Abstract

Very low-frequency (VLF) electric field wavetilt measurements were made continuously over a period of four months at a fixed location. Daytime and nighttime values of the wavetilt were found to be relatively constant. During sunrise and sunset the wavetilt decreased to 20 percent of the daytime value, making wavetilt measurements meaningless for two three-hour intervals during a 24 hour period. A mechanism involving modal interference is postulated.This phenomenon may be a problem in other passive electromagnetic (EM) prospecting systems including audio-frequency magnetic and audiomagnetotelluric.

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