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Teleseismic Location

find direction of signals based on Array algorithms backtrace ray paths through the earth simplifications: flat earth, plane waves usually high or reasonable waveform similarity. Teleseismic Location. Epicentre Location using Arrays.

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Teleseismic Location

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  1. find direction of signals based on Array algorithms backtrace ray paths through the earth simplifications: flat earth, plane waves usually high or reasonable waveform similarity Teleseismic Location

  2. Epicentre Location using Arrays Problem: inaccuracy due to deviations from velocity model at the receiver Solution: array calibration (empirical corrections to direction)

  3. Principle of Array Analysis for a given station geometry: t1, t2, t3 (observed) → plane wave (azimuth and slowness) → t1', t2', t3' (theo)

  4. Validate result apply negative (t1',t2',t3')

  5. In real life ...

  6. Select Picks and measure tn

  7. Check Accuracy (apply -tn')

  8. Larger aperture

  9. Again, select picks and measure tn

  10. Beamforming not satisfying

  11. for appropriate configuration t1, t2,..., tn (observed) → plane wave → t1', t2',..., tn' (theo) (t1, t2, ... , tn) ≈ (t1', t2', ... , tn' )

  12. aperture too large / frequencies too high high veloc. low veloc. t1, t2,..., tn (observed) → plane wave → t1', t2',..., tn' (theo) (t1, t2, ... , tn) ≠ (t1', t2', ... , tn' )

  13. problem with small arrays

  14. Calibration of arrays

  15. Closer look

  16. Plane wave determination without picking FK Algorithm

  17. Two ways of determining the plane wave a) measure t1,t2,t3 directly and invert for slowness,azimuth b) try many plane waves systematically, inversely apply (t1',t2',t3') delays and sum: compare summation amplitudes assume plane wave with slowness and azimuth, compute theoretical delays (t1',t2',t3') and apply, in most cases it looks like this: if you come close the true values of slowness and azimuth you will get aligen signals and constructive summation:

  18. FK diagram destructive summation (wrong t1', t2', t3') 330° 30° azimuth slowness 12 300° 8 60° constructive summation (correct t1', t2', t3') 4 240° 120° 210° 150°

  19. Example: FK analysis, GRF arrayEvent S. XinJiang, 25-Jul-2007, mb 4.6 330° 30° azimuth slowness 12 300° 8 60° 4 240° 120° 210° 150°

  20. Tradeoff: location accuracy and coherency Array aperture no coherency no array features location possible, good array features low coherency low resolution Frequency

  21. Arrays in Germany GERES: aperture ~4km frequencies: 1 - 50 Hz GRF: aperture ~100km frequencies: 0.1 – 5 Hz GRSN: aperture ~1000km frequencies: 0.01 – 0.5 Hz

  22. Resolution of German Arrays Array aperture GRSN no coherency no array features location possible, GRF good array features low coherency GERES low resolution 0.05 1 Frequency (Hz) 50

  23. Benefits of Array Data Processing • Improvement of signal/noise ratio • Determination of slowness and azimuth • Phase identification • Location of remote events • Rupture tracking

  24. XinJiang event, time domainImprovement of signal/noise ratio

  25. Phase Identification

  26. Phase Map, Antofagasta 17-Nov-2007, Chile

  27. Phase Map, Antofagasta 17-Nov-2007, Chile

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