A method and apparatus to calibrate a resistivity measurement taken by a downhole tool in a borehole, wherein the downhole tool estimates the resistivity of an underground formation penetrated by the borehole with at least one sensor situated at a non-zero standoff distance from the borehole, is provided. The method includes taking apparent impedance measurements with the sensor at a set of frequencies and at a first plurality of locations in the borehole, wherein the measurement are uncalibrated measurements. The method also includes identifying a portion of the borehole in which the apparent impedance measurements at least at a first frequency of the set have a predetermined behavior. The predetermined behavior is that the apparent impedance measurements taken in the portion are substantially fitting a linear model when represented in the complex plane. The method also includes using a plurality of measurements obtained at a second plurality of location situated in said portion at the first frequency to determine calibration coefficients for the measurements at said frequency.