Quadrupolar array above an interface separating a half-space with a relative complex permittivity ϵ_cplx and vacuum. T₁ and T₂ are the transmitting electrodes, while R₁ and R₂ are the receiving electrodes. and are the images of the transmitting electrodes by the interface. The parameter r_TnRm is the distance between the T_n and R_m electrodes. The parameter I is the current flowing through the transmitting electrodes.