Abstract

Electromagnetic (EM) exploration for base metals using the natural-source audio-magnetotelluric (AMT) technique has increased significantly during the last five years due to enhancements in all aspects of AMT and to the demand for imaging deeper than conventional controlled-source EM methods. However, regional currents induced by natural sources can be problematic in certain situations, and the appropriate interpretational dimensionality must be known. Herein we demonstrate that a two-dimensional (2D) interpretation is valid for a defined frequency band, but that the effects of large-scale three-dimensional (3D) structures must be considered at lower frequencies. Using an AMT dataset from an area located north of Voisey's Bay, Labrador, Canada, we analyse the responses to determine the appropriate dimensionality and to test them for internal consistency. Maps of the distortion-corrected data identify the lateral extent of connected conducting mineralization intersected by a drilling program. One-dimensional (1D) inversions of the corrected data from those sites on top of the mineralized zone show the resolution properties of the data. We constructed a pseudo-3D model from 2D inversions of the data in the frequency band 1000–10 Hz from all profiles, and this model images the mineralized body sufficiently for exploration purposes. We suggest that the anomalous low-frequency responses observed at sites close to the mineralized zone are possibly due to charges impinged on the mineralized body's boundaries by currents induced in the Atlantic Ocean some 50 km away. Although 3D numerical modeling studies exhibit some of the effects observed, we are unable to reproduce numerically the observed behavior.

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