Guralp Systems CMG-3TB - Manual (Page 47)

Operator's guide
3.6 Downhole orientation
Once the sensor is installed inside the borehole, you will need to
measure its orientation with respect to the compass points. There is no
need to rotate the sensor itself, since the data can be rotated
algorithmically after it is digitised.
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 downhole
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.
Once you know the deviation of the borehole components from the
compass points, you can instruct the digitizer to rotate the signals
algorithmically.
Installing the Scream! extension
The Relative Orientation extension is supplied in the standard
Windows distribution of Scream! 4.2 and later.
The extension uses Matlab libraries, which are currently only available
for Windows. However, you do not need the full Matlab package to use
the extension. The Matlab runtime libraries are also included in the
Scream! distribution.
Installing the reference instrument
To measure the orientation of a sensor, you will need a second
instrument which is known to point precisely North. It should be
located on a solid surface as close to the other instrument as possible.
Most boreholes are constructed with a concrete base around the top of
the borehole. If this is present, we recommend installing the reference
sensor there.
Ideally, the two sensors will be directly connected to the same 6-
channel digitizer. If you are using separate digitizers, you will need to
ensure they are
exactly synchronized. This can be done by connecting
GPS receivers to both digitizers and waiting for the control system of
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