Firn and Shallow Ice Profiling at Jakobshavn Glacier Using Dispersed Seismic Surface Waves

Seismic refraction methods are used commonly to determine firn and shallow ice acoustic properties. However, this practice is hampered by the difficulty in generating shear waves in firn and by the presence of dispersed compressional waves interfering with shear wave arrivals. Conventional reflection surveys used to image the ice-bed interface and subglacial geology contain surface waves that are typically discarded as noise. Surface waves propagate within one wavelength from the surface and their dispersive characteristics can be exploited to estimate shear wave velocities of the near-surface. We employed the Multichannel Analysis of Surface Waves (MASW) method along a 10 km-long reflection line acquired on Jakobshavn Glacier, Greenland, to map the velocity structure of firn and shallow ice. Seismic data were acquired using twenty-four 28 Hz vertical geophones spaced 20 m apart and 0.5 kg of buried explosives at 10 m depth. Rayleigh wave dispersion curves along the 10 km line show phase velocities ranging from 1000 m/s at 100 Hz frequency to 1800 m/s at 8 Hz frequency. The 10 km long continuous surface wave velocity profile indicates near-surface shear wave velocity estimates increasing rapidly with depth, from 1050 m/s to 1950 m/s. Lateral velocity variations reveal low relief structures that correlate with ice internal layering imaged by a coincident ground-penetrating radar profile. Preliminary analysis of surface waves contained in the seismic reflection line yielded usable dispersion curves and a velocity profile consistent with the expected velocity structure near the surface of Jakobshavn Glacier. Use of surface wave methods is suggested as an alternative to refraction surveys for firn and shallow ice imaging.