Seismic Attenuation Illuminates Fluid Pathways in Glacial Ice

Abstract:

Water at the base of ice sheets has a significant impact on the speed at which they can move and on their potential for collapse, and identification of basal water is critical to a full understanding of their stability. In geothermal environments, seismic attenuation is known to be highly sensitive to the presence of fluids along faults and fractures. The goal of this study was to determine whether this can be adapted and applied to the ice-water contact within glaciers.

We used publicly available data archived at IRIS to address two key objectives: first using active source data, to identify the attenuation characteristics of glacial ice and its contrast with fractured and fluid-filled zones, and second to determine whether these features are observable using passive seismic arrays.

The attenuation of seismic energy is highly sensitive to the presence of fractures and water within glacial ice. The Qp/Qs ratio, in particular, is highly sensitive to porous regions where water is free to flow. Qp/Qs is typically larger than unity in hard, rocklike portions of the glacier, but drops below unity where fractures are common. We are able to identify these characteristic features within Antarctic glaciers. We see significant variation along the Beardmore glacier, with high values beneath the slow moving areas where the ice is solid throughout and much lower values within the dynamic region. Upstream, where the glacier is grounded, attenuation appears lowest in large crevasse-like structures, a few of which extend down to the base of the glacier, suggesting regions where the ice is most susceptible to sliding. At the terminus, where the glacier floats freely and is subject to tidal forcing, we see evidence of substantial fracturing extending throughout the glacial column. Significantly, this methodology works even when applied to passive seismic data. At the Whillans Ice Stream, we have excellent resolution of the subglacial lake along with what appear to be channels for the transport of water beneath the glacier.