Seismic Imaging of the Sedimentary and Upper Crustal Structures of the Western Canada Sedimentary Basin

Abstract

Secondary converted waves from receiver functions are highly sensitive to physical properties below the Earth’s surface. When modeled properly, the waveforms of converted waves offer direct constraints on the impedance contrast, depth, and P-to-S velocity ratio pertaining to sedimentary, crustal and mantle interfaces. In this thesis we introduce a nonlinear waveform inversion algorithm that matches the first 5 seconds of receiver functions recorded in the Alberta Basin within the Western Canada Sedimentary Basin (WCSB). Our algorithm searches for the optimal thickness of the sedimentary cover and shear velocities of appropriately selected layers within and below it. Combining inversions with forward simulations, we determine the supracrustal stratigraphy from the regional broadband seismic stations in the WCSB. The inverted models show east tapering sedimentary layers with their thicknesses ranging from ~6 km beneath the foothills of the Rocky Mountains to 3-4 km beneath the Alberta Basin. This finding is consistent with the sedimentary strata determined from regional well-logging data. Our shear velocity models near the top of the basement complement the existing sonic-logs or single component seismic data and offer new constraints on the subsidence history of the WCSB. The resolved range of depths (0-14 km) effectively bridges the gap between the vertical scales of well logging (0-6 km) and those of traditional broadband analysis (> 10 km) involving receiver functions and surface waves. The S-velocity model of the study area reveals (1) the existence of sedimentary low velocity zones of variable thicknesses and amplitudes, (2) the existence of upper-middle crustal anomalous shear velocity zones, and (3) the tectonic evolution history of the subsidence of the Alberta Basin. These findings contribute to better understanding of the tectonic structure and evolution history of the Western Canada Sedimentary Basin.