Reverse-time migration-based reflection tomography using teleseismic free surface multiples

Abstract

Converted and multiply reflected phases from teleseismic events are routinely used to create structural images of the crust-mantle boundary (Moho) and the elasticity contrasts within the crust and upper mantle. The accuracy of these images is to a large extent determined by the background velocity model used to propagate these phases to depth. In order to improve estimates of 3-D velocity variations and, hence, improve imaging, we develop a method of reverse-time migration-based reflection tomography for use with wavefields from teleseismic earthquakes recorded at broad-band seismograph arrays. Reflection tomography makes use of data redundancy -that is, the ability to generate numerous structural images of the subsurface with different parts of the wavefield. In exploration seismology (where it is known as migration velocity analysis) reflection tomography typically involves the generation of an extended image (e.g. offset-or angle-gathers), and the fitness of the background model is evaluated through the application of image-domain annihilators. In regional-scale passive source seismology, however, annihilation-based methods are inadequate because the sparse and irregular distribution of teleseismic sources is not likely to produce illumination over a sufficient range of angles. To overcome this problem we turn towards a source-indexed moveout scheme. Instead of extended image annihilation, we determine the success of the tomographic velocity model by cross correlating images produced with multiply-scattered waves from different teleseismic sources. The optimal velocity model is the one that minimizes correlation power between windowed images away from zero depth shift. We base our inversion scheme on the seismic adjoint method and a conjugate gradient solver. For each image pair, the update direction is determined by correlations between downgoing wavefields with upgoing adjoint wavefields for both images. The sensitivity kernels used in this method is similar to those found in other forms of adjoint tomography, but their shapes are controlled by the spatial distribution of the error function. We present the method and a proof-of-concept with 2-D synthetic data.

Imaging Upper‐Mantle Structure Under USArray Using Long‐Period Reflection Seismology

“…Recently, Tauzin et al (, ) and Cheng et al () used receiver‐function migration to image the subducting Gorda plate to transition zone depths, and hints of Farallon slab fragments are also seen in the plane‐wave migration model PWMIG11 (Pavlis et al, ). In addition, a migration‐based approach to analysis of teleseismic free‐surface multiples was recently described by Burdick et al (). However, to our knowledge, we are the first to analyze topside SH reverberation data by explicitly removing the source‐side terms and to apply this approach to image mantle structure under a regional seismic array.…”

“…Our common‐conversion‐point (CRP) approach for teleseismic SH reverberation analysis is simple to implement and appears to give robust results for the USArray data set. However, our method likely could be improved in a number of ways: Bootstrap resampling of the waveforms would provide a measure of the statistical significance of any observed reflections and could be used to replace our requirement of at least 200 seismograms to display the result. In principle, migration approaches, such as those described by Pavlis (), Shang et al (), Shragge et al (), and Burdick et al () for receiver functions and/or free‐surface multiples, could be applied to better image dipping structures. However, migration methods work best with uniform data coverage, so the very uneven distribution of earthquake sources may present challenges. More comprehensive testing of corrections for 3‐D velocity structure would help to better understand their sensitivity to specific tomography models and how they can change the coherence of the image.…”

A 3‐D spectral‐element and frequency‐wave number hybrid method for high‐resolution seismic array imaging

“…Teleseismic array imaging based on converted and scattered waves is one of the essential tools for investigating the crustal and upper mantle structures, and has contributed significantly over the past three decades to our understanding of tectonic evolution and internal geodynamic processes [e.g., Rondenay, 2009;Kind et al, 2012;Liu and Gu, 2012]. Various methods including receiver function (RF) analysis through single station stacking [e.g., Langston, 1977;Yan and Clayton, 2007], common conversion point (CCP) stacking [e.g., Revenaugh, 1995;Sheehan et al, 2000;Chen et al, 2005], inverse scattering approaches based on asymptotic methods such as generalized Radon transform [e.g., Bostock et al, 2001;Cao et al, 2010;Shang et al, 2014], teleseismic migration [e.g., Shragge et al, 2006;Shang et al, 2012], and teleseismic scattering tomography [e.g., Frederiksen and Revenaugh, 2004;Pageot et al, 2013;Burdick et al, 2014;Tong et al, 2014] have been developed for specific imaging purposes. RF analysis is a routine tool to characterize major discontinuities of the Earth's subsurface such as the Moho, 410 km and 660 km discontinuities [e.g., Rondenay, 2009;Kind et al, 2012].…”

Multimode 3‐D Kirchhoff Migration of Receiver Functions at Continental Scale

“…In CCP stacking, the waveforms are first stacked according to their incoming geometry and then projected in the depth domain, which effectively collapses the 3‐D data on a horizontally layered 1‐D profile. Including lateral variation effects back in the waveform stacking can be partially addressed by performing wave number filtering or designing 2‐D migration schemes (Bostock et al, ; Burdick et al, ; Chen et al, ; Tauzin et al, ). Three‐dimensional elastic parameter variations can be treated to first order by computing the arrival times or full waveforms in a 3‐D reference model.…”