Gary W. McNeice

Multisite, multifrequency tensor decomposition of magnetotelluric data

Accurate interpretation of magnetotelluric data requires an understanding of the directionality and dimensionality inherent in the data, and valid implementation of an appropriate method for removing the effects of shallow, small-scale galvanic scatterers on the data to yield responses representative of regionalscale structures. The galvanic distortion analysis approach advocated by Groom and Bailey has become the most adopted method, rightly so given that the approach decomposes the magnetotelluric impedance tensor into determinable and indeterminable parts, and tests statistically the validity of the galvanic distortion assumption. As proposed by Groom and Bailey, one must determine the appropriate frequency-independent telluric distortion parameters and geoelectric strike by fitting the seven-parameter model on a frequencyby-frequency and site-by-site basis independently. Although this approach has the attraction that one gains a more intimate understanding of the data set, it is rather time-consuming and requires repetitive application. We propose an extension to Groom-Bailey decomposition in which a global minimum is sought to determine the most appropriate strike direction and telluric distortion parameters for a range of frequencies and a set of sites. Also, we show how an analytically-derived approximate Hessian of the objective function can reduce the required computing time. We illustrate application of the analysis to two synthetic data sets and to real data. Finally, we show how the analysis can be extended to cover the case of frequency-dependent distortion caused by the magnetic effects of the galvanic charges.

Electric lithosphere of the Slave craton

The Archean Slave craton in northwestern Canada is an ideal natural laboratory for investigating lithosphere formation and evolution, and has become an international focus of broad geoscientific investigation following the discovery of economic diamondiferous kimberlite pipes. Three deep-probing magnetotelluric surveys have recently been carried out on the craton using novel acquisition procedures. The magnetotelluric responses reveal an unexpected and remarkable anomaly in electrical conductivity, collocated with the kimberlite field that is modeled as a spatially confined upper mantle region of low resistivity (<30 Ω·m) at depths of 80–100+ km, and is interpreted to be due to dissolved hydrogen or carbon in graphite form. This geophysically anomalous upper mantle region is also spatially coincident with a geochemically defined ultradepleted harzburgitic layer. The tectonic processes that emplaced this structure are possibly related to the lithospheric subduction and trapping of overlying oceanic mantle at 2630–2620 Ma.

Electromagnetic constraints on strike-slip fault geometry—The Fraser River fault system

Magnetotelluric data from four profiles crossing the Eocene strike-slip Fraser River fault in southwestern British Columbia suggest that it penetrates the entire crust. This conclusion is supported by seismic reflection observations of a 2-3 km step in the Moho slightly to the east of the surface expression of the fault, but is at variance with an interpretation of the seismic data in which the fault soles into mid-crustal reflectors that seem to be continuous across the fault trace. A crustal-penetrating geometry supports the proposal that the Fraser River fault forms part of a 2500-km-long intracontinental transform fault system in northwestern North America. Modeling studies resolve a thin, highly conducting mid-crustal zone that is connected electrically to the conducting lower crust beneath the Coast belt. Low δ 13 C values close to the Fraser fault system suggest that the electromagnetic signature of this zone may be due to the presence of organic carbon.

North American Central Plains conductivity anomaly within the Trans-Hudson orogen in northern Saskatchewan, Canada

Magnetotelluric data acquired across the Paleoproterozoic Trans-Hudson orogen, northern Saskatchewan, image one of the world9s longest crustal features, the North American Central Plains conductivity anomaly. Modeling shows the anomaly at this latitude to comprise two distinct, westward-dipping bodies of high conductivity lying structurally above a late collisional feature, the Guncoat thrust. The shallower of these bodies correlates with the western part of the La Ronge belt; the deeper body at mid-crustal depths underlies the Wathaman batholith, and its western boundary is close to the inferred subsurface extension of the sub-vertical Needle Falls shear zone. The anomaly is identified with interleaved, biotitic, metasedimentary rocks of the Nemeiben zone and Cree Lake belt and is interpreted to have been thrust beneath the margin during collision of the La Ronge arc with the Rae-Hearne continent via westward-directed subduction.

Electromagnetic images of regional structure in the southern Canadian Cordillera

As part of Lithoprobes Southern Cordilleran transect investigations, magnetotelluric (MT) soundings were made at 160 sites providing unprecedented coverage from the Rockies to the west coast. Striking lateral variation, which spatially correlates with the morphogeological belt boundaries, is apparent at periods sensing the lower crust (≈10 s). For the Rockies, MT phases are around 35°, indicative of a moderately resistive (100s – 1000s Ω·m) North American Basement. Foreland belt phases are transitional and increase from 60° in the east to 70° in the west. Omineca and Coast belt phases are high (75°), implying a conductive (10–30 Ω·m) lower crust, whereas Intermontane belt phases are more than 10° lower (equivalent to ≈150 Ω·m). The regional variation in conductivity correlates to first order with surface heat flow changes along the profile and is also correlative with coincident seismic reflection sections in some aspects.

Multi‐site, multi‐frequency tensor decomposition of magnetotelluric data

Accurate interpretation of magnetotelluric data requires an understanding of the directionality and dimensionality inherent in the data, and valid implementation of an appropriate method for removing the effects of shallow, small-scale galvanic scatterers on the data to yield responses representative of regionalscale structures. The galvanic distortion analysis approach advocated by Groom and Bailey has become the most adopted method, rightly so given that the approach decomposes the magnetotelluric impedance tensor into determinable and indeterminable parts, and tests statistically the validity of the galvanic distortion assumption. As proposed by Groom and Bailey, one must determine the appropriate frequency-independent telluric distortion parameters and geoelectric strike by fitting the seven-parameter model on a frequencyby-frequency and site-by-site basis independently. Although this approach has the attraction that one gains a more intimate understanding of the data set, it is rather time-consuming and requires repetitive application. We propose an extension to Groom-Bailey decomposition in which a global minimum is sought to determine the most appropriate strike direction and telluric distortion parameters for a range of frequencies and a set of sites. Also, we show how an analytically-derived approximate Hessian of the objective function can reduce the required computing time. We illustrate application of the analysis to two synthetic data sets and to real data. Finally, we show how the analysis can be extended to cover the case of frequency-dependent distortion caused by the magnetic effects of the galvanic charges.

Audio-magnetotellurics (AMT) for steeply-dipping mineral targets: importance of multi- component measurements at each site

Summary Steeply-dipping mineralized bodies present a particular problem for audio-magnetotelluric (AMT) exploration. Such targets have little observable effect on currents flowing perpendicular to their dominant strike, thus TM-mode AMT data are not useful for target detection or delineation. The TE-mode and vertical-field transfer function data do detect such targets, and appropriately-designed surveys can define their geometries. The anomalous responses due to such targets are greater in the magnetic fields than in the electric fields. Conducting overburden suppresses the high frequency response, moving the anomaly maxima to lower and lower frequency with increasing overburden conductance. For optimal resolution of target geometries, all five AMT components must be measured at each and every site.

Results of CSAMT and gravity surveys over the Lemieux Dome, Quebec

The Lemieux Dome is a region of circularly uplifted Siluro-Devonian sedimentary and volcanic rocks located in north-central Gasp&, Quebec. The Dome has been intruded by multiple generations of small scale sills and stocks and is cut by mineralized quartz-carbonate veins. The observed structure and mineralization of the Dome have been interpreted to be the result of the upwelling of a large granitic stock. This and indications of mineral zoning similar to that observed at ore deposits in the Gasp6 region suggests the possibility of economic mineralization. However, shallow drilling and surficial mapping have not revealed the presence of significant mineral resources or the location and composition of the postulated hydrothermal heat source.