The present invention relates to a method for determining a representation of multichannel radio-frequency, RF, signals acquired from a physical environment, which comprises a transmitter comprising one or more transmitting elements for emitting ultrasound excitation pressure waves, a propagation medium containing non-uniformities reflecting the pressure waves and a receiver comprising one or more elements for recording the multichannel RF signals from the reflected pressure waves. The method comprises - determining a set of basis functions for representing the received reflected signals by deriving a model, whereby deriving said model comprises * modelling the transmitter as one or more transmitting elements and the receiver as one or more receiving elements, * modelling the propagation medium bounded by said transmitter and said receiver, * modelling individual responses of the non-uniformities to said excitation pressure waves, * deriving the set of basis functions from said individual responses by simulating response to excitation of the non-uniformities in the propagation medium, - determining a representation of the recorded multichannel RF signals using the set of basis functions obtained with the model.