Wide-area debris field and seabed characterization of a deep ocean dump site surveyed by autonomous underwater vehicles

Abstract

Disposal of industrial and hazardous waste in the deep ocean was a pervasive global practice near coastlines in the 20th century. Uncertainty in the quantity, location, and contents of dumped materials at historical disposal sites underscores ongoing risks to marine ecosystems and human health. This study presents analysis of a 150-km2 wide-area sidescan sonar survey conducted in March 2021 with two autonomous underwater vehicles (AUVs) at an offshore dumpsite in San Pedro Basin, California. Previous camera surveys located 60 barrels and other debris; sediment analysis in the region showed varying concentrations of the insecticidal chemical Dichlorodiphenyl-trichloroethane (DDT), for which an estimated 350-700 metric tons was discarded in the San Pedro Basin between 1947 and 1961. A lack of primary historical documents from Montrose Chemical or California Salvage Company that specify the DDT acid waste disposal method has contributed to the ambiguity surrounding whether the dumping method was via bulk discharge or containerized units. Barrels and debris visually detected during previous surveys by Remotely Operated Vehicles (ROVs) Jason and SuBastian and AUV Sentry served as the basis for ground truth testing and object classification using size and acoustic intensity characteristics. Target detection and classification algorithms were developed using image and signal processing techniques that resulted in the identification of over 74,000 debris targets within the survey region. Statistical, spectral, and machine learning techniques are used to characterize seabed variability and for bottom-type classification. These analytical techniques combined with the survey capabilities of AUVs provide a framework for efficient mapping and characterization of uncharted deep-water disposal sites.