Sonar detection and monitoring of sunken heavy fuel oil on the seafloor

The oil products transported at sea that have the potential to become suspended in the water column and sink after weathering or mixing with sediment are quite numerous: asphalt, carbon black oil, bunker C, fuel oil n°5 and 6. In many incidents and accidents part of the spilled heavy oil product has actually sunk and has been difficult to track because of the lack of means of detection. To compensate this gap, a comprehensive sonar experiment has been performed through the use of a large seawater tank on the bottom of which several patches of three different heavy fuel oils have been laid on top of a sandy layer. In this facility several kinds of sonar have been tested as for their response according to their frequency, resolution and type (side scan sonar, multibeam/panoramic sonar, 3D acoustic camera). The results proved to be very valuable according to the acoustic specificity of the products involved. Indeed the acoustic properties are similar to those of the seawater when considering density and sound speed; however the attenuation is much more important and its variation has been measured from 100 kHz to 500 kHz preliminary to the testing. The results of the sonar experiment that have been obtained do confirm the capability of current sonars to detect heavy oil patches over sand seafloors, and should make it possible to select the most adequate survey strategy according to the oil patches dimensions and thickness and to the environmental conditions.