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formation of permafrost. Thus, given a not very thick cover of loose materials with lower thermal conductivity occurring over more heat-conductive crystalline rocks Ah the maximum permafrost thickness (for the period of development) will be greater than that of uniform loose sedimentary formations. And on the contrary, if the rocks with high values of thermal conductivity Ah (for example, effusion rocks) occur over sediments with low thermal conductivity (for example, clays or silty clays) the thickness of the lithologically two-layered permafrost will be less than that of the uniform profiles with Ah. Crystalline bedrock, having greater thermal conductivity than loose materials, at the same depth from the surface (for example, at the depth of 100-200 m) always has higher temperature. In this connection the permafrost thickness (in the conditions of a platform) increases with the dip of the bedrock surface as a whole.

Effect of structure on the permafrost thickness

Redistribution of the geothermal flux as a result of inhomogeneity of the structure and thickness of the sedimentary covers significantly affects the thickness of the permafrost. Increased values of the heat flux are observed above the vaults of anticline structures while there is a decrease above the syncline structures. Such distribution of heat fluxes is explained by the fact that where folding occurs, in addition to general vertical upward heat flux additional heat transfer from depressions to vaults of the anticline structures takes place. Additional heat transfer is associated here with differences in thermal resistance of the sedimentary rocks along the bedding and across it. Values of the geothermal gradient in such situations, for example, on the north of the Western Siberian Plate, can be 4-5 times greater than that where the bedding is horizontal, according to V.V. Baulin. The effect is to decrease permafrost thickness in structural vaults by 100-200 m. Thermal convection in water-bearing horizons and water circulation in strata can increase as well as attenuate thermal anisotropy of local structures. Thus the dependence of the permafrost thickness on the structure and thickness of loose sedimentary strata takes various forms.

Effect of the hydrogeologicalfactor

Development of permafrost always takes place in a dynamic thermal interaction with ground water. The several effects of the latter on the depth of perennial freezing is evident in various hydrogeological structures. The effects are associated with the particular conditions of supply, regime and discharge of the water-bearing horizons, typical of these structures. In the general case, the movement of fresh ground water having positive temperatures creates positive anomalies of the heat flux and increases the geothermal gradient below the permafrost. The change of the geothermal flux on account of the seepage of ground water along a bed can be calculated from M.M. Mitnik's formula:

where Aiw is the temperature deviation of water entering the bed; x is the distance from the place of water entrance to the place of observation; t is the normal temperature of the bed; iob is the observed temperature; n = 2a[mAz(v + ^Jv2 + 4a2/(mAz))])]; Az is the depth of the bed occurrence; m is the bed thickness; v is the seepage rate; a is the thermal diffusivity of the rock.

Accounting for the upward or downward movement of water can be done by way of adding the molecular heat (conductive) flux and the flux due to the seepage v{Cpx (convective), where v( is the seepage rate; Cpx is the heat capacity of water per unit volume. Then when the water is moving upward the geothermal gradient should decrease depending on the rate of water movement, heat capacity, density and thermal conductivity of the waterbearing bed, according to N.A. Ogilvy. It follows that the rate of water exchange plays an important role during the establishment of the permafrost thickness, all other factors being equal. The movement of stagnant ground waters has a minimal effect. High-temperature water associated with deep faults and artesian water rising from great depths have important warming effects on the permafrost. The thickness of permafrost along thick water-saturated zones of faulting is as a rule less than that within adjacent undisturbed rock.

There is a possibility of an abnormally decreased heat flux as a result of horizontal seepage of brines (cryopegs) colder than the surrounding permafrost. Such conditions occur within the Anabar anticline where the maximum thicknesses of permafrost (for the territory of the former USSR) are noted.

Effect of gas reservoirs

The effect of gas reservoirs on the thickness of the permafrost shows itself most often when adiabatic expansion take place, which may lower the rock temperature by up to 5°C. The most favourable conditions for this process occur within the zones of increased jointing, facilitating gas penetration through the joints. This effect is typical, for example, of reservoirs having a bed temperature of 18-21 °C.

Under certain conditions, interaction between natural gases and underground waters occurs in association with formation (and conversely, with disintegration) of natural gas hydrates. As a great amount of heat is released during their formation, while the same amount of heat is absorbed during their disintegration, such thermal effects can cause an increase or decrease of the permafrost thickness as well as appropriate changes in temperature regime of the rocks occurring above and below the zones of hydrate formation. It should be noted that the heat of 'water-ice' phase transformations is 334 x 103 J kg"1, while during the formation of natural gas hydrates 520540 x 103 J kg-1 of heat is released, i.e. the thermal effect of the latter process can be highly significant in the course of formation of the permafrost thickness.

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