Calibration and multiple sound sources in marine geotechnical pollution surveys in rivers

Abstract

During the last two years, detailed acoustic subbottom geotechnical surveys in the Trenton Channel of the Detroit River have resulted in optimized sediment characterization algorithms for the detection and distribution of potential polluted sediments. This work clearly demonstrated the need for a wide variety of sound sources extending in the frequency range of 700 Hz to 20 kHz. The wide frequency range is necessary because of the wide variation in sediment thickness and absorption of the sound waves in highly polluted gaseous sediments. It was determined, through both acoustic and physical cores for this particular site, that this absorption is caused, in part, by entrapped micro-gas bubbles in these particular polluted sediments. This gas causes extreme (8 to 20 decibel) changes in the bottom loss (bottom reflection coefficient) observations over short spatial distances (3 to 5 meters). It was found that with ultra precision source and receiver calibrations for each acoustic system, confirmation of these observed phenomena could be verified analytically. The pollutants in this area comprised a mix of hydrocarbons, PCBs, acids, and heavy metals. This paper presents the procedures and results of the calibration techniques, and shows how these techniques have led to the development of quality control plans for sediment surveys. Further, the operational survey techniques with multi-frequency sub-bottom systems are presented along with specific examples of mapping the spatial distribution of highly varying sediments. In particular, the bottom loss and absorption of the polluted sediments are compared to standard marine non-polluted sediments.