Spectral analysis of the time-varying horizontal magnetic and electric field components yields the magnetotelluric (MT) impedance tensor. This frequency dependent 2x2 complex tensor can be examined for details which axe diagnostic of the electrical conductivity distribution in the Eaxth within the relevant (frequency dependent) inductive scale length of the surface observation point. As such, precise and accurate determination of this tensor from the electromagnetic time series is fundamental to successful interpretation of the derived responses. In this paper, several analysis techniques are applied to the same data set from one of the EMSLAB Lincoln Line sites.Two subsets of the complete data set were selected, on the basis of geomagnetic activity, to test the methods in the presence of differing signal-to-noise ratios for varying signals and noises.Illustrated by this comparison are the effects of both statistical and bias errors on the estimates from the diverse methods. It is concluded that robust processing methods should become adopted for the analysis of MT data, and that whenever possible remote reference fields should be used to avoid bias due to runcorrelated noise contributions.

A geomagnetic depth sounding profile in the western Cordillera has revealed concentrations of electric current related to the tectonics of the region. The east-west profile at approximately 46ø30'N extends from the coast to just east of the Cascade volcanic belt. In the western region of the profile the geomagnetic coast effect at a 50-min period was similar to results in other tectonically active areas such as California and southeastern Australia. Stations near the Cascade volcanic zone had anomalous response functions; a clearcut reversal in both the in-phase and quadrature components was observed in the direction of the induction arrows across the zone. Based on the similarity between the analyzed data curves and the response of a line current model, the anomaly can be approximated by a long, narrow conductor trending north-south in direction. This conductive zone does not extend as far north as a profile previously measured in southern British Columbia.Paper number 80B0813.

Mapping of the regional electrical conductivity structure in western Canada has been carried out with a network of geomagnetic recording stations that cover the frequency range 10 -5-2 X 10 -a Hz (24-hour to 8-min periods). Two main regions have been defined, based mainly oa the amplitudes of the vertical component (Z) variations in the frequency range 10-4-10 -a ttz. The main transition between the western conductive (low Z) region and the eastern (high Z) region has been mapped between latitudes 49øN and 54øN, and the westward extent of the low-Z region has been mapped to the coast. In the northern part the discontinuity trends southeast, following the western flank of the Rocky Mountains; it has been particularly well defined (ia location and strike) at latitude 53.2øN. Near latitude 51ø-52øN the discontinuity swings to a more southerly trend, continuing to 49øN. A second discontinuity has been mapped near latitude 49øN, trending roughly east-west between about 116øW and 118øW. Two other secondorder features have been noted for future investigation: a coastal effect at latitude 55øN and a minor anomaly near Srnithers (126øW, 55øN). The electrical conductivity structure of the earth's crust and upper mantle can be determined by observations of the fluctuations in the natural geomagnetic and geoelectric (telluric) fields at the surface. Two basic methods are in use: magnetotellurics (MT), where the two horizontal magnetic and electric components are recorded, and geomagnetic depth-sounding (GDS), where the three components of the magnetic field are recorded. The two methods are to some extent complementary. In western Canada, the joint University of British Columbia-Dominion Observatory research group has been using GDS primarily to map the areal extent of conductivity regions; MT data from a few selected sites have been used to 'calibrate' this mapping, i.e., to determine the actual conductivity layering for the different regions. Results of the large-scale GDS mapping up to latitude 51 øN have been published [Caner et al., 1967]; the present paper covers work carried out between 1966 and 1969 that extended the GDS mapping to latitude 55øN. • Contribution 341 from the Earth Physics Branch, Department of Energy, Mines, and Resources.

Geomagnetic variation data have been measured along a profile from central Vancouver Island, across the continental shelf to the deep seafloor on the Juan de Fuca plate. As expected, the maximum response in the geomagnetic coast effect occurs near the shelf-edge and the fall-off inland and seaward is observed. The direction and magnitude of the real part of the induction arrows vary slightly with period between 30 and 103 minutes. The induction arrows are co-linear at all sites and are aligned approximately normal to the shelf-break trend. At the seafloor sites, the imaginary part of the induction arrows has a large variation with period, reversing its direction at periods less than 35 minutes. At the land sites, the imaginary part increases in magnitude with increasing period.The responses of several numerical models were calculated using the finite difference algorithm of Brewitt-Taylor and Weaver. The comparison of the calculated and observed responses shows that a good conductor is required at sub-lithospheric depths beneath the profile. The addition of a conductive wedge of sediments beneath the continental shelf improves the fit. The best-fitting model incorporates this wedge and a conductive, shallow-dipping slab representing the Juan de Fuca plate subducting under Vancouver Island.

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