abstract

A critical requirement in the digitization of analogue seismograms is that the y axis of the digitizing device be parallel to the recorded direction of swing of the galvanometer. Small misalignment of the seismogram on the digitizing table can produce major errors in the results of Fourier analysis and other digital techniques. In the examples discussed, the direction of the galvanometer swing was not parallel to the recording drum axis (the nominal direction of swing). Fourier spectra and group velocities are presented for cases in which the seismogram is aligned with the x axis of the digitizer: (1) parallel to the trace; (2) normal to the drum axis; (3) correctly aligned, normal to the direction of galvanometer swing. If the alignment is incorrect, the errors in the results are significant at both short and long periods and lead to erroneous geophysical conclusions. The magnitude of the errors increases with both the degree of misalignment and the amplitude of the recorded signal.

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