P‐wave attenuation anisotropy in fractured media: A seismic physical modelling study

Ekanem, A. M.; Wei, J.X.; Wang, S.; Di, Bangrang; Li, X. ‐Y.

doi:10.1190/1.3603648

Cited by 4 publications

(5 citation statements)

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“…Moreover, in a volcanic environment, diffraction and scattering of the seismic wavefront are expected to be large even at shallow depth (Godfrey, Fry and Savage 2017). Lateral heterogeneity and anelastic attenuation cause volcanic sediments to be generally poor transmitters of seismic waves especially in the highfrequency range (Cas and Wright 1987;Ekanem et al 2013;Di Fiore et al 2015).…”

Section: Figurementioning

confidence: 99%

Using a vibratory source at Mt. Etna (Italy) to investigate the wavefield polarization at Pernicana Fault

Giulio

Punzo

Bruno

et al. 2019

Near Surface Geophysics

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The paper presents the results of a controlled‐source seismic experiment performed using a vibratory source capable of producing harmonic vibrations. The survey aimed at investigating the horizontal directional amplification mechanism observed at the Pernicana Fault (Mt. Etna in Sicily, Italy) along an N150°/N330° orientation. With the vibratory source (a shaker truck) working in shear‐wave mode in the frequency range 5–40 Hz, we recorded seismic data using both three‐component velocimeters and 4.5 Hz geophones. These two types of receivers were arranged in linear configuration along two orthogonal orientations: radial and transverse to the direction of the main polarization observed within the fault zone. The two lines of seismic stations allowed us to investigate the polarization evolution as a function of the distance from the source. Results show that when the shear excitation induced by the source is parallel to the direction of observed polarization of the ground motion, the seismic signal propagates efficiently, maintaining the same horizontal polarization induced by the active source. This polarization is well recorded up to distances as large as 300 m from the source. On the contrary, when the shear excitation is orthogonal to the predominant site polarization, the ground excitation loses its initial polarization in less than 50 m away from the source position. At larger distances, the transmitted energy propagates with the natural site polarization independently from the original source polarization. The observations in terms of polarization were combined with surface‐wave analysis, aimed at investigating the dispersion characteristics of the wavefield on the geophone lines. The data suggest that the experimental Rayleigh‐wave dispersion spectra show a more coherent signal over the frequency range along the same orientation of the observed site polarization, supporting a preferential direction in the propagation of surface waves. From inversion of the Rayleigh dispersion curves, we inferred the shear‐wave velocity (Vs) to be in the range of 120–450 m/s in the first 30 m deep of Quaternary volcanic sediments. The dispersion curves are not constant along transverse and radial directions, suggesting the presence of fracture‐induced seismic anisotropy in the near surface.

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Section: Figurementioning

confidence: 99%

Using a vibratory source at Mt. Etna (Italy) to investigate the wavefield polarization at Pernicana Fault

Giulio

Punzo

Bruno

et al. 2019

Near Surface Geophysics

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“…Subsurface formations that exhibit velocity anisotropy are often characterized by directionally dependent attenuation coefficients Best, Sothcott and McCann 2007). In particular, numerical and laboratory experiments have confirmed the link between attenuation anisotropy and parameters of aligned fractures (Rao and Wang 2009;Ekanem et al 2013;Guo and McMechan 2017). To characterize the anisotropic attenuation coefficients of P-and SV-waves in thinly layered porous rocks, Krzikalla and Müller (2011) combine anisotropic Backus limits (under quasi-static and no-flow assumptions) with interlayer flow models.…”

Section: Introductionmentioning

confidence: 99%

Source‐independent waveform inversion for attenuation estimation in anisotropic media

