2015 JIRP Student Project: Radio-Glaciology measurements of the Juneau Icefield
Faculty experts: Seth Campbell, Shad O’Neel
Overview: Each year, annual “point” measurements of mass gain (accumulation of snow) and mass loss (ablation) are collected across the Juneau Icefield (JIF) to assess whether it is gaining or losing mass (a concept known as mass balance). These measurements are added to a 50+ year continuous record of mass balance on the Juneau Icefield. The primary goal of the radio-glaciology project is to incorporate geophysical measurements from ground-penetrating radar (GPR) into determining spatial variability of glacier snow, firn, and ice as they relate to mass balance of the JIF. We will use GPR to complete several objectives to include:
1. Spatially extrapolating point measurement of winter accumulation across the JIF.
2. Comparing winter accumulation determined from GPR data collected in 2012 with winter accumulation determined from GPR data collected in 2015.
3. Assessing dimension changes in firn layers buried below the winter accumulation by comparing GPR profiles collected in 2015 and 2012.
4. Assessing temporal changes in water content within the snow, firn, and ice
Level 1 students are not expected to continue their work beyond the summer field camp unless computations and write up are not completed during summer. Level 2 students should expect to continue to work on data analysis beyond the summer season, with a more detailed analysis and report turned in near the end of fall semester.
A. Snow accumulation. Snowpits will be excavated at several (15-25) established locations on Taku and Lemon Creek glaciers to the depth of the previous summer surface. In each pit a density profile will be computed and plotted as part of the annual mass balance program. The pits will be used as depth ground-truth for GPR profiles which are collected via snowmobile and/or ski on 5-50 km long transects across the icefield. The GPR profiles will be used to extrapolate point measurement snow pit winter mass balance information across the ice field. (Level 1&2). Level 1 students will provide accumulation thickness estimates from GPR and snow pit ground truth information. And qualitatively compare those measurements with similar data collected in 2012. Level 2 students will convert winter accumulation thicknesses to snow water equivalence while estimating uncertainties in SWE measurements from instrument errors and from melt by using a supplied degree day model.
B. Firn evolution. Approximately 150 km of GPR profiles were collected across Taku Glacier in 2012 and show multiple layers of firn below the winter accumulation. Here we propose to repeat collection of 2012 profiles in 2015 to compare firn layer dimensions and estimate changes relative to time (Level 1&2). Level 1 students will qualitatively infer dimensions changes of firn layers using minimal ground-truth. Level 2 students will attempt to quantitatively infer changes and incorporate multi-year snow pits (digging into and providing ground-truth at least into last year’s firn) into the study.
C. Snow Melt Study. Snow accumulated on a glacier surface in the winter experiences significant melt through the summer season in temperate glacier environments. As the surface snow melts, water percolates into deeper layers. We are interested in determining how much melt occurs and where the melt travels to over the time because water content and snow density both play significant roles in the calculation of snow water equivalence in a snowpack using GPR. Questions remain regarding how much melt stays within the winter snow pack after surface melt occurs and how much melt percolates into deeper firn and ice layers. Here we will use advanced geophysical techniques such as migration, common midpoint (CMP) and Wide angle refraction and reflection (WARR) surveys to estimate changes in water content relative to time. Available meteorological and snowpit data will be incorporated into this study to estimate meteorological impacts on melt and compare radar derived estimates of water content with field observations. (Level 2).
Timeline and logistics: These studies can be completed in conjunction with snowpit excavations during the mass balance studies (with 2-3 days/week spent in the field). The radar teams will use either snowmobile or skis to tow the radar systems for A&B. Study C will be performed at one easily accessible location near Camp 10 through the course of the program. For longer GPR transects, project members will travel to places where most students will not. Students should expect at least 1-2 days per week in camp processing data. New data will be collected, processed and preliminary interpretations made. Additionally, student reports will use other supplied data sets such as prior GPR profiles, meteorological, and snow pit data. Several software programs will be used for analyses including radar processing, GIS (e.g. ArcGIS), and programming software (e.g. MATLAB).
References (numbered by priority, i.e. study #1 first, #10 last):
(1) Woodward J and Burke MJ (2007) Applications of Ground-Penetrating Radar to Glacial and Frozen Materials. J. Environ. Engineering Geophys., 1(12), 69–85
(2) Bingham RG and Siegert MJ (2007) Radio-Echo Sounding Over Polar Ice Masses. J. Environmental and Engineering Geophysics, 1(12), 47–62
(3) Spikes VB, Hamilton GS, Arcone SA, Kaspari S, Mayewski, PA (2004) Variability in accumulation rates from GPR profiling on the West Antarctic plateau. Ann. Glaciol., 39(1), 238-244
(4) Kohler J, Moore J, Kennett M, Engeset R and Elvehoy H (1997) Using ground-penetrating radar to image previous years’ summer surfaces for mass-balance measurements. Ann. Glaciol., 24, 355-360.
(5) Arcone SA (2002) Airborne-radar stratigraphy and electrical structure of temperate firn: Bagley Ice Field, Alaska, U.S.A. J. Glaciol., 48(161), 317-334
(6) Arcone SA and Yankielun NE (2000) 1.4 GHz radar penetration and evidence of drainage structures in temperate ice: Black Rapids Glacier, Alaska, U.S.A. J. Glaciol. 46(154), 477-490
(7) Bradford JH, Harper JT, Brown J (2009) Complex dielectric permittivity measurements from ground-penetrating radar data to estimate snow liquid water content in the pendular regime. Water Resources Research. 45(8), 12 p