Austin Kovacs

The in-situ dielectric constant of polar firn revisited

Abstract The success in using VHF and UHF frequency systems for sounding polar ice sheets has been tempered by an uncertainty in the in-situ dielectric constant which controls the effective velocity of an electromagnetic wave propagating in an air-ice mixture. An empirical equation for determining the relative real dielectric constant ϵ′r vs. density (specific gravity ϱ) of firn or ice was proposed in 1969 by Robin et al. where ϵ′r = (1 + 0.851 ϱ)2. However, this expression has met with uncertainty because wide-angle radar refraction sounding techniques have produced dielectric constant values that are lower than Robins equation predicts. This paper discusses radar soundings made on the McMurdo Ice Shelf, Antarctica, and compares the resulting dielectric constant determinations with Robins equation, laboratory measurements on firn and ice and other expressions given in the literature for determining ϵ′r vs. the specific gravity of dry firn and ice. Our findings indicate that the form of Robins equation is valid. Our analysis also indicates the expression could be slightly improved to read ϵ′r = (1+0.845ϱ)2. Reasons are suggested as to why previous wide-angle radar sounding studies did not reproduce Robins findings.

Shore ice pile-up and ride-up: Field observations, models, theoretical analyses

Abstract A review of the literature on shore ice pile-up and ride-up in arctic and subarctic waters is presented, along with an account of recent observations made by the authors. Cross-sectional profiles of these features are presented from which models and theoretical analyses were made. The expressions derived give the force required to overcome gravitational potential and friction occurring during ice-piling and ride-up. It was estimated that the distributed force required during ice-piling or ride-up was of the order of 10 to 350 kPa (about 1.5 to 50 psi). Field observations revealed that shore ice pile-up or ride-up appears to occur within a period of less than 30 minutes, at any time of year but most often in the spring and fall. Pile-up seldom occurs more than 10 m inland from the sea but ride-up frequently extends 50 m or more inland, regardless of ice thickness. While steeply sloping shores do not favor ice ride-up, sea ice has mounted the steep, 9-m-high bluff at Barrow, Alaska, destroying structures and taking lives.

Electromagnetic properties of sea ice

Abstract Investigations of the in situ complex dielectric constant of sea ice were made using time-domain spectroscopy. It was found that (1) for sea ice with a preferred horizontal crystal c -axis alignment, the anisotropy or polarizing properties of the ice increased with depth, (2) brine inclusion conductivity increased with decreasing temperature down to about −8°C, at which point the conductivity decreased with decreasing temperature, (3) the DC conductivity of sea ice increased with increasing brine volume, (4) the real part of the complex dielectric constant is strongly dependent upon brine volume but less dependent upon the brine inclusion orientation, (5) the imaginary part of the complex dielectric constant was strongly dependent upon brine inclusion orientation but much less dependent upon brine volume. Because the electromagnetic (EM) properties of sea ice are dependent upon the physical state of the ice, which is continually changing, it appears that only trends in the relationships between the EM properties of natural sea ice and its brine volume and brine inclusion microstructure can be established.

Airborne electromagnetic sounding of sea ice thickness and sub-ice bathymetry

Abstract A study was made in May 1985 to determine the feasibility of using an airborne electromagnetic sounding system for profiling sea ice thickness and the sub-ice water depth and conductivity. The study was made in the area of Prudhoe Bay, Alaska. The multifrequency airborne electromagnetic sounding system consisted of control and recording electronics and an antenna. The electronics module was installed in a helicopter, and the 7 m long tubular antenna was towed beneath the helicopter at about 35 m above the ice surface. For this electromagnetic system, both first-year and second-year sea ice could be profiled, but the resolution of ice thickness decreased as the ice became rough. This decrease was associated with the large footprint of the system, which effectively smoothed out the sea ice relief. Under-ice water depth was determined, as was seawater conductivity. The results of the feasibility study were encouraging, and further system development is therefore warranted.

Electromagnetic measurements of multi-year sea ice using impulse radar

Abstract Sounding of multi-year sea ice, using impulse radar operating in the 80- to 500-MHz frequency band, has revealed that the bottom of this ice cannot always be detected. This paper discusses a field program aimed at finding out why this is so, and at determining the electromagnetic (EM) properties of multi-year sea ice. It was found that the bottom of the ice could not be detected when the ice structure had a high brine content. Because of brines high conductivity, brine volume dominates the loss mechanism in first-year sea ice, and the same was found true for multi-year ice. A two-phase dielectric mixing formula, used by the authors to describe the EM properties of first-year sea ice, was modified to include the effects of the gas pockets found in the multi-year ice. This three-phase mixture model was found to estimate the EM properties of the multi-year ice studied over the frequency band of interest. The latter values were determined by: (1) vertical sounding to a subsurface target of known depth, where the two-way travel time of the EM wavelet in the ice is measured; (2) cross-borehole transmission, where the transit time of the EM wavelet is measured through a known thickness of sea ice; and (3) a wide-angle or common-depth-point reflection method. Preliminary findings also indicate that a representative value for the apparent bulk dielectric constant of multi-year sea ice over 2 1 2 m thick is 3.5. This represents an effective EM wavelet velocity of 0.16 m/ns, which may be used to estimate multi-year sea ice thickness in cases where the ice bottom is detected in ice profile data.

