollow areas.

Key words: caisson measurement, multi-beam sonar, dual-axis scanning sonar, three-dimensional point cloud, accuracy, efficiency