Engineering & Mining Journal

AUG 2013

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PRODUCTION BLASTING Figure 4—Comparison of geophone recording with that of an accelerometer recording for the same blast. (Note: accelerometer recordings have been integrated to yield particle velocity.) the geophone and accelerometer recordings for a typical production blast with pyrotecnic delays. The scatter in firing times between the delay rounds is obvious but to be expected. Similarly, the varying particle velocity amplitudes observed with both geophone recording and accelerometer recording among the events is to be expected, but due only partly to varying charge weights and the travel paths for the seismic waves, and related geological factors. The more serious observation is the fact that for the same charge weight per delay and distance, the peak particle velocity obtained from accelerometer recording is often greater than by a factor of two compared to peak particle velocity obtained with geophone recording. For example, the event at approximately 300 ms shows the resultant particle velocity to be 80 mm/s, whereas the data derived from the accelerometer station at the same location shows it be 190 mm/s. Similarly, the event arriving at 500 ms, the geophone data yields a resultant particle velocity of 90 mm/s compared to 220 mm/s for the corresponding accelerometer recording. This difference cannot be explained away simply because of charge weight differences or the geological factors involved, as both accelerometer recording and geophone recordings correspond to 54 E&MJ; • AUGUST 2013 identical charge weights, geological conditions and seismic travel paths. Particle Velocity Amplitudes and Explosive Energy The recorded vibration amplitudes can also be related to the explosive energy yield at the source, as the radiated energy can be shown to be proportional to the square of the amplitude of the particle velocity. Throughout this analysis it is assumed that the seismic energy (i.e., blast vibration energy) is directly related to the total explosive energy released in the borehole. This is a valid argument so long as the source function remains unchanged, i.e., there is no change in the type of explosive used, or the decoupling condition in the borehole is changed, or there is a significant change in the initiation mode employed in the blast that would result in a drastic change in the energy partitioning between shock and gas energy from the explosive (Mohanty, 2009). On this basis, the energy yield from the various explosive loads for the blast in question with accelerometer data is shown against designed delay times is shown in Figure 7. For a true comparison of energy levels, the derived energy values have been linearly scaled (i.e., normalized) with the corresponding charge weight for each delay round, as in earlier works (Mohanty et al., 1997). The effect of varying distances to the corresponding explosive charges on the amplitude of the particle velocity is considered minor in this case because of the relatively large distances involved between the stope blast and the monitoring stations. Therefore, the energy yield values shown in Figure 7 would represent the actual specific energy estimates from each of the explosive columns in the blast, i.e., specific energy per kg of charge. The radius of each circle in Figure 7 represents the relative specific energy/kg Figure 5—Comparison of frequency spectra of particle velocity obtained with geophone recording vs. accelerometer recording at same location along three orthogonal directions for the same production blast (all accelerometer data integrated to yield particle velocity). www.e-mj.com

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