Albuquerque Seismological Laboratory
Problems with STS-1 sensitivities Oct 1, 2011
Recent studies by the Lamont Waveform Quality Center (WQC) (e.g., Ekström et al., 2006) have found temporal changes in some sensors at Global Seismographic Network (GSN) stations. These studies noted an apparent loss of sensitivity in some sensors over time and speculated about aging of the primary GSN sensors.
In contrast, these apparent changes in gain were not observed in other remote calibration studies of the GSN (e.g. Davis et al., 2005; Davis and Berger, 2007) at tidal frequencies nor were they observed in the microseismic band (Hutt, 2007). The apparent gain changes were later confirmed to occur primarily in the period range 50 to 400 seconds (Ringler et al., 2010) and appear to affect only STS-1 seismometers.
Subsequent studies indicated that most of the observed problems are due to humidity in the connectors, cables, or feedback electronics boxes of the STS-1 seismometers (Hutt and Ringler, 2011). The humidity primarily affected the response of the STS-1 in the period range of several hundred seconds by overdamping the response (and decreasing the apparent sensitivity), and at some sites the overdamping is amplitude-dependent. In contrast, the response in the microseismic and tidal bands is generally unchanged or minimal.
To address the data quality issues raised by this problem, the GSN has undertaken a number of measures, including the replacement of all STS-1 cables, connectors, and the feedback electronics boxes during maintenance visits; checking absolute sensor sensitivity and orientation during site visits; and calibrating the stations annually. We are currently processing older calibration data in order to improve the instrument responses. As a result, we are updating our dataless SEED volumes frequently, and users are strongly encouraged to download the latest installment for the CU, IC, and IU networks and for the IW and US networks.
Information on the current status of stations may be found here. Details about the GSN data quality process may be found here. For more information, please contact dcc@usgs.gov.
References
- Davis, P., and J. Berger (2007). Calibration of the global seismographic network using tides, Seis. Res. Lett., 78 (4), 454–459.
- Davis, P., M. Ishii, and G. Masters (2005). An assessment of the accuracy of GSN sensor response information, Seis. Res. Lett., 76 (6), 678-683.
- Ekström, G., C. A. Dalton, and M. Nettles (2006). Observations of time-dependent errors in long-period instrument gain at global seismic stations. Seis. Res. Lett., 77 (1), 12–22.
- Hutt, C. R., personal communication, 2007.
- Hutt, C. R., and A. T. Ringler, Some Possible Causes of and Corrections for STS-1 Response Changes in the Global Seismographic Network, Seis. Res. Lett, 82, 560-571, 2011.
- Ringler, A. T., L. S. Gee, C. R. Hutt, and D. E. McNamara (2010). Temporal variations in global seismic station ambient noise power levels, Seis. Res. Lett, 81 (4), 605-613.
