Aerial Methods of Earth Study
Aerial Methods of Earth Study
the aggregate of aerial methods of research and cartography of the earth’s envelope, its inherent phenomena, and objects of the natural and man-made landscape; their physical properties may be recorded from the air in various zones of the spectrum of electromagnetic waves on various types of instruments. On the basis of this, aerial methods of earth study are subdivided into aerial photography methods, utilized throughout the visible portion of the spectrum (0.4–0.8 microns) and in the near infrared (0.8–1.1 microns); photoelectronic methods, designed for the utilization of narrow zones in those parts of the spectrum and in the ultraviolet (0.01–0.4 microns), far infrared (1.2–25 microns), and radio (1 mm to several meters) wavelengths; aerogeophysical methods, based on the recording of terrestrial gamma radiation and the parameters of its physical fields; and aerovisual methods, limited to the visible portion of the spectrum.
The first stage of aerial methods of earth study consists of the aerial photographing of a locale and the recording of data on the aerial photographs in the form of photographs or recordings. The second stage consists of the study of the contents of the aerial photographs (that is, their interpretation) and of the corresponding measurements, which were made primarily by methods of photogrammetry. The greatest quantity of information about objects and phenomena based on aerial methods of earth study may be obtained when the methods complement one another, taking into account theii peculiarities and the nature of the task. For example, the combination of aerial photography and photoelectronic methods in topographical surveying is very effective, as is the combination of aerial photography, photoelectronic, and aerogeophysical methods in geological surveying and mineral prospecting.
Aerial methods of earth study may be used both self-sufficiently and in combination with earth-based methods of investigation and cartography of a locale. In particular, they may be combined with geodesic determinations in topographical work and with the study of rock exposure, with drilling, and other methods.
Aerial photography methods. The methods of aerial photography that have been utilized since the beginning of the 20th century are the main methods for agricultural, scientific, and military purposes in terms of breadth and volume of usage. Information is recorded with an aerial camera using photographic emulsions of varying light sensitivity. In the 1960’s, along with basic aerial photographic surveying on black-and-white film, color aerial-photographic surveying, with the objects portrayed in natural and transformed colors, also became widespread.
Present-day topographical surveys are entirely based on aerial methods of earth study. These methods are an integral part of the complex of scientific-technical measures on forest inventory; land-use measures; land reclamation; the planning of railroads, highways, power lines, and pipelines; the estimation of industrial resources; the calculation of snow cover; and so on. Aerial photography methods are also used in all aspects of geographical research, in nature conservation, and in various geological works, such as cartography and the study of tectonics (including recent tectonics) and of the structure of marine shoals; in hydrogeological and engineering-geological research and mineral prospecting; in the study of terrain, soils, and vegetation; in the study of continental bodies of water, the processes along the shores of bodies of water, and marine currents and disturbances; in the solution of urban-development and transport problems; in archaeological research; and so on. Aerial photography methods as a whole (aerial photographic surveying and the interpretation and photogrammetric processing of aerial photographs) increase the quality and economic efficiency of this work.
Photoelectronic methods. Photoelectronic methods were in a formative stage in the 1960’s. They are mainly intended to obtain representations of a locale in the visible portion of the spectrum, where they give a significantly higher differentiation of objects by their spectral luminescence (in certain narrow zones), and in those sectors of the spectrum that are not applicable for direct photographing on light-sensitive materials. In this way, aerial methods of earth study provide supplementary information about the physical properties of objects. The information is recorded (by means of special transformers) in the form of an image on the screen of a cathode-ray tube; the image is transferred to a photographic film. The practical applicable photoelectric aerial methods of earth study are spectrometric, ultraviolet, infrathermal, radiothermal, and radar aerial surveying.
Spectrometric aerial surveying makes it possible to obtain spectral luminosity coefficients for objects and an image of the objects in narrow spectral intervals that is selectively amplified by means of signals proportional to the relationship of the luminosities of the objects in two predetermined zones of the spectrum. It is used for determining the zone of the spectrum that is the most effective for the transmission of the peculiarities of a given landscape during aerial photographic surveying and for the direct expansion of information about rocks and vegetation.
