First-order Control Factors for Ocean-bottom Ambient Seismology Interferometric Observations

Adesh Pandey , Aaron Girard , Jeffrey Shragge

Expanding the lower-frequency band of seismic energy sources, particularly below 2.0~Hz, is crucial for improving the stability and effectiveness of full waveform inversion (FWI). Conventional active sources including airguns are ineffective at generating low-frequency wavefields, while ambient seismic wavefields, driven by natural energy sources such as ocean waves, offer a promising alternative. Effectively using ambient wavefield energy for seismic imaging or inversion analyses, though, requires understanding key physical control factors contributing to observations - including ambient source mechanisms and distribution, ocean-bottom bathymetry, and Earth model heterogeneity - which influence wave-mode excitation and partitioning, particularly in the context of ocean-bottom ambient seismology interferometry. This study presents a modelling framework for simulating cross-correlation wavefields generated by ambient seismic sources for dense ocean-bottom sensor arrays within a coupled acoustic-elastic system, without relying on Green's function retrieval assumptions. We model velocity and pressure cross-correlation wavefields to explore the effects of ocean-bottom velocity structure, ambient source distributions, and bathymetric variations on seismic wave excitation and propagation in the low (0.01-2.00~Hz) frequency band. Our results show that the distribution of ambient energy source locations, whether at the seabed or sea surface, significantly affects excited wave-mode characteristics. Love waves are particularly evident in the presence of substantial lateral and vertical bathymetric variations and heterogeneous Earth structure. The distribution of azimuthal ambient energy sources also influences Love-wave excitation, with the most prominent waves observed in the direction of the highest source concentration. Additionally, not all virtual shot-gather components provide unique insights into wave-mode excitation and partitioning. This work improves the understanding of low-frequency ambient seismic wavefields in ocean environments, with potential applications in long-wavelength structural imaging and elastic velocity model estimation from FWI analysis