Larry, I have been trying to finness event detectors for years, and of course have never found one that can sort out earth-related from man-made events, only "something" from "not something". The most common is the "LTA-STA" process that can be implemented either by analog filters or digital means. Basically, a short increase of short period average energy is detected above a running long term average. The STA is long enough to avoid spikes, like from radio telemetry. Some digital detectors manage to use running FFTs before determining the mean amplitudes. The detector parameters must be "tuned" to the local noise situation, ideally not missing any events nor filling the event file with junk. Of course, local and teleseismic detectors need different parameters. I don't know where some code examples could be found, but I would snoop around some major university seismo sites. And of course, all the methods involve a pre-event memory generally long enough to catch the P-wave of a local event that triggers on the S-wave. In the '70s, I used multi-channel analog tape recorders to record the multiplexed FM telemetry carriers on a tape loop to provide the delay; the loop output was written to the event tape (still telemetry carriers) when the LTA-STA detector triggered. The tape loop had to be replaced daily, and the splice in it often made glitches in the data. Obviously digital delays are easier to do, as long as time tagging is preserved in the delayed or pre-event data. The most competent detector I am familiar with is the one used in the IRIS broadband stations. It was developed by Albuquerque, and is called the MHH detector, or Murdock-Hutt-Halbert. It is quite elaborate, and is set up or "built" by 11 parameters specified in the configuration file for the station, with separate detectors for data of different sample rates (derived by decimation and FIR filters from a single instrument). Each MHH detector selects a particular IIR filter at its input. It has proven quite robust in not missing events, and reports detection quality info with each detection along with the period and amplitude of the event and the background average counts. I believe that the code is available in C, and might be found on the ASL web site (aslwww.cr.usgs.gov) . The IRIS stations run OS-9 in 68020/30 multitasking processors, so I don't know if it is PC compatible or if a stand-alone version is available. Regards, Sean-Thomas __________________________________________________________ Public Seismic Network Mailing List (PSN-L)
Larry Cochrane <cochrane@..............>