An apparatus for determining a property of a downhole formation, the apparatus comprising: an array having a plurality of transmitters and receivers capable of propagating electromagnetic waves through the formation; measuring circuitry for measuring an effect of the formation on the propagating waves; control circuitry arranged to vary the propagating waves as a function of at least one of frequency, spacing and polarization; and processing circuitry arranged to combine the effects of the propagating waves that are varied according to frequency, spacing and polarization for determining the property of the downhole formation.