Okak Bay AMT dataset case study: lessons in dimensionality and scale
Alan G. Jones and Xavier Garcia
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 (2-D) interpretation is valid for a defined frequency band, but that the effects of
large-scale three-dimensional (3-D) structures must be considered at lower frequencies. Using an AMT
dataset from an area located north of Voisey’s Bay, Labrador, 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 (1-D) 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-3-D model from 2-D inversions of the data
in the frequency band 1,000 - 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 3-D numerical modeling studies exhibit some of the effects
observed, we are unable to reproduce numerically the observed behavior.
Geophysics, 68, 70-91, 2003.
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Alan G Jones / 10 June 2004 /