It is important for users to understand the procedures we follow to generate the quality assessment metrics and assure proper staton operations.
The GSN calibration policy was established to ensure station instrumentation functions as expected and that the response of the system is well characterized by the metadata. Under nominal conditions, full calibrations will be performed annually. If a station produces anomalous calibration results, or appears to provide rapidly changing response information, we will calibrate stations more frequently until the problem is resolved. General calibration procedures are described in this linked document.
Sensitivity, Orientation and Location (SensOrLoc):
As part of a campaign to perform absolute calibration of the sensor sensitivities and verification of the sensor orientation and location, we have developed a procedure called SensOrLoc (for Sensitivity, Orientation, and Location). This is as close to an "absolute" calibration check as we can get without removing the sensors from the field.
- Sensitivity - This involves the verification of a reference sensor (and recording instrument) at one of the network operations centers, installation of this reference in the field co-located with the GSN sensors for a temporary, co-operation, then a return to the network operations center for verification of the reference sensor response. This allows us to perform an inter-sensor comparison to determine absolute sensitivity and orientation of the in situ sensors relative to a controlled reference.
- Orientation – The orientation of horizontal sensors is being confirmed with each stations upgrade. As there are several techniques available to orient sensors, we have described the techniques we employ in the Sensor Orientation Best Practices document.
- Location – The exact geographical (latitude and longitude) for each site is also being confirmed with GPS techniques for comparison with early map based determinations.
Current QC Practices:
The data collection centers at ASL and IDA both conduct data quality control on a daily basis. (Click on the links on the network operators names for information with respect to their QC practices.) In addition to the Lamont WQC procedures, the IRIS DMC has also created a set of tools to assess data quality, called MUSTANG (for Modular Utility for STAtistical kNowledge Gathering). Probability Density Function analyses for seismic data are provided at the MUSTANG PDF page for data back to 2004, and for all images back to 1972 at the USGS PDF ftp site.