DEPTH-OF-BURIAL AND DECOUPLING EXPLOSION EXPERIMENTS IN ISRAEL : NEAR-SOURCE AND NEAR-REGIONAL SEISMIC ENERGY GENERATION

During the third year of the project, two source phenomenology experiments were conducted at Oron phosphate quarry in Northern Negev. A special complicated technology was used for the creation of large cavities (up to 3.5 m) at different depths (up to 63 m), and accommodation of large near-spherical charges of ANFO explosives. A series of decoupled and fully coupled (reference) explosions with charges of 1240 kg in the cavities was conducted on July 17, 2006. Extensive observations in near-source zone and remote area demonstrated peculiar signal characteristics and energy generation features related to these specific decoupled seismic sources. S/P energy ratios and decoupling factors were estimated at local distances, matching roughly to theoretical conceptions. The project central Depth-of-Burial (DOB) experimental series was conducted on January 2, 2007, and included three equal near-spherical charges of 4,200 kg at depths 26 m, 45 m and 59 m. The main objective of the experiment was to investigate relations between explosion depth and spectral/energetic/magnitude parameters of regional seismic phases. The design and configuration of the Oron DOB experiment were preferable in purity of conditions compared to the previous Balapan DOB experiment (1997): media homogeneity (all charges were placed in similar consolidated sediments), full containment and small separation (~200 m) of the explosions, charge scaled depths in the range of Nevada Test Site (NTS) nuclear tests. An important goal of our experiment was to get rid of the asymmetry effect caused by the difference of lithostatic pressures above and below the vertical lengthy cylinder explosive source, typical for borehole chemical explosions, and to study the generation of shear waves for about spherically symmetric sources, typical for nuclear tests. Numerous good recordings of signals from all shots were obtained at portable near-source accelerometers and close-in 3C seismic stations, permanent local short period (SP) and broad band (BB) stations, and International Monitoring System (IMS) stations EIL and MMAI in Israel, and ASF in Jordan. An important effect observed for near-source accelerometer records was a clear trend of the signal peak amplitude and energy enhancement with the charge depth increase. This effect was accompanied by a significant signal frequency raise, providing empirical verification and estimation of decreasing seismic source size due to increase of overburden lithostatic pressure. Evidently, high frequencies of the radiated signal resulted in rapid attenuation of seismic energy with distance, a drastic drop of amplitude for deeper shots is clearly manifested already at distance of several km, and consequently a clear expected tendency of signal energy and magnitude reduction with depth is observed at regional distances. Analysis of seismic waveforms (in the broad band 1–20 Hz) at close local and regional distances (3.5–240 km) demonstrated a clear decrease of peak amplitude and energy for S-phase and corresponding S/P ratios for deeper shots. The frequency dependence of spectral amplitudes on shot depth was observed at most stations for the both phases: a decrease of amplitudes with increasing shot depth between about 1 and 10 Hz is converted to the opposite trend at higher frequencies ~10–20 Hz. A special feature of Oron DOB experiment the placement of the charges in boreholes, separated by only a few hundred meters, in order to eliminate variations in waveforms and travel times due to different propagation conditions—provided a good opportunity for a comparative location study. Using measured P onsets at the observation range and a local 1D velocity model, the three explosions were located by two procedures: the standard grid-search least square procedure (LSQR), and a new model-free robust “Grid-Sign” (GS) algorithm with essential advantage shown by the latter due to eliminating error of the unknown velocity model misfit. 29th Monitoring Research Review: Ground-Based Nuclear Explosion Monitoring Technologies