Effect of filtering on event picking

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Figure 7.13a shows changes in wave-shape produced by the analog filtering in modern digital instruments. What can you conclude about the effects on picking?


The filtering on traces (b) and (c) is normal alias filtering for 2 ms and 4 ms sampling. Comparison of traces (a), (b), and (c) shows that lowering the high-frequency cut increases the delay. Comparing (c) and (d), we see that increasing the filter slope also increases the delay. Comparing traces (e) and (f) shows that changing the high-frequency slope changes the pulse shape but has little effect on the delay. Changing the low-frequency cut has a similar effect as seen by comparing (c) and (g). Narrowing the bandwidth results in shifting energy backward within the wavelet, decreasing the ability to pick early within the wavelet and increasing the likelihood of misinterpreting polarity.

Figure 7.13a  Far-field air-gun signals through various instrument filters: (a) no extra filtering; (b) out-24 Hz, 72 dB/octave; (c) out-62 Hz, 72 dB/octave; (d) out-62 Hz, 18 dB/octave; (e) 8–124 Hz with slopes of 18 and 72 dB/octave, respectively; (f) 18–124 Hz with 18 and 36 dB/octave slopes; (g) 8–62 Hz with 36 and 72 dB/octave slopes. Timing marks are 10 ms apart.

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