engleski

Multi-array probing of the lower mantle

Two crucial elements in any seismological investigation are the resolution and signal quality. First element controls the size of seismic structure that can be directly imaged and is proportional to the number of source-recorder paths and their geometry. The importance of the second element, signal quality, is most evident when mapping seismic phases emanating from very deep and weak heterogeneities. Usually, these phases are obscured by noise or simply too weak to be detected on a single seismogram. One of the most effective ways in facilitating these difficulties is to use an array of seismic instruments. The combination of results from many sensors allows the enhancement of coherent signals and the suppression of incoherent ”noise”. Furthermore, by applying suitable time delays to enhance weak seismic phases we are providing constraints on the azimuth and inclination of the incoming energy. Signal amplification allows us to use higher frequencies, which effectively increases the vertical resolution. Although array stacking is very 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 (small aperture) to insure the signal coherence thus limiting the size of the area we can probe. Small array size also means that we cannot use energy scattered off great circle path. In this study we addressed above mentioned issues by combing recordings from several short-aperture arrays installed across Australia. By exploiting multiple small aperture arrays of seismic recorders we can pinpoint the source of scattered energy to map out the detailed patterns of heterogeneity in the Earth. Multiple array concept allows us to illuminate specific point in the Earth from many different directions. Method is based on a similar concept as the back-projection technique where the point in time (i.e. part of the seismic wavetrain) is mapped to the specific point in space by raytracing. This allows us to achieve multiple illumination of the same structure using the same event and thereby minimize source effects. Using scattered energy enables us to cover and probe larger area of the Earths interior. We test our method on synthetically created traces (both monochromatic and multi-frequency waves) assuming various source depths and on several recordings from real events. Results show significant improvement both in signal quality and resolution with the additional benefit of being able to accurately locate the source of the scattered energy and map large areas with only limited number of stations.

Seismic array, mantle, deep Earth

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