The level of the Grimsvotn subglacial lake, Vatnajokull, Iceland, monitored with SPOT5 images

We describe the vertical displacement field of an ice shelf floating on a subglacial lake, Grimsvotn, located underneath the Vatnajokull ice cap (Iceland). The uplift is measured using the correlation of two satellite optical SPOT5 images acquired 5 days apart with similar, non-vertical incidence angles. This is the first time correlation of optical images has been used to measure vertical displacements. Our technique is suitable for mapping short-term elevation changes of glaciers. If the surface features are preserved, vertical displacements can be measured every 25 in with an accuracy of about 0.5 m. The uplift map of Grimsvotn shows that 10.9 (+/- 1) km(2) of ice was floating between 11 and 16 August 2004. The ice shelf rose by 1.7 (+/- 0.6) m indicating that the volume of liquid water in the lake increased by 0.018 (+/- 0.007) km(3). Our field observations show that surface melting due to meteorological processes contributed 70% of the accumulated water, hence, the rest originated from ice melted by the subglacial geothermal activity. The power required to melt 0.005 km(3) (water equivalent) of basal ice in 5 days is 4000 MW. The applicability of the technique can be extended to volcanology and seismology, and even landslides or subsidence, when finer-resolution optical images become available. Applied to two pairs of images, it could solve for the 3-dimensional displacements of the Earth's surface. (c) 2005 Elsevier B.V All rights reserved.