Characterization of Electrical Daily Variations Induced by Capillary Flow in the Non-saturated Zone

Electrical daily variations without an associated magnetic field were observed for a period of six weeks on independent parallel dipoles during the one-year electrode inter comparison experiment that took place in Garchy, France, from April 1995 to April 1996. These daily variations appeared suddenly during summer when the soil was dry on the surface. Their amplitude ranged from 0.1 to 1.5 mV for 50-m long dipoles, and characteristic relative phases. The amplitude of the variations was found to be related to soil temperature. These daily variations are accounted for in a model where streaming potentials are produced by coherent capillary flow in the non-saturated zone. Such daily variations are important to control in long-period magnetotelluric sounding and may be of practical use for agricultural applications. As their amplitude strongly depends on time-varying soil conditions, they may also lead to erroneous interpretations as earthquake precursors.