Installing the 3TB in a borehole
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 hand-held 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.
4.6
Down-hole orientation
Once the sensor is installed inside the borehole, you will need to measure its
orientation with respect to True North or Magnetic North. There is no need to rotate
the sensor itself, since the data can be rotated algorithmically after it is digitised.
Note:
The most common problem affecting sensor orientation is the
difficulty of determining an accurate North at the installation site.
Local variations in the Earth's magnetic field affect magnetic
compasses, as do many local structures, including including the
metal bore-hole casing itself.
A simple method for determining the orientation of a sensor package using the
sensor's own horizontal component sensors, has been used effectively by the
Blacknest Seismological Centre, UK, with down-hole and surface equipment from
Güralp Systems (
AWE Report O 10/93, 1993.
)
In this experiment, signals received by the N/S component of the reference sensor
are correlated with those received at the N/S and E/W components of the sensor
being studied, after different amounts of mathematical rotation. The highest
correlation will occur when the N/S component of the reference sensor matches the
rotated N/S component of the borehole sensor.
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Issue G - November 2019
