Installing the 3TB in a borehole
9. A new graph will be displayed showing the results:
Our sample instrument is thus aligned at –90.61 ± 0.07°.
10. The error given is only a rough estimate. For best results, you should repeat
the orientation experiment several times using different data sets. The true
error in the computed orientation can then be determined by observing the
spread of the results.
The Blacknest orientation method generally provides a reliable indication of the
sensor's orientation. In most cases, the greatest source of error is in the installation
of the reference sensor.
If you have particular difficulty in deriving a stable value, additional information is
contained in a third output window, which shows a waterfall plot of Coherence vs
Frequency vs Angle. The frequency range used in the calculations is indicated with a
black outline.
This plot can be used to select a more advantageous frequency range where the
coherence curve is smoother and easier to interpret. In the example above, the
frequency range 0.23 to 0.6 Hertz looks particularly promising: the surface within
this range forms a smooth, symmetrical arch. Ideally, the peak should be very close
to unity and the lowest points, at the margins, should be close to zero.
46
Issue G - November 2019
