Layer 2A structure beneath the Lucky Strike volcano, Mid-Atlantic Ridge, from high-resolution tomography and full waveform inversion.

The surficial layer of the oceanic crust is formed by a heterogeneous process of magmatic eruptions and tectonic deformation. Historically, it has been difficult to obtain seismic images of this region with resolutions comparable to the length scales of the geologic processes. Traditional travel time tomography methods over-smooth the vertical and horizontal velocity structure of the upper crust while multichannel seismic (MCS) processing has concentrated on the wide-angle re?ection imaging of the base of layer 2A with little emphasis on velocity. As a consequence, questions remain about the relationship between seismic descriptions of the upper crust in terms of Layer 2A & B and the geologic observations. In this thesis, the SISMOMAR 3-D multichannel seismic (MCS) dataset, acquired at the center of the Lucky Strike segment of the Mid-Atlantic ridge, is analyzed in a transformative way, using a new method named the Synthetic Ocean Bottom Experiment (SOBE), which can extrapolate seismic sources and receivers down to the seafloor, simulating an on-bottom seismic experiment. This downward extrapolation stage has several advantages: it collapses the sea?oor diffractions and ?lters the background noise, improving the imaging condition, and it unwraps the Layer 2A/2B triplication, which places the refracted energy in front of the water-wave, providing information about upper crustal velocities. The first half of this thesis presents a first application of downward continuation of the seismic wavefield to near seafloor and 3-D high-resolution traveltime tomography to resolve upper crustal structure. New features within the upper crust (Layer 2A) are revealed and interpreted in terms of spatial variations in magmatic, hydrothermal and tectonic processes across the site. The second half of this thesis presents an application of 2-D full waveform inversion (FWI) of part of the previously analyzed dataset to push the spatial resolution down to a few tens of meters. The obtained velocity models reveal even finer-scale structure of the Lucky Strike volcano than had been obtained with the high-resolution tomography method. Several sub-horizontal velocity layers are resolved within the shallow crust that are attributed to different piled up lava sequences, consistent with surface geology that shows diverse lava deposits. The detection of distinct velocity gradient discontinuities within the top kilometer of crust further allows tracing of faults at depth that are well correlated with the surface-expressed faulting. Finally, different characteristics of the FWI models are used to address the geological significance of Layer 2A/2B boundary in this region, which is a long-standing controversy in studies of oceanic crust.