Modeling and Investigation of Airborne Electromagnetic Data, and Reprocessing of Vibroseis Data, from Nechako Basin, BC, Guided by Magnetotelluric Results (NTS 92N, 92O, 93B, 93C, 93F, 93G)
- Colin Farquharson, Memorial University of Newfoundland

This multi-faceted project targets three of the stated areas of interest in the request for proposals for the project "Geoscience to Stimulate Exploration Activity in the Nechako Basin, British Columbia". Specifically: 1. We shall generate synthetic ZTEM data from the conductivity models derived for the Nechako basin from the previously collected magnetotelluric (MT) data, and we shall investigate to which features of the models such airborne electromagnetic (EM) data are sensitive, and to which features they are not. 2. We shall use the MT-derived conductivity models, including depths to interfaces and velocities derived from conductivities, to aid in the reprocessing of the 2008 Geoscience BC vibroseis data. And 3. we shall integrate existing geophysical and geological data with the MT-derived Earth models to develop and refine the tectonic history of the Nechako basin. This multi-component proposal will exploit the unique range of skills of the assembled research team, most notably, expertise in collection, processing and interpretation of MT data, previous experience with the Nechako basin MT data-set, expertise in developing numerical modelling and inversion software for geophysical EM data, expertise in processing land-based seismic data, and experience developing tectonic models for numerous regions of North America.

Posters and Presentations
2012: Three-dimensional inversion of the Nechako Basin magnetotelluric data-set
- Mineral Exploration Roundup Conference 2012 Poster (pdf, 38.6 MB)

2012: A look at magnetotelluric magnetic transfer functions to assess the benefits of an airborne ZTEM survey over the Nechako Basin, British Columbia
- Mineral Exploration Roundup Conference 2012 Poster (pdf, 5.1 MB)

2012: Reprocessing of multichannel seismic reflection profiles across the Nechako Basin, British Columbia: new insights and assessment of potential contribution from MT results
- Mineral Exploration Roundup Conference 2012 Poster (pdf, 6.8 MB)

2011: 3-D Modelling of the Nechako Basin MT Data Set
- Mineral Exploration Roundup Conference 2011 Poster (pdf, 2.5 MB)
Technical Articles
2012: "Analysis of magnetotelluric transfer functions to determine the usefulness of ZTEM data in the Nechako Basin, south-central British Columbia"
- Summary of Activities 2011, Report 2012-1 p. 151-162 (pdf, 4.4 MB)

2011: "Modelling and investigation of airborne electromagnetic data and reprocessing of vibroseis data from the Nechako Basin of south-central British Columbia, guided by magnetotelluric results"
- Summary of Activities 2010, Report 2011-1 p. 275-278 (pdf, 1.0 MB)
Final Deliverables
Geoscience BC Report 2012-14
Modelling and investigation of airborne electromagnetic data, and reprocessing of vibroseis data, from Nechako Basin, B.C., guided by magnetotelluric results

Magnetotelluric (MT) data collected in the Nechako Basin during 2007 have previously been inverted to produce 2D conductivity sections along seven profiles through the region. In the work reported here, these conductivity sections were used to assess the ability of z-axis tipper electromagnetic (ZTEM) data to determine and resolve the conductivity structure of the Nechako Basin, and to investigate the possibility of using the MT-derived conductivity sections to assist in processing the multichannel seismic reflection data collected across the basin in 2008. The results indicated that, although highly dependent on the actual conductivity structure in different parts of the basin, ZTEM data may be capable of identifying the conductive Eocene volcaniclastic rocks to shallow depths but may struggle to differentiate between the Eocene groups and the Cretaceous sedimentary rocks.

Multichannel seismic reflection data were acquired along a number of profiles through the Nechako Basin in 2008. To date, processing of this data-set has proved challenging because of the complex structure of the region, with results not giving as much information about the subsurface as hoped. Here, an attempt was made to use the MT-derived 2D conductivity sections in the processing of the seismic data. The reprocessing of the seismic data resulted in sections with moderately improved reflectivity, although no new structures were revealed that had not been discernible previously. The MT-derived velocity models provided surprisingly decent starting models for the velocity analysis required for stacking the seismic data, but nothing that could not be obtained by conventional velocity analysis of the seismic data alone. Finally, work was performed to integrate the interpretations derived from the MT data with those from other existing geophysical and geological data in order to improve our understanding of the structure and formation of the Nechako Basin. The results of this integration work appear in the special section "New insights in Cordilleran Intermontane geoscience: reducing exploration risk in the mountain pine beetle-affected area, British Columbia" of the Canadian Journal of Earth Sciences (v. 48, n. 6).

M.Sc. Thesis - M. Drew
3D inversion modelling of the Nechako Basin, British Columbia, magnetotelluric data set - Memorial University of Newfoundland, 2012 - Available digitally through Memorial University of Newfoundland (pdf, 46 MB)

Abstract: The Nechako Basin is an intermontane basin situated in south central British Columbia, Canada. Due to a desire for an alternative source of employment stemming from the impact of the mountain pine beetle on the forestry industry, the government of British Columbia has commissioned several geophysical and geological surveys of the area in order to assess its hydrocarbon potential. This sedimentary basin is covered with a layer of volcanic extrusive rocks which renders seismic methods nearly ineffective. As an alternative, a magnetotelluric (MT) survey was performed in order to determine subsurface conductivity and structure. Data from this survey were used in this thesis to produce a 3D conductivity model of the subsurface, by running multiple inversions with varied parameters. Data from four profiles as well as additional sites filling in gaps between profiles were used: Profile A ran west of Nazko. Profile B ran south of Nazko, Profile C ran north-west of Nazko, and Profile D ran east from Nazko toward Quesnel. Models from inversion of this MT survey show sensitivity to thicker surface volcanic rock and show where these bodies may thin or pinch out (e.g. conductivity at the west end of profile A and south end of Profile B). Additionally potential faulting and partial melt is seen in the subsurface, in profile D and at depth between profiles A and C respectively. 3D inversion models have provided information in addition to previous 2D results, giving a different placement of features out of the restricted plane of 2D inversion and showing lateral extent of conductivity anomalies, thus contributing to hydrocarbon exploration in the Nechako Basin.


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