Abstract

To map the internal structure and lower boundary of an alpine rock glacier, we recorded three shallow seismic profiles and drilled four ∼70-m-deep holes through to the underlying bedrock. Although analysis of the seismic data using standard reflection processing schemes did not yield conclusive results because of the dominantly low-frequency returned signals and the presence of strong source-generated noise, tomographic inversions of first-arrival times were successful in mapping several critical subsurface features. A thin, low-velocity layer of loose boulders, air voids, and snow was found to extend across the entire surveyed area. Below this layer, two distinct velocity regimes superimposed on a general increase in velocity with depth were identified. A broad regime of high velocities was interpreted to contain boulders with numerous ice-filled voids, whereas an adjacent regime of relatively low velocities was explained in terms of boulders with air- and water-filled voids. This latter region of degraded permafrost, which was unexpected, may be an early result of global warming. The transition from rock glacier to gneissic bedrock was delineated approximately by the 4300-m/s isovelocity line. Although poorly resolved, its depth varied from ∼35 to ∼70 m beneath the surface of the rock glacier. The tomographic inversion results also provided an explanation for the occurrence of strong source-generated noise in our data.

Using the derived velocity distributions as input, synthetic shot gathers were calculated using a finite-difference viscoelastic scheme that accounted for surface topography. The resultant synthetic data were dominated by the overwhelming effects of back- and side-scattered waves from shallow heterogeneities and high-amplitude guided phases that originated from the thin, surficial low-velocity zone. Had the guided phases not been correctly identified and eliminated during processing, they would have appeared as reflections in the stacked sections. Since low-velocity layers overlying heterogeneous media are common features of the shallow subsurface, our findings may have implications for a broad range of engineering-scale seismic investigations (e.g., studies of landfills, rockslides, moraines, talus slopes, alluvial fans).

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