We call attention here to the limitation of the “Q approximation” in characterizing the complex wave tilt W over a layered half‐space. Using an improved formulation the frequency dependence of W is shown to be a discriminator of the electrical characteristic of the subsurface. The height dependence of the observer is also illustrated.

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The authors consider a two-dimensional model of an aperture excited homogeneous half-space whose admittivity (or complex conductivity) is different in the horizontal and vertical directions. The criterion for focusing the aperture, of finite width, to a point in the half-space is developed. It is demonstrated that, for an electrical field perpendicular to the axis of the aperture, the anisotropy can be allowed for by a suitable scaling of the isotropic model. The numerical results show that there is a significant enhancement of the fields (and the power dissipation) at the focus point provided that the conduction currents are relatively small compared with the displacement currents in the half-space. Also the width of the aperture should be about 3 or 4 wavelengths for effective focusing.