通过记录和分析low-Tc SQUID TEM系统接收曲线基线漂移现象,采用电阻-电容串联结合多级电容并联的吸收电路结构优化发射电流波形,并且进行了系统仿真、实际电路搭建以及野外实验验证.结果表明:经过震荡吸收后,电流尖峰和电压过冲得到明显的抑制,在小电流和大电流条件下,发射线圈中心一次场的变化率峰值均小于所采用的low-Tc SQUID芯片的摆率,基线漂移现象没有再次出现,low-Tc SQUID TEM系统能够长时间稳定工作.%A low-temperature superconducting quantum interference device (low-Tc SQUID) can improve the depth of exploration. However, a low-Tc SQUID may lose its lock owing to oscillations in the current or the occurrence of spikes when the transmitter is switched off. If a low-Tc SQUID loses its lock, it becomes impossible for the low-Tc SQUID TEM system to function normally and stably for a long period of time. This hinders the practical use of the system. In field experiments, the transmitting current is accurately measured, the voltage overshoot and current spike data are recorded, and the gradient of the primary magnetic field at the center of the transmitting loop is calculated. After analyzing the results of field experiments, it was found that when the gradient of the primary magnetic field far exceeds the slew rate of a low-Tc SQUID, the low-Tc SQUID loses its lock. Based on the mechanisms of the transmitting oscillation, an RC serial and multi-parallel capacity snubber circuit used to suppress such oscillation is proposed. The results of simulation and field experiments show that, when using a 100 m×100 m transmitting loop, the gradient of the primary magnetic field is suppressed from 101.4 to 2.4 mT/s with a transmitting current of 40 A, and from 29.6 to 1.4 mT/s with a transmitting current of 20 A. Therefore, it can be concluded that the gradient of the primary magnetic field is below the slew rate of a low-Tc SQUID after adopting the proposed RC serial and multi-parallel capacity snubber circuit. In conclusion, the technique proposed in this paper solves the problem of a lost lock of a low-Tc SQUID, ensuring that the low-Tc SQUID TEM system functions stably for a long period of time, and providing technical assurance for ground TEM exploration at an additional depth.
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