Geological Surveying

the set of geological field investigations conducted for the purpose of compiling geological maps and revealing a given territory’s prospects regarding its mineral resources. Geological surveying involves studying natural and artificial exposures (outcrops) of rocks in order to determine their composition, origin, age, and forms of bedding. Afterward, the borders of distribution of these rocks are entered on a topographical map. While the survey is being conducted, samples of rocks, minerals, and fossils are collected. The survey is performed according to instructions confirmed by the Ministry of Geology of the USSR. The nature of the investigation depends on the scale of the survey. There are small-, medium-, and large-scale surveys as well as detailed surveys.

The small-scale geological survey (1:1,000,000 or 1:500,000) is made by observations along particular courses over the most exposed sectors. The results of the survey are supplemented by geological decoding of aerial photographs.

The medium-scale geological survey (1:200,000 or 1:100,000) is the primary type of geological mapping both for the entire territory of the USSR and for particular regions of economic importance. The purpose of this kind of survey is to study the most important features of the geological structure of the territory and to give a prognostic assessment of minerals. Typically, it involves area investigations accompanied by the exavation of trenches, prospecting shafts, and testholes and by geological decoding of aerial photographs. Comprehensive prospecting for all types of minerals is undertaken.

The large-scale geological survey (1:50,000 or 1:25,000) is made in regions with a mining industry and in regions whose prospects regarding mineral resources have been established by preceding investigations, as well as in regions of agricultural development and of housing and industrial construction. After large-scale geological surveying has been performed, the locations of potential mineral deposits, where subsequent detailed prospecting and comprehensive analytic work will be done, are noted and an initial assessment is provided of the minerals found. Geophysical and geochemical methods, as well as geological decoding of aerial photo-graphs, are used in largescale geological surveying. In regions where natural exposures are inadequate exploratory shafts and boreholes are used to reveal the basic rocks.

The detailed geological survey (1:10,000 and larger) is made in areas containing mineral deposits, as well as in regions where geological engineering investigations and investigations related to water supply and land reclamation are conducted. The surveying is accompanied by the preparation of a large number of cross sections, horizon maps and sketches, models, and block diagrams.

The type of geological surveying employed depends on scale, purposes, and conditions. Traverse, area, and instrument surveys are the most widespread. Route geological surveying involves crisscrossing the work region with routes, most of which are placed so that they transverse the strike of the rocks or folded structures. Route observations are entered on a topographical base or on aerial photographs. The geological structure of the territory enclosed within the routes is established by interpolation of data on adjacent routes and by decoding aerial photographs. The observation points and geological objects are entered on the topographical base by eye, and in forested terrain they are determined by visual surveying. In area geological surveying the entire survey territory is covered with observation points. Their density depends on the scale of the survey, on the complexity of the geological structure, on the degree of exposure, and on the clarity of the representation of geological objects on aerial photographs. Observations are also made along the routes lying both across and along the strike of tectonic structures. Instrument geological surveying is used for scales of 1:10,000 and larger. It differs from area surveying only in that geological objects are entered on the topographical base by means of instruments. Before surveying, all exposures and workings are studied, and datum marks (usually pegs) are set at the control points. The control points may be contacts with intrusive rocks, borders between formations marking horizons, ore bodies, ruptures, and so on. Semi-instrument methods are also used to trace the geological borders on the area between control points and to enter them on the topographical base. Aerial photographs and drawings of mining workings are used here.

Deep-seated geological surveying has also been developed in order to reveal the rock mass structure of a considerable thickness of the earth’s crust and to compile geological maps of particular structural surfaces located at great depths below the earth’s surface-for example, the surfaces of a tectonic unconformity, of the crystalline base of a platform, or of ancient weathering crusts. Deep-seated geological surveying is done with the aid of geophysical and geochemical methods, testholes, geomorphological analysis, and the study of aerial photographs.

Before the Great October Socialist Revolution 10.25 percent of the area of Russia had been covered in geological surveys. During the years of Soviet rule the entire territory of the USSR has been surveyed. Small- and medium-scale geological surveying is done by the Ministry of Geology of the USSR; large-scale surveys are also made by the ministries of the petroleum, coal, and ferrous and nonferrous metals industries and by other organizations.