Abstract

Conventional use of the controlled-source audio-frequency magneto-telluric (CSAMT) method is based on calculating the apparent or Cagniard resistivity from the amplitude ratio of the horizontal electric and magnetic field components. However, direct comparison between these two components shows that the electric field is more sensitive to the underground medium resistivity than its magnetic counterpart. Thus, use of the electric component only should provide adequate information about the electric properties of the subsurface. The measurements typically belong to the far-field zone, but show a non-dipolar characteristic because of the source. In this paper, we therefore propose a simplified CSAMT technique based on measuring the electric field component only. As part of this new formulation, a related theoretical model for the electric field component accounting for the non-dipolar nature of the transmitter antenna is introduced. This is accompanied with a new apparent resistivity definition, including a procedure to transform it into pseudo-phase data, thus removing the static shifts.

The potential of this modified CSAMT method is demonstrated using a field case from the Shanxi province in China. Until recently, it has been thought that no coal deposits exist in this region. However, application of the single-component CSAMT technique as advocated for here, revealed a major coal deposit, which was verified later by drilling.

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