Estimating the full‐scale flexural and compressive strength of first‐year sea ice

Sea ice salinity, density, and temperature data were used to develop new methods for determining the bulk brine volume and porosity of sea ice floes. Methods for estimating full-thickness ice sheet strength, based on large-scale field tests, are presented. The relationships among bulk sea ice properties, strain rate, and strength are illustrated. A new constitutive equation was developed for predicting the full-thickness horizontal compressive strength σc of first-year sea ice as a function of the applied strain rate and bulk porosity in the form σc=B2e˙1/nϕBm where parameters B2, n, and m are about 2.7×103, 3, and −1, respectively, and e˙ and ϕB are the ice strain rate and ice floe bulk porosity of sea ice, respectively. An example of the first-year sea ice indentation force against a 90-m-wide structure is given. Estimating sea ice strength based on remote ice conductivity measurements is also discussed conceptually.

Modeling the electromagnetic property trends in sea ice, part I

Abstract Two phase dielectric mixing model results are presented showing the electromagnetic (EM) properties of sea ice vs. depth. The modeled data are compared with field measurements and show comparable results. It is also shown how the model data can be used in support of impulse radar remote sensing of sea ice. Examples of the remote measurement of sea ice thickness using impulse radar operating in the 80 to 300 MHz frequency band are presented and discussed.

The Brine Zone in the McMurdo Ice Shelf, Antarctica

Abstract : Observations of a 4.4-m-high brine step in the McMurdo Ice Shelf, Antarctica, show that it has migrated about 1.2 km in 4 years. The present brine wave is overriding an older brine-soaked layer. This migration is proof of the dynamic nature of the step, which is the leading edge of a brine wave that originated at the shelf edge after a major break-out of the McMurdo Ice Shelf. The inland boundary of brine penetration is characterized by a series of descending steps that are believed to represent terminal positions of separate intrusions of brine of similar origin. The inland boundary of brine percolation is probably controlled largely by the depth at which brine encounters the firn/ ice transition (43 m). However, this boundary is not fixed by permeability considerations alone, since measurable movement of brine is still occurring at the inland boundary. Freeze-fractionation of the seawater as it migrates throught the ice shelf preferentially precipitates virtually all sodium sulfate, and concomitant removal of water by freezing in the pore spaces of the infiltrated firm produces residual brines approximately six times more concentrated than the original seawater.

Chemical fractionation of brine in the McMurdo Ice Shelf, Antarctica

Abstract : During the austral summers of 1976-77 and 1978-79, several ice cores were taken from the McMurdo Ice Shelf brine zone to investigate its thermal, physical and chemical properties. This brine zone consists of a series of superimposed brine layers (waves) that originate at the seaward edge of the ice shelf and migrate at various rates, depending upon their age and position in the ice shelf. The brine in these layers becomes increasingly concentrated as the waves migrate inland through the permeable ice shelf firn. Chemical analyses of brine samples from the youngest (uppermost) brine wave show that it contains sea salts in normal seawater proportions. Further inland, deeper and older brine layers, though slightly highly saline (S200%), are severely depleted in (S04)2- /Na+ ratio being an order of magnitude less than that of normal seawater. Analyses of Na+, K+, Ca2+, Mg2+, (So4)2- and C1-, together with solubility and temperature considerations, show that the sulfate depletion is due to selective precipitation of mirabilite, Na2S041OH2O. The location of the inland boundry of brine penetration is closely related to the depth at which the brine encounters the firn/ice transition. However, a small but measurable migration of brine is still occurring in otherwise impermeable ice; this is attributed to eutectic dissolution of the ice by concentrated brine as it moves into deeper and warmer parts of the McMurdo Ice Shelf.

Sea ice thickness versus impulse radar time-of-flight data

Abstract Two second-year sea ice floes were probed using “impulse” radar sounding and direct drilling methods. The resulting two-way time-of-flight of the impulse radar EM wavelet, traveling from the surface to the ice “bottom” and back to the surface, was compared with snow and ice thickness data obtained from a drill hole. From this comparison, simple relationships are presented that provide an estimate of the thickness of sea ice, between about 1 and 8 m thick, with or without a snow cover. Relations are also presented that show the bulk or apparent dielectric constant of the ice floes versus ice thickness, again with or without the snow cover. The data revealed that the apparent dielectric constant of the sea ice decreased with increasing ice thickness from a value of about 7 for ice 1 m thick, to about 3.5 for ice 6 m thick.