In magnetotelluric (MT) soundings, apparent resistivity curves for the two orthogonal polarizations (ρxy and ρyx) coincide when the subsurface is 1D (horizontally layered) beneath the sounding point, but diverge (split) when 2D/3D lateral heterogeneity, such as a vertical contact, exists nearby. The period at which this divergence appears indicates the depth (via skin-depth penetration) at which the lateral heterogeneity becomes electromagnetically 'visible' — divergence at short periods (shallow skin depth) indicates a shallow/near-surface contact, while a contact directly beneath the sounding point (versus one located laterally away from it) produces subtly different diagnostic patterns in how the curves separate and behave at intermediate periods.