Propagation Experiment of VLF Electromagnetic Pulses in the Deep Underground

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This research has been carried out over two years on the basis of following two categories :(1) Development of a wave form capture with a wide (100 dB) dynamic range to observe weak electromagnetic echoes from the deep underground(2) Computer simulation on the propagation of a VLF electromagnetic pulse in the deep undergroundAs for item (1), we have developed a wave form capture with a wide dynamic range to measure weak VLF echoes reflected from the deep underground which would be received simultaneously with a much stronger electromagnetic pulse directly coming from the transmitter. By using an automatic gain controlling circuit to select the gain (from 0 to 36 dB) of an signal amplifier, a total 100-dB dynamic range has been realized with a single 12-bit (66 dB) A/D converter. The developed wave form capture has been confirmed to work well for several test signals.As for item (2), a three-dimensional computer simulation code using a finite-difference timedomain (FDTD) method, has been developed to compute underground propagation of an electromagnetic wave radiated from a loop antenna on the ground. Using this simulation code, we have evaluated VLF scattering cross sections of metal plates of various sizes buried in the deep (-800m) underground. The results have been applied to estimate the loop current necessary to radiate a VLF radar pulse which produces its echo from the deep underground strong enough to be received by a search coil magnetometer on the ground.