Seismic noise and mantle discontinuity reflections – insight gained and future directions

The fine structure of the upper and lower limits of the mantle’s transition zone (so-called 410 and 660 discontinuities) enlightens us on the temperature and composition of the mantle with, consequently, major impact on our modelling of mantle dynamics. We demonstrated in 2012 that it is possible to extract subvertically reflected waves (Pv410P and Pv660P) on the discontinuities from the seismic noise. We review how our knowledge has progressed since then, for better imaging of the 410 and 660 discontinuities. Rather than recovering an approximate Green’s function, based on high levels of diffraction and/or an even source distribution, Pv410P and Pv660P result from cross terms of a limited geographical coverage of P-wave sources from distant storms. It is possible to identify, in the data or with models of seismic noise, the time windows which effectively contribute to Pv410P and Pv660P, but the methods carry inherent risk of source location dependent time and amplitude bias which can be quantified through numerical modeling. Together, these studies indicate practical ways forward for being able to extract mantle discontinuities reflections with higher signal to noise ratios than previously, and for adequately interpreting them through a deeper understanding of potential bias and uncertainties.