2. Turn the seismometer until the north-south guide on the top of the pressure vessel is
parallel to the line at the top of the hole.
2.8.2 Aligning the Trillium Compact Posthole using a Surface
seismometer
You can use a seismometer installed at the surface to determine the orientation of the
horizontal (X and Y) components of the Trillium Compact Posthole once installed in the hole.
This method involves comparing the recorded output of both seismometers and computing
the relative direction of seismic wave motion to determine the relative azimuth of the down-
hole Trillium Compact Posthole compared to the surface seismometer.
When you install the reference seismometer on the surface, ensure that you align it carefully in
a known orientation. After both seismometers/instruments have been installed, leave the
installations undisturbed for at least one hour while collecting data from both. When you are
ready to perform your data analysis, ensure that your post-processing software is equipped to
apply a rotation transformation, allowing it to measure and correct the relative azimuth. Take
note of the values and enter them on the Centaur in the Digitizer Orientation Correction
configuration feature. See the
Centaur user guide (17935)
for instructions.
Orientation correction.
You should record the actual azimuth. This value can be used
to perform field data rotation to correct the sensor orientation for instances where the
physical orientation of a deployed three-component geophysical sensor is different
than what is desired, resulting in output X, Y and Z signals that do not represent the
desired directions of sensitivity (typically East, North and Vertical). See the
Orientation
correction
sections in the
Centaur User Guide (17935)
for more detail.
One advantage of this method of alignment is that it allows you to verify the performance of
the down-hole instrument: it should be quieter than the surface instrument.
Contact Nanometrics (see
"Contact Technical Support " on page 103
) for more information on
using this method of alignment.
Chapter 2 - Installing a Trillium Compact Posthole Seismometer
16889R10 • 2020-11-12
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