Materiały konferencyjne SEP 1992

462 Underground Exploitation School '92 mining demands. It łs not possible, for example» to install geo- phones In the face. For the measurement, this article refers to, the geophone probes were installed with a horizontał distaoice of 50 m in 5 m deep boreholes in the middle of the seam in the roadways of a longwall retreat mining area (see Figurę 10). During the advance of the face the probes were destroyed successively amd other geophones had to be connected to the registration 1ine. Usually the probes nearest to the working face were connected for registration with at least one 3-compo- nent probe in each roadway. Two different trigger conditions were set up for signal registration and on-1ine statistics. An event was recorded on magtape, when a minimum amplitudę (amplitudę trigger) at least 5 probe-locations (coincidence trigger) had been exceeded in a pre- defined time window. For statistical aims a coincidence of 2 was satisfactory for an up count. Dur i ng the who1e measur i ng period (43 weeks) of the shown example about 60 000 events were detected and used for statisti- cal evaluations, and seismograms of about 36 000 events were stored on 112 magtapes for further investigatłons. The first approach to evaluated the informations obtained from seismoacoustic measurements is a simple statistical one. With t he i dea t hat t he event -ratę corre 1 ates with the generał stress conditions, on-line plots at the measuring site with count rates versus hours were drawn. These plots give an immediate overview of the seismic actłvities. As an exELmple figurę 11 shows a "normal" event count rate from Monday to Wed- nesday without any indications for increased stresses and an increased event rate on Thursday and Friday, One feature is sig- nificant: during the mining shifts the "normal" event rate reaches about 30 to 40 events per hour, whereas during the shifts without mining the rate goes down to about 5 events per hour, 1 ike on Saturday or Sunday. This was also found by measurements in other seams, and it shows that most of the events are directly induced by the coal mining. With the test drilling method significantly increased stres- ses were detected especially under a residual pi 1lar situated 165 m above the actual mining depth. A higher event rate than