VLF?EM prospecting equipment usually measure the tilt angle of the major axis of polarization from horizontal either in degrees or in percent and the ellipticity (in percent) of the polarization ellipse of magnetic field. However, the structure of the elliptically polarized field in the presence of local conductor allows the establishment of inter-relationship of the space components (amplitudes of the vertical and horizontal magnetic field components) and the tilt angle with the phase components (ellipticity or imaginary component), providing an opportunity to obtain the ellipticity without actually measuring with a phase shifting circuitary provision in the equipment. The validity of the inter-relationship under real field conditions thus obtained is demonstrated by field examples. Further, the field results show the applicability of the VLF?EM technique in detecting resistive targets also when electrical field components alone or the impedance measurements are made. However, the magnetic field components alone do not reflect the resistive target well.

On the inter-relationship between the VLF?EM prospecting parameters

The very low frequency electromagnetic prospecting equipment measures different space and phase components of the electromagnetic field in the frequency range 15–25 kHz emanating from distant naval radio stations. The field application of the equipment designed and constructed for this purpose has demonstrated the usefulness of the investigations in groundwater and geological mapping. The amplitude measurements necessitate correction of the diurnal variations of the electromagnetic field at the observation point.

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I B Rama Prasada Rao

Papers

Very low frequency electromagnetic prospecting method—A few field applications in India