OKAK BAY: A LESSON IN DIMENSIONALITY FOR US ALL

Alan G. Jones, Xavier Garcia
Geological Survey of Canada
615 Booth St., Ottawa, Ontario, K1A 0E9, Canada
ajones@cg.nrcan.gc.ca

A large aeromagnetic anomaly at Okak Bay, north of Voisey's Bay, 
Labrador, prompted exploration activity to determine if there existed 
another nickel body like Voisey's Bay. Exploration activities included 
undertaking audio-magnetotelluric, AMT, data acquisition at almost 
50 sites with one third centred on the magnetic high. The AMT dataset 
was interpreted in terms of a small shallow conductor at 600 m and a 
major deeper conductor at 1-3 km. Drilling intersected a body of 
sub-economic grade ore at 454-470 m, consistent with 1D inversion of 
the distortion-corrected data, but failed to intersect any ore down to 
base of drilling at 1.5 km.  Examination of the responses showed a 
non-inductive effect at frequencies less than 10 Hz. We undertook 2D 
inversion of the data from all profiles in the restricted frequency 
range of 1,000 - 10 Hz. A pseudo-3D model constructed from the 2D 
models infers the geometry of the body sufficient information for 
exploration purposes.  The response below 10 Hz we modelled in 3D, 
using the extended Born approximation of Habashy et al. We found that 
the Atlantic Ocean, some 25-50 km distant, interacts with the shallow 
body to produce a galvanic response that distorts the apparent 
resistivity curves at the sites close to the body, but not the phase 
curves. Including these distorted rho_a curves in an inversion leads 
to a mis-interpretation of the deep conductivity below the shallow 
body.