Guralp Systems CMG-1T - Manual (Page 15)

Operator’s Guide CMG-1T Ocean Bottom
Seismometer
September, 1999, Issue C Section 2,
Page 3
The positions of the masses of all sensors are monitored by identical
capacitative position sensors and it is the signals from these which form the
basis of the seismometers’ mode of operation. The electronic processing of the
mass position signals is carried out on circuit boards mounted in a screened
compartment above the mechanical components. The main output voltages,
proportional to ground-velocity, are transmitted out of the case on differential
balanced lines, while the mass position signals are sent as single-ended circuits
referred to analogue ground via the output plug. Coming into the case are the
control signals which signal the control circuit to clamp or unclamp the masses
and to start the levelling and zeroing sequences, running the motors to null the
offset voltages from the transducers. There is also provision for applying a
calibrating voltage to the force transducers to allow the deflection sensitivity to
be measured.
The outer case is completely hermetically-sealed to the base by a compression
‘O’ ring joint and all electrical connections pass to the exterior via a glass sealed
plug. The only mechanical adjustments on the case are the three levelling feet
which can be locked in position without disturbing the setting. The tips of the
feet carry insulating beads which prevent electrical ground loops from being set
up inadvertently.