Integrated Interpretation and First Arrival Tomography of Reflection Surveys in Nechako Basin (NTS 92N, 92O, 93B, 93C, 93F, 93G)
- Andy Calvert, Simon Fraser University

The Nechako Basin, which is located in the interior plateau of British Columbia between the Coast Mountains and the Rocky Mountains, has seen very little exploration for hydrocarbons, in marked contrast to the western Canadian sedimentary basin. The structure and hydrocarbon potential of the basin is poorly understood. Project area

As part of a previous Geoscience BC project at Simon Fraser University, 1650 km of seismic data acquired in the Nechako basin by Canadian Hunter have been interpreted and integrated with magnetic and gravity data. In addition, shallow (<1000 m depth) P wave velocity models have been derived along the linear survey profiles using tomographic inversion of first arrival travel times.

This new project extends the current work to the new seismic survey that will be acquired by Geoscience BC in Nechako basin in 2008. The new survey will provide a regional context to the older, more focussed seismic surveys, including a better defined relation between the sedimentary sub-basins and the evolution of the underlying igneous crust, which should be better imaged. The longer recording spreads of the 2008 survey, which will have offsets up to 14.4 km, should also allow discrimination of sedimentary lithologies from igneous rocks to depths of approximately 2 km, complementing the interpretation of the reflection images. With integration of the 1980s and 2008 seismic data with existing gravity/magnetic data, the along-strike extent of the prospective NNW-trending Cretaceous rocks exposed in the Nazko river valley, and drilled by Canadian Hunter, should be better defined.

Posters and Presentations
2011: 3-D refraction tomography in the eastern Nechako Basin, BC: recovering intra- and sub-volcanic velocity structure in a complex volcanic and sedimentary system
- Mineral Exploration Roundup Conference 2011 Poster (pdf, 12.4 MB)
Technical Articles
2011: "Seismic reflection constraints on upper crustal structures in the volcanic-covered central Nechako basin, British Columbia"
- Canadian Journal of Earth Sciences, Volume 48(6) p.1021-1037

2011: "Velocity models from three-dimensional traveltime tomography in the Nechako Basin, south-central British Columbia"
- Summary of Activities 2010, Report 2011-1 p. 255-264 (pdf, 5.5 MB)

2010: "Near-surface volcanic rocks in the southeastern Nechako Basin, south-central British Columbia (parts of NTS 092N, O, 093B, C): interpretation of the Canadian Hunter seismic reflection surveys and first-arrival tomographic inversion"
- Summary of Activities 2009, Report 2010-1 p.203-226 (pdf, 20.5 MB)

2009: "Preliminary first-arrival modelling constraints on the character, thickness and distribution of Neogene and Eocene volcanic rocks in the southeastern Nechako Basin, south-central British Columbia"
- Summary of Activities 2008, Report 2009-1 p.151-156 (pdf, 3.03MB)

Final Deliverables
Geoscience BC Report 2012-13
Subsurface structure of the eastern Nechako Basin from coincident three-dimensional tomographic velocity and reflection data (Parts of NTS 093B, C, F, G)

Extensive Tertiary basalt flows of the Chilcotin, Endako, and Ootsa Lake groups cover most of the Nechako Basin, and up to date, no discoveries have been made in this region. The inability to define prospects results from the lack of clear reflections in many areas due to the heterogeneous subsurface geology and the effects of near-surface volcanic rocks on seismic imaging. Basin analysis identified that petroleum systems in the area reside mainly within Jurassic and Cretaceous lithologies, which remain poorly imaged and in many instances unknown. Due to the complexity of the velocity model, post-stack time migrations of the 2008 Geoscience BC long-offset two-dimensional (2-D) seismic data show no dramatic improvement, with almost no coherent data below the basaltic lavas. However, significant progress in deciphering the complex geology can be made using 3-D tomographic imaging of velocity structures, when correlated with surface geology, seismic data, borehole information and laboratory velocity measurements. Tomographic velocity modelling suggests the existence of faults and velocity inversions, and a number of several buried intrusive bodies of various ages have been identified in the area.


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