NORSAR Basic Seismological Research
NORWEGIAN SEISMIC ARRAY (NORSAR) KJELLER
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This annual report describes the work accomplished during the period 1 October 1990-30 September 1991. The inversion of NORSAR travel time data constituted one of the first examples of seismic tomography. It is now becoming apparent that these data are influenced by wave diffraction effects, and hence that ordinary ray based tomography may be inadequate. A newly developed tomographic method that takes diffraction effects into account is applied. Data from the NORSAR array in its original form 22 subarrays aperture 100 km have been used and results are compared with those of the ray-based inversion experiments undertaken around 15 years ago. To facilitate comparison with earlier results, we inverted for velocity structure in the same boxlike structure as was used on the previous investigations. Discriminating between earthquakes and underground explosions is formulated as an exercise in pattern recognition approach analysis. The analysis has been applied to a learning set of 44 nuclear explosions 8 test sites and 35 earthquakes in Eurasia recorded at the NORESS array. The signal features considered were the normalized power in 8 spectral bands in the 0.2-5.0 Hz range of the P-wave 6 sec and the P-coda 30 sec. Physically, it means that we exploit potential differences in the shape of earthquake and explosion spectra, respectively. Other features includes are peak P and P-coda amplitude frequencies and relative PP-coda power.
- Nuclear Explosions and Devices (Non-Military)