Abstract

The ratio of the horizontal electric and magnetic components of the electromagnetic field generated by a radio transmitter in the very low frequency (VLF) range is used in MT-VLF resistivity mapping to determine the apparent resistivity of the ground. A theoretical calculation of the responses of simple 2-D and 3-D prismatic bodies shows that the measurable lateral variations in both fields are independent, uncoupled, and correspond to frequency-independent, static variations. It is advantageous therefore to process and map the fields separately. Processing methods used in gravimetry and magnetics are especially appropriate in correcting the apparent anisotropy related to the horizontal polarization of the primary field and in integrating (upward continuation) the high-frequency spatial variations. The different processes tested on a synthetic case and on data obtained at the Centre de Recherches Geophysiques (CRG) test site showed how to eliminate shallow features and a current channeling conductor to favor the anomaly of a fault. A third example with a hydrogeological application shows the advantages of this method over Wenner direct-current resistivity measurements.

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