Observation of the dispersion-relation of T waves propagating as modes using ocean bottom distributed acoustic sensing.

Ocean-bottom distributed acoustic sensing (DAS) can measure strain induced by seismo-acoustic waves. Among them, T waves travel in the water column at the speed of sound and are expected to propagate as modes. DAS dense time and space sampling gives access to dispersion curves in the frequency-wavenumber domain. In DAS data acquired by the Ocean Observatory Initiative on Oregon shore, Scholte waves dominate the 0.1-1.5 Hz frequency band. We identify a cable section where Scholte-wave energy forms marked power-law shape regions in the frequency-wavenumber domain. We use these dispersion curves for deterministic inversion of a power-law shear-speed profile and of a constant seabed density based on an analytic model. We focus on T waves from a regional earthquake in the 2-30 Hz frequency band. Far offshore, the dispersion curves of four T-wave modes are visible, but only one mode is visible where seabed inversion is possible. This T-wave mode is used for compressional-wave speed inversion based on analytical modes in a fluid-fluid-solid waveguide and using seabed density from Scholte-wave inversion. We discuss the differences between theoretical modes in a fluid-fluid-solid waveguide and in a fluid-solid-solid waveguide with a sediment shear-speed lower than the ocean sound speed.