Snow streamer (Photo: Olaf Eisen, Alfred-Wegener-Institut)

Subglacial conditions play a key role in the most dynamic parts of the ice sheet: ice streams, outlet glaciers and tidewater glaciers. These fast glaciers drain large quantities (90%) of the ice sheet in to the ocean. Seismic data provides the subglacial boundary condition of these dynamic parts of ice sheets. We also successfully use seismics to map and characterize conditions below ice shelves. This is an area where marine seismic data and land seismic data connect. The seismic acquisitions set-up developed by AWI fills this important niche in seismic data.

The terrain and target depth pretty much dictates the use of the seismic equipment. Since 2009 we have been experimenting and constantly improving our equipment. For larger surveys at the ice sheet or shelf where the ice is thick and surface conditions flat we use the larger 1500m streamer and a larger vibrator towed by a tracked vehicle, a Pisten-Bully. Like this we collected more than 400km of seismic data on the sheet-shelf transition of Antarctica. In more difficult terrain like the ablation zone of the Greenland Ice Sheet equipment often has to be transported by a helicopter or plane. We used the 300m snowstreamer and mini-vibrator or explosives for deeper targets. We pulled the streamer by skidoo or ourselves depending on the difficulty of the terrain. Using the mini-vibrator and spiked geophones we occasionally work on Alpine Glaciers, the most spectacular being a 4500m high Alpine saddle, Colle Gnifetti.

Gimballed geophone (Photo: Johannes Bondzio, Alfred-Wegener-Institut)

Seismic data acquisition (seismics) is a technique to generate and record elastic or acoustic waves in the subsurface. By determining the location of seismic reflections, we create a structural map of the subsurface. We also determine physical properties of the subsurface materials, partly from reflections, partly from refractions. For this you need a source and receivers. As we work on snow and ice we use specialized seismic equipment. We use explosives or a vibrator as a source. We have a large force P-wave vibrator, a Failing Y1100, for deeper targets, a medium force P-wave vibrator buggy, the Envirovib and a small force mini-vibrator Elvis generating both P and S-waves. For receivers we use a streamer (transmission cable) consisting of gimballed (cardanic) mounted receivers (geophones). This streamer can be towed and the receivers automatically position themselves correctly. Next to this we occasionally use spiked geophones. We have geophones to record either P or S waves both have their own advantage. For deeper targets we have a 1500m long streamer and for shallower targets we have a 300m streamer. The streamer we pull either by skidoo or snow cat. This makes the seismic recording operation similar to marine seismic data collection.

Seismics in west Greenland (Photo: Johannes Bondzio, Alfred-Wegener-Institut)
Difficult terrain (Photo: Coen Hofstede, Alfred-Wegener-Institut)

Seismic waves penetrate through the ice sheet or shelf and are as such able to provide information on the subglacial conditions. This is a big unknown of the large ice sheets Antarctica and Greenland where up to recently only small seismic surveys have been performed. AWI developed a set-up allowing us to collect long transects of seismic data easy, fast and with a small team.