ALBERT

Notes: Near-source data from the nuclear explosion Coalora detonated at Yucca Flats, Nevada Test Site (NTS), are utilized to constrain the seismic-source function. The equivalent seismic source is interpreted in terms of physical processes in the source region with the aid of data from within the explosion's non-linear region. The isotropic, deviatoric and spall contributions are separated and quantified. Standard spectral interpretations of the radiated wavefield for source parameterization are contrasted with complete waveform modelling with moment tensor determination. Individual waveform spectra (source-receiver offsets 〈 2 km) can be interpreted in terms of an isotropic source model, which is in agreement with a Mueller-Murphy model, including f−2 high-frequency decay and source corner frequency of 1.8 Hz. The deviatoric component of the moment tensor is a factor of 5–10 times smaller than the isotropic component. Deviatoric source radius, as estimated from the spectral data, is 125 m, smaller than the equivalent elastic source radius, which is bounded between 133 and 202 m. Stress drop estimated with the Brune source model is 45 bars with an average slip of 17 cm. Moment tensor inversion produces an isotropic source strength of 8 × 1020 dyne cm, while scalar moments from the spectral interpretation are a factor of 2.5 larger. This difference is attributed to the application of whole-space propagation path corrections with a free-surface amplification to the spectral interpretation. The spall source is longer period and delayed in time from the initial explosion. Its contribution to the diagonal elements of the moment tensor is dominant on the Mzz component, a factor of 3 larger than the Myy and Mxx components. Spall source strength from waveform inversion is within a factor of 2 of forward models developed from acceleration data within the spall zone. It is longer in duration than the forward prediction, reflecting the effect of a quasi-point source assumption in the forward model. Complex propagation effects extend in time and homogenize the data beyond 2 km as exemplified by wave trains at 5 km that are 20 s in duration and similarity of radial, vertical and transverse acceleration spectra. In contrast, observations at 2 km or less are short in duration with strong differences between transverse and radial-vertical spectra. These apparent propagation path effects suggest that source biases can develop at ranges as close as 2–5 km.

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.