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We are developing a new type of near-surface electromagnetic (EM) survey based on measurements of the magnetic field and its derivatives in the megahertz frequency range. The vertical derivatives of the horizontal magnetic fields can replace the electric fields in plane-wave methods. These derivatives are difficult to determine at low frequencies, but are large enough to be measured in the frequency range considered.

We used 3-D modeling to answer several questions concerning this technique. Because the derivative is to be determined in a vertical hole, the measuring procedure itself may introduce a distortion of the fields. It was shown that this effect is significant and has to be taken into account in the design of the sensor or the measuring procedure. Other modeling projects included: (1) the resolution of the method as compared with more traditional techniques of resistivity determination, (2) the influence of small-scale inhomogeneities, and (3) the use of tensorial measurements in complicated areas.

Under realistic conditions the derivatives to be determined are never larger than a few percent of the total field. The modeling requires high accuracy that can be reached only with very dense discretizations and an optimized iteration strategy.

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