Guralp Systems CMG-1T - Manual (Page 18)

CMG-1T Ocean Bottom Seismometer Operator’s
Guide
Section 3, Page 2 September, 1999,
Issue C
100 SEC AND 30 SEC HYBRID VELOCITY SENSOR
The block diagram for both of the hybrid systems is given below.
In comparison to a conventional feedback accelerometer the hybrid velocity
response is obtained with a dominant single pole which is set at either 0.1 Hz or
at 0.033 Hz. The dominant pole breaks the system response into two sections,
the first one being flat to acceleration from dc to the dominant corner frequency
(0.1 Hz or 0.033 Hz) and the second response flat to velocity starting from the
dominant corner frequency until the high frequency cut off.
The high frequency cut off is realised with an active low pass filter. Without
this low pass filter the sensor velocity response is flat up to 100 Hz.
As the hybrid system is dc coupled to remove any dc offsets a 0.01 Hz (100 sec)
or 0.005 Hz (200 sec) active high pass filters are used. These filters are external
to the sensor feedback loop.
The main advantage of such a system is to provide a stable response,
particularly for portable systems, with a high saturation level at high frequencies
and high dynamic range at the long periods.