Underwater structure inspection with laser light stripes

Abstract

As international trade is getting prosperous, commercial ports need to accommodate hundreds of vessels everyday. When a vessel berths the wharf carelessly, it is possible to cause some damage to the pier beneath the waterline without leaving any evidence. Generally, scuba divers are sent down with camera to take photos or video tapes of the underwater structure for further studies of the damage. We propose using a laser light stripe technique to overcome the problem of turbid water. From geometry we know that a straight line projected to a plane remains a straight line. On the other hand, if there are any concave or convex features on the plane, the projected line will deform accordingly. So we can extract information, such as position or depth of the concavity, from the deformation of the projected line. In our study, we project a laser beam stripe on the target and use a CCD camera to capture the image. With image processing, we can extract the profile of the cross section, thus, the overall 3D measurement of the target. We verify this idea with different shapes of objects submerged in different levels of turbid water in a test tank. Preliminary results indicate that this device can detect narrow gaps of 1 cm in NTU level 2.5 foul water from a distance of 60 cm with coverage of 60 cm wide.