For the prevention and control of rockburst in underground coal mines, a detailed assessment of a rockburst hazard area is crucial. In this study, the dependence between stress and elastic wave velocity of axially-loaded coal and rock samples was tested in a laboratory. The results show that P-wave velocity in coal and rock is positively related to axial stress and can be expressed by a power function. The relationship showed that high stress and a potential rockburst area in coal mines can be determined by the elastic wave velocity anomaly assessment with passive seismic velocity tomography. The principle and implementation procedure of passive seismic velocity tomography for elastic wave velocity were introduced, and the assessment model of rockburst hazard using elastic wave velocity anomaly was built. A case study of a deep longwall panel affected by rockbursts was introduced to demonstrate the effectiveness of tomography. The rockburst prediction results by passive velocity tomography closely match the dynamic phenomenon in the field, which indicates the feasibility of elastic wave velocity anomaly for rockburst hazard prediction in coal mines.

This article is devoted to the German classes of particles: Gradpartikel (degree particle) and Fokuspartikel (focus particle), which are assigned divergent contents in different grammars and lexicons. In addition to this, a problem with didactic implications is the inconsistency of terminology. For example, Helbig/Buscha (2001) apply the term Gradpartikel to expressions that Hentschel/Weydt (2013) classify as Fokuspartikel. Engel (2009), on the other hand, refers to the expressions as Gradpartikel, which in Hentschel/Weydt (2013) represent the classes Fokuspartikel and Intensivpartikel. There are more similar inconsistencies in the literature with regard to class names and the inclusion of particles within them. The aim of this article is to revise the classification criteria and analyse the distinguishing characteristics of these classes and to put the terminology in order. The choice of terms in this article was determined by the primary function of the class: Fokuspartikel is named according to the function of focusing attention on something (German: fokussieren), while Gradpartikel is named according to the function of reinforcing (German: gradieren).

The paper analyzes the impact of potential changes in the price relation between domestic and imported coal and its influence on the volume of coal imported to Poland. The study is carried out with the application of a computable model of the Polish energy system. The model reflects fundamental relations between coal suppliers (domestic coal mines, importers) and key coal consumers (power plants, combined heat and power plants, heat plants, industrial power plants). The model is run under thirteen scenarios, differentiated by the ratio of the imported coal price versus the domestic coal price for 2020–2030. The results of the scenario in which the prices of imported and domestic coal, expressed in PLN/GJ, are equal, indicate that the volume of supplies of imported coal is in the range of 8.3–11.5 million Mg (depending on the year). In the case of an increase in prices of imported coal with respect to the domestic one, supplies of imported coal are at the level of 0.4–4.1 million Mg (depending on the year). With a decrease in the price of imported coal, there is a gradual increase in the supply of coal imports. For the scenario in which a 30% lower imported coal price is assumed, the level of imported coal almost doubles (180%), while the supply from domestic mines is reduced by around 28%, when compared to the levels observed in the reference scenario. The obtained results also allow for the development of an analysis of the range of coal imports depending on domestic versus imported coal price relations in the form of cartograms.