Underwater Monocular Image Depth Estimation using Single-beam Echosounder

This paper proposes a methodology for real-time depth estimation of underwater monocular camera images, fusing measurements from a single-beam echosounder. Our system exploits the echosounder's detection cone to match its measurements with the detected feature points from a monocular SLAM system. Such measurements are integrated in a monocular SLAM system to adjust the visible map points and the scale. We also provide a novel calibration process to determine the extrinsic between camera and echosounder to have reliable matching. Our proposed approach is implemented within ORB-SLAM2 and evaluated in a swimming pool and in the ocean to validate image depth estimation improvement. In addition, we demonstrate its applicability for improved underwater color correction. Overall, the proposed sensor fusion system enables inexpensive underwater robots with a monocular camera and echosounder to correct the depth estimation and scale in visual SLAM, leading to interesting future applications, such as underwater exploration and mapping.