Guralp Systems CMG-3TB - Manual (Page 46)

CMG-3TB
3.5 Levelling and centring
Once it is installed, you should level and centre the instrument ready
for use. This can be done using the various surface control units:
1. Connect an inclinometer monitor unit to the breakout box.
2. Turn on the borehole control unit using the ON/OFF switch
under the transparent flap.
3. Turn on the inclinometer monitor unit using its ON/OFF switch,
and read off the X and Y components of the tilt from the
analogue meters.
4. If both tilts are within the green shaded region, the instrument is
close enough to vertical that it can be levelled and centred
successfully. If either output is in the red shaded region, you
should
not attempt to unlock or centre the sensor masses.
Instead, if possible, you should move the instrument within the
borehole to a place where it can lie closer to vertical.
5. Connect a handheld control unit (HCU) to the sensor control
unit, if you have one.
6. Unlock the sensor masses, either by pushing the UNLOCK
buttons of the borehole control unit, or by holding down the
ENABLE and UNLOCK switches of the HCU together for at least
six seconds.
7. When you press the switches, the BUSY LED will come on.
After a while, the unlocking process will be completed, and the
instrument will start centring itself. Whilst this happens, the
BUSY LED will flash.
8. Monitor the outputs of the mass positions, either using the HCU
or your recording system. The microcontroller inside the unit
will zero the outputs from the vertical, N/S and E/W sensors in
sequence. After successful centring, the mass position outputs
should be in the range 0.1 – 0.8 V.
9. If the centring process leaves the mass position outputs above
±1.1 V, repeat steps 4 and 5. You will probably need to initiate
the centring process several times before the masses are
adequately centred.
46 Issue C