Fine-scale velocity structure of upper oceanic crust from full waveform inversion of downward continued seismic reflection data at the Lucky Strike Volcano, Mid-Atlantic Ridge

We present a fine-scale 2D velocity structure beneath the Lucky Strike Volcano on the Mid-Atlantic Ridge (MAR) using an elastic full waveform inversion (FWI) method. The FWI is a data driven procedure that allows simultaneous exploitation of both reflections and refractions energy in multi-channel seismic data to create a single self-consistent, high-resolution velocity image of the upper crust that can be used for geologic interpretation. The long-wavelength background P-wave velocity model required by the local optimization approach was created using a combination of downward continuation and 3D first-arrival travel-time tomography. The elastic waveform inversion was applied to carefully windowed downward continued data, where wide-angle reflections and refractions arrive in front of the water-wave and are thus isolated from the high-amplitude seafloor scattering energy that is particularly acute in areas of rough igneous seafloor. Waveform inversion reduces the misfit of the initial model by 76% after 19 iterations and strongly reduced the size of the residuals relative to the signal size. The final model shows fine scale structure beneath the northern part of the Lucky Strike volcano on a resolution of tens of meters. Evidence for successive lava sequences testifies to the constructional origin of the upper section of layer 2A. Normal faults are revealed within the shallow crust and are strongly correlated with seafloor observations.