Regionalization in geocryological mapping

Geocryological maps are compiled to study geocryological conditions in two cases: at the stage of completion of a geocryological survey at any scale, and when summarizing the geocryological and other materials at the general scales (without conducting the geocryological surveys).

The procedure of compilation of geocryological maps at surveying scales is based on the integrated study of two groups of cryoforming factors and conditions: 1) zonal landscape-climatic and 2) regional structural-geological, hydrogeological and orographic. In accordance with the surveying procedure the maps are compiled first of all for well-studied 'key areas' and then on the basis of landscape regionalization for less-studied areas situated between them. The maps are the main result of regional integrated surveys or thematic study of geocryological conditions in the course of surveys within the permafrost as well as within the area of seasonal ground freezing.

Geocryological maps at surveying scales compiled as a result of field investigations are always included - as along with permafrost characteristics the geocryological structure of the territory is one of the main components. The geocryological characteristics revealed and studied within the key areas compiled with the help of various techniques of investigation are distributed in the course of compilation of geocryological maps on the basis of landscape microregionalization which takes into account topographic elements and relationships between them, rock composition, moisture content and cryogenic structure, ground microtopography, swampiness, vegetation, insolation conditions and microclimate. As this takes place, the analyses of natural conditions of the areas and distribution of geocryologi-cal characteristics within them should be made using aerial and satellite photographs.

With respect to interrelations between the factors of the natural environment and the geocryological characteristics established in the course of the survey, a set of geocryological maps at the surveying scale (or larger than surveying scale) is compiled on the basis of geocryological classifications of seasonally and perennially frozen ground. For example, on the map of geocryological conditions a single geocryological characteristic (for instance, mean annual ground temperatures) or a few (for instance, mean annual ground temperatures, permafrost thickness and taliks) can be presented by appropriate graphics reflecting their spatial variability.

The main method for presenting the geocryological conditions on maps is separate mapping of each geocryological characteristic and each permafrost-forming component of the natural environment over all the topographic elements within the map. Such an approach allows one to get information on one or another parameter of the geocryological situation with the help of the map and also to reveal the spatial variation and association with natural environmental factors and to make a preliminary engineering-geocryological forecast, on the basis of these interrelations, for the regions open for economic development.

Geocryological maps are accompanied by geocryological sections on which, in addition to geological structure and rock composition the permafrost thickness and distribution, temperature field below the level of zero annual amplitude (in the form of lines of equal temperatures), moisture and ice content in a profile in depth with indication of the type of cryogenesis (syn- and epigenetic), etc. are shown. The content of a map should be in agreement with its name and function. The main mapping means - colour -showing clearly the spatial variation of the major (for the purpose of investigation) geocryological characteristic in the region is used for the main content of the map. Characteristics secondary in importance (for the given map) are mapped with the help of hatching and symbols.

Field maps are mainly compiled as particular work maps at the surveying or larger than surveying scale, i.e. reflect only one characteristic of the natural environment (for example, topographic elements or Quaternary deposits) or one geocryological characteristic (for example, distribution of thawed and frozen ground). Maps of observational material, Quaternary deposits, vegetation, and swampiness, topographic elements, cryogenic phenomena and technological formations, etc. are such maps. The maps com pleting the geocryological survey or constituting those for a report are compiled during the office work period. The compilation of the geocryological maps at all surveying scales is fundamentally unified but has its special features.

The large-scale maps

The large-scale geocryological maps are compiled mainly on the basis of data obtained within each area under study and are designed for the solution of concrete engineering-geological problems. Therefore the large-scale geocryological map is always engineering-geocryological while its main content is associated with geocryolithogenetic peculiarities of area deposits (structure, composition, cryogenic structures and ice content of frozen ground, their properties, stability of temperature regime to change of natural conditions in the course of development, etc.).

Separation of the areas with various cryolithologic peculiarities is associated with the detail of the landscape microregionalization when each topographic element is divided into a few landscape areas (microregions) with respect to natural characteristics.

