A magnetotelluric (MT) survey was carried out on the western flank of the Sudbury Structure (Ontario, Canada) over a recently discovered ore body to test the potential of the method as a reconnaissance tool for deep mineral exploration. The observed responses exhibit a large phase anomaly (exceeding 75° in some places) centered over the ore body, which is interpreted as arising from a combination of 3-D induction and current channeling. Impedance decomposition methods were used to define frequency ranges in which the response is quasi 2-D with geologically plausible strike values. Two‐dimensional rapid relaxation inversions of the data, where appropriate, generated quantitative models in good agreement with known parameters of the ore body. Two‐dimensional inversions, however, cannot replicate either the size or the peculiar nature of the observed phase anomaly. Therefore 3-D modeling trials were undertaken, which best explain the MT responses in terms of 3-D induction within the Trillabelle body coupled with current channeling through neighboring faults. An important conclusion of these trials is that isolated, deep, highly conductive ore bodies may not be observable using MT. Prospects for their detection appear to improve when they are linked with regional, current gathering features such as conductive faults, or a larger‐scale, alteration or dissemination halo.

Reprocessing of part of a Lithoprobe high-resolution seismic reflection line across the southern part of the Abitibi Belt has improved the imaging of shallow reflections and allowed correlation of the data with surface geology. Enhancement of early reflections was accomplished by focusing on the high-frequency content of the data. This improved resolution of reflections at two-way traveltime as early as 0.3 s and attenuated noise such as shear waves. The shallow reflections are interpreted as impedance contrasts at the contact between a metadiabase–diorite body and metavolcanics rocks. Offsets of the reflectors correlate with faults mapped at the surface and indicate a downdropped block, which may be of interest for mineral exploration.

Galvanic distortion of the regional electric field bY near-surface heterogeneities leads to disturbance in the measured MT impedance tensor. This tensor is expressed as the product of a local galvanic distortion tensor and a two-dimensional regional impedance tensor. In this paper, we present a method to remove the effect of galvanic distortion and extract the regional impedances. This method is based essentially on single value decomposition of the distortion tensor and can be used for two or three-dimensional surface heterogeneities. The recovered regional impedances are shifted by a real constant in such a way that neither the shape of regional apparent resistivity curve nor the phases are altered. A synthetic model with conductive hemispherical heterogeneity embedded in a two-dimensional regional structure (Groom and Bailey, 1988) demonstrates the utility of the method.

Published K indices defined from ground-based magnetic variations are shown to give reliable upper bounds to the mid-latitude, external transient field recorded by Magsat (350 km altitude). The argument relies on model calculations of three-dimensional current systems known to be representative of auroral activity and gives a simple method for estimating source contamination in the magnetic data used for main field and crustal anomalies calculations.