The methods and carryingout of geocryological surveys

Carrying out geocryological surveys at any scale includes three stages: preliminary, field and office work.

The preliminary stage begins with the drawing up of the work program, laying out the main problems of the survey to be conducted. For its realization field observations and published materials such as topographic maps, aerial photographs at large and small scale (black and white, colour and spectral range) and satellite photographs (when the small-scale survey is conducted), geological and tectonic reports and maps, descriptions of geological sections and exposures, geomorphological maps and Quaternary deposit maps, geobotanical maps, climatic and hydrogeological data are gathered and studied. The study of earlier geocryological, hydrogeological, engineering-geological and geophysical investigations showing the character of the geocryological surroundings, plays a special role. At the same time the reliability and representativeness of the materials of the previous investigations carried out during various periods of time, at various scales and usually using various methods, are established.

On the basis of this analysis and preliminary interpretation of aerial photos and satellite photos (for the small-scale survey) the preliminary (geological-geographical) map of the landscape region is compiled. Then the factual data characterizing distribution, structure, occurrence, thickness, temperature regime of the permafrost, etc. are entered on the landscape map. All this allows one to get an idea of geocryological conditions and of the level of knowledge about the area, to determine the possible interval of change of geocryological characteristics, the program of field-work necessary and the most rational methods and degree of complexity of investigations as well as the plan for, and the problems of, reconnaissance, aerial-visual and correlation (control) exploration during the preliminary period.

In the absence of such data it is necessary to study general maps to identify the regional geocryological background and to get an idea about the geocryological conditions of the territory under study. The preliminary assessment of the effect of natural factors on the ground temperature regime contributes to this.

On the basis of the maps obtained and materials studied the key areas are considered and the requirements for drilling and sampling, for geophysical observations and for observations in the laboratory and under special conditions, and other types of work within key areas are drawn up. A study of experience with development is also drawn up; the extent of ground sampling, the volume of surface and ground waters and the content and extent of calculation work using a computer are determined.

The main problems of the field work period are to study the dependence of each geocryological characteristic on components of the natural environment and to determine general and regional characteristics of the formation and spatial variability of geocryological conditions. The main study is carried out within key areas and then it is supplemented with the help of exploratory investigations using aerial photos, geocryological, geological, geomorphological, geobotanic, microclimatic and geophysical methods for special purposes, etc. As the materials are obtained the preliminary map of the landscape regions is improved, columnar sections are made from bore holes, exposures and mine openings, field geocryological profiles are drawn and calculations for revealing the interdependence of factors in the natural environment and geocryological characteristics are performed. All the data are plotted on the map of record or on the map of landscape regionalization. The latter is more rational because the map of regionalization takes geocryological content into account and becomes the basis for geocryological mapping.

Within the key areas geological formations and distribution of geological-genetic rock complexes, their composition and properties are studied. Ice content and study of cryogenic structure, revealing syn- and epicryogenic ground freezing are a major concern. In addition, ground temperature measurements and observations of seasonal freezing and thawing depth are carried out, the permafrost thickness is determined, and the regime and temperature of groundwater in bore holes and in springs are studied. Water sampling for chemical and spectral analyses, and sampling of rocks with disrupted and undisrupted textures (including frozen rock massifs) for laboratory studies of their composition, thermal physical, physical-mechanical and permeability properties are performed. Observations of special regimes, thermal-moisture field dynamics and of the development of geocryological processes are carried out.

Studies of experience with construction, with thermal amelioration of the ground and of the efficiency of environmental protection measures are a major preoccupation. Disturbance of natural conditions and consequences of the associated processes are thoroughly studied and charted. All these measures help to improve the calculation of forecasts for the conditions.

The result of exploratory and detailed investigations within key areas are used for the regional classification of seasonally and perennially frozen ground, of taliks and exogenous geological processes and phenomena and of groundwater. Then on the basis of these classifications and geocryological sections the geocryological maps on a larger scale than that of the survey are compiled for the key areas.

