scholarly journals Azimuthal anisotropy beneath north central Africa from shear wave splitting analyses

2015 ◽  
Vol 16 (4) ◽  
pp. 1105-1114 ◽  
Author(s):  
Awad A. Lemnifi ◽  
Kelly H. Liu ◽  
Stephen S. Gao ◽  
Cory A. Reed ◽  
Ahmed A. Elsheikh ◽  
...  
Keyword(s):  
Shear Wave ◽  
Central Africa ◽  
Shear Wave Splitting ◽  
Azimuthal Anisotropy ◽  
North Central ◽  
Wave Splitting

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

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

<p>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.</p><p>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.</p><p>This work has received funding from the NERC (CHIMNEY; NE/N016130/1) and EU Horizon 2020 programme (STEMM-CCS; No.654462).</p>

Shear‐wave splitting in Quaternary sediments: Neotectonic implications in the central New Madrid seismic zone

Geophysics ◽  
1996 ◽  
Vol 61 (6) ◽  
pp. 1871-1882 ◽  
Author(s):  
James B. Harris
Keyword(s):  
Shear Wave ◽  
Shear Wave Splitting ◽  
Azimuthal Anisotropy ◽  
Data Matrix ◽  
New Madrid Seismic Zone ◽  
Seismic Zone ◽  
Earthquake Hazards ◽  
Near Surface ◽  
Neotectonic Deformation ◽  
Wave Splitting

Determining the extent and location of surface/near‐surface structural deformation in the New Madrid seismic zone (NMSZ) is very important for evaluating earthquake hazards. A shallow shear‐wave splitting experiment, located near the crest of the Lake County uplift (LCU) in the central NMSZ, shows the presence of near‐surface azimuthal anisotropy believed to be associated with neotectonic deformation. A shallow four‐component data set, recorded using a hammer and mass source, displayed abundant shallow reflection energy on records made with orthogonal source‐receiver orientations, an indicator of shear‐wave splitting. Following rotation of the data matrix by 40°, the [Formula: see text] and [Formula: see text] sections (principal components of the data matrix) were aligned with the natural coordinate system at orientations of N35°W and N55°E, respectively. A dynamic mis‐tie of 8 ms at a two‐way traveltime of 375 ms produced an average azimuthal anisotropy of ≈2% between the target reflector (top of Quaternary gravel at a depth of 35 m) and the surface. Based on the shear‐wave polarization data, two explanations for the azimuthal anisotropy in the study area are (1) fractures/cracks aligned in response to near‐surface tensional stress produced by uplift of the LCU, and (2) faults/fractures oriented parallel to the Kentucky Bend scarp, a recently identified surface deformation feature believed to be associated with contemporary seismicity in the central NMSZ. In addition to increased seismic resolution by the use of shear‐wave methods in unconsolidated, water‐saturated sediments, measurement of near‐surface directional polarizations, produced by shear‐wave splitting, may provide valuable information for identifying neotectonic deformation and evaluating associated earthquake hazards.

scholarly journals A potential post-perovskite province in D″ beneath the Eastern Pacific: evidence from new analysis of discrepant SKS–SKKS shear-wave splitting

2020 ◽  
Vol 221 (3) ◽  
pp. 2075-2090 ◽  
Author(s):  
Joseph Asplet ◽  
James Wookey ◽  
Michael Kendall
Keyword(s):  
Shear Wave ◽  
Seismic Anisotropy ◽  
Shear Wave Splitting ◽  
Azimuthal Anisotropy ◽  
Eastern Pacific ◽  
Core Mantle Boundary ◽  
The Core ◽  
Wave Splitting ◽  
Mantle Anisotropy ◽  
Lowermost Mantle

SUMMARY Observations of seismic anisotropy in the lowermost mantle—D″—are abundant. As seismic anisotropy is known to develop as a response to plastic flow in the mantle, constraining lowermost mantle anisotropy allows us to better understand mantle dynamics. Measuring shear-wave splitting in body wave phases which traverse the lowermost mantle is a powerful tool to constrain this anisotropy. Isolating a signal from lowermost mantle anisotropy requires the use of multiple shear-wave phases, such as SKS and SKKS. These phases can also be used to constrain azimuthal anisotropy in D″: the ray paths of SKS and SKKS are nearly coincident in the upper mantle but diverge significantly at the core–mantle boundary. Any significant discrepancy in the shear-wave splitting measured for each phase can be ascribed to anisotropy in D″. We search for statistically significant discrepancies in shear-wave splitting measured for a data set of 420 SKS–SKKS event–station pairs that sample D″ beneath the Eastern Pacific. To ensure robust results, we develop a new multiparameter approach which combines a measure derived from the eigenvalue minimization approach for measuring shear-wave splitting with an existing splitting intensity method. This combined approach allows for easier automation of discrepant shear-wave splitting analysis. Using this approach we identify 30 SKS–SKKS event–station pairs as discrepant. These predominantly sit along a backazimuth range of 260°–290°. From our results we interpret a region of azimuthal anisotropy in D″ beneath the Eastern Pacific, characterized by null SKS splitting, and mean delay time of $1.15 \, \mathrm{ s}$ in SKKS. These measurements corroborate and expand upon previous observations made using SKS–SKKS and S–ScS phases in this region. Our preferred explanation for this anisotropy is the lattice-preferred orientation of post-perovskite. A plausible mechanism for the deformation causing this anisotropy is the impingement of subducted material from the Farallon slab at the core–mantle boundary.

