Research Projects

Imaging the Earth’s interior structure is crucial for solving many practical problems such as carbon sequestration, metal mining, ground water studies, exploration of mineral oils, and natural gases. The primary objective of this research is to extract useful information about the Earth’s interior structure in seismically stable regions where earthquakes do not occur frequently. In such regions, seismic ambient noise is processed to perform tomography (i.e., imaging of Earth’s interior structure). Ambient noise is generated mainly by the interaction of ocean swells with the seafloor. Ambient noise generated in this manner is comprised of primary and secondary microseisms. Ambient noise tomography (ANT) relies on cross-correlating long duration ambient noise data recorded at pairs of seismic stations, estimating the empirical Green’s function (EGF). Theoretically, ambient noise has the potential to obtain complete Green’s functions (i.e. the path effect due to a point source in ideal case), provided that noise sources are distributed uniformly in both space and time. Real-world complexities such as anisotropic and non-stationary noise sources undermine the effectiveness of conventional methods of extracting EGF, therefore in practical seismic studies, retrieval of seismic waves in EGF is often restricted to surface waves. Unequal energy distribution of ambient noise can amplify incoherent noise relative to coherent signal components, causing bias in the estimated EGF. These biases impact the subsequent analyses like dispersion curve extraction and their inversion, eventually leading to limited resolution and inaccuracies in imaging the Earth’s subsurface structure. Additionally, retrieving surface waves across all frequencies is challenging due to highly directional dominant noise sources, and the recovery of higher-order modes of surface wave propagation is even harder due to multipath fading. High frequency surface waves are useful for sampling the shallow subsurface, while low-frequency surface waves are more effective for probing deeper layers. Relying on incomplete observed bandwidth of surface waves is insufficient for effectively constraining velocity interfaces. Therefore, extraction of the body wave signals, along with the complete bandwidth of surface waves, is essential for accurately imaging subsurface structure.