Imaging the upper mantle transition zone with a generalized Radon transform of SS precursors

Abstract

We demonstrate the feasibility of using inverse scattering for high-resolution imaging of discontinuities in the upper mantle beneath oceanic regions (far from sources and receivers) using broadband wavefield observations consisting of SS and its precursors. The generalized Radon transform (GRT) that we developed for this purpose detects (in the broadband data) signals due to scattering from elasticity contrasts in Earth's mantle. Synthetic tests with realistic source–receiver distributions demonstrate that the GRT is able to detect and image deep mantle interfaces, even in the presence of noise, depth phases (‘ghosts’), phase conversions, and multiples generated by reverberation within the transition zone. As a proof of concept, we apply the GRT to ∼50,000 broadband seismograms to delineate interfaces in the depth range from 300 to 1000 km beneath the northwest Pacific. We account for smooth 3D mantle heterogeneity using first-order perturbation theory and independently obtained global tomography models. The preliminary results reveal laterally continuous (but undulating) scatter zones near 410 and 660 km depth and a weaker, broader, and more complex structure near 520 km depth. The images also suggest the presence of multiple, laterally intermittent interfaces near 350 km and between 800 and 1000 km depth, that is, above and below the transition zone sensu stricto. Filtering of the data (we consider four pass-bands: 20–50 s, 10–50 s, 5–50 s, and 2–50 s) reveals a prominent frequency dependence of the magnitude, width, and complexity of the interfaces, in particular of the scatter zone near 520 km depth; such dependencies may put important constraints on the mineralogy and phase chemistry of the transition zone.