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Correction of seismograms for the transfer function of the seismometer

BELL TELEPHONE LABORATORIES, INCORPORATED, MURRAY HILL, NEW JERSEY

Abstract

If digitized seismic data is collected through a seismometer whose transfer function is known, it is not difficult to design a digital filter which will substitute a given band-pass transfer function for that of the seismometer. Various filters of this kind have been applied to digitized records of teleseisms registered on a Benioff short-period vertical seismometer at Chester, New Jersey. After digital filtering on a computer, the resulting data is converted to analog form and recorded on a seismograph drum for visual examination. These records indicate, (1) over the frequency range of interest for teleseismic registration, the seismograph may be regarded as a constant leading phase shift of 270 degrees, a 0.35 second time delay, and a phaseless amplitude function, (2) that it may not be easy to improve on the transfer function of the Benioff instrument for the detection of first motion, and (3) that some additional high-pass filtering might improve the signal-to-microseism ratio at times when the 2-4 second microseism level is high.