Window length selection for optimum slowness resolution of the local-slant-stack transform

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In order to study fine scale structure of the Earth's deep interior, it is necessary to extract generally weak body wave phases from seismograms that interact with various discontinuities and heterogeneities. The recent deployment of large-scale dense arrays providing high-quality data, in combination with efficient seismic data processing techniques, may provide important and accurate observations over large portions of the globe poorly sampled until now. Major challenges are low signal-to-noise ratios (SNR) and interference with unwanted neighbouring phases. We address these problems by introducing scale-dependent slowness filters that preserve time-space resolution. We combine complex wavelet and slant-stack transforms to obtain the slant-stacklet transform. This is a redundant high-resolution directional wavelet transform with a direction (here slowness) resolution that can be adapted to the signal requirements. To illustrate this approach, we use this expansion to design coherence-driven filters that allow us to obtain clean PcP observations (a weak phase often hidden in the coda of the P wave), for events with magnitude M w > 5.4 and distances up to 80 •. In this context, we then minimize a linear misfit between P and PcP waveforms to improve the quality of PcP-P traveltime measurements as compared to a standard cross-correlation method. This significantly increases both the quantity and the quality of PcP-P differential traveltime measurements available for the modelling of structure near the core-mantle boundary. The accuracy of our measurements is limited mainly by the highest frequencies of the signals used and the level of noise. We apply this methodology to two examples of high-quality data from dense arrays located in north America. While focusing here on body-wave separation, the tools we propose are more general and may contribute to enhancing seismic signal observations in global seismology in situations of low SNR and high signal interference.

Section: Discussionmentioning

confidence: 99%

Section: A P P E N D I X a : R E S O L U T I O N A N A Ly S I Smentioning

confidence: 97%

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Extraction of weak PcP phases using the slant-stacklet transform — I: method and examples

Ventosa

Romanowicz

2015

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“…2 shows an example from an exploration seismology setting, and the theory is briefly described below. Following Ventosa et al (2012), the LSST of a seismic record section u (x, t) is defined as:…”

Section: The Lssfmentioning

confidence: 99%

High resolution upper mantle discontinuity images across the Pacific Ocean from SS precursors using local slant stack filters

Zheng¹,

Ventosa

Romanowicz

2015

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SS precursor observations are a powerful tool to study the topography and character of transition zone discontinuities, especially in regions such as ocean basins where few seismic stations exist, precluding other high resolution approaches. Still, the available coverage is limited by the distribution of sources and stations, but also by the level of noise and by the fact that, in some distance ranges, interfering seismic phases mask the weak signal from the SS precursors. We introduce an array data processing tool, the local slant-stack filter, to address these challenges and clean up the otherwise noisy SS precursor record sections. We show that these filters are a powerful tool for extracting the weak yet coherent SS precursor signals while removing interfering seismic phases as well as random noise, yielding robust precursor traveltime measurements with spatial resolution higher than what can be achieved by the conventional common midpoint stacking method. The effectiveness of the filters are demonstrated by application to synthetic and real data. We systematically apply this filtering method to an SS precursor data set recorded by the U.S. Transportable Array that samples a vast region of the Pacific Ocean and its northwest margin, and present maps of 410 and 660 discontinuity topography. We discuss correlations observed between our discontinuity images and several fine-scale heterogeneities revealed by mantle shear wave tomography in the vicinity of Hawaii and the Pacific Superswell.

“…The most commonly adopted approach is stacking (delay-andsum) due to its simplicity and effectiveness in suppressing incoherent (though not necessarily low-amplitude) arrivals. Recently, more sophisticated resolution enhancement approaches such as Radon transform (Gu & Sacchi 2009;Schultz & Gu 2013), local-slantstack transform (Ventosa et al 2012) and slant-stacklet transform have been proposed. The localslant-stack transform was successfully applied to long-period SS precursors (Zheng et al 2015), which effectively differentiates between seismic arrivals based on moveout or epicentral distance.…”

mentioning

confidence: 99%

Improvement of coda phase detectability and reconstruction of global seismic data using frequency–wavenumber methods

Schneider¹,

Thomas²,

Dokht

et al. 2017

Due to uneven earthquake source and receiver distributions, our abilities to isolate weak signals from interfering phases and reconstruct missing data are fundamental to improving the resolution of seismic imaging techniques. In this study, we introduce a modified frequencywavenumber (fk) domain based approach using a 'Projection Onto Convex Sets' (POCS) algorithm. POCS takes advantage of the sparsity of the dominating energies of phase arrivals in the fk domain, which enables an effective detection and reconstruction of the weak seismic signals. Moreover, our algorithm utilizes the 2-D Fourier transform to perform noise removal, interpolation and weak-phase extraction. To improve the directional resolution of the reconstructed data, we introduce a band-stop 2-D Fourier filter to remove the energy of unwanted, interfering phases in the fk domain, which significantly increases the robustness of the signal of interest. The effectiveness and benefits of this method are clearly demonstrated using both simulated and actual broadband recordings of PP precursors from an array located in Tanzania. When used properly, this method could significantly enhance the resolution of weak crust and mantle seismic phases.