Fig. 8.1. Temperature dependence of specific electrical resistance, of frozen earth materials: (a) - rocks (according to M.S. King): 1-3 - sandstones of different moisture contents; 4 - clayey schists; (b) - soils (according to A.D. Frolov): 1 - sand with massive cryotexture; 2 - the same, with schlieren structure;
3 - sandy-silty materials and silty-clay materials with massive cryotexture;
state is the decreasing amount of the current-conducting component - the unfrozen water, with stretching and narrowing of conducting paths and their intermittent nature because of ice segregation in the material. Thus, the resistivity value of frozen ground is dependent on factors that determine the amount and pattern of unfrozen water distribution i.e., on composition, temperature, water (ice) content, salinity, cryogenic texture and the like (Fig. 8.1). With negative temperatures the resistivity may increase by several orders in a narrow temperature interval; the most drastic increase of values is observed in the interval of essential phase transition.
Dielectric constant (e) of soils determines their capacity to polarize under the impact of an alternating electromagnetic field on account of the ordered orientation of bound electric charges available. Permittivity of the frozen materials is dependent on dielectric properties of their components. Relative dielectric permittivity of a gas component is equal to 1, as is that of a vacuum, while for the majority of rock-forming minerals it does not exceed 10. For free pure water e is equal to about 80, i.e. an order higher than that of minerals, which explains the substantial influence of moisture content on the permittivity of earth materials. The dielectric constant of ice within the range of high frequencies (over 104-105 Hz) can be much lower than that of water.
With lower (negative) temperatures soil permittivity diminishes in general (Fig. 8.2) in conformity with the lower content of unfrozen water at lower temperature and with reduced e values of the bound unfrozen water.
Frozen earth materials show imperfect elasticity. Under external e e
Fig. 8.2. Temperature dependence of dielectric constant of frozen soils of different composition and moisture content at/ = 106 Hz (according to B.N. Dostovalov): 1 - clay WM = 35.5%; 2-3 - sand with fVtot = 9% and 3%, respectively.
dynamic loads various elastic vibrations arise in them including longitudinal, transverse, surface-type and the like, differing by the pattern of displacement of surrounding particles. One of the dynamic methods to study elastic properties of frozen soils in natural situations and, mainly, in samples, is the acoustic (ultra-sound) method.
The most important acoustic parameters of rocks are the velocities of elastic wave propagation: Vp - longitudinal, Vs - transverse, VR - surface. Velocities of longitudinal wave propagation have been studied most thoroughly.
As earth materials progress into the frozen state, velocities of propagation of elastic waves increase, being determined primarily by phase transitions at freezing and the development of the new component - ice. Ice is characterized by much greater velocities (Vp = 3500-4000m s_1) than water in a liquid phase (V = 1450m s_1). The velocity characteristics of frozen soils are also influenced by the type of ice segregation, although to a lesser degree than electric properties.
Taking into account the above velocities of elastic wave propagation in the frozen soils one may conclude that these are dependent on all the factors determining the amount and form of ice segregation and content of unfrozen water, namely, mineral composition and granulometry, porosity, water content, salinity of interstitial moisture, temperature, etc. (Fig. 8.3).
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