Guralp Systems CMG-6TD - Manual (Page 75)

CMG-6TD Calibrating the 6TD
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6.5 Calibration methods
Velocity sensors such as the 6TD are not sensitive to constant DC levels,
either as a result of their design or because of an interposed high-pass filter.
Instead, three common calibration techniques are used.
•
Injecting a step current allows the system response to be determined in
the time domain. The amplitude and phase response can then be
calculated using a Fourier transform. Because the input signal has
predominantly low-frequency components, this method generally gives
poor results. However, it is simple enough to be performed daily.
•
Injecting a sinusoidal current of known amplitude and frequency
allows the system response to be determined at a spot frequency.
However, before the calibration measurement can be made the system
must be allowed to reach a steady state; for low frequencies, this may
take a long time. In addition, several measurements must be made to
determine the response over the full frequency spectrum.
•
Injecting white noise into the calibration coil gives the response of the
whole system, which can be measured using a spectrum analyser.
You can calibrate a 6TD sensor using any of these methods, using its built-in
signal generator.
6.5.1 Noise calibration with Scream!
The most convenient way to calibrate a 6TD instrument is to use its
pseudo-random broadband noise generator with Scream!'s noise calibration
extension. The extension is part of the standard distribution of Scream!, and
contains all the algorithms needed to determine the complete sensor response
in a single experiment.
Information on other calibration methods is available on the Güralp Systems
Web site.
1. In Scream!'s main window, right-click on the digitiser's icon ( ) and
select Control…. Open the Calibration pane.
2. Select the calibration channel corresponding to the instrument, and
choose Broadband Noise. Select the component you wish to calibrate,
together with a suitable duration and amplitude, and click
. A new data stream, with a stream ID ending Cn (where
n is 0, 2, 4 or 6, depending on the selected tap), should appear in
Scream!'s main window. This stream contains the returned calibration
signal.
MAN-T60-0002 75 Issue F - February 2014