The seismometers of the SIL array can be used as accelerometers by
modifying the transfer function slightly. The frequency response
described by equation 29 and shown in Figure 4 is
proportional to the velocity of the ground motion at the geophone site.
Acceleration is the time derivative of velocity. In the frequency domain
this corresponds to multiplying the input signal (i.e. ground velocity) by *s*.
To find the response of the
SIL system to ground acceleration the velocity proportional transfer function
is divided by frequency, i.e. one extra pole is added at (0.0, 0.0).
Equation 29 then becomes:

To obtain the acceleration at the recording station, the spectra of the seismograms are divided by . Figure 5 shows a plot of versus frequency for the six types of geophones used in the SIL system. For the two short period instruments, the response has a peak at their respective eigen frequencies. For the broadband instruments the acceleration response is flat from about 0.2 Hz (determined by the digitizer) down to the respective eigen frequency of each geophone type.

The
ground displacement at the geophone sites can be obtained from the
velocity proportional seismograms by integration. In the frequency
domain this corresponds to dividing the input of the system by *s* or,
equivalently, multiplying the transfer function by *s*. This implies adding
one zero at (0.0, 0.0). Equation
29 then gives the displacement proportional transfer
function as:

To obtain the ground motion, the spectra of the seismograms are divided by . Figure 6 shows as a function of frequency for the six seismometer types.

Wed Mar 19 12:54:50 GMT 1997