Monitoring water accumulation in a glacier using magnetic resonance imaging

Abstract

Abstract. Tête Rousse is a small polythermal glacier located in the Mont Blanc area (French Alps) at an altitude of 3100 to 3300 m. In 1892, an outburst flood from this glacier released about 200 000 m 3 of water mixed with ice, causing much damage. A new accumulation of melt water in the glacier was not excluded. The uncertainty related to such glacier conditions initiated an extensive geophysical study for evaluating the hazard. Using three-dimensional surface nuclear magnetic resonance imaging (3-D-SNMR), we showed that the temperate part of the Tête Rousse glacier contains two separate water-filled caverns (central and upper caverns). In 2009, the central cavern contained about 55 000 m 3 of water. Since 2010, the cavern is drained every year. We monitored the changes caused by this pumping in the water distribution within the glacier body. Twice a year, we carried out magnetic resonance imaging of the entire glacier and estimated the volume of water accumulated in the central cavern. Our results show changes in cavern geometry and recharge rate: in two years, the central cavern lost about 73 % of its initial volume, but 65 % was lost in one year after the first pumping. We also observed that, after being drained, the cavern was recharged at an average rate of 20 to 25 m 3 d −1 during the winter months and 120 to 180 m 3 d −1 in summer. These observations illustrate how ice, water and air may refill englacial volume being emptied by artificial draining. Comparison of the 3-D-SNMR results with those obtained by drilling and pumping showed a very good correspondence, confirming the high reliability of 3-D-SNMR imaging.

Subseasonal changes observed in subglacial channel pressure, size, and sediment transport

“…Pressurization of distributed (e.g., linked cavity) systems with inefficient drainage is thought to cause enhanced glacier sliding [Lliboutry, 1968;Iken, 1981;Kamb et al, 1985;Bartholomaus et al, 2008], while well-connected (e.g., channelized) systems forming during the melt season are thought to seasonally prevent sustained overpressurization [Röthlisberger, 1972;Schoof, 2010] and thus reduce sliding [Mair et al, 2002]. These conceptual models, though, are difficult to test since flow tracers [e.g., Stenborg, 1969;Hooke et al, 1988;Kohler, 1995], borehole pressure sensors [e.g., Mathews, 1964;Iken and Bindschadler, 1986;Hubbard et al, 1995;Murray and Clarke, 1995;Andrews et al, 2014;Schoof et al, 2014], and active seismic, resistivity, and radar imaging measurements [e.g., Vincent et al, 2012;Legchenko et al, 2014] only provide temporally and spatially limited observations. There are presently no observational methods that enable simultaneous constraints on channel geometry and water pressure.…”

Mechanisms of subglacial cavity filling in Glacier de Tête Rousse, French Alps

“…In September 2013, another intraglacial reservoir was found in the upper part of the glacier (Legchenko and others, 2014) in the vicinity of large crevasses located between 3220 and 3230 m a.s.l. (Fig.…”

“…Geophysical measurements were performed using surface nuclear magnetic resonance (SNMR; Legchenko and others, 2014). This method has been widely used in the exploration of groundwater (Legchenko and Valla, 2002) and twodimensional water-saturated formations (Boucher and others, 2006;Girard and others, 2007;Legchenko and others, 2008;Hertrich and others, 2009).…”

“…Nevertheless, using the 3-D SNMR measurements before pumping (Legchenko and others, 2014), the total volume is estimated and a residual volume after pumping is deduced. In this way and assuming a linear change of the cavity volume with time, the evolution of the volume of water within the cavity can be inferred from water-level changes.…”

“…Against all expectations, these studies revealed a subglacial lake (Vincent and others, 2012;Legchenko and others, 2014). The volume of water contained in the glacier was assessed at 53 500 m 3 , and data obtained from the boreholes and from surface nuclear magnetic resonance (SNMR) and ground-penetrating radar (GPR) measurements indicated that this water was contained in a single subglacial cavity.…”