Multi-Array Probing of Lower Mantle Structure

Abstract:

Array processing of seismic waveforms from many sensors allows the enhancement of coherent signals and the suppression of incoherent "noise". Time correction of a waveform enhances weak seismic phases and provides constraints on the azimuth and inclination of the incoming energy. Furthermore, signal amplification allows the use of higher frequencies, which effectively increases the imaging resolution. Although array stacking is effective in amplifying weak seismic signals, its inherent weakness lies in the assumption of the instantaneous plane wave arriving at the array. This assumption limits the size of the array (short aperture) to insure the signal coherence, which in turn limits the size of the area within the Earth’s interior that we can probe. Small array size also means that we cannot use energy scattered off a great circle path.

 In this study we address the above-mentioned issues by installing new and combing the existing several short-aperture arrays in Australia. By exploiting these multiple short aperture arrays we can pinpoint the source of scattered energy to map detailed patterns of heterogeneity in the lower mantle. The concept of multiple arrays allows us to illuminate specific point in the Earth from many different directions. The method is based on a similar approach to that the back-projection technique uses, where the point in time (i.e. part of the seismic wavetrain) is mapped onto the specific point in space through ray tracing. This allows us to achieve multiple illumination of the same structure and thereby minimize source effects. Using scattered energy enables us to cover and probe a larger area of the Earth’s interior.

We test the feasibility of our approach deploying the precursors to PcP and PKP seismic phases. We also test our method on synthetically created traces (both monochromatic and multi-frequency waves) varying the depths of the scatterers. We detect significant improvements, both in the signal quality and resolution, with an additional benefit of being able to accurately locate the source of the scattered energy.