ALBERT

Beschreibung: After almost three decades of study, from its initial discovery in the 1960s to laboratory analyses of rock samples last year, we can now identify the most probable cause of the North American Central Plains (NACP) conductivity anomaly for much of its 1, 500-km strike extent. Tectonic processes operating during Paleoproterozoic Trans-Hudson orogenesis, with closure of the 5, 000-km-wide Manikewan ocean, included subduction and compression of sediments deposited during a hiatus in volcanism as the first of the advancing arcs approached the Archean continental margin to the west (Wyoming and Rae/Hearne cratons). These sediments were folded, and syngenetic sulphides within them migrated to concentrate along fold hinges, preferentially along strike, leading to high anisotropy in electrical conductivity (over 2-3 orders of magnitude). Mapping of the anomaly in similar tectonic environments, from the southern Dakotas to northern Manitoba, suggests that these processes were active along the whole western and northern margin of the orogen. However, other processes, possibly invoking graphitic emplacement in a foredeep, more likely account for the southern terminus of the anomaly from the Black Hills to southeastern Wyoming. © 1997, Society of Geomagnetism and Earth, Planetary and Space Sciences. All rights reserved.

Beschreibung: The electrical structure of the crust and upper mantle beneath three regions of Scandinavia has been delineated by the magnetotelluric (MT) and the horizontal spatial gradient (HSG) techniques. The analyses were applied to data recorded by the Scandinavian IMS magnetometer array complimented by telluric observations. Models compatible with the response functions observed in northern Sweden and northeastern Norway/northern Finland are distinctive by exhibiting: (i) a negligibly small resistivity contrast across the seismic Moho; and (ii) the unequivocal existence of an electrical asthenosphere beneath both regions. In definite contrast, the response function observed in southern Finland demands a highly conducting layer (of resistivity around 10-50 Ωm) in the lower crust, and an order of magnitude increase in resistivity on entering the mantle. This increase is at a depth compatible with the known seismic Moho for the region. It is not possible to make a quantitative estimate of the depth to the electrical asthenosphere beneath southern Finland, due to lack of long period information, but a qualitative measure indicates that the asthenosphere depth increases with increasing distance towards the centre of the north European craton. © 1983, Society of Geomagnetism and Earth, Planetary and Space Sciences. All rights reserved.

Beschreibung: To facilitate a comparative study of approaches and algorithms for two-dimensional interpretation of magnetotelluric (MT) data, a dataset of thirty-five sites, called Coprod2, was distributed to the electromagnetic induction community. The data are from stations along a 400 km east-west profile in southern Saskatchewan and Manitoba, Canada, crossing the thick Paleozoic sediments of the Williston basin. Within the basement beneath the sediments lies one of the world's longest and most enigmatic crustal conductivity features-the North American Central Plains (NACP) conductivity anomaly. Also, at the eastern extremity of the profile is a second basement anomaly (TOBE) interpreted to be associated with the Thompson Nickel Belt at the Superior-Churchill boundary. The MT data were corrected for static shifts, and only the off diagonal impedance data together with the transfer function data, and their errors, were made available. These MT data are of wide bandwidth (384 Hz–1820 s) and high quality (impedance errors typically 〈2%), and require sophisticated modelling and inversion in order to extract as much meaningful information as possible from them. A challenge for those interpreting these data is that there is a very small, but critical, response in the B-polarization data to the presence of the basement anomalies. In this introductory paper, the previous electromagnetic, and other geophysical, studies of the NACP anomaly, and its tectonic setting within the Trans-Hudson orogen, are described. Representative data from the Coprod2 data are illustrated, and models derived by various groups are shown. Comparisons of these models reveals that a single measure of misfit is an inadequate description of how well a model fits a dataset; one must compare the model fit to the data at virtually each datum to avoid trends in the misfit residuals. Additionally, reliable and consistent error and static shift estimates are essential in order to obtain high resolution images of the Earth's conductivity structure. Finally, the Coprod2 data highlight the sensitivity in B-polarization mode data to breaks in the resistive layer, whereas the E-polarization data sense regions of enhanced conductivity. © 1993, Society of Geomagnetism and Earth, Planetary and Space Sciences. All rights reserved.

