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10 st / June 2016 www.sea-technology.com and easily map large areas in a range of environmental conditions, but require spe- cialized knowledge to op- erate and interpret the data and verifcation of acoustic signatures with bottom ob- servations or samples. In a fast-paced spill emergency, immediate surveys are es- sential for sunken oil detec- tion and recovery; therefore, a coordinated effort combin- ing all appropriate methods is crucial. During a recent riverine spill incident, sci- entists working with NOAA in support of the U.S. Coast Guard were faced with this challenge. T/B Apex 3508 Sunken Oil Response, Assessment On September 2, 2015, a collision occurred between northbound and southbound barges on the Mississippi River near river mile marker 938 (in the vicinity of Colum- bus, Kentucky). The collision compromised the watertight integrity of the #3 starboard tank of T/B Apex 3508, causing the release of 2,870 barrels (120,540 gallons) of clarifed slurry oil (CSO) into the river. CSO is a heavy oil byproduct of the petroleum refning process. The CSO being transport- ed by the T/B Apex 3508 had a specifc gravity of 1.14 (API gravity of -7.4), properties which suggested a high probabil- ity of sinking. The U.S. Coast Guard and the NOAA scien- tifc support team were mobilized for spill assessment and response, operating out of an Incident Command Post in nearby Paducah, Kentucky. No oil was observed on shore- lines downstream of the collision location; aerial overfights indicated only isolated light sheening. A "tailgate test" was performed using samples of the CSO and river water, which confrmed that the CSO substance sank nearly immediately T he majority of oil spill re- sponse strategies are fo- cused on foating oil. However, in certain situations, specif- cally releases of heavy petro- leum products (e.g., very heavy crude oil, slurry oil, heavy fuel oil), there is an increased risk of oil sinking and accumulat- ing on the bottom. If the densi- ty of the oil is greater than that of the receiving water, the oil will submerge in the water col- umn (if turbulence or currents are high) or sink to the bottom (if turbulence or currents are low). Oil that initially foats can increase in density as the oil weathers or mixes with sed- iment and become submerged or sink. A foating oil can mix with sand in the surf zone or af- ter stranding on a beach, then sink offshore. Turbulence can entrain the oil in the water col- umn, where the oil can inter- act with suspended sediments and organic matter, forming oil-particle aggregates that can become heavier than the re- ceiving water and settle out as turbulence and currents decrease. Because sunken oil can be remobilized or buried by changes in currents or wave ac- tion, there is a need for rapid detection and recovery. Identifcation and mapping of sunken oil has historically been diffcult due to logistical challenges and a signifcant gap in the scale and extent of bottom observations. Diver observations and underwater cameras provide the most detailed perspective, but operations are slow, labor-inten- sive and provide limited spatial coverage, as well as lim- ited effectiveness under low-visibility conditions. Sorbents dragged along the bottom are often used, but there is no calibration between the amount of oil on the sorbent and on the bottom. Bottom sampling is slow and only provides data for a very small area. Sonar instruments can quickly Application of Sonar For Oil Spill Response Acoustic Detection, Evaluation and Monitoring of Sunken Oil Spills By Dr. Tim McClinton • Lt. Greg Schweitzer • Dr. Jacqueline Michel Setting of the T/B Apex 3508 incident on the Lower Mississippi River. The thin black lines are tracklines of initial SSS surveys.