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

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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.