In 2007, we conducted a systematic study of tides observed on GSN instruments with the goal of validating the instrument response information distributed to users. This work was completed and published in Seismological Research Letters. See citation below. In that study, we investigated the variation of the M2 lunar tide as measured at GSN sites. Although this signal is outside the normal frequency band of interest to seismologists, the M2 tidal line is well recorded at many GSN stations with very good signal-to-noise ratio (SNR) on the vertical principal seismometer and is a reliable indicator of sensor sensitivity.
The above figure is drawn from that study. If the instrument response were perfectly characterized throughout the deployment period, we would expect the ratio of observed to theoretical amplitudes to be unity in the top plot and the phase difference to be nil in the lower plot. That they are not always so exposes a problem in what we report for II.NNA and points us to what time periods we need to investigate further. When the current in situ recalibration program (see sidebar for explanation) has covered all stations once, we will be in a position to use calibrated measurements from the current period to extrapolate expected values back in time and correct many of the metadata inaccuracies uncovered by the tidal observations.
Davis, P., and J. Berger (2007). Calibration of the Global Seismographic Network using tides, Seismol. Res. Lett. 78, 454–459.