Abstract
Abstract. Ambient noise seismology has revolutionized seismic characterization of the Earth's crust from local to global scales. The estimate of Green's function (GF) between two receivers, representing the impulse response of elastic media, can be reconstructed via cross-correlation of the ambient noise seismograms. A homogenized wave field illuminating the propagation medium in all directions is a prerequisite for obtaining an accurate GF. For seismic data recorded on glaciers, this condition imposes strong limitations on GF convergence because of minimal seismic scattering in homogeneous ice and limitations in network coverage. We address this difficulty by investigating three patterns of seismic wave fields: a favorable distribution of icequakes and noise sources recorded on a dense array of 98 sensors on Glacier d'Argentière (France), a dominant noise source constituted by a moulin within a smaller seismic array on the Greenland Ice Sheet, and crevasse-generated scattering at Gornergletscher (Switzerland). In Glacier d'Argentière, surface melt routing through englacial channels produces turbulent water flow, creating sustained ambient seismic sources and thus favorable conditions for GF estimates. Analysis of the cross-correlation functions reveals non-equally distributed noise sources outside and within the recording network. The dense sampling of sensors allows for spatial averaging and accurate GF estimates when stacked on lines of receivers. The averaged GFs contain high-frequency (>30 Hz) direct and refracted P waves in addition to the fundamental mode of dispersive Rayleigh waves above 1 Hz. From seismic velocity measurements, we invert bed properties and depth profiles and map seismic anisotropy, which is likely introduced by crevassing. In Greenland, we employ an advanced preprocessing scheme which includes match-field processing and eigenspectral equalization of the cross spectra to remove the moulin source signature and reduce the effect of inhomogeneous wave fields on the GFs. At Gornergletscher, cross-correlations of icequake coda waves show evidence for homogenized incident directions of the scattered wave field. Optimization of coda correlation windows via a Bayesian inversion based on the GF cross coherency and symmetry further promotes the GF estimate convergence. This study presents new processing schemes on suitable array geometries for passive seismic imaging and monitoring of glaciers and ice sheets.
Highlights
Passive seismic techniques have proven efficient to better understand and monitor glacier processes on a wide range of time and spatial scales
The underlying seismic interferometry techniques used in ambient noise studies are rooted in the fact that the elastic impulse response between two receivers, Green’s function (GF), can be approximated via cross-correlation of a diffuse wave field recorded at the two sites (Lobkis and Weaver, 2001; Campillo et al, 2014)
In order to separate the contribution of these noise sources, we first perform singular value decomposition (SVD) of the cross-spectral density matrix (CSDM), and we use a selection of eigenvectors and eigenspectral equalization (Seydoux et al, 2017) to improve the convergence of noise cross-correlation (NCC) towards an estimate of the GF
Summary
Passive seismic techniques have proven efficient to better understand and monitor glacier processes on a wide range of time and spatial scales. The underlying seismic interferometry techniques used in ambient noise studies are rooted in the fact that the elastic impulse response between two receivers, Green’s function (GF), can be approximated via cross-correlation of a diffuse wave field recorded at the two sites (Lobkis and Weaver, 2001; Campillo et al, 2014). As an alternative to continuous ambient noise, Walter et al (2015) used crevassing icequakes recorded during a 1-month seismic deployment at Gornergletscher (Switzerland) They recorded thousands of point source events which offered an idealized spatial source distribution around one pair of seismic sensors and could obtain accurate GF estimates. In light of our analysis, we discuss suitable array geometries and measurement types for future applications of passive seismic imaging and monitoring studies on glaciers
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