The present invention relates to a method of investigating the wall (2) of a borehole in a geological formation by means of two injectors (4, 5) with a potential V being applied between them and in measuring the potential difference δV between two electrically isolated measurement electrodes (6) situated between the two injectors (4, 5) and spaced apart from the formation by an insulating layer (1), in which the resistivity of the formation is obtained from the measured values of δV, V and I, with corrections being made for the effects due to the nature and the thickness of the insulating layer (1). In a first variant these corrections are based on a correction factor obtained from curves of K=Rt.I/δV as a function of the impedance V/I of the injector. In a second variant these corrections are obtained by estimating the current IF actually injected into the formation while taking account of leakage current IL, and calculating the resistivity of the formation from the equation (I). The invention also provides an investigation device enabling the values of IF and/or IL to be estimated. Rt = k c ⁢ δ ⁢   ⁢ V I F = k c ⁢ δ ⁢   ⁢ V I - I L ( I )