Elektor's article (1987) about ionic detection for water, somebody maybe has in English. The article refers you can use a plate or telescopic antenna for this. The designers of this device assume that can find water IN MOVEMENT (subsurface), the only kind of water detectable by INTERCHANGING of ions.
The oscillator, through the 4066 transmit the frequency to the antenna for to discharge excesive ions arrival. The 10 k pot provides a kind of threshold and works as sensibility. Key is for detect water and other for electricity in walls. At more low conmutation via 2M2 preset the sensibility is better, this is at more low frequency. C9 is a kind of memory for the signal to compare, require good quality capacitor. Downloadble complete article (Zahori.zip) here:
http://www.mytempdir.com/472871
Maybe somebody can translate for all, maybe somebody has the English version.
Here the schematic: