Acoustical Imaging and Surface Reconstru-ction Methods for Ship-hull Inspection

In this work we present our method for ship-hull inspection by using acoustical imaging. Ship-hull inspection is usually done in dirty harbour waters, which makes an acoustic-vision system more suitable than a camera-vision system. The wet-end of our acoustic-vision system consists of a vertically positioned SONAR transmit and receive antenna, with 128 equidistantly spaced transducer elements. Signals were recorded in a domain of interest ranging from 5 to 10 meters. For the SONAR pulse we use a high-frequency continuous wave pulse, with a carrier frequency of 1 MHz and a bandwidth of 125 kHz. We implemented different broad-band beam formers in time domain for near-field and far-field applications. Each measurement provides us with a 2D image slice. By moving the SONAR antenna, a 3D volume is covered by the successive 2D slices. In order to get the information about the surface of the scanned object, we reconstruct the surface of interest by finding the location of the maximum value in each beam signal of the 2D slices. 3D image noise is removed from the surface by employing a threshold filtering technique. The overall outcome of the acoustical inspection system is the visualized reconstructed surface of the scanned object.