Sean,

Thanks for the input.  I haven't tried anything very fancy to date.  The
scheme I'm using at the moment is reminiscent of the LTA-STA idea.  I keep a
running "LTA" with a shorter "STA" centered in the LTA window, and trigger
on a specified number of samples that exceed a defined threshold value.
I've played back the few event files that I have from my system along with
some no-event files and it does an OK job of distinguishing between them.
BUT several of my event files do not include the P-wave
onset so I'm a long way from claiming any sort of victory.  In the mean
time, I'm willing to tolerate a high false alarm rate.  If the false alarm
rate is high enough, you wind up with a round the clock record (chuckle).

My data collector does have some pre-detection memory, but it's only 5
minutes at the moment.  Not long enough I've concluded.  Disk space is
cheap, missing the start of the show is disappointing.

Larry

----- Original Message -----
From: 
To: 
Sent: Saturday, August 05, 2000 3:45 PM
Subject: event detection


> 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

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