BOGOSLOVSKY, V. V. and A. A. OGILYY, 1973. Deformations of Natural Electric Fields Near Drainage Structures, Geophysical Prospecting 2 I, 7 16-723, The paper deals with the electro-filtrational fields formed near the drainage structures. Main laws of deformation of these fields above vertical and horizontal drains are considered and practical examples supplied. Deformation of seepage flow by drainage structures increases the groundwater flow velocities and, consequently, intensifies the electric fields of filtrational origin. The values of anomalies (amounting to tens and some hundreds of millivolts) mainly depend on the pressure drop, lithological and granulometric composition of the rocks, and on the salinity and chemical composition of the ground water. Laboratory investigations show that at a given pressure drop maximum electro-filtrational fields are observed for particles ranging from 150 to 250 lo (Ogilvy, Ayed, and Bogoslovsky 1969). Water flow in open fissures is characterized by smaller values of streaming potentials decreasing with the opening of fissures. The presence of a sand filler in the fissures up to 40% brings about an increase in the SP values. Further increase in the amount of filler results in somewhat reduced SP values.The magnitude of the electric fields under observation is sharply reduced by the presence of clay fractions both in sandy grounds and fissure filling material On the one hand, the clay causes a decrease of the permeability of the medium, and on the other the appearance of potentials of adsorptional origin.Since the streaming potentials are directly proportional to the potential difference between the immobile part of the electric double layer and the free solution (E potential), they decrease with an increase of the electrolyte concentration.Therefore, maximum electro-filtrational fields are induced by the *