Surface wave tomography of the Nechako Basin, British Columbia, using ambient seismic noise

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 (&sim1.8 km), volcanic rock (&sim0.6 km), the sedimentary basin (&sim2.0 km), the Precambrian basement (&sim9.1 km), the lower crust (&sim17.0 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 (&sim5km) 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.