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.
GRAM can be obtained free for 10 days, after which it stops working. The purchase
price is very modest, however, unlike its earlier versions the new GRAM is not usable
at ULF any more. It may be worth trying to find an older version, which has still the
0 - 86Hz range selectable. There is also a freeware program called SPECTRAN
It is very easy to drive and also includes an automatic screen capture tool.
The most versatile and extensive program is SPECTRUM LAB, but it is also by far
the most difficult one to operate and needs a lot of practice to get fluent in. It is also
freeware and gets frequently revised.
If you want to explore frequencies below ~5Hz, you'll discover most sound card's poor 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.
I've also tried an A/D card which is being normally 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. Recordings using
this software of ULF spectra 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.
