Monitoring water accumulation in a glacier using magnetic resonance imaging

Abstract

Tête Rousse is a small polythermal glacier located in the Mont Blanc area (French Alps) at an altitude of 3100 to 3300 m. Recent accumulation of melt water in the glacier was assumed to occur, but such accumulation had yet to be confirmed. Using 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 m³ of water. Since 2010, the cavern is drained every year. Using 3-D-SNMR, 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 the changes in cavern geometry and recharge rate: in two years, the central cavern lost about 73% of its initial volume, but 65% were 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 m³ d⁻¹ over the winter months and 120 to 180 m³ d⁻¹ in summer. These observations illustrate how ice and water 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

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

“…Between 2007 and 2010, new investigations were performed to check the potential existence of a subglacial water cavity in Glacier de Tête Rousse. 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.…”

“…Haeberli and others, 1989; Fountain and Walder, 1998; Richardson and Reynolds, 2000; Roberts, 2005; Bajracharya and Mool, 2009; Dobhal and others, 2013). Unlike proglacial and supraglacial lakes, englacial and subglacial water cavities cannot be detected easily (Legchenko and others, 2011, 2014; Vincent and others, 2012). For this reason, they can represent a major invisible threat in densely populated mountainous areas.…”

Seasonal Subsurface Thaw Dynamics of an Aufeis Feature Inferred From Geophysical Methods

“…Surface NMR has been used for almost three decades study groundwater (e.g., Trushkin et al, ) and cryosphere systems (e.g., Legchenko et al, ; Lehmann‐Horn et al, ), as well as freeze‐thaw dynamics in permafrost (Keating et al, ; Parsekian et al, , ). While GPR measurements allow fast acquisition of high‐resolution profile lines, surface NMR is typically implemented as a static measurement, with data averaging at one location to generate an individual sounding.…”

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 , ; Iken , ; Kamb et al , ; Bartholomaus et al , ], while well‐connected (e.g., channelized) systems forming during the melt season are thought to seasonally prevent sustained overpressurization [ Röthlisberger , ; Schoof , ] and thus reduce sliding [ Mair et al , ]. These conceptual models, though, are difficult to test since flow tracers [e.g., Stenborg , ; Hooke et al , ; Kohler , ], borehole pressure sensors [e.g., Mathews , ; Iken and Bindschadler , ; Hubbard et al , ; Murray and Clarke , ; Andrews et al , ; Schoof et al , ], and active seismic, resistivity, and radar imaging measurements [e.g., Vincent et al , ; Legchenko et al , ] only provide temporally and spatially limited observations. There are presently no observational methods that enable simultaneous constraints on channel geometry and water pressure.…”