Engineering and Mining Journal - Whether the market is copper, gold, nickel, iron ore, lead/zinc, PGM, diamonds or other commodities, E&MJ takes the lead in projecting trends, following development and reporting on the most efficient operating pr
Issue link: https://emj.epubxp.com/i/148853
PRODUCTION BLASTING Figure 2—Placement of vibration monitors (i.e. geophones and accelerometers) in Stope 4263 at 6600 level for comparison of blast records. ufacturer of this product was 96, with a VOD of up to 4,900 m/s. Each explosive column is bottom initiated with a 450 g Pentolite booster. Blast monitoring at the mine is usually carried out with a commercially available monitor (Instantel Minimate) employing three-component geophones. The first step in this study was to ascertain the adequacy of this vibration monitor in assessing the production blasts at the mine. For this purpose, a comparative study was carried out with high-frequency accelerometers placed side by side with Minimate. The usual precautions were taken in mounting the geophones and the accelerometers (Farnfield, 1996) on to the surface of the rock underground. The accelerometers had an upper frequency range in excess of 10 kHz and the usual acceleration limit was 100 g. The placement of the two types of monitors (i.e., geophones and accelerometers) is shown in Figure 2. The data for the accelerometer recording was carried out with both an analog data acquisition system with a bandwidth of DC to 45 kHz, and a digital data acquisition system with a sampling rate of 1MHz. tudes among the delay rounds are clearly evident. The data highlights the need for multi-axial recording of vibrations, but the pronounced variations along each axis can be attributed only partly to the varying charge weights in the holes. A more diagnostic comparison between high-frequency accelerometer recordings and those by geophones is illustrated in Figure 4 for the same blast. Closer exami- nation of the two records, i.e., particle velocity records from the geophones and integrated accelerometer data, from the same blast shows only a qualitative agreement between the two types of recordings. The peak particle velocity values obtained by integration of the accelerometer data show consistently higher amplitudes than those obtained with direct geophone recordings, despite both geophones and accelerometers placed in the same location. The difference between the two types of recordings is further amplified when one examines the frequency content of the individual wavelets corresponding to each delay round. The respective frequency spectra of particle velocity from the geophone and accelerometer recordings are shown in Figure 5, for a single delay round. Whereas, the energy content in the geophone recording is seen to be confined below 300 Hz, the same particle velocity values obtained from integration of accelerometer data shows the maximum amplitude closer to 1 kHz or higher. In most cases the peak particle velocity derived from accelerometer data greatly exceeds those obtained by geophone recordings. These differences have serious implications when one compares the resultant particle velocity records obtained by the two modes of recording. This is illustrated in Figure 6, which shows the comparison between the resultant particle velocity for Analysis A typical vibration record obtained with the geophones (i.e., Minimate) along longitudinal, transverse and vertical directions is shown in Figure 3. Individual delay rounds are clearly seen in the vibration record, despite the usual scatter in firing times. The highly unequal particle velocity ampliwww.e-mj.com Figure 3—Particle velocity records obtained with geophones from a production blast. AUGUST 2013 • E&MJ; 53