Imaging mantle plumes with instantaneous phase measurements of diffracted waves

Abstract

In a synthetic tomographic experiment, we succeeded to recover an idealized narrow mantle plume reaching deep into the lower mantle by using a misfit based on the instantaneous phase difference. A misfit based on simple cross-correlation traveltime shifts leaves the lower mantle part of the plume largely unresolved, despite the use of finite-frequency sensitivity kernels. The time-continuous and amplitude-independent instantaneous phase misfit allows us to measure the interaction between direct and diffracted waves as a function of time, which is difficult to capture by simple cross-correlation traveltime measurements. The diffracted waves arriving later than the main phase are essential to improve the tomographic result. The measurement of diffracted waves yields the necessary information to recover the plume correctly even in the lower mantle. The instantaneous phase measurement is ideal to capture this interaction, but other time- or frequency-dependent measurements may give similar results. We also investigated the effect of wavefront healing on cross-correlation traveltime shifts for a range of differently sized idealized mantle plumes.We confirm that wavefront healing severely reduces traveltime shifts when the plume conduit is considerably thinner than the width of the first Fresnel zone. For plume conduits with a diameter on the order of 100 km, even traveltime shifts measured at periods as short as T = 1 s are affected.