Estimating interval shear-wave splitting from multicomponent virtual shear check shots

Geophysics ◽  
2008 ◽  
Vol 73 (5) ◽  
pp. A39-A43 ◽  
Author(s):  
Andrey Bakulin ◽  
Albena Mateeva
Keyword(s):  
Shear Wave ◽  
Shear Wave Splitting ◽  
Azimuthal Anisotropy ◽  
Data Set ◽  
Vertical Seismic Profiling ◽  
Wave Splitting ◽  
Vsp Data ◽  
Classic Approach ◽  
A New Technique

Measuring shear-wave splitting from vertical seismic profiling (VSP) data can benefit fracture and stress characterization as well as seismic processing and interpretation. The classic approach to measuring azimuthal anisotropy at depth involves layer stripping. Its inherent weakness is the need to measure and undo overburden effects before arriving at an anisotropy estimate at depth. That task is challenging when the overburden is complex and varies quickly with depth. Moreover, VSP receivers are rarely present all the way from the surface to the target. That necessitates the use of simplistic assumptions about the uninstrumented part of the overburden that limit the quality of the result. We propose a new technique for measuring shear-wave splitting at depth that does not require any knowledge of the overburden. It is based on a multicomponent version of the virtual source method in which each two-component (2-C) VSP receiver is turned into a 2-C shear source and recorded at deeper geophones. The resulting virtual data set is affected only by the properties of the medium between the receivers. A simple Alford rotation transforms the data set into fast and slow shear virtual check shots from which shear-wave splitting can be measured easily and accurately under arbitrarily complex overburden.

Thin layers and shear‐wave splitting

Geophysics ◽  
1993 ◽  
Vol 58 (10) ◽  
pp. 1468-1480 ◽  
Author(s):  
Richard D. Slack ◽  
Daniel A. Ebrom ◽  
John A. McDonald ◽  
Robert H. Tatham
Keyword(s):  
Time Delay ◽  
Shear Wave ◽  
Shear Wave Splitting ◽  
Dominant Frequency ◽  
Azimuthal Anisotropy ◽  
Thin Layers ◽  
Anisotropic Layer ◽  
Near Surface ◽  
Wave Signal ◽  
Wave Splitting

The near‐surface weathering layer is considered by many to be strongly anisotropic. Any shear‐wave signal passing through this low‐velocity layer will inherit, to some degree, the anisotropic response of this layer. For thin weathering layers, information about previous anisotropic events may be distorted; when the thickness of this layer approaches some physically defined limit, however, a previous layer’s anisotropic signature is completely overwritten. Hodograms and Alford rotations are typically used to analyze shear‐wave splitting in the presence of azimuthal anisotropy. When the time‐delay generated by an azimuthally anisotropic layer is ⩾τ/8, where τ = one period of the wavelet’s dominant frequency, distortion of a shear‐wave signal is great enough to degrade the accuracy of the interpretation in hodogram analysis. We found that Alford rotations are superior to visual hodogram analysis when the time delay between the fast and slow shear‐waves is less than τ/8. When two azimuthally anisotropic layers with different symmetry axes exist, however, interpretations generated through both hodogram analysis and Alford rotations begin to deteriorate when the time‐delay generated by the second layer is ⩾τ/8. Recent field work has shown that the weathering layer may possess differential shear‐wave birefringence in excess of 25 percent. If we assume a dominant frequency of 40 Hz and shear‐wave velocities of [Formula: see text] and [Formula: see text], then an azimuthally anisotropic weathering layer may be as little as 5.8 m (19 ft) thick when it begins to overwrite a previous layer’s anisotropic response. When the time delay generated by a second anisotropic layer is ⩾τ (46.4 m, 152 ft thick), information about earlier anisotropic events are completely overwritten.

Solving Shear-Wave Splitting in PS Data Processing of 3D 4C OBN Seismic Survey Offshore Nunukan, Indonesia

2020 ◽  
Author(s):  
A. Waluyo
Keyword(s):  
Shear Wave ◽  
Shear Wave Splitting ◽  
Azimuthal Anisotropy ◽  
Seismic Survey ◽  
Tectonic Activity ◽  
Ocean Bottom ◽  
Survey Area ◽  
Seismic Image ◽  
Wave Splitting ◽  
Data Continuity

A 350-km2 3D 4C ocean-bottom node (OBN) offshore survey was acquired in December 2018, in Nunukan, North Kalimantan, Indonesia (Figure 1) with the processing work completed during 2019. Indonesia as a whole and the Nunukan survey area are situated in an active tectonic area where the Pacific Plate in the east and the Australian Plate in the south are actively pressing toward the Asian Plate. The tectonic activity is the main source of regional and local stress in the survey area. The dominant stress direction is Northeast-Southwest (NE-SW) (Figure 1). This stress exhibits azimuthal anisotropy in seismic waves, defined as the dependence of seismic wave speeds on propagation azimuth. In homogenous media, once the two horizontal geophone components of OBN acquisition have been properly rotated to the source-detector direction (radial) and a direction perpendicular to it (transverse), the converted waves (PS) energy will be maximum at the radial and minimum at the transverse directions. However, in azimuthal anisotropy media, shear waves split into fast and slow velocity components and the transverse data can appear to have more energy than the radial, as we observed clearly in the Nunukan OBN PS data. Ignoring the shear-wave splitting can result in degrading the PS seismic image. Herein, we outline how we addressed the azimuthal anisotropy in the Nunukan 3D OBN data. Without a shear-wave splitting correction, the PS seismic image lost data continuity of target horizons, making any attempt to correlate it with the PP image extremely difficult. The shear-wave splitting correction provided a much better PS image with improved structural data continuity and higher vertical resolution, giving greater confidence for PP-PS event correlation.

Sign in / Sign up

Export Citation Format

Share Document