Abstract:

Based on the knowledge of the TEM theory, the secondary field direction generated by the heterogeneities in the position of the receive loop is different when the local heterogeneities exist below large fixed transmit loop, which may result in attenuation curve deviating from the response curve of the homogeneous body. In order to analyze the impact of heterogeneities to the attenuation curve, we adopts large fixed loop equipment whose area is 150 m×150 m and emission current is trapezoidal wave with the amplitude of 10 A and turn-off time  of  176 μs. We use the electromagnetic signal attenuation rule of multi-turn and close-wound coil, namely anomaly loop to simulate the response curve of  shallow heterogeneities whose time constant is respectively 1 ms, 3 ms or 5 ms.When the anomaly loop is located below the transmit loop or its frame edge, with the different relative position between the transmit loop, anomaly loop and receive loop, that will change the mutual inductance of them and cause the response of the anomaly loop negatively and lead to the integrated response attenuation curve of the receive loop appearing zero-crossing condition. So we need to attract attention of the negative response in the subsequent processing when the heterogeneities exist below the large fixed loop.

Key words: TEM, large fixed loop, local non-homogeneous body, anomaly-loop, attenuation curve