In vertical electrical sounding (VES) interpretation of a K-type resistivity curve (a resistive middle layer sandwiched between two less resistive layers), the principle of equivalence applies to a thin, relatively resistive middle layer: because current preferentially flows through the more conductive bounding layers, the sounding curve is primarily sensitive to the product of the middle layer's resistivity and thickness (its transverse resistance, T = R x h), rather than to resistivity and thickness independently; consequently, if independent information (such as drilling) reveals the true thickness of this layer differs from the curve-interpreted value, the true resistivity can be recovered by holding the transverse resistance T constant and solving for the corrected resistivity.