More Than the Sum of Its Parts: Increased Information Content through a Combination of Ground-Penetrating-Radar and Seismic Methods on Temperate Glaciers.

Abstract:

Over temperate glaciers images obtained from ground-penetrating-radar (GPR) are often blurred because electromagnetic waves are scattered on water pockets or by complex glacial bed topography, or damped due to a higher overall water content within the glacier. A combination of seismic and GPR surveying can increase the data information content and aid interpretation of subsurface structure.

In September 2012 we acquired surface and borehole GPR and seismic data in the ablation zone of the Rhone Glacier located in central Switzerland. GPR data were acquired using antenna frequencies of 25, 50 and 100 MHz. Active reflection seismic data were recorded along a coincident profile across the glacier. Seismic waves were generated with small explosive sources spaced at 4m, and recorded on 30 Hz geophones at 2 m spacing. Both methods resulted in images showing a maximum depth of the glacier of approximately 130 m. However, the seismic image of the glacier bed was of much higher resolution and showed a clear primary reflection from the base, whereas the GPR image often showed several reflections of similar amplitude, above and from the bedrock interface, or no reflection at all. We interpreted a series of crenulations along the glacier bed reflector in the seismic image as melt water channels. This interpretation was supported by the intermittent nature of GPR glacier bed reflections, which are expected to be more sensitive to changes in water content than to the ice-rock interface.

First break travel time inversions of the surface seismic data yielded velocities of 3320 m/s near the top of the glacier, and remarkably constant values of 3720 m/s at depths below 4.5m. However, travel time inversion of seismic data between boreholes which penetrated as far as the glacier bed, indicate a 3D anisotropy of seismic velocity, ranging from 3650 m/s horizontally across the glacier to 3850 m/s horizontally along the line of the glacier. Vertical seismic velocity was found to lie between these values. The seismic anisotropy is most likely caused by preferred c-axis orientations of the ice crystals.