ALBERT

Notes: A series of controlled seismic experiments performed in a limestone quarry demonstrate the utility of high-precision electronic detonators in studying source characteristics of multiple explosive arrays. At near-source ranges (80-130m), where source dimensions are on the same order as source-receiver distances, the influence of the difference in travel path-length among individual explosions on the seismograms is significant. Focusing of the seismic energy is observed as a function of station location with respect to the source array and is attributed to the extended source length (68-94 m) and firing time of the source (380-544 ms).We examine two methods for modelling ripple-fired explosions at near-source ranges using the principles of superpositioning. The first method is based primarily on acquisition of an adequate single shot signal and requires well-constrained shot times. Amplitude variations which result from travel path differences are not modelled, which restricts use of this technique for purposes of blast vibration reduction to larger distances (〉2-3 source dimensions) where the spatial finiteness effects of the source begin to diminish. For near-source distances (≤2 source dimensions), we successfully model multiple-source seismograms by convolving a synthetic seismic source signal for a single explosion with individual half-space Green's functions calculated for each explosion in the array. Our single-source model for a cylindrically shaped single charge (borehole length of 17.5 m and diameter of 90 mm) of 68 kg consists of a modified Mueller-Murphy approximation which utilizes source parameter estimates taken from chemical explosion study results. Model parameters include a final cavity radius of 0.25 m and an elastic radius of 18 m. The final model is obtained by convolving the simulated single-source time series with half-space Green's functions calculated for several source depths and superposed to approximate the spatial extent of the borehole. The relative amplitude and phase characteristics of the observed single-source signal at the same distance (80.6 m) are reproduced by this model.Multiple-source synthetic seismograms contain individual Green's functions for each source-receiver distance but utilize identical sources for the explosive array. Focusing effects are shown to be due to the effect of propagation path differences between individual explosions in agreement with the results of Anderson & Stump (1989) in simulating multiple-source seismograms. Good fits to the measured production shot amplitude spectra are obtained with the synthetic spectra. Spectral peaks are well-matched due to precision of the firing times which were controlled by electronic detonators. Our example of delay time variances for 32 ms production shot (Appendix) argues for better constraint of firing times for contolled seismic experiments. Such constraint requires a 1 per cent error or less in cap firing times which can be realized by the use of firing systems with an order-of-magnitude increase in precision compared to pyrotechnic detonators.