The frequency response or transfer function of a system relates the input signal, x(s), to the output y(s) of the system by an equation of the form:
By convention the frequency variable s is the complex frequency, i.e.
.
At present the SIL seismic data acquisition system consists of 28 remote seismic stations. Each station is equipped with an RD3/OSD3 digitizer and a seismometer. The digitizer is composed of a fourth order low-pass filter, a second order anti-alias filter, a 56-coefficient FIR filter and amplifiers. The response of the FIR filter is not included in the following description. It is flat up to approximately 40 Hz and falls off abruptly for higher frequencies (Nanometrics 1990).
At present six types of seismometers have been implemented, the 1 Hz Lennartz LE-3D, the 5 sec Lennartz LE-3D/5s (Lennartz 1990), the Streckheisen STS-2 used at the IRIS station in Borgarfjöršur, Guralp CMG-3T, provided by the University of Cambridge and Guralp CMG-3T and CMG-3ESP from the Passcal instrument pool.
The velocity response of the RD3/OSD3 digitizer is independent of frequency from approximately 1 Hz up to about 40 Hz. All the seismometer types in use in the SIL network also have flat response curves in this frequency band. However, their sensitivity differs by more than a factor of five and for the broadband instruments the passband extends down to 0.01 Hz. The difference in amplification has to be accurately accounted for in the analysis of the data, especially since estimates of absolute spectral amplitudes are used to constrain the focal mechanisms of earthquakes recorded by the SIL system.
This report documents the frequency response of the digitizer and geophones currently in use in the SIL network and describes the implementation of instrument calibration data in the system.
