Abstract: The invention relates to a method of surveying a geological formation through which a cased borehole passes, said method being characterized by the fact that: in a first step, current is injected into the casing (11) at a first point (In1) spaced apart longitudinally from said formation so as to cause current to leak into said formation, and electrodes (a, b, c) defining two consecutive sections of casing situated at the level of said formation ,ire used to measure the respective potential drops along said sections; in a second step, current is injected into the casing at a second point (In2) spaced apart longitudinally from the formation and situated on the opposite side thereof from said first point, so as to cause current to leak into the formation, and said electrodes are used to measure the potential drops along said sections; the corresponding measurements of the two steps are combined to obtain the values corresponding to a circuit formed by the casing between the two injection points is and essenti

Abstract: The invention relates to a method of surveying the resistivity of a geological formation through which a borehole provided with metal casing passes, in which method a leakage current is caused to leak into the formation outside the casing, and the leakage current is determined on a casing section at a certain level, the leakage current being indicative of the resistivity of the formation. According to a characteristic of the invention, the resistivity is determined on the basis of the leakage current by applying a factor that depends on the distance z between the level and the surface.

Abstract: The invention relates to a logging method and apparatus for determining the electrical resistivity of an earth formation traversed by a borehole. The resistivity is determined using a measuring pad which has a central electrode operable to deliver a measuring current. The measuring current is focused radially by means of two focusing systems: an active focusing system which focuses the measuring current in a first direction; and a passive focusing system which focuses the measuring current in a second direction perpendicular to said first direction.