Radial anisotropy in Valhall: ambient noise-based studies of Scholte and Love waves

Abstract

We perform the ambient noise Scholte and Love waves phase-velocity tomography to image the shallow subsurface (a few hundreds of metres) at the Valhall oil field. Seismic noise was recorded by multicomponent (north, east and vertical) ocean bottom cable from the Valhall life of field seismic network. We cross-correlate six and a half hours of continuous recording of noise between all possible pairs of receivers. The vertical-vertical and the transversetransverse components cross-correlations are used to extract the Scholte and Love waves, respectively. We combine more than 10 millions of interstation correlations to compute the average phase-velocity dispersion curves for fundamental mode and first overtone. Then, a Monte Carlo inversion method is used to compute average 1-D profiles of V SV and V SH down to 600 m depth. In the next step, we construct 2-D Scholte and Love waves phase-velocity maps for fundamental mode using the eikonal tomography method. These maps are then jointly inverted to get the 3-D distribution of V SV and V SH from which the radial anisotropy and the isotropic velocity (V S) are estimated. The final model includes two layers of anisotropy: one in the shallow part (above 220 m) with a significant negative radial anisotropy (V SH < V SV) due to vertical cracks because of subsidence and another in the deeper part (between 220 and 600 m) with a positive radial anisotropy (V SH > V SV) due to the stratification at that depth.

Shallow three-dimensional structure of the San Jacinto fault zone revealed from ambient noise imaging with a dense seismic array

“…Resolving radial anisotropy (e.g. Tomar et al 2016) expressing the difference in propagation speed between horizontally and vertically polarized waves requires three-component data. An analysis of azimuthal anisotropy through an eikonal tomography approach (Mordret et al 2013b;Zigone et al 2015) will be the subject of future work.…”

Surface Wave Tomography of Northeastern Tibetan Plateau Using Beamforming of Seismic Noise at a Dense Array

“…An approach completely different from the tomographic one is the local tracking of the phase variation of a wavefront, also called eikonal tomography. Originally applied to earthquake data (Alsina et al, ; Friederich, ; Weidle, ), this approach has also been used more recently with the Green's functions obtained from noise cross‐correlation (Lin et al, ; Ritzwoller et al, ; Tomar et al, ). This methodology has been boosted in recent years by the deployment of large and dense networks and can in theory provide models with high lateral resolution, up to the interstation distance.…”

Rayleigh‐Love Discrepancy Highlights Temporal Changes in Near‐Surface Radial Anisotropy After the 2004 Great Sumatra Earthquake

“…It is also understood that strong anisotropy, either in the form of azimuthal anisotropy (L. W. Chen et al., 2017; T. Y. Huang et al., 2015; Mordret et al., 2013) or radial anisotropy (Jeng et al., 2020; Naghavi et al., 2019; Shirzad et al., 2017; Tomar et al., 2017) have been detected in the near surface. Typically, azimuthal anisotropy describes the azimuthal dependence of S ‐wave velocity (denoted as V S or β ) and particle motion within the horizontal plane, assuming a horizontal symmetry axis (i.e., horizontal transverse isotropy, abbreviated as “HTI”) (Crampin, 1975).…”

“…Early observations of the “Rayleigh‐Love discrepancy” typically referred to the fact that the phase (or group) velocity of Rayleigh and Love waves cannot be reconciled by a simple isotropic velocity model (Anderson, 1961; Babuska & Cara, 1991; Maupin & Cara, 1992), indicative of radial anisotropy. While crustal radial anisotropy was extensively investigated (Hu et al., 2020; H. Huang et al., 2010; Jaxybulatov et al., 2014; Moschetti et al., 2010; Shapiro et al., 2004), only a handful of studies investigated near‐surface radial anisotropy (Jeng et al., 2020; Shirzad et al., 2017; Tomar et al., 2017). To our knowledge, no reports documented key observations indicative of changes in radial anisotropy in the near surface.…”

“…While crustal radial anisotropy was extensively investigated (Hu et al, 2020;H. Huang et al, 2010;Jaxybulatov et al, 2014;Moschetti et al, 2010;Shapiro et al, 2004), only a handful of studies investigated near-surface radial anisotropy (Jeng et al, 2020;Shirzad et al, 2017;Tomar et al, 2017). To our knowledge, no reports documented key observations indicative of changes in radial anisotropy in the near surface.…”