Effects of scattering parameters on seismic observables of PKP precursors and apparent D″ reflections – A parameter study

Abstract

The lowermost mantle has been studied with a variety of seismic waves which show that different structures on many length-scales exist there but it is possible that several seismic phases could be generated by one structure. Specifically, we use a randomly distributed scattering layer that is able to generate precursors to the core phase PKPdf as well as waves that resemble reflections off the D″ layer (PdP wave) and here we present a parameter study to understand how scattering parameters in this layer affect the two tested seismic waves and whether it is possible to discriminate between a reflector or a scattering layer as cause for apparent D″-reflected waves. We compute synthetic data for several source-receiver combinations and vary the parameters of the heterogeneity model above the core-mantle boundary: correlation length, velocity (and density) perturbation, taper to soften the transition, thickness, location of the scattering layer and the randomness of the models. We measure the effects on the amplitudes and arrival times of the seismic waves, and in addition for the apparent D″ reflection, we also record the number of reflected waves, their polarity and slowness. We find that the precursor and reflection amplitudes are influenced by all scattering parameters and that trade-offs exist. One conclusion therefore is that it is difficult to match local scattering parameters to fit observations with a unique model since several combinations of scattering parameters result in similar amplitudes and travel times. Interestingly, amplitudes for different correlation lengths are affected differently for larger and smaller epicentral distances, therefore testing the amplitudes of precursors at different distance ranges could potentially help to distinguish between correlation length and perturbation effects. We were not able to further narrow down the scattering parameters of the layer when using a combination of PKP precursors and PdP-like reflections due to the non-uniqueness. Analyzing the effects on PdP arrivals that arrive between P and PcP, we find influences on all seismic observables albeit without clear trends. But based on our modeling it seems prudent to assume that the waves are caused by a scattering layer if 1) the observations show strong scatter in arrival time, slowness and amplitude and 2) if more than one arrival is present between P and PcP.