Sensitivity and Orientation
To permit our field crews to conduct the required surveys as easily and rapidly as possible without sacrificing accuracy and to verify sensor calibration at the same time, we have purchased a compact broadband sensor and DAS for short deployments at the station whose sensor orientation and calibration is to be checked. We selected the Nanometrics Compact Trillium/Taurus digitizer hardware for its compactness, portability and performance. These devices can be operated independently of the existing GSN system and are small enough to be shipped in checked luggage.
After orienting the portable sensor to true North (see illustration courtesy of Nanometrics) and gathering several hours data (such as shown here) while operating in parallel to, but electronically separate from, the GSN system, the sensor will be retrieved and returned to San Diego where its calibration can be verified on a shaketable.
Random Binary Calibrations
At the core of a GSN station is the Quanterra Q330HR data logger. For calibration tests on IRIS/IDA systems, the Q330HR DAS generates the random binary signal and also provides a record of the input to the sensor calibration coils. Because this input has been processed by the DAS with the same digital filters as the output data stream, comparison of the input and output permits a direct estimate of the pure sensor transfer function.
Due to a design limitation, the Q330HR will not generate a random binary series longer than 4.5 hours (16383 sec) nor with a frequency lower than 0.49 Hz. The series shown in Figure 1 and Figure 2 was generated with the following parameters: Signal attnenuation -12 dB (+/- 2.5 V), frequency .4902 Hz, duration = 4 hours.
The response of the Z, N and E components of the seismometer are shown in the top three traces, the input to the coils, on the bottom trace. Note that the polarity of the Z signal is different from the N and E components. This is always the case for the STS1 sensors.
REFERENCES
Berger, J., D.C. Agnew, R.L. Parker and W.E. Farrell, Seismic system calibration: 2. Cross-spectral calibration using random binary signals, BSSA, 69, 271-288, 1979.
Davis, P. and J. Berger, Calibration of the Global Seismographic Network using tides, SRL, 78, 454-459, 2007.
Davis, P., M. Ishii and G. Masters, An assessment of the accuracy of GSN sensor response information, SRL, 76, 678-683, 2005.
Fels, J.-F. and J. Berger, Parametric analysis and calibration of the STS-1 seismometer of the IRIS/IDA seismographic network, BSSA, 84, 1580-1592, 1994.