For every portion of the map the following characteristics are shown throughout its area: 1) mean annual ground temperatures with gradations of 0.5 and 1°C; 2) depths of seasonal thawing and freezing (mean, with gradations of 0.5 m and extreme values); 3) thicknesses of permafrost and taliks (within the southern geocryological zone - with gradations of 1025 m; within the northern one - of 50-100 m); 4) cryogenic phenomena in their specific areas of development. Such mapping is conducted against the geological background - shown on the map at the level of suites, subsuites and facies with particular outlining of their boundaries and reflection of their physico-mechanical and thermal-physical properties in the legend. In addition, such maps must reflect 1) areas of thawed and frozen ground within their natural boundaries which, on the maps of smaller scale, are shown by symbols for permafrost islands and discontinuous permafrost distribution; 2) the depths of seasonal ground thawing for warm years and of seasonal freezing for cold ones along with the mean perennial depths; 3) intensity and stage of development (initial, mature, dying out) of cryogenic phenomena within the areas of their distribution; 4) areas of periodic separation of permafrost from the layer of seasonal freezing. For the purpose of engineering-geological assessment it is necessary to show the value of ground heaving in the course of freezing for every cryolithogenetic complex under the natural conditions as well as its change in the course of the planned economic development. Every area must be assessed also with respect to possible development of the thermokarst process when icy ground layers (or pure ice) occur near the surface; and with respect to development of thermal erosion etc.

According to the complexity of the natural terrain conditions, the information necessary for solving the engineering-geological problems can be presented on one integrated map or on a few maps. Usually a set of maps includes cryolithological and geocryological maps, a map of seasonal ground freezing and thawing types, maps assessing development of cryogenic processes, etc.

The importance of engineering-geocryological maps becomes greater during the period of structural design as well as during the period of post-construction investigation, provided the mapping was carried out with small intervals of gradation of the geocryological characteristics. The engineering-geocryological large-scale map must be supplemented with forecast maps for the construction period and with advice how to control the geocryological situation and how to improve it.

The medium-scale maps

As well as the large-scale maps, medium-scale geocryological maps are compiled for the purpose of the particular type of economic development but can include larger areas under study, especially for the purposes of hydro-technical constructions.

Maps at such scales are optimal for reflection of the geocryological conditions within the boundaries of all the topographic elements taking into account their landscape specific features and cryogenic structure (Fig. 16.1) and do not require additional partitioning of geomorphological elements with respect to landscape-lithological microconditions. The special features of the medium-scale map are : 1) agreement between boundaries of geomorphological elements and their typical geocryological characteristics; 2) the necessity to reflect quantitative parameters on the map and in sections not only for engineering-geocryological conditions but also for hydrogeological ones; 3) reflection of deeper investigation of the interaction between the permafrost and the groundwater.

The slightly less detail of mapping on the medium-scale maps compared with the large-scale ones can be illustrated by the following example. On the large-scale maps within the swampy ground of flood-plains, terraces, watersheds and other elements the chains of frost mounds, individual frozen peatlands and thawed intermound depressions are shown. On the medium-scale maps such an area (and other ones with close alternation of small patches of frozen and thawed ground) can be shown only by the symbol for

Fig. 16.1. The medium scale geotemperature map pattern: Mean annual ground temperatures, °C: 1 - from 0 to - 0.5 °C; 2 - from - 0.5 to - 1 °C; 3 - from - 1 to

— 1.5°C; 4 - from -1.5 to -2°C;5-from -2to -2.5°C; 6-from -2.5 to

— 3.7°C; 7 - from -3to -3.5°C; 8-from -3.5 to -4°C;9-from -4 to -5C; 10-from —5 to -6°C; 11-from -6to -7°C; 12-below -7°C. Materials:

13 - medium-grained sands; 14 - boulder-pebble deposits; 15 - sandy-silty-clay with rock waste of diabase and sand; 16 - sandy-silty with rock debris of diabase;

17 - 18 - silty-clay-rich material (17 - with rock debris of tulfas and diabase;

18 - with rock debris of diabase); 19-20 - boundaries (19 - of lithologic variations; 20 - of areas with various temperatures); 21 - key bore holes; 22 - bedrock precipices.

Fig. 16.1. The medium scale geotemperature map pattern: Mean annual ground temperatures, °C: 1 - from 0 to - 0.5 °C; 2 - from - 0.5 to - 1 °C; 3 - from - 1 to

— 1.5°C; 4 - from -1.5 to -2°C;5-from -2to -2.5°C; 6-from -2.5 to

— 3.7°C; 7 - from -3to -3.5°C; 8-from -3.5 to -4°C;9-from -4 to -5C; 10-from —5 to -6°C; 11-from -6to -7°C; 12-below -7°C. Materials:

13 - medium-grained sands; 14 - boulder-pebble deposits; 15 - sandy-silty-clay with rock waste of diabase and sand; 16 - sandy-silty with rock debris of diabase;

17 - 18 - silty-clay-rich material (17 - with rock debris of tulfas and diabase;

18 - with rock debris of diabase); 19-20 - boundaries (19 - of lithologic variations; 20 - of areas with various temperatures); 21 - key bore holes; 22 - bedrock precipices.

permafrost islands, with a description of the proportions of thawed and frozen ground in the legend.

