Solid, water, gas, and ice in frozen porous media: Measurements and implications.

摘要

Porous media such as soils, rocks or sediments are multiphase systems characterized by three phases: solid, gas and liquid. A fourth phase, ice, is add to the system when the material is in a frozen state. Knowledge of the dielectric properties of frozen porous material is important for mapping and exploration in many areas of the world, situated at high latitudes or altitudes, where soils can be frozen for a substantial part of the year. Moreover the relationship between the unfrozen liquid content in a porous medium and its temperature, the freezing characteristic, can be used to derive the moisture characteristic, which is an important hydraulic property, both above and below 0°C. An instrument, which measures the freezing characteristic, was developed and tested as part of this study. Temperatures and unfrozen water contents were measured, and the temperatures were converted to water potentials. Results showed good agreement between the developed method and vapor pressure methods used for comparison and validation. To measure the water content of undisturbed samples an alternative, non-invasive, spiral-shaped Time Domain Reflectometry (TDR) probe was designed and tested. The probe was tested (1) experimentally using a Tektronix cable tester and media of different dielectric properties and (2) numerically by simulating the electromagnetic field using the Laplace equation.; The dielectric behavior of frozen material is highly affected by its ice content. The measurement of ice content using TDR technique is complicated, because at the operational frequencies of the TDR, ice has dielectric permittivity values which are very close to the values for the solid phase, making ice partially invisible to TDR analysis. At lower frequency, however, ice has higher dielectric permittivity because it undergoes relaxation in the kHz range. To elucidate the possibility of measuring ice content in porous material, dielectric spectroscopy of three frozen soils were performed in the low frequency range. An AC bridge was used for measurement of the soil dielectric properties and a nitrogen cryostat was employed for temperature control. The higher dielectric permittivity of ice at low frequencies allowed us to differentiate between liquid water and ice in the three soils tested.