Abstract-The presence of the primary field in the helicopter transient electromagnetic system makes the dynamic range of the response signal so large that it is difficult to observe the secondary field. Therefore, a bucking coil is usually introduced to eliminate the primary field. However, in a traditional design, the size of the bucking coil increases with the size of the system, which makes the bucking coil hard to install, and opposite magnetic moment is large in huge systems. In this paper, a new bucking coil design for a helicopter transient electromagnetic system is proposed. Compared with the traditional design, the bucking coil diameter, total weight and total magnetic moment in two designs are calculated. The results show that the bucking coil we designed is more than 8 times smaller and 5 times lighter than that in the traditional design, which is easier for installation. The bucking moment impact is reduced to 0.03% of the total magnetic moment when the diameter of the transmitting coil increases to 35 m, which improves the efficiency of the system. Then we analyze the requirement of manufactory precision and installation accuracy for the bucking coil in our design to get the best bucking result.

An adaptive sharp boundary inversion scheme is developed to improve resolution with feasibility for transient electromagnetic (TEM) data inversion. By using weighted minimum gradient support (WMGS) constraint, this method focuses the resistivity change areas on layer boundary locations. Prior information describing roughness can be added into the constraint to improve resolution. Furthermore, even though no prior information about layer boundaries is available, it can still reconstruct models with geo-electrical interfaces. Synthetic models prove that this method has a better performance in presenting layer boundaries than smooth-model inversion. Field data of a TEM test line are inverted using this method, which makes the basement layer visualized easily.

Abstract-Magnetic anomaly detection (MAD) is to find hidden ferromagnetic objects, and a hidden object is often described as a magnetostatic dipole. Many detection methods are based on the orthonormal basis functions when the target moves along a straight line relatively to the magnetometer. A new kind of parabolic trail orthonormal basis functions (PTOBF) method is proposed to detect the magnetic target when the trajectory of the target is parabola. The simulation experiment confirms that the proposed method can detect the magnetic anomaly signals in white Gaussian noise when SNR is −15.56 dB. The proposed method is sensitive to the characteristic time and curvature. High detection probability and simple implementation of proposed method make it attractive for the real-time applications.

A triple-section arm structure is proposed for designing a planar spiral antenna. All three sections are designed by combining logarithmic, rooted, and sine equations. The slowly outstretched and contractive structure is innovatively realized. According to the radiation characteristics of the spiral antenna, each section corresponds to different in-band enhancement effects. Numerical simulation in the frequency domain and experiments using two different baluns are carried out. The results show that the novel spiral topology could simultaneously achieve improved axial ratio, low cross-polarized gain, and excellent impedance matching throughout the whole band. The axial ratio is reduced by 1.5 dB at midfrequencies and more at low frequencies, comparing the proposed arm with a sinusoid-added equiangular spiral arm. Without applying the resistive loading method, a lower cutoff frequency of 750 MHz is still realized both in impedance bandwidth and axial ratio bandwidth. The low cutoff frequency of the proposed arm is 30.2% lower than the conventional Equiangular spiral arm. Besides, the polarization isolation is significantly improved, especially at low frequencies. Therefore, the proposed miniaturized spiral arm structure could be a competitive form for designing spiral antennas.