Taking a Seismic Peek Below the Gargantuan Glacier

Situated in the West Antarctic, the Thwaites Glacier covers an area of over 192,000 square kilometres, more than twice the size of Austria. As numerous measuring campaigns conducted in the past several years confirm, climate change has made the frozen giant worryingly unstable. For example, the ever-warmer seawater from the adjacent Amundsen Sea has now penetrated below the glacier and melted it to such an extent that the grounding line – that is, the last point at which the ice still rests on the ground – has retreated 14 kilometres inland since 1992. Consequently, due to meltwater and calving icebergs, today Thwaites Glacier is losing ice at twice the rate it did 30 years ago and, together with its neighbouring glaciers in the West Antarctic, is currently responsible for over 10 percent of global sea-level rise. If all the ice in the West Antarctic were to melt, the global sea level would rise by more than three metres.

“That makes it abundantly clear that the future development of Thwaites Glacier is of global importance,” says Prof Olaf Eisen, a glaciologist at the Alfred Wegener Institute. “To make better forecasts, we need to know more about the glacier’s features. And that includes the underside of the ice. What does the ground it’s resting on look like? Is it hard and rocky, or does it consist of sediments? Is the underside of the ice still frozen, or is there a liquid film? All of these factors are critical when it comes to how quickly the glacier flows out to sea.”

Olaf Eisen and the AWI are part of the International Thwaites Glacier Collaboration (ITGC), which is investigating the glacier under the leadership of the National Science Foundation (USA) and the Natural Environment Research Council (UK). In the context of the subproject GHOST (Geophysical Habitat of Subglacial Thwaites), Eisen and his fellow AWI researcher Dr Coen Hofstede now plan to mount a measuring campaign that will gather essential information on the ground below the glacier. To do so, Hofstede and two other members of AWI staff will journey to the West Antarctic and use the vibroseis technique to peer below its kilometre-thick ice.

“Currently, we’re the only ones in the world capable of employing this technique in the Antarctic,” says Eisen. How it works: an all-terrain buggy on a large metal plate is hauled over the ice like a giant sledge. Every 50 to 100 metres, the sledge is stopped and a seismic vibrator (‘shaker’) extending from the bottom of the buggy generates low-frequency vibrations in the ground. These seismic waves penetrate the ice of the glacier and the underlying ground. Certain boundary layers reflect the waves back, and the reflections are registered by devices known as geophones. “There are a total of 480 geophones linked to 60 channels and spread at regular intervals along a 1.5-kilometre-long cable – the streamer – that we haul behind the buggy,” explains glaciologist Hofstede. “The data recorded allows us to draw conclusions on the features of the ground beneath the glacier.”

Drawing on this data, the subproject GHOST is intended to e.g. clarify how the glacier will behave with regard to a specific ridge below the ice, located ca. 70 kilometres inland from the current location of the grounding line. If the ever-faster-flowing glacier could be slowed or even stopped by the ridge, it could potentially delay the disintegration of Thwaites Glacier.

Video about the Thwaites Glacier