Ultraviolet aerial surveying is based on the fact that some rocks and vegetation fluoresce under the effect of ultraviolet irradiation (in this case, from the air); this makes it possible to record their outline on the aerial photograph. Positive results have been obtained in prospecting for oil, gas, and uranium and in the separation of infected areas of crops.
Infrathermal and radiothermal aerial surveying make it possible to record differences in objects by their temperature characteristics. Receivers of the required radiation on board aircraft make it possible to detect temperature differences on land and water with an accuracy of up to 1°C; thus, it is possible to reveal water currents under the vegetation cover, currents and schools of fish in bodies of water, taliki (islands of thawed soil in areas of permafrost) and islands of sporadic permafrost, geothermal anomalies of a volcanic nature, junctions of certain rocks, fire and smoke contours of forest fires, and others on “thermal” aerial photographs.
Radar aerial surveying is carried out with various wavelengths, frequencies, and pulse shapes. This makes it possible to obtain, at any time of day and independent of atmospheric conditions, an image of the locale such that the material composition and the structure and moistness of the surface rock, sea ice, and so on can be partially interpreted. The scanning radar ray of definite parameters makes it possible to penetrate snow, surface vegetation, and the shell of covering deposits to a depth of several meters. A particular type of radar aerial surveying is aerial radio altimetry, which is used in combination with aerial photographic surveying for topographical purposes.
Methods based on the study of the polarization of light by various objects (in order to determine the spatial orientation of their microstructure) and the application of optical quantum generators (lasers) in the capacity of scanning devices (“feeling” the earth’s surface with radioelectric rays) are among the most promising. Research is being done on the possibility of combining the photoelectronic and aerial photography aerial methods of earth study (multichannel surveying) with a view toward simultaneously obtaining combined information from an airplane or artificial earth satellite.
Aerogeophysical methods. Aerogeophysical methods, which appeared in the mid-20th century and are based on the recording and measurement of gamma radiation of the earth and also of the parameters of its magnetic, gravitational, and electrical fields, make it possible to attain greater depth of study of the earth’s crust than with other methods. Aerogeophysical methods include aerial magnetic, radiometric, and gravimetric surveying, aerial electric prospecting, and aerial seismic prospecting (the last is as yet less developed).
The task of aerial magnetic surveying includes the measurement of the components of the magnetic field with special instruments—aeromagnetometers. The analysis (according to the data obtained) of the structure of this field and the establishment of its connection to the geology of the region make it possible to reveal the presence and essential features of a number of deposits, particularly those that create magnetic anomalies.
Aerial radiometric surveying is used to record the intensity of natural gamma radiation of the earth’s surface. The use of instruments—aerial radiometers and gamma spectrometers—makes it possible to establish the prospects of the areas under study with regard to radioactive elements (uranium, thorium, and others) and also the spectral composition of the radiation, which is important for the definition of rocks in regional geological cartography.
Aerial gravimetric surveying, which consists of measurements from an aircraft of the force of gravity by means of gravimeters, is carried out mainly for the study of the shape of the earth and in order to reveal anomalies of the gravitational field that are connected with large geological structures.
Aerial electric prospecting is based on the measurement from the air of secondary electrical fields that are created by rocks with various electrical conductivity. It is used in prospecting for certain minerals.
Aerovisual methods. Aerovisual methods use as their receiver the human eye, which distinguishes objects by their brightness and color contrast in the visual portion of the electromagnetic wave spectrum. In spite of the supplementary intent of these observations, they principally permit, as distinct from other aerial methods of earth study, the study from the air of any object on the earth in its natural form, varying with the observation conditions. Aerovisual observations are used partly as a supplement, but also partly instead of earth-based observations (although primarily on sparsely settled territories), with the goal of increasing the efficiency of topographical, forest valuation, geological, and other types of work.
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