Propagation Experiment of VLF Electromagnetic Pulses in the Uuderground for Development of Deep Earth Radar

本课题的研究是在以下两个方面进行的：（1）大功率甚低频电磁脉冲发生器和信号接收系统的改进;（2）甚低频在地下传播的理论分析。针对第（1）项，我们研制了一种波形捕获接收机，它能以12位的精度和60 kHz的采样频率存储500毫秒以上的两路接收波形，以减少商用电力线噪声的影响。一种防水探测线圈已经开发出来，即使在地下的水中也将用于信号探测。为了提高脉冲传输的高频（100 kHz以下）响应性，我们将驱动SIT的晶体管电路替换为二极管电路。对于第（2）项，我们开发了Sommerfeld积分法，用于计算由放置在地面上的水平环形天线辐射的地下球形电磁场。利用这一点，我们已经评估了衰减的辐射VLF波在一个水平分层的地下，也从一个对象在地下的物理光学近似的基础上的反射波的强度。另一方面，一维和二维的计算机模拟程序已经开发出来，以分析波在更复杂的地下介质中的传播，这是传统的理论计算无法处理的。

This research has been carried out over two years on the basis of following two categories :(1) Improvement of a high-power VLF electromagnetic pulse generator and a signal reception system(2) Theoretical analysis of VLF propagation in the undergroundAs for the item (1) , we have developed a wave form capture receiver which can memorize two channels of received waveforms over 500 msec at the 12-bit accuracy and at the sampling frequency of 60kHz, to reduce the effect of commercial power line noise. A waterproof search coil has been developed which will be used for signal detection even in the water in the underground. We have replaced a transistor circuit driving SIT for high-power radiation, with a diode circuit to improve high-frequency (up to 100kHz) response of pulse transmission.As for the item (2) , a Sommerfeld integration method has been developed to calculate spherical electromagnetic fields in the underground which is radiated by a horizontal loop antenna placed on the ground surface. Using this, we have evaluated attenuation of a radiated VLF wave in a horizontally stratified underground and also intensity of reflected waves from an object in the underground on the basis of the physical optics approximation. On the other hand, one-and two-dimensional computer simulation codes have been developed to analyze wave propagation in a more complicated underground medium, which conventional theoretical calculations fail to deal with.