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The study compares the three deep learning approaches and assesses their relative performance solving the 1-D magnetotellurics (MT) inverse problem. MT data from a 1-D geothermal-type structure are used as an example to examine Variational Autoencoder (VAE), Residual Network (Res-Net), and U-Net architectures, adapted for 1-D MT inversion. Root Mean Square Error (RMSE) and Pearson correlation coefficient are applied as misfit measure and similarity criterion, and box plot tools are used to parameterize individual model parameters. The results show that the U-Net provides the most successful recovery of the 1-D resistivity models, even though all three approaches can produce accurate inversions of MT data. To investigate applicability of results to real data sets, the models performance are examined for the case of data containing noise. Three deep learning algorithms are robust with respect to data noise, although the U-Net is relatively superior. The study results provide a platform for more complex magnetotelluric inverse problems and ones involving real data sets.

Electromagnetic induction surveys are commonly used to assess soil salinity. In this study, a DualEM 1S instrument was used to survey an area in southern Manitoba to characterize sulphate-dominated salinity in clay-rich alluvial soils. The efficacy of predicting apparent conductivity (corrected for departure from low induction number responses) was determined by regression analysis of parameters including saturated-paste salinity, pore-water salinity, volumetric water content, porosity and combinations of these terms determined from 542 soil samples at 65 sampling sites. Predictors were depth-weighted using the V–H mode (exploration depth of 0.5 m) and the V–V mode (exploration depth of 1.5 m) of the electromagnetic instrument. The analysis yielded strong correlations of apparent conductivity (ECa) with saturated-paste or pore water conductivity, with Pearson r2 correlation coefficients exceeding 0.75 for the V–H mode and 0.86 for the V–V mode. The strong correlations of ECa with salinity are explained by moisture conditions being close to field capacity during the survey. These results are similar to those for chloride-dominated salinity. The study yielded less commonly observed results related to the clay-rich soils, including a threshold value of ~ 10% below which volumetric water content does not affect ECa, and an improved prediction of ECa by dividing the porosity data based on clay content. The study results demonstrate the efficacy of using ECa to parameterize sulphate-dominated salinity in clay-rich soils. They also show that salinity can be estimated relatively accurately from saturated paste conductivity at near-field capacity without considering moisture content.

In 2015, we mapped surface and near-surface physical properties of a Paleo-Inuit lithic quarry site, LbDt-1, located in the interior of southern Baffin Island, Nunavut, Canada using a multi-method approach. The survey site is characterised by a dense chert flake deposit. The purpose of the survey was to document this survey site's surface features using three-dimensional laser scanning and to investigate the utility of active remote sensing and geophysical methodologies at prehistoric lithic quarry sites. Manual and automated data reduction, interpretation and inversion methods were applied across each dataset to isolate the surface and subsurface distribution of flakes. Laser scanning results demonstrate a remarkable dispersal of surface chert flakes confined to a general area of the geophysical survey. To define the base of the lithic deposit layer, a combination of enhanced radar reflections and two-layer inversion models of magnetic responses obtained using electromagnetic measurements was used. Radar results suggest the deposit has a thickness of around 10–20 cm and indicate that there are no additional parts of the deposit masked by soil in this area. The magnetic susceptibility data define an upper layer of ~ 20 cm thickness and susceptibility (0.004–0.008 SI) overlying a less magnetic (< 0.004 SI) lower layer, with the spatial variations in the upper layer suggesting debitage and gravel deposits have lower magnetisation than the topsoil at the site. Overall, this study demonstrates the capacity of remote sensing and geophysical methods to non-invasively investigate some prehistoric activities without the need for full-scale excavation and the collection of large material assemblage characteristic of lithic quarry sites.