The set of medium-scale maps completing the geocryological survey usually include: a geocryological map, a map of ground freezing and thawing types, engineering-geocryological and hydrogeocryological maps com piled for natural conditions (for the period of investigations) and a forecast geocryological map with regard to the general plan of construction and the means for protecting the natural environment.

The small-scale maps

These geocryological maps are always integrated as they are compiled at the pre-design stages of investigations and must provide answers to problems on the many aspects of development of an area and on environmental protection. Their problem is to reflect the complex of geocryological, hydrogeological and engineering-geocryological conditions over large areas, often undeveloped but having potential for exploration and exploitation of mineral resources, for linear construction, etc.

Field maps are compiled in the course of small-scale surveys first of all for the key areas at a scale 2-3 times larger than that of the survey. The purpose of compiling working maps for key areas is to make clear the main geocryological conditions in each microregion (particular geocryological characteristics) and their variation over the area from one microregion to another (the general geocryological characteristics). Preparation of the integrated geocryological maps for the whole region is based on the maps of the areas and on the analyses of relationships between natural environmental factors and geocryological characteristics.

As the regions under small-scale mapping are large and often complex as far as their geocryological structure is concerned, it is profitable to compile the integrated maps separately for each characteristic taking into account every particular prepared map in succession. The final integration of all the geocryological characteristics and permafrost-forming factors of the natural environment on one or a few maps reflects regional geocryological characteristics.

The small-scale integrated surveys as a whole are completed with the preparation of a set of integrated maps such as a geocryological map (map of permafrost distribution and mean annual ground temperatures, of permafrost thickness and structure, of cryogenic phenomena and taliks); a map of seasonal ground freezing and thawing types; an engineering-geocryologi-cal map; and a hydrogeocryological map. The set can include a map of Quaternary deposits, a cryolithological map and geocryological maps for assessment (by stability and variation of cryogenic phenomena and cryogenic conditions as a whole), etc. Such a set of maps facilitates the preliminary assessment of competing areas to be developed, to select the most favourable one as far as engineering-geocryological or hydrogeocryological conditions are concerned. Overall the wide integration and great depth of investigations in the course of the small-scale geocryological mapping are based also on the geocryological development of the region in the Late Cenozoic and during each of its stages.

Forecast and assessment maps

Compilation of such maps at the large and medium scales is dictated mainly by the need to solve various practical problems.

The aim of the compilation of forecast geocryological maps is mapping of such geocryological characteristics which can develop in the course of the proposed development, or through the natural dynamics of climate. Their basis is the geocryological forecasting through all the key parameters of the geocryological environment and the existing map of currently researched geocryological conditions. Lines of demarcation on a forecast geocryological map differ essentially from the map of the existing situation only within the areas of intensive change of natural conditions.

The aim of the assessment maps is the reflection of engineering-geo-cryological information obtained in the course of survey in a form suitable for use when making projections and when solving other practical problems. Selection of the features for the assessment depends on the specific engineering use of the natural environment, the geocryological forecast and engineering-geocryological characteristics.

The forecast and assessment maps and the map of existing geocryological conditions are the basis for compilation of a map showing tolerance of natural conditions to technological changes or a map for environmental protection. This map includes the subdivision of terrain with respect to degree of inertia (or degree of reaction) to geocryological change following various disruptions, and recommendation on trends in the designated geocryological conditions.

General mapping and regionalization

Up to now the geocryological maps covering the whole territory of the former USSR have been compiled at the general scales (1:40000000 -1:5 000 000) mainly because of rather limited data.

The first maps of the permafrost regions (beginning from the end of the eighteenth century up to the 1940s were very simple as far as their content and illustration were concerned and included only one or two characteristics represented schematically: at first only the conventional line of the southern limit of permafrost, then conventional lines of mean annual ground temperature with values of —1, —3, —5 and — 10°C. In spite of their simplicity, the content of those maps was rather progressive for that time (maps of A.F. Middendorf, G. Wild, A.V. L'vov, L.A. Yachevskiy, V.B. Shostakovich, M.I. Sumgin and V.F. Tumel') as they gave the idea of zonal permafrost variation. In the 1950s as observational material was accumulated and the science of permafrost developed for separate regions and then for the whole permafrost zone of the former USSR, there appear the permafrost maps by V.A. Kudryavtsev, I.Ya. Baranov, I.A. Nekrasov, A.I. Popov, A.I. Kalabin, P.I. Melnikov, and others at the 1:40000000 -1:20000000 scales. In the 1960s I.Ya. Baranov compiled the first geo-cryological map of the former USSR at the 1:10000000 scale, which generalized in somewhat schematic form the data on seasonally and perennially frozen ground, the mean annual temperatures and on physical-geographical phenomena accumulated by that time. Then Baranov published in 1977 the new geocryological map of the USSR at the 1:5000000 scale, which was substantially more detailed with respect to the concept of types of cryogenesis and the main characteristics of seasonally and perennially frozen ground. In those years geocryological maps were published, at the general scale, varying in content and in detail, for regions showing promise for economic development. Thus a great stride forward in geocryological mapping is a set of three maps at the 1:1 500 000 scale, compiled for the region of permafrost within the Western Siberian Plate published in 1976 (edited by Ye.M. Sergeyev) and a set of larger scale maps published in 1986 (edited by V.T. Trofimov).

