BANDPASS FILTERING TO REDUCE NOISE ON SEISMOGRAMS - IS THERE A BETTERWAY

Short-period (SP) P waves at low signal-to-noise ratios (S/Ns) usually appear as signals of about 1 Hz or higher frequencies riding on backg round noise with frequencies of 0.5 Hz or less. Often bandpass filteri ng is used to further attenuate the noise relative to the signal to im prove the S/N. However, bandpass filtering introduces significant dist ortion into the signal. Here the use of optimum filters is described, such filters being constructed using autocorrelation functions of the signal and noise derived from noise and signal models. The autocorrela tion of the impulse response of the recording system is used to descri be the signal. For the noise, a tapered cosine wave is used that has t he frequency of the main noise component. To compute the filter, the r elative sizes of the signal and noise correlations should be proportio nal to the square of the S/N. Now, the noise can be estimated by subtr acting the estimated signal from the observed seismogram. This propert y can be used to decide by trial and error on the S/N. The optimum S/N is found by gradually reducing the trial ratio starting from too high a value, until the noise has the amplitude expected and there is no e vidence of significant signal in the noise estimate. Examples are give n showing how the method can lift observed signals off the background noise with less distortion to the signal than usually occurs with band pass filtering.