GEOPHYSICS, VOL. 68, NO. 1 (JANUARY-FEBRUARY 2003); P. 70–91, 23 FIGS., 4 TABLES. 10.1190/1.1543195 Case History Okak Bay AMT data-set case study: Lessons in dimensionality and scale Alan G. Jones ∗ and Xavier Garcia ‡ 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 cur- rents 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 consid- ered 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 lat- eral extent of connected conducting mineralization in- tersected 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. INTRODUCTION Exploration for economic mineralization in mature prospects in Canada is focused on using geophysical meth- ods that probe deeper than conventional techniques (i.e., >500 m). For electromagnetic surveys, the natural-source audio-magnetotelluric method (AMT) has become an attrac- tive option with its ability to image conductivity structures in resistive basement from about 300 m to a few kilometers. Controlled-source AMT (CSAMT), proposed in the mid-1970s (Goldstein and Strangway, 1975) to counter the poor and unre- liable natural-source AMT data acquired at that time, has been used extensively in the past but suffers from more involved and complex logistical requirements (a bipole source), typi- cally lack of tensor information, fewer processing, analysis, and Manuscript received by the Editor September 18, 2000; revised manuscript received January 02, 2002. ∗ Geological Survey of Canada, Continental Geoscience Division, 615 Booth Street, Room 218, Ottawa, Ontario K1A 0E9, Canada. E-mail: ajones@nrcan.gc.ca. ‡Formerly Geological Survey of Canada, Continental Geoscience Division, Ottawa, Ontario, Canada; presently Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543. E-mail: xgarcia@whoi.edu. c  2003 Society of Exploration Geophysicists. All rights reserved. modeling tools, and a restricted physical range of valid ap- plication. Understanding the nature of the natural source fields (Garcia and Jones, 2002) together with developments in instrumentation, processing, analysis, modeling, and in- version have recently led to preferential use of AMT over CSAMT in Canada for both regional exploration activities and for anomaly delineation. Large AMT surveys, principally for base metals, have been conducted since 1996 in Newfound- land (Voisey’s Bay), Ontario (Sudbury basin), and Manitoba (Thompson nickel belt), with more than 15 000 AMT sites ac- quired in Canada (see, e.g., Livelybrooks et al., 1996; Chouteau et al., 1997; Balch et al., 1998; Stevens and McNeice, 1998; Zhang et al., 1998). A relatively small (46 sites) AMT survey was conducted over a magnetic high north of the renowned Voisey’s Bay deposit, 70