Imaging the lowermost mantle (D″) and the core-mantle boundary withSKKScoda waves

Abstract

SUMMARY In our previous studies we developed a method for imaging heterogeneity at and near the core– mantle boundary (CMB) with a generalized Radon transform (GRT) of (transverse component, broad-band) ScS data, and we developed a statistical model for producing images of the D �� discontinuity with variable confidence levels. In these applications, the background is smooth and perturbations are represented as contrasts. Here we extend the theory to allow (known) discontinuities, such as the CMB, in the background model. The resulting imaging operator, which is formally not a GRT, can be used, either alone or along with ScS, for the imaging of lowermost mantle structure and, in particular, the D �� discontinuity with the scattered SKKS wavefield. Synthetic seismograms calculated with the WKBJ method are used to test the performance of our approach. As a proof of concept, we transform ∼38 000 radial component SKKS waveforms into image gathers of a CMB patch beneath Central America. The SKKS image gathers and image traces are in good agreement with the image traces obtained from the GRT transform of ScS data.