Abstract

With all due respect to Singh and Rankin (1983), I believe that their criticism of the paper by Thiel and Chant (cited above) needs clarification. Contrary to the assertion by Singh and Rankin, the quoted formula for the function We (incorrectly called a wavetilt) can indeed depend upon the mode of propagation. The factor sin θ, appearing in the quoted expressions, is the sine of the complex angle of the incident wave whether it be a ground wave or a downgoing ionospherically reflected wave. This point is central to the long-standing dispute between the “Cagniard school” of magnetotelluric sounding and those who believe that source field characteristics can play a role. Thiel and Chant correctly pointed out that the elliptical polarization of the downgoing wave can mess up the results. But possibly they overlooked that this effect, in a surface impedance measurement, is minimized compared with a true wavetilt measurement. The latter senses the vertical electric field with a vertical (i.e., whip) antenna, whereas in a true surface impedance measuring scheme both horizontal E and horizontal H fields are sensed. Actually Thiel and Chant measured surface impedance, so the elliptical polarization of the downgoing wave probably plays a minor role.

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