Steven A. Arcone

Glaciohydraulic supercooling : a freeze-on mechanism to create stratified, debris-rich basal ice : I. Field evidence

Debris-laden ice accretes to the base of Matanuska Glacier, Alaska, U.S.A., from water that supercools while flowing in a distributed drainage system up the adverse slope of an overdeepening. Frazil ice grows in the water column and forms aggregates, while other ice grows on the glacier sole or on substrate materials. Sediment is trapped by this growing ice, forming stratified debris-laden basal ice. Growth rates of >0.1 m a -1 of debris-rich basal ice are possible. The large sediment fluxes that this mechanism allows may have implications for interpretation of the widespread deposits from ice that flowed through other overdeepenings, including Heinrich events and the till sheets south of the Laurentian Great Lakes.

Ground-penetrating radar reflection profiling of groundwater and bedrock in an area of discontinuous permafrost

We have used ground‐penetrating radar to profile the depth of 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 permafro...

Numerical studies of the radiation patterns of resistively loaded dipoles

Abstract The far field radiation patterns of finite-size resistively loaded horizontal electric dipoles lying on a low-loss dielectric half-space are computed. The patterns are a superposition of the solutions for transient infinitesimal dipole elements, where the transient waveform for each element is synthesized from the exact steady-state solution. The current excitation for each dipole element is a half cycle of a sine squared waveform that propagates along the line of elements at a speed that can be varied. No reflections from the antenna ends are assumed. The time duration of the current half cycle governs the full period of the dominant part of the transient radiated waveform. The amplitude of the dipole excitation current is determined by a cosine distribution that accurately simulates the resistive loading. The transient radiation patterns differ from those of a steady-state dipole mainly by being more narrow in the E-plane, as is true for a finite-size steady-state dipole radiating in air. In addition, the far field waveforms are slightly distorted in directions off vertical in the E-plane. Two-way power radiation patterns are presented for both conductive and non-conductive dielectric media in a footprint mode, i.e. the power response to an isotropic point scatterer on a subsurface flat plane. The radar footprint in a conductive dielectric shows very narrow beamwidth due to the added conductive attenuation along the longer paths off the vertical direction. Field tests in water and glacial ice and laboratory observations show good agreement with the E-plane model results but suggest that the H-plane directivity is strongly affected by the separation between transmit and receive antennas and by the range.

Variability in Accumulation Rates from GPR Profiling on the West Antarctic Plateau

Abstract Isochronal layers in firn detected with ground-penetrating radar (GPR) and dated using results from ice-core analyses are used to calculate accumulation rates along a 100 km across-flow profile in West Antarctica. Accumulation rates are shown to be highly variable over short distances. Elevation measurements from global positioning system surveys show that accumulation rates derived from shallow horizons correlate well with surface undulations, which implies that wind redistribution of snow is the leading cause of this variability. Temporal changes in accumulation rate over 25–185 year intervals are smoothed to along-track length scales comparable to surface undulations in order to identify trends in accumulation that are likely related to changes in climate. Results show that accumulation rates along this profile have decreased in recent decades, which is consistent with core-derived time series of annual accumulation rates measured at the two ends of the radar profile. These results suggest that temporal variability observed in accumulation-rate records from ice cores and GPR profiles can be obscured by spatial influences, although it is possible to resolve temporal signals if the effects of local topography and ice flow are quantified and removed.

Field observations of electromagnetic pulse propagation in dielectric slabs

The propagation of electromagnetic pulses in naturally occurring dielectric surface layers has been examined. Pulse duration used in field experiments reported here has been on the order of nanoseconds with pulse bandwidths in the high VHF to low UHF band. The layers were sheets of fresh water ice and granite at thicknesses ranging between .4 and 4 m. Both transverse electric (TE) and transverse magnetic (TM) modes were attempted but only the TE propagation could be interpreted. Analog recordings of wide-angle reflection and refraction (WARR) profiles were taken and recorded in a continuous graphic display. The displays allowed easy identification of phase fronts thereby facilitating study of of the dispersion of the pulses. The phase and group velocities of the wave-group packets agree well with the velocities predicted from dispersion curves derived from the modal waveguide equation. In one case the Airy phase of wave-packet propagation occurred. The best measure of the dielectric constant of the layer was the frequency of the air wave.

Propagation of a ground-penetrating radar (GPR) pulse in a thin-surface waveguide

Field observations are tested against modal propagation theory to find the practical limitations upon derivation of layer permittivities and signal attenuation rates from a radar moveout profile over two‐layer ground. A 65‐MHz GPR pulse was transmitted into a 30‐60‐cm‐thick surface waveguide of wet, organic silty to gravelly soil overlying a drier refracting layer of sand and gravel. Reflection profiles, trench stratigraphy, resistivity measurements, and sediment analysis were used to quantify the propagation medium and possible attenuation mechanisms.Highly dispersive modal propagation occurred within the waveguide through 35 m of observation. The fastest phase velocity occurred at the waveguide cutoff frequency of 30 MHz, which was well received by 100‐MHz antennas. This speed provides the refractive index of the lower layer, so the near‐cutoff frequencies must match a lower layer refraction. A slower, lower frequency phase of the dispersed pulse occurred at about 60–70 MHz, with an average attenuation ...

