Source phenomenology experiments with borehole explosions of special design in Israel

Two source phenomenology explosion experiments-decoupling and depth-of-burial (DOB)-were conducted by the Geophysical Institute of Israel (GII) in July 2006 and January 2007 in Negev, Israel. A special blast design and technology were utilized to form large 1240 kg (decoupling) and 4200 kg (DOB) nearspherical Ammonium Nitrate and fuel oil charges in cavities at different depths (26-63 m), in marls and phosphates, created beforehand by small shots in boreholes. The main goal of the experiments was investigation of empirical relationships between the contained spherical source and energy/magnitude parameters of different seismic phases at near-source and regional distances. Small decoupling factors for the heavily overdriven tests were estimated as 2.8 and 6.4 for charge/volume ratios 250 and 70 (kg=m³), respectively. Very high signal frequencies (40-90 Hz) accompanied by the highest peak accelerations were observed at near-source distances for the deep (63 m) decoupled explosion. A clear magnitude/energy reduction with increasing depth of contained sources was obtained at regional distances (similar to the 1997 Balapan DOB experiment), complemented by near-source recordings of higher frequencies and larger amplitude/energy for deeper charges. Observations of the Negev DOB experiment are remarkably consistent with the Mueller and Murphy (1971) model predictions of source spectra features. The same conversion point of spectra dominance at f_c ~ 10 Hz for different depth shots, predicted by the model, was observed over a broad distance range (0.2-230 km). The novel aspects of the experiments include (1) investigating seismic coupling in a source medium (soft sediments) that was not previously studied to a large extent, (2) using a homogeneous source medium for all the tests, (3) using nearly spherical explosive sources as opposed to the long cylindrical charges used in most previous large-scale high explosives tests, and (4) full containment of all explosions. The experiments, designed to simulate nuclear sources, provide data and knowledge for improvement of nuclear test monitoring for low-yield explosions.