Advancement Of Passive Seismic Methods For Seismic Site Characterization And Resource Exploration

Hema Sandeep Sharma Ms, Western University

Abstract

Passive seismic methods use background low frequency ambient vibrations produced by natural and artificial sources. Advancements that enable the utilization of passive seismic signals are required in several areas, including understanding the wave content of the recorded passive seismic wavefield, and seismic site characterization and near-surface seismic tomography. This thesis utilizes case studies to tackle aspects of these three passive seismic research areas. My first case study determines the possible wave type contributions, where the microtremor horizontal to vertical spectral ratios (MHVSRs) were quantified in terms of their peak broadness and their fitness using mathematical functions. We developed an approach that determines the predominant wave types contributing to the microtremor wavefield using particle motion plots of cross-correlated microtremor recordings. Furthermore, the impact of a Rayleigh wave ellipticity assumption of the forward amplification model for MHVSR inversion to estimate the shear wave velocity (V_S) depth profile was also evaluated. It is found that incorrect wave contribution assumptions lead to up to 30% error in prediction of soil thickness and bedrock velocity. My second case study closes some of the gaps in seismic site characterization in eastern Canada, where we applied both non-invasive (passive and active source seismic) and invasive methods to characterize 172 sites in Essex County, southwestern Ontario. The findings indicate that the soils in Essex County are softer than Montreal, but stiffer than other regions in eastern Canada. We also found greater soil thickness in northern and eastern Essex County, which could make it more susceptible to seismic damage in the case of an earthquake. By combining non-invasive and invasive techniques, we developed region-specific relationships between important seismic site characterization parameters. The third case study of this thesis advances the use of passive seismic recordings for ambient noise tomography to develop a three-dimensional (3D) velocity model of an area of mineral exploration interest near Marathon, Ontario. The novel aspect was to extract surface wave dispersion estimates by cross-correlating passive seismic recordings to achieve a data-driven high-resolution 3D velocity model, which is useful to improve mineral generation models and identify exploration targets.