Oil beneath Arctic Ice: Predicting Under-Ice Storage Capacity as a Means to Better Anticipate Oil Slick Spreading under Ice

摘要

In the event of an Arctic oil spill, ice in the water is a complicating factor. The presence of ice complicates the forecasting of the movement and spreading of oil as well as the planning of the oil spill clean-up process. The underside of Arctic Sea ice is not flat, rather it presents non-geometric, unpredictable protrusions into the water column. The depth to which these protrusions grow relates to the longevity of the ice itself. The challenge faced by the oil spill forecaster is that information on the under-ice storage capacity is not readily available. Hence, we explored how to estimate under-ice storage capacity based on the ice stage. Historical reports on Arctic sea ice stage (e.g., first year, thin ice) were obtained from the Alaska Ocean Observing System. Ice stage data is available daily in the form of an ice 'egg' code, similar to the CIS/WMO egg. Next, we acquired historic data on under-ice storage capacity. During the winters of 2010-2013, Shell deployed upward looking sonar at several sites in the Beaufort and Chukchi Seas. The sonar made direct measurements of ice draft. The under-ice storage capacity-defined as the volume of pore space above the average ice draft level-was estimated based on the ice draft data. Analyzing the ice draft alongside the ice stage data, we developed low, medium, and high estimates of oil storage capacity. When the primary ice stage is assigned an ice egg code of 1 to 3, storage capacity is deemed 'low,' identified by less than 15,000 m~3/km~2. For primary ice stage with a 7 to 10 ice egg code, storage capacity is 'medium,' between 15,000 and 50,000 m~3/km~2. Older ice with an ice egg code of 11 or greater has a 'high' storage capacity of 50,000 to 80,000 m~3/km~2.