This signals collection follows the first "RDF earth monitoring" article on this web site. A couple of orthogonal earth dipole, 25m long, placed in a very quiet place in the country (in Noth West ItalY), a standard Sound Blaster card, two cheap low noise preamplifier, an old PC Pentium 120 and a Freeware software for acquire and store spectrograms in unattended operations: this is all you need for start in this fascinating kind of monitoring.

Wolfgang Büscher give us a big free resources with his last Spectrum Lab releases (see at http://www.qsl.net/dl4yhf for free download) with the RDF function. A couple of orthogonal antennas (they can be a couple of earth dipoles or a couple of loops) and we can store with the spectrogram pictures the incoming direction of recorded signals . I wanted to deepen what signals can be received with earth dipoles in the 1 to 100 Hz frequency range, and their direction coming. Here are the principal results in a couple of months of "24 hours a day" monitoring.
 

		A quiet day In a "normal" day we can usually receive these signals: 1) 50 Hz main frequency, always presents, everywhere 2) 50 Hz signal changes in intensity in the time 3) 100 Hz (50 Hz x 2) changes direction in the time 4) some tones travel as LSB of 100 Hz tone 5) a weak static 6) noise coming from mains: it appears as AM modulation of 50 Hz tone: upper part is specular to lower part 7) same kind of noise of point 6: two different colors identify two different source direction 8) too strong to be a static: probably a mains stroke 9) a green hiss below 10 Hz: its amplitude is correlated to 50 Hz tone intensity, then brought by mains
		Mirrored signals A big amount of mirrored signal comes by mains: A-A') and so on for B-B', C-C', D-D', E-E': many strong tones are present in the ground. They are normally 50 dB below the 50 Hz tone and comes from different directions.  F) 100 Hz tone seems to be a carrier of violet noise, not detected in correspondence of 50 Hz G) two LSB subtone from different directions (green a nd blue) H) fascinating: a strange noise below 12 Hz. Acquired in a wave file and accelerated 63 times it sounds similar to an human lament .... someone ask help from underground? :-) Look at this sequence: it appears to be correlated sometimes with start/stop mirrored sequences. I suppose it comes from mains.
		Others mirrored signals Not always they are mirrored to 50 Hz, sometimes to first mains harmonic: A-A') and so on for B-B', C-C', D-D'. Various and variable tones. Perhaps generated by electric engines. All mirrored to 50 Hz. E) strong hiss below 10 Hz: its intensity is correlated to 50 Hz one F) weak tone at 16.6 Hz, not mirrored (perhaps a real signal). Norwegian railways or simply 50 Hz divided for 3? G-G') blue wide band noise, mirrored to 100 Hz. In this case the first harmonic seems to be the noise carrier
		First contact! An alien code appears from noise background: A) 16.6 Hz short tone sequence (mirrored at 83.4 Hz) B) a lighting static or a mains stroke: it is strange because changes direction with frequencies C) below 12 Hz, down to 1 Hz, a very strange signal draw an 8 8 series. Time sequence is different from "A" signal. This signal is not mirrored to 50 Hz. Have we discovered a secret gate access of "Matrix"? :-)  Note for skeptic investigator: there is a railroad line close to me, at about 5 km; perhaps it came from there.
		Worms Every healthy ground is reach of worms: In the pictures a worm, or better a not stable in frequency carrier, at 14.9 Hz. Below 100 Hz many single (not mirrored) tone are present: different width, from different directions, different frequencies and usually unstable in frequency. Probably generated by electric engines.  Below 12 Hz another "lament" sequence.
		Bugs They "fly" from 85 Hz to 95 Hz. These strange forms of life... hops, sorry: these strange signals are normally strong, come from different direction (or better: have different colors) and are not a pure tone but an amusing draw. They are normally rare (one or two for week). This picture is obtained with Photo Shop building a collage of different receptions: frequency of original signals and time scroll is respected. They are never mirrored: I have not idea of their possible origin.
		Below 12 Hz Another interesting "laments" sequence This sequence is interesting: signal changes direction. Arrives in blue, became violet, then red, yellow and disappear as green. Sequence is similar to the statics color during a storm. May be it a natural origin signal?
		Earthquake precursor A very strong wide band signal below 40 Hz. This signal unfortunately is not a seismic signal, but from theories explained in the article "ON THE POSSIBLE ORIGIN, PROPAGATION AND DETECTABILITY OF ELECTROMAGNETIC PRECURSORS OF EARTHQUAKES" in this web site by Prof. Ezio Mognaschi,  they can be very similar. I don't know what caused this strong event.

It's impressive the quantity and the variety of signals received by ground. At a first and quick analysis they don't come from the same direction, then not from the local mains line. If they came from the house mains they would have the whole same color.

But "at these low frequencies" a different color don't means necessarily different directions. If, for example, we put a large open coil powered with 50 Hz in our house, and slowly rotate it, the color tone in the spectrogram will change - just like in the experiment described in the article "RDF earth monitoring"  with the audio generator and the earth stakes in some distance from the earth dipoles RX antenna. We can do the same on VLF, as long as it's not a true electromagnetic field but a "stray field" : Some turns of wire around a ferrite rod, connected to a generator at some kHz, we can "transmit" (well, not really, it's just a magnetic stray field) which looks like a moving transmitter - but in fact its just a ferrite rod, swiveled on the workbench, ten meters away from the crossed receiving loop.

The impression remains that most of listened signals with earth dipoles are not natural origin, or better: are domestic origin. A lot of electric devices, like kitchen machines, petrol engines, and electric generators in cars and even bicycles, produce a lot of characteristic sounds. With the VLF RDF loop, we can "hear" cars 30/40 meters far from the antenna much earlier than we hear the audio wave of the car itself. I remember some test done with the "easyloop" (only 19 turns of 75 cm diameter) in my garden, where a fuel injection car was "receivable" up to 200 m of distance. These are always short events on the waterfall, some kilohertz higher than the bugs over presented, but the mechanism looks very similar.

These receptions confirm partly the loop model as correct way to understand how an earth dipole works. But a big doubt remains on the real nature of the received signals: if an earth dipole works as a vertical underground loop, why Schumann resonances are not evident in receptions? Why ELF component of static discharge (clearly receivable with a short stylus) disappear with the earth dipole? How deep is the ground loop, and how deep it can be? A long earth dipole gives always more signal than a short one?

Some of these question will be faced in a next article.