A system comprising: a transmit coil and a receive coil configured to receive a structure under test therebetween; a signal generator configured to drive the transmit coil with a radio frequency signal over a plurality of frequencies, and thereby cause the transmit electromagnetic coil to produce a magnetic field that by magnetic coupling through the SUT induces a voltage in the receive coil; a pair of receivers (110, 112) and signal processing circuitry coupled to the transmit coil and receive coil, and configured to measure the current through the transmit coil and the voltage in the receive coil, and from the voltage and the current produce a measurement of attenuation of the RF signal over the plurality of frequencies caused by the SUT between the transmit coil and receive coil; and an analysis system coupled to the signal processing circuitry and configured to calculate an effective conductivity of the SUT from the measurement of attenuation, wherein the effective conductivity has components in respective orthogonal axes of a global coordinate system, and the analysis system is configured to calculate specifically one or more of the components; wherein the transmit coil and receive coil form an arrangement and include a first arrangement in which the transmit coil and receive coil have a first diameter, and a second arrangement in which the transmit coil and receive coil have a different, second diameter, wherein the signal generator is configured to separately drive the transmit coil (102a, 102b) of the first arrangement and second arrangement, and the pair of receivers (110, 112) and signal processing circuitry are configured to separately produce a first measurement of attenuation of the RF signal for the first arrangement, and a second measurement of attenuation of the RF signal for the second arrangement, and wherein the analysis system being configured to calculate specifically one or more of the components includes being configured to calculate specifically one or more of the components from the first measurement of attenuation and second measurement of attenuation. a transmit coil and a receive coil configured to receive a structure under test therebetween; a signal generator configured to drive the transmit coil with a radio frequency signal over a plurality of frequencies, and thereby cause the transmit electromagnetic coil to produce a magnetic field that by magnetic coupling through the SUT induces a voltage in the receive coil; a pair of receivers (110, 112) and signal processing circuitry coupled to the transmit coil and receive coil, and configured to measure the current through the transmit coil and the voltage in the receive coil, and from the voltage and the current produce a measurement of attenuation of the RF signal over the plurality of frequencies caused by the SUT between the transmit coil and receive coil; and an analysis system coupled to the signal processing circuitry and configured to calculate an effective conductivity of the SUT from the measurement of attenuation, wherein the effective conductivity has components in respective orthogonal axes of a global coordinate system, and the analysis system is configured to calculate specifically one or more of the components; wherein the transmit coil and receive coil form an arrangement and include a first arrangement in which the transmit coil and receive coil have a first diameter, and a second arrangement in which the transmit coil and receive coil have a different, second diameter, wherein the signal generator is configured to separately drive the transmit coil (102a, 102b) of the first arrangement and second arrangement, and the pair of receivers (110, 112) and signal processing circuitry are configured to separately produce a first measurement of attenuation of the RF signal for the first arrangement, and a second measurement of attenuation of the RF signal for the second arrangement, and wherein the analysis system being configured to calculate specifically one or more of the components includes being configured to calculate specifically one or more of the components from the first measurement of attenuation and second measurement of attenuation.