![]() ![]() Such behavior was recognized by Paal as being consistent with the response from a current, induced by the radio field, running along the top edge of the target. A large rotation in the compass bearing of the horizontal magnetic field coincided with the location of the maximum in this component. ![]() By surveying over known shallow orebodies in Sweden with a calibrated field-intensity meter tuned to VLF stations, Paal showed that the horizontal VLF magnetic field was greatly enhanced over subsurface conductors at exactly the same location where the modulus of the vertical magnetic field component became a minimum. It was not until 1963 that Paal (1965) observed that radio waves at VLF frequencies (technically the 3–30 kHz band, but in fact limited to 15–25 kHz by the available high powered transmitters) could be used to prospect for electrically conductive orebodies. ![]() These early measurements were, however, made at relatively high frequencies which resulted in a shallow depth of penetration. Indeed measurements of the conductivity and dielectric constant of the earth using “wave-tilt” techniques were first performed in the 1930's. The fact that electrical properties of the ground affect the behavior of radio waves has been known for years. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |