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Broadening the Response of Short-Period Sensors: An Application to the Sprengnether S-7000 Seismometer

¹ Signal Processing Laboratory
University Mental Health Research Institute
P.O. Box 66517
GR-156 01 Papagou, Athens, Greece
E-mail: elkouts1{at}otenet.gr
 (E.K.)
² Institute of Geodynamics
National Observatory of Athens
Lofos Nimfon, Thisio, GR-118 10 Athens, Greece
E-mail: nmelis{at}gein.noa.gr
 (N.S.M.)

In this study the application of active damping on a short-period Sprengnether S-7000 passive sensor is presented. This sensor uses a unique magnetic suspension mechanism that eliminates spring resonance effects and ensures improved thermal behavior. The resilient properties of the magnetic spring are devoid of nonlinearities seen in the metallic springs. Although the magnetic suspension is highly susceptible in external magnetic fields, the magnetic scheme applied on the S-7000 seismometer ensures reliable magnetic shielding and it can be considered as a stable suspension. The multiple frequency-dependent feedback, which is the dominant technique in the design of broadband sensors, modified the original velocity response envelop of the old instrument. The theoretical response obtained from the new transfer function, in accordance with the experimental response obtained from calibration data, characterizes the modified S-7000 seismometer as a flat velocity (0.011 to 20 Hz) sensor with top sensitivity 1046 V/m/sec. Amplitude and spectral analyses of datasets obtained from local ground noise, as well as from local, regional, and teleseismic earthquake activity, have been shown to be comparable with respective recordings of other standard broadband seismometers operated at the Athens (ATH) seismographic station vault.