Magnetotelluric studies of the Trans-Hudson orogen over the last two decades, prompted by the discovery of a significant conductivity anomaly beneath the North American Central Plains (NACP), from over 300 sites yield an extensive database for interrogation and enable three-dimensional information to be obtained about the geometry of the orogen from southern North Dakota to northern Saskatchewan. The NACP anomaly is remarkable in its continuity along strike, testimony to along-strike similarity of orogenic processes. Where bedrock is exposed, the anomaly can be associated with sulphides that were metamorphosed during subduction and compression and penetratively emplaced deep within the crust of the internides of the orogen to the boundary of the Hearne margin. A new result from this compilation is the discovery of an anomaly within the upper mantle beginning at depths of ∼80-100 km. This lithospheric mantle conductor has electrical properties similar to those for the central Slave craton mantle conductor, which lies directly beneath the major diamond-producing Lac de Gras kimberlite field. While the Saskatchewan mantle conductor does not directly underlie the Fort a la Corne kimberlite, which is associated with the Sask craton, the spatial correspondence is close.

Since Mohorovičić 1 discovered a dramatic increase in compressional seismic velocity at a depth of 54 km beneath the Kulpa Valley in Croatia, the ‘Moho’ has become arguably the most important seismological horizon in Earth owing to its role in defining the crust–mantle boundary. It is now known to be a ubiquitous feature of the Earth, being found beneath both the continents and the oceans, and is commonly assumed to separate lower-crustal mafic rocks from upper-mantle ultramafic rocks. Electromagnetic experiments conducted to date, however, have failed to detect a corresponding change in electrical conductivity at the base of the crust, although one might be expected on the basis of laboratory measurements 2 . Here we report electromagnetic data from the Slave craton, northern Canada, which show a step-change in conductivity at Moho depths. Such resolution is possible because the Slave craton is highly anomalous, exhibiting a total crustal conductance of less than 1 Siemens—more than an order of magnitude smaller than other Archaean cratons. We also found that the conductivity of the uppermost continental mantle directly beneath the Moho is two orders of magnitude more conducting than laboratory studies on olivine would suggest, inferring that there must be a connected conducting phase.

Magnetotelluric (MT) measurements to image the three-dimensional resistivity structure of the North American continent from an Archean core to a region of Tertiary assembly were recorded at almost 300 sites along 3200 km of profiles on the Lithoprobe Slave-Northern Cordillera Lithospheric Evolution (SNORCLE) transect in northwestern Canada. At the largest scale, the MT results indicate significant lithospheric thickness variation, from 260 km at the southwest margin of the Slave craton to significantly <100 km at the southwestern end of the SNORCLE transect in the Cordillera. At intermediate scale, the resistivity results allow broad terrane subdivisions to be made. Several anomalously conductive zones along the SNORCLE transect, in rocks ranging in age from Archean to Tertiary, are attributed to the introduction of either water or carbon into the crust and mantle during subduction processes. At the local scale, the MT data image two major faults crossing the study area, the Great Slave Lake shear zone and the Tintina Fault. The resistivity images show that both the Tintina Fault and Great Slave Lake shear zone form crustal-scale features, and that the Tintina Fault has a remarkably uniform resistivity signature over a 400 km strike length in the study area. Arguably the most controversial conclusion reached is that the MT data do not support the western extension of North American autochthonous basement suggested from interpretation of the seismic reflection data.

Magnetotelluric (MT) soundings were made along a transect in northern Canada crossing the Proterozoic Wopmay Orogen, Fort Simpson basin, and adjacent parts of the Slave craton and the Nahanni terrane. The results are used to define the geoelectric structure and constrain the crustal and lithospheric structure and evolution. Across the Wopmay Orogen, geoelectric strikes at crustal depths average N34°E and are interpreted to be related to transcurrent faulting that occurred during late distal collisions at the western margin of the orogen. Weak two-dimensionality in the Fort Simpson basin is interpreted to be due to the sedimentary rocks in the basin. At longer periods, geoelectric strikes across the Wopmay Orogen rotate from ∼N43°E at uppermost mantle penetration to ∼N62°E at a depth of 100 km. The uppermost mantle strikes are interpreted to be due to ductile shearing linked to the transcurrent faulting in the overlying crust. The deeper strikes may be caused by shearing at the base of the present-day lithosphere. Within the Wopmay Orogen, the MT results image a conductor at the margin of the Fort Simpson and Hottah terranes interpreted to be related to the collision of these terranes. Conductive crust beneath the western margin of the Great Bear magmatic arc suggests correlative rocks of the Coronation margin extend south of the Slave craton. Lastly, decreased resistivity in the Hottah terrane at mantle depths is interpreted to be caused by the introduction of graphitic or sulphidic rocks during subduction prior to the Hottah-Slave and Fort Simpson-Hottah collisions.

