116 / 2021-09-30 19:46:33

Comprehensive Use of Gutenberg-Richter Distribution and Geotomography Results to Improve Seismic Hazard Assessment in Hard Coal Mines

seismicity,geotomography

In many underground coal mines around the world, which operate at great depths or in areas of complex tectonics, the mining of coal seams is accompanied by seismic events. These are referred to as mining-induced seismicity. Under certain geo-mechanical conditions, some of the tremors may cause rockbursts, as a result of which mine workings lose their functionality and miners may suffer serious injuries, including fatal ones. This is especially true for tremors with high seismic energy (magnitude more than 2.0) and with their focal points located at short distances from active mine workings. The threat of rockbursts causes that the problem of continuous seismic hazard assessment is crucial both for the safety of personnel working underground and for continuous coal production. In many countries in the world, research is being conducted on the use of various methodological solutions in this field, including those existing in the area of natural earthquakes occurrence.

Polish coal basin, located in the Upper Silesia area, is a classic example of a basin with a high level of induced seismicity, which has taken place here for over 70 years. Exploitation of the multi-seam coal deposit, which has been continued for over 200 years, has resulted in the fact that for several decades the exploitation has been taking place at increasing depths and surrounded by thick and compact sandstone rocks (at present, the average depth of exploitation is almost 800 m and the greatest depth approaches 1300 m).      

The basic method of continuous observation of the seismicity in question is the seismological method, with a developed methodology for recording, interpretation and use of results for the daily assessment of the hazard state. One of the interpretation solutions is based on use of energy-quantity distribution of registered seismic phenomena, known in global seismology under the name of Gutenberg-Richter distribution.  Periodic measurements of the velocity of seismic wave propagation are also carried out in the observed rock mass fragment using seismic geotomography. In the active variant of this method, seismic waves are induced by initiating small charges of explosives and recorded by a suitably designed network of geophone receivers. The obtained distributions of seismic waves propagation velocity  in roof rocks of a coal bed enables locating the zones of  stress anomaly and determination of the maximum value of velocity in the observed panel.