A unique high-resolution electromagnetic sounding system, utilizing a four-axis coil and a new calibration technique, has been designed, constructed, and tested. The resulting measurements have an accuracy of approximately ±0.1%. The four-axis coil, which includes three approximately orthogonal coils and a calibration coil, is designed to automatically collect ellipticity data at 20 discrete frequencies in the range 30 Hz-30 kHz. The approximately orthogonal coils detect three components of the magnetic field and the calibration coil injects a calibration signal into the receiving system at the same time that the data are being recorded. A three-dimensional mathematical rotation program was developed to calculate ellipticity, removing the need to know the absolute coil orientation. Equations were formulated for the angular relationships between the approximately orthogonal coils. The actual intercoil angles were computed using a nonlinear optimization analysis applied to the measured data. The system was tested by measuring the ellipticity at one point on the earth's surface for various orientations of the coil. In addition, a close match between calculated and measured ellipticity profiles was obtained in a field test over a known target.
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