Rexford M. Morey

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.

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.

Sounding sea ice thickness using a portable electromagnetic induction instrument

Field trials using a man‐portable, commercially available, electromagnetic induction (EMI) sounding instrument, with a plug‐in data processing module for the remote measurement of sea ice thickness, are discussed. The processing module was made to allow for the direct determination of sea ice thickness and to show the result in a numerical display. The processing module system was capable of estimating ice thickness within 10 percent of the the true ice value for ice from about 0.7 to 3.5 m thick, the thickest of undeformed ice in our study area. However, since seawater under the Arctic pack ice has relatively uniform conductivity (2.55 ± 0.05 S/m), a simplified method can be used for estimating sea ice thickness using just an EMI instrument. This technique uses only the EMI conductivity measurement, is easy to put into use, and does not rely on theoretically derived look‐up tables or phasor diagrams, which may not be accurate for the conditions of the area.

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.

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.

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.