ULF Spectra
The data acquisition , recording and display can be done in many ways, each seems
to have their special strong and weak sides, as I discovered .
For instance GRAM , in conjunction with the PC sound card is very powerful and may
be the most useful and versatile tool for lower VLF, ELF and the higher ULF bands.
But many parameters are fixed, you are not allowed to just select any frequency as
the start and stop spectrum window. For example the lowest frequency window which
is possible is 0-86Hz. So if you wish to display a band from 5.....45Hz ,you can't do
that. SPECTRAN is restricted to audio frequencies only. And if you wish to ex-
plore frequencies below 10Hz, you discover limitations with most sound card's low
frequency response.
No low frequency limits exist with standard ADC devices, the less expensive types
are usually having high frequency limitations instead. I have used the ADC-11 from
PICO Technology, which is quite versatile in spectrum mode. You can select several
FFT modes and from 256 to 4096 data points.
Recently I have obtained the A/D card which is being used by the seismic data
amateurs, which can be seen at the ELFRAD site. This gives you up to 6 channels
and is of course directly coupled (dc capability). The software which comes with
the card ("SDR") is extremely versatile and therefore takes a few "driving lessons"
before one is proficient in using it. The raw data files produced by SDR are then
processed in software called WinQuake, and displayed in either time domain or
frequency domain (spectrum display). The spectrum display is in log.frequ. form.
A local engineer some time ago has written very useful additional software for this
card, making it possible to record continuous 24hr ULF spectra. I'm using this
software at the moment to record spectra like below:
0-40Hz (8,14,20&26Hz Schumann resonances) using the induction coil
0-10Hz (8 Hz Schumann resonance) induction coil.
0- 5Hz this one using the large under ground coil.
As can be seen from the spectra, there are clearly many hours during each 24hr cycle
which are badly affected by various interferences. The 'good" ones are easily found,
the spectrum baseline being much quieter than during the noisy times.
But it also shows the intensity variations of Schumann resonances over 24hrs., which
is neatly presented all on one page.