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This document outlines the traditional calibration procedures for the Global Seismographic Network (GSN) sensors, focusing on the accuracy of measurement and sensitivity through various tests. It discusses the importance of sample rate correction, handling sensor failures, and ensuring correct metadata. The methods described include comparing observed outputs with modeled responses and identifying pitfalls in testing techniques. Additionally, it highlights the benefits of using tides for continuous signal monitoring and the significance of comparing co-located broadband sensors for reliable data.
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II GSN CalibrationProceduresNovember 16, 2011Peter DavisProject IDAScripps Institution of OceanographyUniversity of California, San Diego
Traditional calibration procedures verified the DAS sensitivity and measured shape of system response.
Convolve model of sensor response with input • Compare observed output with model • Iterate until fit acceptable
What these tests measure: • Random binary test determines shape of response curve within 1% • Step input to digitizer determines sensitivity to > 0.1% • Manufacturer provides value of sensor generator constant to within 1%
What these tests measure: • Random binary test determines shape of response curve within 1% • Pitfall – capturing the input signal, test only ‘snapshot’ • Step input to digitizer determines sensitivity to > 0.1% • Pitfall – compensate for system load • Manufacturer provides value of sensor generator constant to within 1% • Pitfall – not verified
Sumatra-Andaman earthquake excited many low frequency normal modes Figure from Park et al. (2004), courtesy G. Roult
Portable sensors used to check sensitivity of permanent sensors:
What this test measures: • Relative performance of GSN as a network • Pitfall – no measure of absolute sensitivity • Pitfall – accuracy limited by mode coupling • Pitfall – need a very large earthquake to excite the mode to observable amplitude
Why use tides? • Tides are a continuous, constant signal • Tidal amplitudes can be predicted to 1% accuracy globally • Tides are observed at all times at most GSN stations • Pitfall – must include effects of ocean loading • Pitfall – horizontal sensors effected too strongly by local tilts • Pitfall – tidal periods outside band of seismic interest
What comparing co-located broadband sensors measures: • Accurate determination of relative response of the common instruments • Accurate determination of relative orientation