Magnetotelluric (MT) measurements were made on a profile across the Trans-Hudson orogen in 1992 as part of the Lithoprobe transect. The present study includes analysis of results from a 300 km-long section of the profile in which allocthonous Paleoproterozoic juvenile terranes and arc rocks of the western Trans-Hudson orogen have been juxtaposed against the Archean Sask craton. Impedance tensor decomposition of data from the 40 MT sites in the area indicates a geoelectric strike of N28°E. Two-dimensional inversion of the data using a non-linear conjugate gradient algorithm provided images of the resistivity structure. Resistivity images reveal that the crust of the Sask craton is relatively resistive (>2000 Ω.m). In contrast, the rocks of the Flin Flon belt, Glennie domain, and La Ronge domain are mostly relatively conductive (<100-1000 Ω.m). In the east of the study area, the images suggest that the Tabbernor fault juxtaposes more conductive rocks of the Glennie domain in the west against more resistive Archean rocks in the east in the upper 20 km of the crust. In the west of the study area, the images confirm that the North American Central Plains conductor occurs within westward-dipping rocks of the La Ronge domain. The resistivity images also reveal that the lower crust beneath the west of the Glennie domain, within a crustal culmination defined by seismic reflection data, is electrically conductive (<100 Ω.m). An explanation for the enhanced conductivity is that part of the lower crust beneath the western Glennie domain is of Proterozoic age. In this case, a possible source for the enhanced conductivity, based on its location at the edge of the Sask continental block, is Proterozoic ocean margin rocks.

Resume : Afin d'obtenir une image de la structure de resistivite en trois dimensions du continent nord-americain, d'un noyau archeen a une region d'assemblage tertiaire, des mesures magnetotelluriques (MT) ont ete enregistrees a pres de 300 sites le long de 3200 km de profils de la geotraverse SNORCLE du projet Lithoprobe dans le nord-ouest du Canada. A la plus grande echelle, les resultats magnetotelluriques indiquent des variations importantes d'epaisseur de la lithosphere, de 260 km a la bordure sud-ouest du craton des Esclaves a passablement moins de 100 km a l'extremite sud-ouest de la geotraverse SNORCLE, dans la Cordillere. A une echelle intermediaire, les resultats des donnees de resistivite permettent de subdiviser le terrain en de larges terranes. Plusieurs anomalies de zones conductrices le long de la geotraverse SNORCLE, dans des roches datant de l'Archeen au Tertiaire, sont attribuees a l'introduction de carbone ou d'eau dans la croute et le manteau durant les processus de subduction. A une echelle locale, deux failles majeures traversent la region a l'etude, la zone de cisaillement du Grand lac des Esclaves et la faille Tintina. Les images de resistivite montrent que la faille Tintina et la zone de cisaillement du Grand lac des Esclaves sont des elements d'une echelle crustale et que la faille Tintina a une signature de resistivite remarquablement uniforme sur une longueur de 400 km. selon la direction, dans la region a l'etude. Tel que suggere par l'interpretation des donnees de sismique reflexion, il est permis de croire que la conclusion la plus controversee est que les donnees MT ne supportent pas l'extension vers l'ouest du socle autochtone de l'Amerique du Nord.