A diagnostic system for monitoring changes in a medium is disclosed. Thesystem includes atransmitter configured to generate and transmit a time-varying magnetic fieldinto a mediumresponsive to a first signal. The system also includes a receiver positionedon an opposite sideof the medium from the transmitter and configured to generate a second signalresponsive to areceived magnetic field at the receiver. The system also includes a processingunit configuredto determine a phase shift between the transmitted magnetic field and thereceived magneticfield for a plurality of frequencies of the transmitted time-varying magneticfield.
