f Measurements of D.C. small signals masked by noises

The measurement of a man made signal S, masked by natural noises of mean strenght N, is common to many geophysical methods; but the Author will take into account only the case of the geoelectrical D.C. current surveys about which he reached a sufficient experience. The problem arose when deep sounding where carried out by means of Dipole-Dipole arrays and with long distances between the two dipoles,up to twenty kilometers.