How to detect oil spills under sea ice

As petrochemical companies prospect for oil in the crust beneath the Arctic Ocean, the threat of oil spills looms. Compounding the threat is the possibility of a spill happening underneath sea ice, where it could spread unseen. To meet the challenge of detecting such spills, Christopher Bassett of Woods Hole Oceanographic Institution and his collaborators are investigating the feasibility of using autonomous underwater vehicles equipped with sonar.

Given that seawater, crude oil, and ice have different acoustical properties, the approach might seem straightforward. It isn’t. The resulting ice contains a network of brine channels and a water–ice interface with small dendritic structures. If an oil spill occurs underneath growing ice, the oil gets trapped under the ice and between the finger-like structures to form a complex multilayer system. Bassett and his colleagues re-created that process in a tennis-court-sized tank of seawater (shown here) at the Cold Regions Research and Environmental Laboratory in Hanover, New Hampshire. Specifically, they grew six patches of ice and pumped in adjustable amounts of Alaskan crude oil from below. A cart equipped with six transducers ran on rails beneath each patch in turn to record the echoes of acoustic pulses of various frequencies and bandwidths.

The researchers found that the channel with the highest nominal frequency (500 kHz) and the largest bandwidth (370–590 kHz) was best able to resolve oil layers thinner than 1 cm, but at the cost of lower penetration and less sensitivity to ice’s complex structure. By contrast, the channel with the lowest nominal frequency (100 kHz) and the smallest bandwidth (75–130 kHz) could resolve only thick layers of oil, but it could detect oil that had been newly encapsulated by an underlayer of ice. (C. Bassett et al., J. Acoust. Soc. Am. 140, 2274, 2016, doi:10.1121/1.4963876 .)