MAXIMUM-LIKELIHOOD FILTERING OF LASA NOISE SEISMOGRAMS.
TELEDYNE INDUSTRIES INC ALEXANDRIA VA EARTH SCIENCES DIV
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Realizable and symmetrical maximum-likelihood filters, two seconds long and based on fitting intervals consisting of 75, 150, 180, and 300 seconds of both raw and bandpass filtered inputs, were computed using seismic noise recorded during the night of 25 March 1966, by 19 sensors in the AO subarray at the Montana LASA. Excluding the realizable filter based on raw data, the effect o maximum-likelihood filtering noise samples lying outside the fitting interval is to produce noise levels 1 to 3 db lower than the corresponding direct sum of bandpass filtered data. Moreover, the filters lower the rms noise level slightly more than the square-root-of-N in the data sample lying outside of, but immediately adjacent to, the fitting interval, but fail to yield the square-root-of-N noise reduction if applied to noise fields which are several minutes removed from the interval. Depending upon the nature of the input data raw or prefiltered, the symmetrical filter is 1 to 5 db better than the corresponding realizable filter, but its output appears to be approximately 180 degrees out-of-phase with its realizable counterpart. Author