The field work period is completed with preparation of the geocryological maps of the whole area under study on the basis of the landscape regionalization maps for key areas. These maps are supplemented with summary tables showing characteristics of permafrost, taliks and the sea sonally thawed layer, profiles, plots representing the dependence of certain characteristics on each factor of the natural environment and other explanatory materials.

Special methods are used to get specific geocryologicalinformation during the field work period. Thus, for example, the palaeopermafrost analysis and palaeopermafrost reconstruction method is used for studying the formation, evolution and dynamics of the permafrost. The method of permafrost facies (cryolithological) analysis combined with the geological survey facilitates study of the Quaternary deposits in the permafrost parts. The landscape indicators help to establish the dynamic series of landscapes associated with development of specific complexes of geocryological processes.

During the office work period researchers complete the processing and analysis of field and laboratory investigations and of the observations of regime, classify the permafrost-geological sections, use laboratory data to establish characteristics of the composition and properties of the permafrost of different origin and age and determine the absolute cryogenic age of syn-and epicryogenic materials, and make clear the genesis and special features of talik zones, and the development of various cryogenic phenomena. Using computer calculations they make clear the effect of each factor of the natural environment on the mean annual temperature and depth of seasonal ground thawing and freezing, assess the interaction of the permafrost with groundwater and the spatial variability of the permafrost thickness, and make the natural-historical geocryological forecasts. Then they improve regional classifications of permafrost and taliks, compile the final geocryological maps for natural and predicted conditions, perform final geocryological (engineering-geological) regionalization of the territory and give the region-to-region description of geocryological conditions. In conclusion the engineering-geocryological assessment of the territory is performed and recommendations for deliberate changes of natural conditions for optimizing the regime of the geocryological environment are prepared. The results of the investigations, their substantiation and scientific conclusions are given in the scientific report including a text, maps, graphical and textual data supplements. The text of the report should include also the methods of map compilation and analysis, permafrost forecasting, engineering-geological assessment of the area and recommended measures for the control of permafrost processes.

The investigation techniques

Regional geocryological investigations techniques can be combined into three groups each of which include particular types of work.

I. Determination and measurement of various indices of geocryological characteristics (factual data): 1) mining-drilling; 2) geophysical; 3) experimental-instrumental; 4) photography (ground-based as well as aerial and satellite); 5) visual and aerovisual observations (description, drawing).

II. Primary processing and interpretation of geocryological data: 1) statistical (analysis of the law of distribution of exponents); 2) graphical (plots, diagrams, histograms, etc.); 3) tabulation; 4) interpretation; 5) logical, physical-mathematical correlations; 6) statistical simulation (trend analysis, cartographic-statistical, regression, correlation, factor analyses, etc.).

III. Establishment of natural characteristics: 1) comparison, conjugate analysis (through natural components and conditions) of the territory and landscape indicators; 2) systematization and classification (preparation of particular and general regional diagrams and classifications); 3) charting and integrated regionalization; 4) mathematical and physical simulation.

The main aims of the drilling work are: study of permafrost parameters (composition, ground temperature, thickness, occurrence, cryogenic structure, ice content, etc.), sampling of frozen and thawed materials for laboratory analyses; thermometric and hydrogeological observations, integrated well hole logging. The specific nature of the permafrost study imposes particular demands on the design and technology of drilling. Thus bore-hole drilling is profitably carried out with air blowing of the bottom of the hole, providing for the smallest disruption of the natural ground temperature regime the most complete conservation of the permafrost core and the most exact determination of the permafrost thickness and of the depth of the water-bearing horizon.

