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Published byDarlene Ford Modified over 8 years ago
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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
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Epicentre Location using Arrays Problem: inaccuracy due to deviations from velocity model at the receiver Solution: array calibration (empirical corrections to direction)
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Principle of Array Analysis for a given station geometry: t 1, t 2, t 3 (observed) → plane wave (azimuth and slowness) → t 1 ', t 2 ', t 3 ' (theo)
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Validate result apply negative (t 1 ',t 2 ',t 3 ')
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In real life...
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Select Picks and measure t n
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Check Accuracy (apply -t n ')
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Larger aperture
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Again, select picks and measure t n
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Beamforming not satisfying
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for appropriate configuration t 1, t 2,..., t n (observed) → plane wave → t 1 ', t 2 ',..., t n ' (theo) (t 1, t 2,..., t n ) ≈ (t 1 ', t 2 ',..., t n ' )
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aperture too large / frequencies too high t 1, t 2,..., t n (observed) → plane wave → t 1 ', t 2 ',..., t n ' (theo) (t 1, t 2,..., t n ) ≠ (t 1 ', t 2 ',..., t n ' ) high veloc. low veloc.
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problem with small arrays
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Calibration of arrays
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Closer look
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FK Algorithm Plane wave determination without picking
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Two ways of determining the plane wave a) measure t 1,t 2,t 3 directly and invert for slowness,azimuth b) try many plane waves systematically, inversely apply (t 1 ',t 2 ',t 3 ') delays and sum: assume plane wave with slowness and azimuth, compute theoretical delays (t 1 ',t 2 ',t 3 ') 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: compare summation amplitudes
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FK diagram 30° 60° 120° 150°210° 240° 300° 330° 4 8 12 slowness azimuth constructive summation (correct t 1 ', t 2 ', t 3 ') destructive summation (wrong t 1 ', t 2 ', t 3 ')
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Example: FK analysis, GRF array Event S. XinJiang, 25-Jul-2007, mb 4.6 30° 60° 120° 150°210° 240° 300° 330° 4 8 12 slowness azimuth
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Tradeoff: location accuracy and coherency Frequency Array aperture no coherency no array features low resolution good array features location possible, low coherency
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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
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Array aperture no coherency no array features low resolution good array features location possible, low coherency 0.05 50 1 GRSN GRF GERES Frequency (Hz) Resolution of German Arrays
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Benefits of Array Data Processing Improvement of signal/noise ratio Determination of slowness and azimuth Phase identification Location of remote events Rupture tracking
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XinJiang event, time domain Improvement of signal/noise ratio
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Phase Identification
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Phase Map, Antofagasta 17-Nov-2007, Chile
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Rupture Tracking
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