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Summary

ARLETT is a three-component surface-to-borehole harmonic electromagnetic (EM) system developed by BRGM mainly for deep mineral exploration. The probe contains three magnetometers and two inclinometers, which makes it possible to project the EM fields onto a fixed coordinate system and to independently calculate the borehole trajectory. The system has a low internal noise level, high spectral resolution, and effective noise-rejection and signal-processing capabilities.

Conductive targets can be detected directly on the recorded logs of the total EM field, particularly using the phases of the field components and the ellipticity. Locating these targets requires reduction of the data to remove the primary field (of the transmitting loop) and retain the (usually much smaller) secondary field of the target. The reduced data then are matched to the field of a simple 3-D current filament.

The ARLETT system has been tested in the Sudbury Basin, which has highly conductive targets in a resistive environment, and in the Iberian Pyrite Belt, which has moderately conductive targets in a (less) conductive host. The frequency range appears sufficiently wide to distinguish between low-, intermediate-, and high-frequency ranges of the induction response.

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