The use of galvanometers as band-rejection filters in electromagnetic seismographs

Abstract

The large-amplitude intermediate-period, 4-9 seconds, microseisms are effectively removed from high-magnification long-period seismograms with maximum magnification near 50 seconds period. This has been accomplished by the use of a galvanometer of natural period in the microseism range as a band-rejection filter. Addition of the filter galvanometer greatly increases the usefulness of these instruments for studies of long-period microseisms and for clear detection and resolution of the long-period components of both body and surface waves from small shocks at very great distances. Magnification equations and theoretical magnification curves show that extremely high sensitivities for desired periods and low sensitivities for undesired periods can be obtained by combining one or more filter galvanometers in seismograph systems, varying the damping constants and natural periods of the components, and varying the coupling between the components. Also, by proper choice of instrument parameters, undesired periods can be rejected without appreciable modification of the seismograph magnification for periods outside the rejection band.