No abstract

ABSTRACT. W ave lets transmitted by short-pulse radar a re recovered from co ntinu ous profiles a nd used to d etermin e interfacial dielectric contrasts within the englacial a nd basal ice at the terminus area ofMatanuska Glacier, A laska, U.s.A. The field studies were in the abla tion regio n, where radar horizons co uld , at some point , be id entified with interfaces between clear ice and air, water or basal ice, and were perform ed in ea rl y spring before drain age full y deve loped. Th e profiles used closely spaced antennas with bandwidths ce ntered nea r 50 and 400 MHz. Transmitted wavelets reflected from interfaces of known dielec tri c contrasts are used to esta blish a phase reference for other events from interfaces between unknown contrasts. Migratio n and Fourier-transform filtering a re then applied to th e profil es a nd shown to recover th ese wavelets from diffractions a nd reflections . Interfacial dielec tric co ntras ts are d etermined from the relative phase of the wavelets. Near the terminus la ke, some basal ice events a bove 8 m depth are interpreted as voids. Further up-glacier, most englacial events are interpreted as voids, but d eeper localized reflectors a nd h orizons to 90 m d epth a nd within th e ba. a l zone are interpreted as voids , water or d eb ris. Ph ase cannot be determin ed for the basal-substr ate transition reflections. R eco mmendations are m ade for improving the wavelet recovery process a nd the qu ali ty of GPR migration.

We have used ground-penetrating radar to profile the depth to permafrost, to groundwater beneath permafrost, and to bedrock within permafrost in alluvial sediments of interior Alaska. We used well log data to aid the interpretations and to calculate dielectric permittivities for frozen and unfrozen materials. Interfaces between unfrozen and frozen sediments above permafrost were best resolved with wavelet bandwidths centered at and above 100 MHz. The resolution also required consideration of antenna configuration, season, and surface conditions. Depths to subpermafrost groundwater were profiled where it was in continuous contact with the bottom of the permafrost, except near transitions to unfrozen zones, where the contact appeared to dip steeply. The complexity of the responses to intrapermafrost bedrock, detected at a maximum depth of 47 m, appears to distinguish these events from those of subpermafrost saturated sediments. The relative dielectric permittivity ranged between 4.4 and 8.3 for the permafrost, and between 12 and 45 for partially to fully saturated, unfrozen silts and sands. Scattering losses are evident from intrapermafrost diffractions and from the improved penetration achieved by lowering the midband radar frequency from 100 to 50 MHz.