Mine gases cause major hazards to underground coal mining in many parts of the world. Among them, instantaneous outbursts of coal and gas as well as gas explosions are the most severe. During the formation of coal from vegetation, water, carbon dioxide and methane are produced in varying quantity. At the later stages of coalification, methane is produced in larger quantities and trapped within the coal.
It is commonly accepted that most of the amount of gases up till 80% - 90% of total quantity are deposited in coal or rocks as adsorbed ones, while the remaining 10% - 15% of the gas is recognized as the free gas. In theory, up to 200 m3 (Roberts 1983) or even up to 465 m3 (Kozlowski 1986) could be formed from 1 tonne of coal during coalification. The sorptive capacity depends on the internal area which in turnes is related to rank, to the nature of gas and to the pressure (Jackson 1984).
Gas hazard is a serious problem in Polish mining. Out of 70 coal mines operating in that country 44 are gasous mines giving 53% of total national production of coal. Accordingly, proper knowledge of gas content and its space structure in the deposit is of greate importance.
Very little has been done till now on study of space structure of gas content in deposits. Classifications of seams bases mainly on gas content hand produced maps with the use of simple algorithms of interpolation between data points (Kozlowski 1986). It has also been recognized as not creating truly reproducible results, i.e. results which are free from personal bias. Accordingly, to increase safety, high, single, occasional readings of gas content are considered decisive in the process of classification of gas hazard areas. Since the gas content data are very scattered, statistical methods to study the data should be employed. Among many of such methods (Agterberg 1974) geostatistics originating from the Matheron's concept of regionalized variable (Matheron 1903) has been successfully applied in many engineering areas (Journel and Huijbregts 1978). Structural analysis is one of the most important steps of geostatistical procedure.
2 GAS CONTENT AS A REGIONALIZED VARIABLE
There are many methods employed in determination of gas content in the deposit (Kozlowski 1986, Lama and Bartosiewicz 1983). The common future of the methods is their local character. Resulting from each method a numerical value of gas content is prescribed to a point in the deposit. The nature of gas distribution in the deposit as seen in Fig. 1 is typical to many geological values, for example : mineral grade, thickness of the seam, density etc. Regionalized variable theory as proposed by Matheron (Matheron 1963) is the basis for the ensuring geostatistical analysis.
Figure 1. Sample record of the gas content in the Lenin mine in E-W direction. (available in full paper)
A regionalized variable is any numerical function with a spatial distribution that varies from one place to another with apparent continuity whose changes cannot be represented by any workable function (Matheron 1970).