Picks the best channels at a station, avoiding processing 00.BHZ and 01.BHZ at the same station.
This can often happen when there are multiple sensors at a station, perhaps a surface seismometer and
a borehole seismometer. The processor decides on best based on a simple hueristic based on site
and channel codes. The defaults, each specified as a comma separated list, are:
site Code Heuristic = 00, ,01,02,03,04,05,06,07,08,09,10
gain Code Heuristic = H,L
band Code Heuristic = B,H,L,M,S,V,E,U
orientation Code Heuristic = Z,N,E,1,2,3,U,V,W
All three components are considered equally best, so in a typical broadband station the best channels might be 00.BHE, 00.BHN, 00.BHZ, and this subsetter will pass all three, while failing every other channel at this station. This means that any channel that is part of a three components of motion group will be considered "better" than any lone single component.
See edu.sc.seis.fissuresUtil.chooser.BestChannelUtil for the underlying code. The reason this is an event-channel subsetter is because the "best" channel at a station may change with time, for example a short period only station may be replaced by a broadband seismometer. Before the switch SHZ may be the best vertical, but afterwards it would be BHZ and SHZ would no longer exist.