ABSTRACT

Laser line scan is an optical system for observing the seafloor. A rotating prism in a towed fish sweeps a laser beam across the bottom. The return signal is received at the fish, and can be enhanced digitally using sophisticated image-processing techniques. Laser line scan's coverage rate is much higher than video mounted on an ROV, and approaches that of side-scan sonar. The images are of higher quality than side-scan, and are easier to interpret because they are optical rather than acoustic. The high power of the laser allows the system to be three to five times further away from the target object than video, and it has been demonstrated to be effective even in murky water Laser line scan was developed for military purposes, and has been applied in underwater search, pipeline surveys, geotechnics, archaeology and biology.

INTRODUCTION

SAlC has recently completed a series of experimental tests and shipborne operational deployments of a new sensor system for underwater optical imaging. The Laser Line Scan (LLS) system produces high resolution," picture quality" panoramic surveys at rapid area coverage rates, real tune data monitoring and storage of digital images for post processing, analysis, and accurate survey area reconstruction. Applications to mine detection and specia1 purpose surveillance is possible in both towed systems as well as UUV's.

The system has produced high-resolution images with 70° swath widths at altitudes from 2.5 to 40 m (8 ft to 130 feet) above the sea floor at speeds up to 6 knots in water depths up to 700 m. To date, operations have taken place over a wide range of coastal and open ocean environments from the Pacific Northwest to Southern California, and from New England to the Gulf of Mexico This paper consists of a brief LLS system description and provides selected images acquired by the LLS system This imagery demonstrates the value of this system for search and locate operations, in-situ monitoring of subsurface structures and analysis of environmental parameters.

The LLS imaging sensor is installed in a hydrodynamic tow body which is connected to the tow vehicle by 900 m of electro-optic cable. Tile sensor system is based on a synchronously scanned blue/green laser transmitter and photomultiplier tube (PMT) receiver which laterally scans as the tow body moves above the bottom. The image is formed by assembling successive laser line scans into a two-dimensional pushbroom "image". This system utilizes state-of-the art technology to provide a user-friendly operator interface, user selectable resolution of up to 2048 pixels across a fixed 70° field-of-view, fibre optic data and telemetry, and 12-bit digital signal dynamic range.

The LLs system has demonstrate useful operating platform altitude of up to 5 optical attenuation lengths. For example, I typical Southern California water clarity, tow body altitudes of 12 m with image swathes 17 m wide are consistently recorded at high resolution of "a few mm" scale.

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