Guralp Systems CMG-6T - Manual (Page 23)

Operator's Guide
3.5 Calibration methods
Velocity sensors such as the 6T 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 6T sensor using any of these methods, using the
signal generator built in to all Güralp Systems' digitisers.
3.6 Noise calibration with Scream!
The most convenient way to calibrate a 6TD instrument is to use a
CMG-DM24's pseudo-random broadband noise generator along 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 digitizer'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 Inject now. A new data stream ending Cn (n = 0 – 7)
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