Electric Logging (ELOG)
http://mindtattoos.com/?okychyruvsja=%D8%AF%D9%84%D8%AA%D8%A7-%D8%AE%D9%8A%D8%A7%D8%B1-%D8%AB%D9%86%D8%A7%D8%A6%D9%8A&638=56 The Elog is the classic water-well combination probe combining shallow, medium and deep penetrating resistivity measurements with self-potential (SP).
Principle of Measurement:
http://psychtherapy.co.uk/?kindor=%D8%A7%D9%84%D8%AA%D8%B9%D9%84%D9%8A%D9%85-%D8%AE%D9%8A%D8%A7%D8%B1-%D8%AB%D9%86%D8%A7%D8%A6%D9%8A&151=19 A low-frequency bi-directional electric current from a source electrode on the probe returns through the formation to the cable armour above an insulated bridle. Potentials due to this current flow are measured on various sense electrodes on the probe with respect to a voltage reference ‘fish’ normally located at the surface. These measurements are converted to apparent formation resistivities within the probe and digitally transmitted to the surface.