with anoxic high-salinity water which are death traps for small marine organisms. However, much of it appears to have been documented from an offshore engineering perspective, for the purpose of oil exploration. Seabed survey for gouges Seabed gouging by ice is an eminently discreet phenomenon: little sign of it can be observed from above the water surface – the odd evidence includes sea floor sediments incorporated into the ice. Information of interest on these gouges includes: depth, width, length and orientation. Gouging frequency – the number of gouges produced at a given location per unit time – is another important parameter. This kind of information has been gathered by means of seabed mapping with ship-borne instrumentation, typically a fathometer: echo sounding devices such as a side-scan and a multi-beam sonar systems. Repetitive mapping involves repeating these surveys a number of times, at an interval ranging from a few to several years, as a means of estimating gouging frequency. Gouge characteristics Seabed gouges produced by drifting ice features can be many kilometers in length. In Northern Canada and Alaska, gouge depths may reach . Most, however, do not exceed 1 meter (3 feet). Anything deeper than 2 meters is referred to by the offshore engineering community as an extreme event. Gouge widths range from a few meters to a few hundred meters. The maximum water depths at which gouges have been reported range from , northwest of Svalbard in the Arctic Ocean. These are thought to be remnant traces left by icebergs during the Pleistocene, thousands of years ago, when the sea level was lower than what it is today. In the Beaufort Sea, Northern Canada, a 50 km (30 mi) long gouge was shown to exist, with a maximum depth of and in water depths ranging from . The gouge is not always straight but varies in orientation. This event is thought to be about 2000 years old. Recent episodes of grounding, gouging and fragmentation of large Antarctic icebergs have been observed to produce powerful hydroacoustic and seismic signals that further illuminate the dynamics of the process. The ice features In the offshore environment, the gouging features are made up of two kinds of ice: glacial ice and sea ice. Glacial ice Physically and mechanically, glacial ice is akin to lake ice, river ice and icicles. The reason is that they all form from freshwater (non saline water). Ice sheets,