Beschreibung: Two major factors hinder mineral exploration at this present time: (1) a conventional resistivity borehole log is often more indicative of the resistivity of the pore-filled fractured rock in the close locale (〈 1 m) of the borehole than the actual resistivity of the layer in which the probe is located; and (2) ground-based em techniques, both natural and controlled source, are often unable to locate a mineralized zone beneath another mineralized zone. In this paper, the theory is presented for the basis of a conceptually new type of borehole technique based on the ratio of the measurement of the natural horizontal magnetic field variation to its gradient with depth, down the hole, viz. [formula omitted]. Defining the “downhole apparent resistivity”, ρa(ω, d) by it is shown that, for a 1D earth structure, as ω tends to infinity, then ρa(ω,d) tends to the actual resistivity of the layer in which the probe is located. Also, ρa(ω,d) is independent, in the 1D case, of any structure above it, and weakly dependent in the 2D case. The technique has the benefit of most borehole methods of being far superior at resolving structure at depth below it, e.g., a second good-conducting zone, than equivalent ground-based methods (e.g., MT and /or GDS). Application of the technique to some theoretical 1D and 2D structures is presented, as well as a discussion of the feasibility of constructing the necessary sensor for the proposed technique. © 1983, Society of Geomagnetism and Earth, Planetary and Space Sciences. All rights reserved.

Beschreibung: The BC87 magnetotelluric data exhibit distortions due to three-dimensional structures at all scale sizes, from very local to regional. Previous work has shown that these distortions can sometimes, but not always, be described by an electric field galvanic distortion model, and hence can be removed using a tensor decomposition approach. This work extends the analysis to include galvanic distortion of the magnetic field, and shows its importance for many of the BC87 sites. However, even a combined electric and magnetic field galvanic distortion approach fails for quite a few of the sites studied. This is pervasive for periods shorter than 10 s, suggesting local 3D inductive effects, but model inadequacy at longer periods may indicate a breakdown of the model assumptions, particularly the requirement that the regional electric field be both uniform across the distorting body and comparable to that at the observation point.

Beschreibung: The BC87 data exhibit distortions due to three-dimensional structures at almost all scale sizes from that of the electrode array (〈100 m) to that of the 150×50 km plutonic Nelson batholith. These distortion effects must be identified and removed, as far as possible, prior to interpretation. A galvanic model of these distortions is shown to be valid for most of the frequency range of observation, but 3D induction is significant at certain sites for some bands. A first-order regression was fit to the decomposition recovered long period E-polarization apparent resistivities to correct for the remnant local site “static shifts”. We model the distortion- and level-corrected data using both 2D forward trial-and-error fitting and Occam2 smooth inversion. The crustal section of the resulting models correlates well with some geological features. However, the long period phase difference between the E- and B-polarization lasting for more than a decade is difficult to interpret without recourse to introduction of a layer with differing electrical conductivity in the two orthogonal horizontal directions, i.e., an anisotropic layer. There is greater than an order of magnitude difference in the conductivities, with the higher value in conductivity being in a direction which is commensurate with the Juan de Fuca plate-push direction. © 1993, Society of Geomagnetism and Earth, Planetary and Space Sciences. All rights reserved.

Beschreibung: As part of project Lithoprobe, magnetotelluric data were acquired at twenty-seven sites along a 150 km east-west profile in southeastern British Columbia straddling the complex tectonic boundary between ancestral North American rocks and those of the easternmost accreted terranes. These data, named the BC87 dataset, display complex effects due to both three-dimensional induction and galvanic scattering at virtually all scale sizes, from that of the electrode array (〈50 m) to that of the large plutonic Nelson batholith (150×50 km surface extent). The dataset was distributed widely in order to compare and contrast different schemes for extracting the underlying predominant two-dimensional structure, and to test differing interpretational algorithms on data from a complex crystalline terrane. The MT data are shown, and the tectonic setting described. Also, seismic and previous EM studies of the region are described. © 1993, Society of Geomagnetism and Earth, Planetary and Space Sciences. All rights reserved.