Mapping the Structure of the Nechako Basin Using Passive Source Seismology (NTS 92N, 92O, 93B, 93C, 93F, 93G)
- John Cassidy, Natural Resources Canada

Passive Sources Seismology Project Area The goal of this project is to help assess the hydrocarbon and mineral potential of the Nechako Basin in the Interior Plateau region of south-central BC. This will be accomplished by mapping the structure of the Nechako Basin using passive source seismology. Specifically, waveforms from distant earthquakes will be used to resolve the shear-wave velocity structure beneath the basin and provide information on the thickness and nature of the basin-fill sedimentary rocks and also on the thickness of the underlying crust.

The advantages of this method are numerous:

  • using natural earthquake sources means that permitting/explosions are unnecessary;
  • the energy is "coming from below", enabling us to see "below the basalts" that blanket the underlying prospective units and have degraded the quality of active source seismic data; and
  • the S-wave information obtained complements the P-wave information obtained from active source seismic studies.

The combination of P- and S-wave velocity information provides constraints on Poisson's ratio, which is indicative of lithologic composition. By integrating S-velocity information with gravity and MT data, results can be verified and constraints on pore-fill fluids and fracture characteristics can be estimated. This information can be used to develop and constrain models of basin architecture, identify pathways for fluid migration and provide insight into basin composition, all of which are required to further our understanding of the basin's resource potential.

This project represents a unique, and very cost-effective means to provide regional-scale structural information (at seven sites situated over an area of approximately 33,000 square kilometers) that will help direct future targeted studies (including MT, active source seismic, and drilling) by industry and other groups. A key component of this project will be training a graduate student in multidisciplinary geophysical research.

Go to Earthquakes Canada: http://earthquakescanada.nrcan.gc.ca

Nechako Seismic Station RAMB, showing typical station layout, with solar panels and satellite dish.  Photo courtesy of J. Cassidy.

Posters and Presentations
2009: Seismic Tomography of the Nechako Basin, British Columbia, Using Ambient Seismic Noise
- Mineral Exploration Roundup Poster (pdf, 1.93MB)

2009: Mapping Crustal Structures of the Nechako Basin Using Teleseismic Receiver Functions
- Mineral Exploration Roundup Poster (pdf, 6.34MB)

2008: Mapping the Structure of the Nechako Basin using Earthquake Waves
- Mineral Exploration Roundup Poster (pdf, 1.52MB)

Technical Articles
2011: "Imaging the Nechako Basin, British Columbia, using ambient seismic noise"
- Canadian Journal of Earth Sciences, Volume 48(6) p.1038-1049

2010: "Passive source seismic studies of the sediments, crust and mantle beneath the Nechako Basin, south-central British Columbia (NTS 092O, 093B, C, F, G)"
- Summary of Activities 2009, Report 2010-1 p.235-244 (pdf, 7.3 MB)

2009: "Mapping the sedimentary rocks and crustal structure of the Nechako Basin, south-central British Columbia using teleseismic receiver functions"
- Summary of Activities 2008, Report 2009-1 p.163-170 (pdf, 5.29MB)

2009: "Seismic tomography of the Nechako Basin, south-central British Columbia, using ambient seismic noise"
- Summary of Activities 2008, Report 2009-1 p.171-174 (pdf, 4.62MB)

2008: "Seismic tomography of the Nechako Basin, south-central British Columbia, using ambient seismic noise"
- Summary of Activities 2007, Report 2008-1 p.115-120 (pdf, 5.39MB)

Final Deliverables
M.Sc. Thesis - O.A. Idowu
Surface wave tomography of the Nechako Basin, British Columbia, using ambient seismic noise - University of Manitoba, 2009 - Available through the University of Manitoba

Abstract: The Nechako Basin in British Columbia, Canada has been a difficult basin to explore due to the presence of Tertiary volcanic outcrop. The volcanic outcrop makes the use of conventional seismic methods difficult due to a strong velocity inversion at its base. An alternative to active source methods is the passive source method known as ambient noise surface wave tomography. The method, which examines the high-frequency surface wave field that is obtained from noise analysis, is sensitive to large-scale crustal structure and has been successfully applied to measuring the depths of sedimentary basins. Ambient noise surface wave tomography will thus help to unravel the structural composition of the Nechako Basin. We estimated station-to station Green's functions within the basin, by cross-correlating the vertical components of the seismic noise data recorded by 12 POLARIS and CNSN seismic stations between September 2006 and November 2007, using a two-station method. The resulting Green's functions are dominated by Rayleigh waves. The dispersion characteristics of the Rayleigh waveforms were measured within the micro-seismic band. Inversion of the dispersion curves produced 2-D group velocity maps between 0.55 Hz and 0.03 Hz, and 1-D S velocity models for Nechako Basin and its surrounding region. The average 1-D model within the basin suggested a six layered medium; surface/near surface sediment (~1.8 km), volcanic rock (~0.6 km), the sedimentary basin (~2.0 km), the Precambrian basement (~9.1 km), the lower crust (~17.0 km), and the upper mantle. The average 1-D model outside the basin is similar to the model within the basin, except that the volcanic and sedimentary layers are absent. The 2-D group velocity maps show the lateral variations of rock composition within the Nechako region. High lateral variations are observed at frequencies between 0.55 Hz and 0.3 Hz, a band which is believed to be sensitive to the depth of the sedimentary basin (~5km) within the region. Zones of low group velocity structures within the area suggest that the region consists of a major deep and laterally extensive sedimentary package at the basin centre, and a shallow sedimentary package at the southern edge of the Nechako Basin.

M.Sc. Thesis - H-S. Kim
Mapping crustal structure of the Nechako Basin using teleseismic receiver functions - University of Victoria, 2010 - Available digitally through University of Victoria (pdf, 9.9MB)

Abstract: This thesis describes a passive-source seismic mapping project in the Nechako Basin of central British Columbia (BC), Canada, with the ultimate goal of assessing the hydrocarbon and mineral potential of the region. The Nechako Basin has been the focus of limited hydrocarbon exploration since the 1930s. Twelve exploratory wells were drilled; oil stains on drill chip samples and the evidence of gas in drill stem tests attest to some hydrocarbon potential. Seismic data collected in the 1980s were of variable quality due mainly to effects of volcanic cover in this region. For the present study, an array of nine seismic stations was deployed in 2006 and 2007 to sample a wide area of the Nechako Basin and map the sediment thickness, crustal thickness, and overall geometry of the basin. This study utilizes recordings of about 40 distant earthquakes from 2006 to 2008 to calculate receiver functions, and construct S-wave velocity models for each station using the Neighbourhood Algorithm inversion. The surface sediments are found to range in thickness from about 0.8 to 2.7 km, and the volcanic layer below ranges in thickness from 2.3 to 4.7 km. Both sediments and volcanic cover are thickest in the central part of the basin. The average crustal thickness across the basin is about 30-32 km; it is thicker in the northern and western parts of the basin, and thinner in the southern and eastern parts. This study complements other research in this region, such as independent active-source seismic studies and magnetotelluric measurements, by providing site-specific images of the crustal structure down to the Moho and detailed constraints on the S-wave velocity structure.

 


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