2 research outputs found
Wave separation in ambient seismic noise using intrinsic coherence and polarization filtering
International audienceThis paper deals with passive ambient seismic noise processing. A new wave separation method is proposed when several polarized waves are recorded by a pair of multicomponent sensors. Although it is well known that the information given by the auto-covariance matrix at one sensor is not sufficient for estimating the mixing matrix, it is shown in this paper that with two multicomponent sensors, the identification of the mixing matrix can be completed. In addition to the mathematical proof and synthetic example, an application to a real geophysical data set is also presented. A previous analysis of these data revealed the presence of surface waves. The application of the proposed algorithm leads to the separation of Rayleigh and Love waves
Imaging New Zealand's Crustal Structure Using Ambient Seismic Noise Recordings from Permanent and Temporary Instruments
We use ambient seismic noise to image the crust and uppermost mantle, and to determine the spatiotemporal characteristics of the noise field itself, and examine the way in which those characteristics may influence imaging results. Surface wave information extracted
from ambient seismic noise using cross-correlation methods significantly enhances our knowledge of the crustal and uppermost mantle shear-velocity structure of New Zealand. We assemble a large dataset of three-component broadband continuous seismic data from
temporary and permanent seismic stations, increasing the achievable resolution of surface wave velocity maps in comparison to a previous study.
Three-component data enables us to examine both Rayleigh and Love waves using noise cross-correlation functions. Employing a Monte Carlo inversion method, we invert Rayleigh and Love wave phase and group velocity dispersion curves separately for spatially
averaged isotropic shear velocity models beneath the Northland Peninsula. The results yield first-order radial anisotropy estimates of 2% in the upper crust and up to 15% in the lower crust, and estimates of Moho depth and uppermost mantle velocity compatible with previous studies.
We also construct a high-resolution, pseudo-3D image of the shear-velocity distribution in the crust and uppermost mantle beneath the central North Island using Rayleigh and Love waves. We document, for the first time, the lateral extent of low shear-velocity zones in
the upper and mid-crust beneath the highly active Taupo Volcanic Zone, which have been reported previously based on spatially confined 1D shear-velocity profiles. Attributing these low shear-velocities to the presence of partial melt, we use an empirical relation to
estimate an average percentage of partial melt of < 4:2% in the upper and middle crust.
Analysis of the ambient seismic noise field in the North Island using plane wave beamforming
and slant stacking indicates that higher mode Rayleigh waves can be detected, in
addition to the fundamental mode. The azimuthal distributions of seismic noise sources
inferred from beamforming are compatible with high near-coastal ocean wave heights in
the period band of the secondary microseism (~7 s). Averaged over 130 days, the distribution
of seismic noise sources is azimuthally homogeneous, indicating that the seismic
noise field is well-suited to noise cross-correlation studies. This is underpinned by the
good agreement of our results with those from previous studies. The effective homogeneity
of the seismic noise field and the large dataset of noise cross-correlation functions we
here compiled, provide the cornerstone for future studies of ambient seismic noise and
crustal shear velocity structure in New Zealand