...
For security reasons, the military occasionally diverts seismic and acoustic data. Often the divert consists of splitting the data into Over time how this diversion is performed has changed. Currently, when diverted the entire data set is removed. Diverted data is then reviewed by military authorities, if it does not contain sensitive recordings it is returned to the ONC archive.
Prior to August 2016, instead of diverting the entire frequency range of the data, the military diverted only a low frequency band of the data. When this filtering occurred, the file-name was appended with 'LPF' or 'HPF' for high-pass or low-pass filtering a low and high pass filtered components with the file names appended with "-LPF" or "-HPF", respectively. Sometimes one-half of this diverted is available live, while the other component is withheld for review. Usually that withheld data is returned, after a delay of 3 days to 2 months. Once filtered, this data remained in the LPF/HPF frequency bands.
Withheld data is usually altered by band-pass filtering, often with decimation or resampling. In rare circumstances, withheld data is returned whole or only the missing LPF/HPF data is returned (this data is not labelled "-LPF" or "-HPF", but it should be). This returned data is not usually compatible with original/non-diverted data. Data diversion is further explained in the data diversion page.
...