Closure of deep boreholes in ice sheets: a discussion

• Published in: Annals of glaciology Vol. 47; pp. 125 - 133
• Main Authors: Talalay, P.G., Hooke, Roger LeB
• Format: Journal Article
• Language: English
• Published: Cambridge, UK Cambridge University Press 2007
• ISSN: 0260-3055; 1727-5644
• DOI: 10.3189/172756407786857794

The most frequently used relation between ice deformation rate, έ, and stress, ፐ, is the power law, commonly called Glen’s flow law, έ = Aፐn , in which A is an ice stiffness parameter and n is an empirical constant. A can be estimated from the simple exponential relation where A 0 is a constant independent of temperature; E, commonly called the enhancement factor, depends on ice crystal orientation, impurity content and other factors; Q is the activation energy for creep; R is the universal gas constant; and T is the absolute temperature. Laboratory experiments yield values of A 0 = 9.514MPa–3a–1 for secondary creep. Typical borehole closure experiments then give E ≈ 0.16. This low value probably results from the fact that, when deforming into a borehole, ice is subject to stresses that are inconsistent with the preferred orientation of c axes that has developed over many years under a stress configuration with no borehole present. Closure data from Vostok hole 3G yield E ≈ 0.7. This higher value may reflect a unique stress environment yielding fabrics that are somewhat better oriented for borehole closure.