On the Interpretation of Magnetotelluric Data in the Plains of Alberta

Abstract

Tensor apparent resistivity values obtained from the magnetotelluric field measurements, in the plains of Alberta indicate a major regional anisotropy. The direction of the major axis of the anisotropy is parallel to the strike of the Rocky Mountains and the sedimentary basin of Western Canada. Results obtained by numerical methods for a two-dimensional geoelectric model support an interpretation of the observed magnetotelluric results, depending on regional structures. The observed polarization parameters of the magnetotelluric fields are also in good agreement with those computed for the two-dimensional model. This study shows that the conclusions based on some earlier magnetotelluric results, which postulated an upper mantle conductivity layer must be treated with strong reservations.

Electrical resistivity structure of the Flathead Basin in southeastern British Columbia, Canada

“…These strike directions justify rotating the resistivity and phase data into the dominant geological trends for the region (-N30°W). In the Plains of Alberta, Reddy and Rankin (1972) also found the major axis of resistivity anisotropy to be in a direction N30°W.…”

The problem of current channelling: A critical review

“…If, in the centre ofa graben, a 1D model closely resembles the Epolarisation mode (as suggested by Reddy and Rankin, 1972), then inspection of the attenuation of the horizontal magnetic field as it diffuses downwards through a 1D model of the sediments into the poorly conducting basement would indicate how the phase of Hy varies with depth. Figure 17 illustrates the resistivity-depth profile of the centre of the Rhinegraben (taken from Babour and Mosnier, 1980), and also the attenuation in amplitude, and change in phase (both relative to the surface horizontal magnetic field), with depth at the two bounding periods of the study of Babour and Mosnier (1980) 20 s and 7200 s (2 hr).…”

“…This has had the rather adverse and detrimental effect that many investigators are now somewhat reluctant to interpret their data by choice of suitable 1D or 2D models. As shown in the section on numerical modelling, provided that the observational profile is conducted sufficiently distant from any 3D structurebe it the 'ends' of the body or a coastline, where 'sufficiently distant' is the appropriate 3D 'adjustment distance' -then a 2D interpretation of the 'E-polarisation' induction mode response is valid (and, as shown by Reddy and Rankin (1972), in certain circumstances a 1D interpretation of the 2D E-polarisation data yields a sufficiently close approximation to the true conductivity-depth structure).…”

A synthesis of electromagnetic studies in the Lithoprobe Alberta Basement Transect: constraints on Paleoproterozoic indentation tectonics

“…Some of these early studies (e.g., Srivastava et al 1963;Rankin and Reddy 1969) showed that structures within the crystalline crust were aligned approximately 45°from true north. Importantly for subsequent studies, Vozoff and Ellis (1966) showed that Reddy and Rankin (1972) that suggest the presence of a strong conductivity anomaly are BVL, Bonnyville; TWH, Two Hills; RDR, Red Deer; and DLD, Donalda. Thin black lines indicate the location of seismic reflection profiles.…”

“…Using 2-D forward modelling to interpret their data, Reddy and Rankin (1972) suggested that the crust under Alberta was strongly 2-D and that previous interpretations of deep-mantle conductivity structures using 1-D models for MT data from central Alberta should be viewed with caution. Although not specifically discussed in the original papers, data, published by Reddy and Rankin (1972) from the nearly collinear stations of Red Deer, Two Hills, Donalda, and Bonnyville (Fig. 3), contain the unmistakable response of a significant 2-D conductor in the upper crust, forming a roughly northeasttrending anomaly.…”