A great number of maps at small and general scales was compiled for particular regions of the permafrost under intensive development: for Southern Yakutiya, Western Siberia, southern part of Central and Eastern Siberia and Zabaykal'ye.

In 1986 the Geocryological Map of the former USSR at the 1:4000000 scale, edited by A.I. Popov, was published. The main content of this map was the distribution of syn- and epigenetic types of frozen strata, the cryolithological characteristics of which were presented in columns typical of the permafrost sections. In the early 1970s the Department of Geocryol-ogy (Geological Faculty, Moscow State Unuversity) began the integrated small-, medium- and large-scale geocryological surveys within the large and various lands of the permafrost regions and this was completed with the 1986 compilation of the new geocryological maps for the whole territory of the former USSR at the 1:2 500 000 scale, including the Geotemperature Map of the USSR, the map of the permafrost thickness, and the Geocryological Map of the USSR. Geocryological characteristics on all the maps cited above have been correlated (as far as the scale permitted) with the homogeneous natural conditions within each of the areas considered.

The Geocryological Map of the former USSR at the 1:2 500 000 scale

This integrated map is compiled on a geologic-genetic and forma-tional basis and reflects the relationships between the perennially frozen strata and the geological structure and the composition of bedrock, the landscape-climatic conditions and the topography, as well as the neotec-tonic, hydrogeological and deep geothermal conditions, by means of separate mapping of the main geocryological characteristics within the topographic elements and groups of elements.

Such a method allows representation of the regular change of geocryological conditions from south northward (in connection with the latitudinal zonation of heat exchange on the surface), from west eastward (in connection with variations of sectorial structure and continentality of climate), from low topographic elements to high altitude ones and from slightly sloping topographic elements to steep ones (in connection with altitudinal zonation of heat exchange and with topography), from stable tectonic structures to mobile ones (in connection with geothermal flux increase), and from young deposits to more ancient ones, etc.

Analysis and correlation of geocryological characteristics within each area with the permafrost-forming factors of the natural environment, were carried out on the basis of the method of key landscape maps. Maps of large-, medium- and small-scale geocryological surveys and thematic investigations were used as key ones.

The main content of the map includes the distribution of frozen and unfrozen ground, spatial variations of their mean annual temperatures (see Fig. 15.2), thickness and structure of the permafrost, the relationship between frozen, cooled and relict layers over the territory and in section (see Fig. 15.1), permafrost-geological phenomena and taliks as well as geological structure of the territory reflecting the real composition of permafrost, its cryogenic structure and ice content which are shown on the map through geologic-genetic complexes of Quaternary deposits and geological formations of pre-Quaternary rocks. The type of freezing, macro-ice content and cryogenic structures are also shown.

Comprehensive representation of the geocryological characteristics, especially within the southern zone, facilitates the map's use as the basis for regional geocryological forecasting after taking into account the general construction procedures (deforestation, removal of moss-turfed soils, partial removal of snow, etc.).

The content of each geocryological map at the 1:2 500 000 scale is supplemented and generalized by general maps at the 1:25 000000 scale: 1)

of the distribution of frozen and unfrozen ground; 2) of the mean annual ground temperatures; 3) of permafrost thickness; 4) of cryogenic age of the ground and type of cryogenesis; 5) of hydrogeocryology; and 6) of the engineering-geocryological regional characteristics of the former USSR territory.

The Geocryological Map of the former USSR at the 1 : 2 500 000 scale can be used together with much specialized data extending the characteristics and assessment of geocryological conditions and their monitoring as the new natural data arrive. For this purpose we can use a computer, the memory of which can store and analyze all the information about each element of relief recognized on the map in digitized form.

Rational use of frozen ground and environmental protection in the course of economic development of the permafrost regions

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