Characteristics of seismoacoustic processes due to rock fracture in Dzhezkazgan pits

Abstract

The seismoacoustic method [1-5, etc.] is one means of predicting dangerous states of rocks which are liable to spontaneous caving. Musin et al. [4] describe the use of this method for determining the size of stable roof spans of rooms in Dzhezkazgan ore mines. In this article we determine the characteristics of seismoacoustic processes arising during rock fracture in the Dzhezkazgan deposit, with the aim of obtaining data for developing apparatus and methods of observation for detecting rock fall hazards in pits in this deposit.Our observations were made in Pit 31-32 on the 180 m horizon in rooms 11 and 16, as well as in the pillar between rooms 15 and 16. Elastic pulses arising in the rock during fracture were detected by a seismic receiver. The sensitive element of the resonance receiver [4] is an X-cut 45* plate of Rochelle salt, 65 X 25 x 6 mm in size. The receiver has sharp resonances at 450 and 2300 Hz. At 450 Hz the sensitivity is approximately 220 mV/g (g = 981 cm 9 sec'Z). The receiver was used to listen for sound pulses arising in the solid rock owing to spontaneous cracking of the roof.By means of ShShG seismometers [3] we determined the predominant frequency spectra and other parameters of the elastic pulses arising by breakage of the rock. The sensitive element of this seismometer is a pack of 45* xcut plates of Rochelle salt, 20 X20 X 20 mm in size. The working frequency range is 400-2000 Hz. The sensitivity in this range is 70 mV/g.The seismometers were used in conjunction with an amplifier. The effective frequency range of the amplifier was 200-3000 Hz. Its input sensitivity was at least 200 vV, and it was supplied with power from dry batteries. The signals were recorded on magnetic tape. At a tape speed of 9.53 cm/sec, the working frequency range of the tape recorder was 100-8000 Hz.The seismometerswere set up in boreholes drilled in the pillar or roof, 1.5-2.5 m deep. Contact between the seismometer and the side of the hole was effected with the aid of a wedge rod. To attenuate the influence of airtransmitted sound waves on the gage, the hole was filled with sound-absorbing packing.Natural cracks arising during slow spontaneous breakage of the pillar and roof were listened to inRoom 16 (roadway 9, horizon 180). During the observations (at which time there were no production operations) we measured the total noise levels in 10-rain periods, and later in 1-minute periods. A number of natural cracks were recorded on the magnetic tape in room 8.The elastic pulses which accompanied intense breakage of the rocks were recorded as follows. In a zone of marked strata separation, observed in the pillar of room 8, weexploded surface charges weighing about 900 g. As a result, chips parted from the rock (the areas of the individual breakage surfaces were up to 0.5-1 mZ). The explosion and rock cracking were recorded on magnetic tape; the seismometer was an ShShG geophone which was set up in a hole at a depth of 1.5 m about 4 m from the explosion zone. The geophone was wedged in place and the hole filled w...