Flexural borehole modes and measurement of shear‐wave azimuthal anisotropy

Aligned fractures/cracks in rocks are a primary source of elastic anisotropy. In an azimuthally anisotropic formation surrounding a borehole, shear waves split into fast and slow waves that propagate along the borehole and are recorded by a borehole logging tool. However, when the formation has conjugate fractures with orthogonal strike directions, the azimuthal anisotropy vanishes. Hence, azimuthal anisotropy measurements may not be adequate to detect orthogonal fracture sets. We develop a method for obtaining azimuthal and radial shear-wave anisotropy parameters simultaneously from four-component array waveforms. The method utilizes a velocity tomogram around the borehole. Azimuthal and radial anisotropy were determined by integrating shear velocity radiation profile along the radial direction at different azimuthal angles. Results indicate that this approach is reliable for estimating anisotropy properties in aligned crack systems. The advantage of this interpretation method is shown in multiple conjugate crack systems. Field data processing examples are used to verify the application of the proposed technique. Comparison of results against those obtained with a conventional procedure shows that the new method can not only provide estimates of azimuthal anisotropy, but also of the radial anisotropy parameter, which is important in fracture network evaluation.

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Rotation Analysis of Shear Wave Polarizations in the Presence of Depth-variant Azimuthal Anisotropy

Proceedings 76th EAGE Conference and Exhibition 2014 ◽

10.3997/2214-4609.20141614 ◽

2014 ◽

Author(s):

J. Maleski ◽

R.H. Tatham

Keyword(s):

Shear Wave ◽

Azimuthal Anisotropy

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Direct inversion of 3-D shear wave speed azimuthal and radial anisotropy from surface-wave traveltime data: methodology and applications

10.5194/egusphere-egu2020-22100 ◽

2020 ◽

Author(s):

Yao Huajian ◽

Liu Chuanming ◽

Hu Shaoqian

Keyword(s):

Surface Wave ◽

Shear Wave ◽

Wave Speed ◽

Joint Inversion ◽

Azimuthal Anisotropy ◽

Crust And Upper Mantle ◽

Shear Wave Speed ◽

Dispersion Data ◽

Radial Anisotropy ◽

Deformation Patterns

Seismic anisotropy plays a key role in understanding deformation patterns of Earth&#8217;s material. &#160;Surface wave dispersion data have been widely used to invert for azimuthal and radial anisotropy of shear wave speeds in the crust and upper mantle typically based on a 1-D pointwise inversion scheme. Here we present new methods of inverting for 3-D shear wave speed azimuthal and radial anisotropy directly from surface-wave traveltime data with the consideration of period-dependent surface wave raytracing.&#160;For the inversion of 3-D azimuthal anisotropy, our new method includes two steps: (1) inversion for the 3-D isotropic Vsv model directly from Rayleigh wave traveltime data (DSurfTomo; Fang et al., 2015, GJI); (2) joint inversion for both 3-D Vsv azimuthal anisotropy and additional 3-D isotropic Vsv perturbation. The joint inversion can significantly mitigatethe trade-off between the strong heterogeneity and azimuthal anisotropy. We apply the new method (DAzimSurfTomo) (Liu et al., 2019, JGR)to a regional array in Yunnan, southwestern China using the Rayleigh-wave phase velocity dispersion data in the period band of 5-40 s extracted from ambient noise interferometry. The obtained 3-D model of shear wave speed and azimuthal anisotropy indicates differentdeformation styles between the crust and upper mantle insouthern Yunnan.&#160;For the inversion of 3-D radial anisotropy, we presented a new inversion matrix that directly inverts Rayleigh and Love wave traveltime data jointly for 3-D Vsv and radial anisotropy parameters (Vsh/Vsv) simultaneously without intermediate steps (Hu et al., submitted to JGR). &#160;The new approach allows for adding the smoothing or model regularization terms directly on the radial anisotropy parameters, which helps to obtain more reliable radial anisotropy structures compared to the previous division approach (Vsh/Vsv) from separate inversion of Vsv and Vsh structures. We apply this new approach (DRadiSurfTomo) to the region around the eastern Himalayan syntaxis using ambient noise dispersion data (5-40s). The obtained 3-D Vs and radial anisotropy models reveals complex distribution of crustal low velocity zones and spatial variation of deformation patterns around the eastern syntaxis region.

