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The information on what type of instrument is in operation at a given SIL station is stored in the master configuration file /usr/sil/etc/ at each station. Calibration data (poles, zeros and gain) for all implemented geophone and digitizer types are stored in separate files, called NAME.resp, on the /usr/sil/etc directory. Here NAME is the instrument type, e.g.\ LE1 for the Lennartz LE-3D geophone, RD3 for the RD3/OSD3 digitizer, etc. The calibration data file for Guralp CMG-3T GURALP.resp is printed below.

#	GURALP.resp:
#		The gain, poles and zeros of the transfer function for the 
#		Guralp CMG-3T broadband geophone.  Data from "Test and calibration
#		data, CMG-3T Serial No:  T3192, T3193, T3194, T3196 and T3197",
#		Guralp Systems report.
#	Author:
#		Sigurdur Th. Rognvaldsson
#	Date:
#		24-06-1995
#	Note:
#		The gain quoted here is the multiple of output sensitivity (G in 
#		the Guralp report) and the normalizing factor (A in Guralp report).
#		A factor of 2pi is also included to convert from s=if used in the
#		calibration report to s=2pi*if used in IMO report.  Thus:
#		gain = 2pi*G*A.  The output sensitivity or velocity output, G, is
#		taken as the mean of all 15 channels which have been calibrated,
#		i.e. G=2*756 +- 5 [V/m/s].  A = -49.5.  Hence
#		gain = 2*pi*2*756*(-49.5) = -4.702587e+05
-4.702587e+05					# Guralp geophone gain.
4	 # Number of poles.
-4.44221e-02 4.44221e-02 	# Poles
-4.44221e-02 -4.44221e-02 
-5.057964e+02 1.935221e+02
-5.057964e+02 -1.935221e+02
3 # Number of zeros.
0.0 0.0 # Zeros
0.0 0.0
9.456194e+02 0.0
The calibration information in the RD3.resp file contains one additional zero to transform the data from ground velocity to displacement.

At start-up, the data acquisition software reads the instrument type from the configuration file, looks for the appropriate NAME file and reads the necessary calibration data. This is needed to estimate the absolute spectral amplitudes of triggering phases and to produce normalized amplitude data for use by the central selection software.

At the center similar calibration data files are used by the central processing software to remove the instrument response from the recorded waveforms. When the waveforms arrive from the site stations, appropriate calibration data for the recording station is written in the ah header before the data is stored on disk. The transfer function is specified by its poles and zeros in the s-domain. The calibration information in the ah header also contains one additional zero to transform the data from ground velocity to displacement. The coefficient of the highest power of s in equation 16 is used as a normalization factor in the ah programmes.

Storing the calibration data in every waveform data file is a waste of disk space and in future versions of the database calibration data will be kept in separate tables linked to the compactly stored waveforms.

Sigurdur Th. Rognvaldsson
Wed Mar 19 12:54:50 GMT 1997