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The first step in the interpretation of magnetotelluric (MT) data involves estimating [Formula: see text] frequency‐domain impedances, Z(w), from the raw electric and magnetic field time series e(t), h(t) (approx. [Formula: see text] real numbers/site) [e.g., Swift, 1967, Sims et al., 1971]. Superficially, this initial data reduction step is almost trivial. Making the usual MT assumptions that the external sources are spatially uniform, and allowing for noise in the simplest way, e and h are related in the frequency domain via the linear statistical model e = Zh + ε, (1) where ε represents noise. The impedance Z can then be estimated quite simply by Fourier transforming the time series, and using linear least squares (LS] to minimize the misfit to equation (1) [Sims et al., 1971]. Unfortunately, this simple approach can fail catastrophically for noisy data, producing estimates that are heavily biased or wildly oscillatory [e.g., Gamble et al., 1979a; Jones et al., 1989; Figure 1]. As a consequence a number of refinements to the simple LS approach have been proposed in an effort to guarantee impedance estimates that are useful for subsequent stages in the interpretation process.

S U M M A R YTwo magnetic variation anomalies in southern Scotland and northern England have been linked to the position of the Iapetus suture. Previous magnetotelluric measurements along a 140 km profile that crosses both anomalies have been supplemented by new high-quality broad-band observations. Groom-Bailey decomposition of the impedance and hypothetical event analysis of the magnetic variation data agree in defining different strike directions for the two halves of the MT profile: N50"E for the northern part over the Southern Uplands, and N90"E for the southern part over the Northumberland Trough.TE and TM mode apparent resistivity and phase data at representative frequencies have been inverted for the two sub-profiles separately using the smooth inverse of Smith & Booker (1990). The uppermost layer has a low resistivity, is variable in thickness, and correlates well with the known position and thickness of Carboniferous sedimentary rocks. The second layer has a very high resistivity (thousands of a m ) , and reaches the surface where the Lower Palaeozoic metamorphic rocks of the Southern Uplands crop out. A relatively rapid transition to low resistivities occurs at depths of between 8 and 16 km. The conducting 'layer' appears to be quasi-continuous, but where the profile crosses the anomalies identified by magnetic variation measurements, the conductance increases, and the upper surface is shallower.The spatial coverage of the magnetic variation data has enabled us to extrapolate conductive features away from the line of section and project the electrical image onto the NEC vertical-incidence seismic reflection profile in the North Sea. There is excellent agreement between a number of features in the acoustical and electrical images. The shallowing of the low-resistivity layer to form a narrow wedge-like feature corresponds to the offshore position of the Stublick fault, while, to the north of the fault, the top of the layer coincides with a south-dipping reflector thought to be a thrust. However, the zones of high conductance and high lower-crustal reflectivity do not in general correlate. The good conductor beneath the Northumberland Trough spans two zones which were differentiated on the NEC profile in terms of their reflectivity.The shape of the conductor's upper surface in the vicinity of the Stublick fault agrees well with the model of Chadwick & Holliday (1991), who proposed that the Iapetus suture was a whole-crustal shear with a gently dipping central ramp. The coincidence between the (present-day) low-resistivity layer and a surface of weakness that was active 300-400 Myr ago is much more readily explained in terms of mineralogy (the presence of graphite) than the presence of fluids.There is also strong support for interpreting the northern conductor in the same way. Its upper surface is relatively flat and occupies the position predicted as the horizontal detachment surface, over which wedges of the Southern Uplands rocks were thrust. Its northeastward extension is sampled by a group of Carbonif...

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In an effort to reduce costs and increase revenues at mines, there is a strong incentive to develop highresolution techniques both for near-mine exploration and for delineation of known orebodies. To investigate the potential of high-frequency EM techniques for exploration and delineation of massive sulfide orebodies, radio frequency electromagnetic (RFEM) and ground-penetrating radar (GPR) surveys were conducted in boreholes through the McConnell massive nickel-copper sulfide body near Sudbury, Ontario, from 1993Ontario, from -1996 Crosshole RFEM data were acquired with a JW-4 electric dipole system between two boreholes on section 2720W. Ten frequencies between 0.5 and 5.0 MHz were recorded. Radio signals propagated through the Sudbury Breccia over ranges of at least 150 m at all frequencies. The resulting radio absorption tomogram clearly imaged the McConnell deposit over 110 m downdip. Signal was extinguished when either antenna entered the sulfide body. However, the expected radio shadow did not eventuate when transmitter and receiver were on opposite sides of the deposit. Two-dimensional modeling suggested that diffraction around the edges of the sulfide body could not account for the observed field amplitudes. It was concluded at the time that the sulfide body is discontinuous; according to modeling, a gap as small as 5 m could have explained the observations. Subsequent investigations by INCO established that pick-up in the metal-cored downhole cables was actually responsible for the elevated signal levels.Both single-hole reflection profiles and crosshole measurements were acquired using RAMAC borehole radar systems, operating at 60 MHz. Detection of radar reflections from the sulfide contact was problematic. One coherent reflection was observed from the hanging-wall contact in single-hole reflection mode. This reflection could be traced about 25 m uphole from the contact. In addition to unfavorable survey geometry, factors which may have suppressed reflections included host rock heterogeneity, disseminated sulfides, and contact irregularity.Velocity and absorption tomograms were generated in the Sudbury Breccia host rock from the crosshole radar. Radar velocity was variable, averaging 125 m/µs, while absorption was typically 0.8 dB/m at 60 MHz. Kirchhoffstyle 2-D migration of later arrivals in the crosshole radargrams defined reflective zones that roughly parallel the inferred edge of the sulfide body.The McConnell high-frequency EM surveys established that radio tomography and simple radio shadowing are potentially valuable for near-and in-mine exploration and orebody delineation in the Sudbury Breccia. The effectiveness of borehole radar in this particular environment is less certain.