Fundamental and Higher Mode Inversion of Dispersed GPR Waves Propagating in an Ice Layer

Dispersion of ground-penetrating radar (GPR) waves can occur when they are trapped in a layer. In this paper, we analyze the modal propagation of GPR pulses through a layer of ice that is overlying water. Dispersed transverse electric (TE) waves that are trapped in the waveguide have larger amplitudes than the critically refracted waves that travel through air, whereas the transverse magnetic (TM) critically refracted waves traveling through air are more dominant than the trapped dispersed TM waves. This can be explained by the leaky waveguide behavior of the ice layer. The reflection coefficients for the waves incident on the ice-water interface show that the TM modes are more leaky than the TE modes. Still, clear dispersion is observed in both cases, which depends on the permittivity and thickness of the ice. Similar to inversion of dispersed Rayleigh waves, these parameters can be estimated by calculating phase-velocity spectra, picking dispersion curves, and inverting the dispersion curves using a combined local and global minimization procedure. Synthetic data show several higher order modes of which separate and combined inversions return the input modeling parameters accurately. Experimental data acquired on a frozen lake show strong dispersion for the TE and TM modes. The phase-velocity spectra of the field data show three TE and four TM modes of which separate and combined inversion of different modes return similar values for the ice thickness and known permittivity of ice. Due to the more leaky behavior of the TM modes, the TE inversion is better constrained and more suitable for inversion.

High resolution of glacial ice stratigraphy : A ground-penetrating radar study of Pegasus Runway, McMurdo Station, Antarctica

Ground-penetrating radar (GPR) has been used to detect areas of present or potential structural weakness beneath a 3.2-km snow-covered ice runway on the Ross Ice Shelf, Antarctica. The bandwidths of the transmitted wavelets were centred near 500 MHz. The data show many horizons up to tens of metres long and occurring to about a 9-m depth, below which a brine intrusion limits penetration. The horizons are interpreted as discrete scatterers because of their diffraction nature and loss of higher frequencies with depth. The presence of porous ice or dispersed water is interpreted from wavelet phase. The water may be associated with apparent deepening and fading of the brine horizon. If the above interpretation is correct, water occurs at depths to 3.5m and extends as much as 40 m horizontally, which is greater and deeper than known previously. At 3.5 m depth, the water may be adsorbed on mineral particles rather than remain free. Migration of the diffractions with a single-layer migration scheme shows all horizons above the brine layer to be small dielectric perturbations within the ice. Stacking and Hilbert transformation of the data reveal slight folding along the length of the runway. Loss of high-frequency amplitude in the wavelets suggests that higher frequency radar might improve resolution only in the top few metres.

Stratigraphic Variation Within Polar Firn Caused by Differential Accumulation and Ice Flow: Interpretation of a 400 Mhz Short-Pulse Radar Profile from West Antarctica

We investigate causes of the stratigraphic variation revealed in a 177 km, 400 MHz short- pulse radar profile of firn from West Antarctica. The profile covers 56 m depth, and its direction was close to those of the ice flow and mean wind. The average, near-surface accumulation rates calculated from the time delays of one radar horizon consistently show minima on leeward slopes and maxima on windward slopes, confirming an earlier study based on stake observations. The stratigraphic variation includes up to 30 m depth variation in individual horizons over tens of km, fold limbs that become progressively steeper with depth, and fold-hinge loci that change direction or propagate down-ice with depth over distances far less than predicted by the ice speeds. We use an accumulation rate model to show how local rate anomalies and the effect of ice speed upon a periodic variation in accumulation rate cause these phenomena, and we reproduce two key features seen in the stratigraphic variations. We conclude that the model provides an explanation of changes in spatial stratigraphy and local measures of accumulation history given the constraints of surface topography, ice and wind velocities, and a general accumulation rate for an area.

Stratigraphic Continuity in 400 Mhz Short-Pulse Radar Profiles of Firn in West Antarctica

Abstract We track dated firn horizons within 400 MHz short-pulse radar profiles to find the continuous extent over which they can be used as historical benchmarks to study past accumulation rates in West Antarctica. The 30–40cm pulse resolution compares with the accumulation rates of most areas. We tracked a particular set that varied from 30 to 90 m in depth over a distance of 600 km. The main limitations to continuity are fading at depth, pinching associated with accumulation rate differences within hills and valleys, and artificial fading caused by stacking along dips. The latter two may be overcome through multi-kilometer distances by matching the relative amplitude and spacing of several close horizons, along with their pulse forms and phases. Modeling of reflections from thin layers suggests that the – 37 to – 50 dB range of reflectivity and the pulse waveforms we observed are caused by the numerous thin ice layers observed in core stratigraphy. Constructive interference between reflections from these close, high-density layers can explain the maintenance of reflective strength throughout the depth of the firn despite the effects of compaction. The continuity suggests that these layers formed throughout West Antarctica and possibly into East Antarctica as well.