Never before have I ever seen a seismogram/spectrogram signal like GS.LA03 on the 24th between about 1500 UTC and 1800 UTC.
The linear waveform of that looks like this:
And the spectrogram looks like this:
And it sounds like this:
436 KB FLAC file (You can play this with WinAmp or Audacity – both free.
Initially when I looked at them they seemed like helicopter traces but I have never heard them make a sound like this. That does not mean to say they are not however. This is a confirmed helicopter trace.
There is a trace on the 25th (today) on LA10.10 which looks like a helicopter, and sounds more like one. Here is the trace of today:
The confirmed Mt St Helens helicopter was on a 100 samples per second instrument like LA10, where as LA03 is 40 samples per second. Since I have never come across this signal on a 40 Hz instrument I do not know what to expect. It still seems very odd that it is so perfectly stepped. Maybe it is a signal introduced for testing? I have not really got a clue other than that, but it is quite interesting.
It is also showing on the borehole seismo and I am not convinced that the signal from a chopper would go down 6000ft!
The sound is the same as LA03. Stepped and NOT like the sound of the helicopter. This same signal appears on the 25th (today) at LA10.10, but more importantly it is at exactly the same time band.
No ideas now. Interference or a test signal of some sort? Who knows!