The transient electromagnetic (TEM) method is a widely used nonintrusive geophysical method for ground exploration. Due to the mutual inductance between the transmitter (TX) coil and the receiver (RX) coil, the primary field generated by the emission current reduces the accuracy of the near-surface detection. Because the feature signal [Formula: see text] carrying the target information is mainly distributed in the early secondary-field response, the expanded detection signal will reduce the near-surface investigation capability of the TEM device due to the following three aspects: the loss of the proportion of [Formula: see text] in the detection signal due to the excessively high primary-field response, the loss of [Formula: see text] due to the clipping loss, and the reduction of the noise margin in the case in which the detection signal is magnified. These problems are particularly significant in small-loop devices due to the tight coil distribution. The mutual inductance can be reduced by adjusting the relative positions of the TX and RX coils, a configuration called the weak-coupling coil design. We have analyzed the design principle of the weak-coupling coil design and developed a new design scheme — the crossing-loop design. Simulation results indicate that the crossing-loop design performs superiorly in terms of the detection sensitivity and the investigation depth, compared with the nonweak-coupling coil design and other weak-coupling coil designs such as the gradient coils, opposing coils, and the bucking coil design. The experimental results indicate that the crossing-loop design provides much better near-surface investigation capability than the central-loop device with the same TX coil, which is a typical nonweak-coupling coil design.
The pulsed eddy current (PEC) inspection is considered a versatile non-destructive evaluation technique, and it is widely used in metal thickness quantifications for structural health monitoring and target recognition. However, for non-ferromagnetic conductors covered with non-uniform thick insulating layers, there are still deficiencies in the current schemes. The main purpose of this study is to find an effective feature, to measure wall thinning under the large lift-off variations, and further expand application of the PEC technology. Therefore, a novel method named the dynamic apparent time constant (D-ATC) is proposed based on the coil-coupling model. It associates the dynamic behavior of the induced eddy current with the geometric dimensions of the non-ferromagnetic metallic component by the time and amplitude features of the D-ATC curve. Numeral calculations and experiments show that the time signature is immune to large lift-off variations.
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