Summary
The GSN Dead Channel Metric is a variant of dead channel detection based on a low signal threshold in the microseism range of 4 to 8 seconds period. The values shown with this metric indicate a TRUE or FALSE condition, depicted by a ‘1’ or ‘0’ value respectively. A TRUE value, meaning that the channel is dead, occurs when the median probability value of a day’s worth of instrument-corrected power spectral density (PSD) values are 5 dB below the New Low Noise Model (NLNM) when averaged over the 4 to 8 second band.
Uses
A ‘1’ value is a good indication of a channel that is not responding to ground motion signals. This can also appear where the response metadata for the channel is faulty. Using a boolean value for dead_channel_gsn allows users to scale metrics such as percent_availability whenever a channel is dead. For instance, multiplying percent_availability by 0 whenever dead_channel_gsn is TRUE is one way to prevent dead channels within the archive from being included as “available” in reports. Using rrds to request broadband channels with dead_channel_gsn=0 should omit dead channels from the request.
Data Analyzed
Traces – one N.S.L.C (Network.Station.Location.Channel) per measurement
Window – up to 24 hours depending on the completeness of data
Data Source – IRIS miniSEED archive or IRIS PH5 archive
SEED Channel Types – [BCDFHLM]H? | High Gain
Algorithm
- Request N.S.L.C data having sample rate 1 SPS or greater.
- Generate the PSD matrix for the selected channel, midnight to midnight (as per McNamara, et. al.).
- Slice the PSD matrix, retaining values between 4 to 8 seconds period.
- Calculate the median power in dB at each bin frequency (4-8 seconds), writing results to a vector Y(x).
- Calculate the differences in median vector from the equivalent NLNM vector and take the average: mean[ NLNM – Y(x)].
- If the dB deviation from the NLNM is greater than 5.0, set dead_channel_gsn value = 1 (TRUE), else set value = 0 (FALSE):
value = '1': deviation from NLNM > 5.0 | '0':default
Metric Values Returned
value – ‘1’ if TRUE (dead), ‘0’ if FALSE (ok)
target – the trace analyzed, labeled as N.S.L.C.Q (Network.Station.Location.Channel.Quality)
start – time of the first sample of time series data for that day in UTC
end – time of the last sample of time series data for that day in UTC
lddate – date/time the measurement was made and loaded into the MUSTANG database (UTC)
Notes
References:
McNamara, D.E. and R.P. Buland, Ambient Noise Levels in the Continental United States, Bull. Seism. Soc. Am., 94, 4, 1517-1527, 2004.
Ringler, A.T., M.T. Hagerty, J. Holland, A. Gonzales, L.S. Gee, J.D. Edwards, D. Wilson, and A.M. Baker. 2015. The data quality analyzer. Comput. Geosci. 76, C (March 2015), 96-111. DOI=http://dx.doi.org/10.1016/j.cageo.2014.12.006
