The detection of the goaf area is the key to the treatment of the coal mine goaf. The underwater ultrasonic imaging technology is applied to detect the shape of the water filled goaf of coal mine for the first time in China. The basic principle of ultrasonic imaging technology is explained. The work process of the ultrasonic detection is introduced with a detection case of a coal mine goaf under a railway bridge in Shanxi, China. The ultrasonic reflection signatures of wall, fork and ground obstacle of the mine roadway have been discussed. The post-processing techniques of the ultrasonic detection are proposed to reconstruct the goaf shape from cloud data. Finally, the detection results are compared with the known exploratory drilling data. It shows that the ultrasonic goaf detection has a high accuracy, which can satisfy the requirement of the actual engineering. The detection results provides the key data for the treatment of the goaf area.

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