Remote thickness measurement of oil slicks on water by laser-ultrasonics

At the National Research Council of Canada Industrial Materials Institute, research is in progress on the application of laser-ultrasonics to remote measurement of the thickness of oil on water. Laser-ultrasonics is a novel technique developed for the nondestructive inspection of materials. It uses a short pulse laser for the generation of ultrasonic waves in the oil layer and a second laser, coupled to an optical interferometer, for the remote detection of the ultrasonic surface motion. Direct measurement of the time of flight of the ultrasonic wave provides the value of the thickness of the oil layer. Application of this technique to thickness measurement of oil on water has been studied in small and large scale laboratory tests. Small scale tests demonstrate the direct and unambiguous determination of the oil layer thickness. Accuracy is essentially limited by the knowledge of the acoustic properties of the oil. Large scale tests show that a distance of almost 37 meters does not severely impede the method, so air borne application appears possible. Surface motion such as that caused by sea waves does not reduce the accuracy of the thickness determination but does limit the measurement rate. Preliminary airborne tests with a single laser probe confirm that laser-ultrasonics monitoring of the thickness of an oil spill is feasible