1. A sonar system comprising: a transmitter configured to direct multi-frequency transmit signals into a propagation channel, each multi-frequency transmit signal including a first frequency and a second frequency; a receiver tuned to receive multi-frequency return signals propagating in the propagation channel, each multi-frequency return signal including a first frequency return signal and a second frequency return signal, the first frequency return signal having a first phase and the second frequency return signal having a second phase; a signal processor coupled to the receiver, the signal processor being configured to extract the first phase and the second phase from the multi-frequency return signal and calculate a phase difference, the phase difference being the difference between the first phase and the second phase, the phase difference corresponding to an acoustic impedance value of a material disposed in the propagation channel.