A diagnostic system for monitoring changes in a medium is disclosed. The system includes a transmitter configured to generate and transmit a time-varying magnetic field into a medium responsive to a first signal. The system also includes a receiver positioned on an opposite side of the medium from the transmitter and configured to generate a second signal responsive to a received magnetic field at the receiver. The system also includes a processing unit configured to determine a phase shift between the transmitted magnetic field and the received magnetic field for a plurality of frequencies of the transmitted time-varying magnetic field.
