Methods based on the STA/LTA ratio and the modified energy ratio (MER) are studied for their efficacy in automatic first-arrival time picking on high-noise microseismograms. Testing of the two methods on both field and synthetic data indicate that they can pick arrival times for signal-to-noise (SNR) levels as low as 1.7, but that MER time picking yields more consistent results. Moreover, MER time picking is significantly faster than STA/LTA time picking.
For many seismic processing procedures, reducing the random noise on the input seismograms will lead to much improved results. Trace averaging (i.e., stacking) over a group of seismograms aligned in phase is a standard method for reducing random noise. Time shifts for trace alignment can be found using one of two methods: finding the average trace that gives minimum variance to the shifted input traces, and using the time picks from the MER method. A noise-signal separation (NSS) technique, developed as an extension to simple trace averaging, separates a noisy seismogram into a noisereduced signal component and a random noise component while preserving the relative signal amplitudes on the input seismograms.
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