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Fracture characterisation using frequency-dependent shear-wave splitting analysis of azimuthal anisotropy: application to fluid flow pathways at the Scanner Pockmark area, North Sea

10.5194/egusphere-egu2020-6669 ◽

2020 ◽

Author(s):

Adam Robinson ◽

Gaye Bayracki ◽

Calum MacDonald ◽

Ben Callow ◽

Giuseppe Provenzano ◽

...

Keyword(s):

Shear Wave ◽

North Sea ◽

Seismic Anisotropy ◽

Shear Wave Splitting ◽

Azimuthal Anisotropy ◽

Geophysical Data ◽

Fluid Migration ◽

Ocean Bottom ◽

Air Gun ◽

Wave Splitting

Scanner pockmark, located in the Witch Ground Graben region of the North Sea, is a ~900 m by 450 m, ~22 m-deep elliptical seafloor depression at which vigorous and persistent methane venting is observed. Previous studies here have indicated the presence of chimney structures which extend to depths of several hundred meters, and which may represent the pathways along which upwards fluid migration occurs. A proposed geometry for the crack networks associated with such chimney structures comprises a background pattern outside the chimney with unconnected vertical fractures preferentially aligned with the regional stress field, and a more connected, possibly concentric fracture system within the chimney. The measurement of seismic anisotropy using shear-wave splitting (SWS) allows the presence, orientation and density of subsurface fracture networks to be determined. If the proposed model for the fracture structure of a chimney feature is correct, we would expect, therefore, to be able to observe variations in the anisotropy measured inside and outside of the chimney.Here we test this hypothesis, using observations of SWS recorded on ocean bottom seismographs (OBS), with the arrivals generated using two different air gun seismic sources with a frequency range of ~10-200 Hz. We apply a layer-stripping approach based on observations of SWS events and shallow subsurface structures mapped using additional geophysical data to progressively determine and correct for the orientations of anisotropy for individual layers. The resulting patterns are then interpreted in the context of the chimney structure as mapped using other geophysical data. By comparing observations both at the Scanner pockmark and at a nearby reference site, we aim to further contribute to the understanding of the structures and their role in governing fluid migration. Our interpretation will additionally be informed by combining the field observations with analogue laboratory measurements and new and existing rock physics models.This work has received funding from the NERC (CHIMNEY; NE/N016130/1) and EU Horizon 2020 programme (STEMM-CCS; No.654462).

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Vertical open fractures and shear‐wave velocities derived from VSPs, full waveform acoustic logs, and televiewer data

Geophysics ◽

10.1190/1.1443467 ◽

1993 ◽

Vol 58 (6) ◽

pp. 818-834 ◽

Cited By ~ 6

Author(s):

Frédéric Lefeuvre ◽

Roger Turpening ◽

Carol Caravana ◽

Andrea Born ◽

Laurence Nicoletis

Keyword(s):

Shear Wave ◽

Azimuthal Anisotropy ◽

Stoneley Wave ◽

Correlation Technique ◽

Seismic Profiles ◽

Vertical Seismic Profiles ◽

Wave Velocities ◽

Shear Wave Velocities ◽

Full Waveform ◽

Zero Offset

Fracture or stress‐related shear‐wave birefringence (or azimuthal anisotropy) from vertical seismic profiles (VSPs) is commonly observed today, but no attempt is made to fit the observations with observed in‐situ fractures and velocities. With data from a hard rock (limestones, dolomites, and anhydrites) region of Michigan, fast and slow shear‐wave velocities have been derived from a nine‐component zero offset VSP and compared to shear‐wave velocities from two full waveform acoustic logs. To represent the shear‐wave birefringence that affects the shear wave’s vertical propagation, a propagator matrix technique is used allowing a local measurement independent of the overburden layers. The picked times obtained by using a correlation technique have been corrected in the birefringent regions before we compute the fast and slow velocities. Although there are some differences between the three velocity sets, there is a good fit between the velocities from the shear‐wave VSP and those from the two logs. We suspect the formations showing birefringence to be vertically fractured. To support this, we examine the behavior of the Stoneley wave on the full waveform acoustic logs in the formations. In addition, we analyze the borehole televiewer data from a nearby well. There is a good fit between the fractures seen from the VSP data and those seen from the borehole.

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