Bai

Tsvankin

2019

Geophysical Prospecting

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In previous publications, we presented a waveform‐inversion algorithm for attenuation analysis in heterogeneous anisotropic media. However, waveform inversion requires an accurate estimate of the source wavelet, which is often difficult to obtain from field data. To address this problem, here we adopt a source‐independent waveform‐inversion algorithm that obviates the need for joint estimation of the source signal and attenuation coefficients. The key operations in that algorithm are the convolutions (1) of the observed wavefield with a reference trace from the modelled data and (2) of the modelled wavefield with a reference trace from the observed data. The influence of the source signature on attenuation estimation is mitigated by defining the objective function as the ℓ2‐norm of the difference between the two convolved data sets. The inversion gradients for the medium parameters are similar to those for conventional waveform‐inversion techniques, with the exception of the adjoint sources computed by convolution and cross‐correlation operations. To make the source‐independent inversion methodology more stable in the presence of velocity errors, we combine it with the local‐similarity technique. The proposed algorithm is validated using transmission tests for a homogeneous transversely isotropic model with a vertical symmetry axis that contains a Gaussian anomaly in the shear‐wave vertical attenuation coefficient. Then the method is applied to the inversion of reflection data for a modified transversely isotropic model from Hess. It should be noted that due to the increased nonlinearity of the inverse problem, the source‐independent algorithm requires a more accurate initial model to obtain inversion results comparable to those produced by conventional waveform inversion with the actual wavelet.

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“…Q is particularly useful in the characterization of rock and fluid properties and is sensitive to a number of factors, such as disruption in the medium due to faults, fractures, and weathering, which all act to reduce Q (Rodríguez‐Pradilla, ; Rubino et al, ), porosity and pore fluid parameters, temperature, density, pressure, lithology, and others (Gei & Carcione, ; Prasad & Manghnani, ; Quan & Harris, ; Sain et al, ; Yıldırım et al, ). When used in addition to seismic velocity analysis, Q can provide complimentary information on rock properties (Ekanem et al, ; E. Liu et al, ; Maultzsch et al, ; Quan & Harris, ), and Prasad and Manghnani () found that Q was more sensitive than compressional wave seismic velocity to effects of pore pressure and pore space deformation.…”

Section: Introductionmentioning

confidence: 99%

Seismic Attenuation From Ambient Noise Across the North Sea Ekofisk Permanent Array

et al. 2018

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Quality factor (Q) or equivalently attenuation α=1Q describes the amount of energy lost per cycle as a wave travels through a medium. This is important to correct seismic data amplitudes for near‐surface effects, to locate subsurface voids or porosity, to aid seismic interpretation, or for characterizing other rock and fluid properties. Seismic attenuation can be variable even when there are no discernible changes in seismic velocity or density (Yıldırım et al., 2017, https://doi.org/10.1016/j.jappgeo.2016.11.010) and so provides independent information about subsurface heterogeneity. This study uses ambient noise recordings made on the Ekofisk Life of Field Seismic array to estimate Q structure in the near surface. We employ the method of X. Liu et al. (2015, https://doi.org/10.1093/gji/ggv357), which uses linear triplets of receivers to estimate Q—ours is the first known application of the method to estimate the Q structure tomographically. Estimating Q requires an estimate of phase velocity which we obtain using the method of Bloch and Hales (1968, https://pubs.geoscienceworld.org/ssa/bssa/article-abstract/58/3/1021/116607/) followed by traveltime tomography. The Q structure at Ekofisk has features which can be related to local geology, showing that surface ambient noise recordings may provide a new and robust method to image Q. Our results suggest that there is a nonlinear relationship between Q and compression. They also may explain why it has been found that in the period range of 1 to 2 s considered here, ambient noise cross correlations along paths that span the North Sea Basin are unreliable: Such Q values would attenuate almost all ambient seismic energy during such a traverse.

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P‐wave attenuation anisotropy in fractured media: A seismic physical modelling study

Cited by 4 publications

References 0 publications

Using a vibratory source at Mt. Etna (Italy) to investigate the wavefield polarization at Pernicana Fault

Using a vibratory source at Mt. Etna (Italy) to investigate the wavefield polarization at Pernicana Fault

Source‐independent waveform inversion for attenuation estimation in anisotropic media

Seismic Attenuation From Ambient Noise Across the North Sea Ekofisk Permanent Array

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