Geocryological test holes should be equipped with moisture-insulated observation pipes. Temperature observations should be carried out after the natural temperature field is restored. According to various investigations the time of such restoration varies from 10 days to a few years depending on the hole depth, its construction, type and time of drilling. Geocryological bore holes are divided by their depth into two groups: mapping, through the layer of annual temperature fluctuations (10-25 m); and key geothermal holes through the whole permafrost. In the course of survey the mapping holes are positioned at two to three points within each type of landscape (type of area, microregion). Key holes are positioned usually taking into account changes in the geological structure of the territory that are causing regional change of the permafrost thickness. Supporting bore holes are used also to study the interactions between the permafrost and the groundwater.

Geophysical methods are used to study temperature fields, the state (frozen or thawed), composition, structure and mode of occurrence of various genetic ground types in section and in plan. For these purposes thermomet-ric, electrical, acoustic and nuclear logging of holes, electrical profiling and vertical electrical soundings are carried out, infra-red, thermomicrowave, radiolocation and aerial photography are conducted. Particular attention is given to the method of measurement of parameters of physical fields. The widest experience is gained in this regard when using the integrated bore hole logging and direct current (electrical) prospecting methods. The geophysical techniques and particular geophysical investigations used in the course of the permafrost survey depend on a number of factors, mainly: 1) special features of geological and permafrost structure of the territory under study; 2) the scale of researches; 3) the end use of the survey being conducted.

Thus, for example, in the course of engineering-geocryological and hy-drogeocryological survey of hard-to-reach poorly known areas in mountain regions with discontinuous permafrost the most productive are electrical prospecting techniques such as electrical profiling (EP) and vertical electrical sounding (VES).

Geophysical techniques used in small-scale surveys present the most complete characterization of, overall, the given region (or of some parts of this region such as topographic elements and associated landscapes) with values of such parameters as permafrost thickness, ice content and its most typical variability in the section, thickness of sedimentary deposits, microinclusions and ice bodies, thickness of the fractured zone in rock massifs with close crystal bounds, degree of Assuring of rocks within these zones, talik zones with the outlines and extent of thawed materials and water saturation. The specific aims in this case (revealing the most typical background values of the parameters under study) lie in the investigation method: first of all VES points are installed in the best-known typical undisrupted conditions, and only then over the rest of the area. The VES curves obtained within the typical areas are interpreted first, the special features of which in the permafrost section allow quantitative interpretation on the basis of parametric VES in the best understood holes. In the second stage, the remaining VES curves are interpreted. However, in this case the data on specific resistance of the thawed and frozen sections obtained at the first stage of quantitative interpretation are used as key ones.

The results of the interpretation of the small-scale survey are presented in the form of a section as a rule. Typical of this section are generalized data, the values of which are calculated by averaging all the special determinations of these characteristics made using the electrical prospecting techniques.

The geophysical investigations in the course of the large-scale surveys are designed to reveal the inter-landscape variability of the values of the features studied, with the consequent necessity of substantially increasing the density of observational points. The large scale of the investigations require also detailed study of the whole territory, including objects with steeply falling boundaries. For these purposes the range of geophysical techniques used is substantially extended. Various types of logging, of refraction-seismic exploration (RSE), of vertical seismic profiling (VSP); with laboratory measurements of specific resistance and elastic wave velocities (SR) are used, as well as acoustic and penetrational logging to study composition, moisture, density, porosity and ice content of sandy, clay-rich, coarse silty and rudaceous grounds and also the composition, state and properties of bedrock (porosity and degree of fracturing, static elastic modulus, modulus of strain, of short-term uniaxial compression resistance, the stress state coefficient, etc.).

To study such geocryological phenomena as karst, thermokarst, icing, occurrence of ice bodies in frost mounds and elsewhere, ground veins, pseudomorphs of ice wedges etc., various kinds of logging, electrical profiling, VES, resistivity measurements in bore holes and water basins, RSE, SR, gravimetry, VSP are used.

Specific presentation of the geophysical investigations in the course of the large-scale survey, except for the detailed geoelectrical sections constructed with the help of particular profiles, is by the compilation of special-purpose maps (for example, the map of various depths important for a particular kind of construction within the area under study).

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