Azimuthal anisotropy analysis of multiazimuth P-wave seismic data — An example from the Rock Springs Uplift, Wyoming, USA

2018 ◽  
Vol 6 (3) ◽  
pp. T649-T666 ◽  
Author(s):  
Hema S. Sharma ◽  
Subhashis Mallick ◽  
Sumit Verma ◽  
Erin Campbell
Keyword(s):  
Carbon Sequestration ◽  
Seismic Anisotropy ◽  
Waveform Inversion ◽  
Azimuthal Angle ◽  
Azimuthal Anisotropy ◽  
P Wave ◽  
Fracture Density ◽  
Fracture Orientation ◽  
Well Data

Our study area in Rock Sprigs Uplift, Wyoming, lies close to the carbon dioxide ([Formula: see text])-producing Jim Bridger power plant, and hence it is a good site for carbon sequestration. Two subsurface reservoirs within this area are being analyzed for their capability of long-term carbon storage. The presence and orientation of fractures within a reservoir and the associated seal govern the efficiency and long-term effectiveness of [Formula: see text] storage. The presence of natural fractures gives rise to seismic anisotropy that is related to the fracture orientation and density. This work analyzed P-wave multiazimuth seismic amplitude and well data from a potential carbon sequestration site for the anisotropy analysis. Using prestack waveform inversion, accurate azimuthal velocities were obtained for offset-to-angle transformation and to compute azimuthal angle gathers. These angle gathers were then stacked for each azimuth and analyzed for azimuthal anisotropy to estimate the fracture orientation and relative fracture density. Finally, by corroborating the results of the seismic azimuthal analysis with well data, it was confirmed that the results from the azimuthal analysis of the angle stacks are related to the fracture orientation and density.

Azimuthal anisotropy analysis to estimate the fracture orientation and the relative fracture density

2017 ◽  
Author(s):  
Hema Sharma ◽  
Subhashis Mallick ◽  
Sumit Verma ◽  
Erin Campbell
Keyword(s):  
Azimuthal Anisotropy ◽  
Fracture Density ◽  
Fracture Orientation

P-wave and S-wave azimuthal anisotropy at a naturally fractured gas reservoir, Bluebell‐Altamont Field, Utah

Geophysics ◽  
1999 ◽  
Vol 64 (4) ◽  
pp. 1312-1328 ◽  
Author(s):  
Heloise B. Lynn ◽  
Wallace E. Beckham ◽  
K. Michele Simon ◽  
C. Richard Bates ◽  
M. Layman ◽  
...  
Keyword(s):  
Azimuthal Anisotropy ◽  
P Wave ◽  
Fracture Density ◽  
Wave Reflections ◽  
S Wave ◽  
Gas Reservoir ◽  
Green River ◽  
Wave Data ◽  
Reflection Data ◽  
Naturally Fractured

Reflection P- and S-wave data were used in an investigation to determine the relative merits and strengths of these two data sets to characterize a naturally fractured gas reservoir in the Tertiary Upper Green River formation. The objective is to evaluate the viability of P-wave seismic to detect the presence of gas‐filled fractures, estimate fracture density and orientation, and compare the results with estimates obtained from the S-wave data. The P-wave response to vertical fractures must be evaluated at different source‐receiver azimuths (travelpaths) relative to fracture strike. Two perpendicular lines of multicomponent reflection data were acquired approximately parallel and normal to the dominant strike of Upper Green River fractures as obtained from outcrop, core analysis, and borehole image logs. The P-wave amplitude response is extracted from prestack amplitude variation with offset (AVO) analysis, which is compared to isotropic‐model AVO responses of gas sand versus brine sand in the Upper Green River. A nine‐component vertical seismic profile (VSP) was also obtained for calibration of S-wave reflections with P-wave reflections, and support of reflection S-wave results. The direction of the fast (S1) shear‐wave component from the reflection data and the VSP coincides with the northwest orientation of Upper Green River fractures, and the direction of maximum horizontal in‐situ stress as determined from borehole ellipticity logs. Significant differences were observed in the P-wave AVO gradient measured parallel and perpendicular to the orientation of Upper Green River fractures. Positive AVO gradients were associated with gas‐producing fractured intervals for propagation normal to fractures. AVO gradients measured normal to fractures at known waterwet zones were near zero or negative. A proportional relationship was observed between the azimuthal variation of the P-wave AVO gradient as measured at the tops of fractured intervals, and the fractional difference between the vertical traveltimes of split S-waves (the “S-wave anisotropy”) of the intervals.

Can Teleseismic Travel-Times Constrain 3D Anisotropic Structure in Subduction Zones? Insights from Realistic Synthetic Experiments

2020 ◽  
Author(s):  
Brandon VanderBeek ◽  
Manuele Faccenda
Keyword(s):  
Upper Mantle ◽  
Subduction Zones ◽  
Seismic Anisotropy ◽  
Joint Inversion ◽  
Azimuthal Anisotropy ◽  
P Wave ◽  
Elastic Model ◽  
Anisotropic Structure ◽  
Travel Times ◽  
Delay Times

<p>Despite the well-established anisotropic nature of Earth’s upper mantle, the influence of elastic anisotropy on teleseismic tomographic images remains largely ignored. In subduction zones, unmodeled anisotropic heterogeneity can lead to substantial isotropic velocity artefacts that may be misinterpreted as compositional heterogeneities (e.g. Bezada et al., 2016). Recent studies have demonstrated the possibility of inverting P-wave delay times for the strength and orientation of seismic anisotropy assuming a hexagonal symmetry system (e.g. Huang et al., 2015; Munzarová et al., 2018). However, the ability of P-wave delay times to constrain complex anisotropic patterns, such as those expected in subduction settings, remains unclear as the aforementioned methods are tested using ideal self-consistent data (i.e. data produced using the assumptions built into the tomography algorithm) generated from simplified synthetic models. Here, we test anisotropic P-wave imaging methods on data generated from geodynamic simulations of subduction. Micromechanical models of polymineralic aggregates advected through the simulated flow field are used to create an elastic model with up to 21 independent coefficients. We then model the teleseismic wavefield through this fully anisotropic model using SPECFEM3D coupled with AxiSEM. P-wave delay times across a synthetic seismic array are measured using conventional cross-correlation techniques and inverted for isotropic velocity and the strength and orientation of anisotropy using travel-time tomography methods. We propose and validate approximate analytic finite-frequency sensitivity kernels for the simplified anisotropic parameters. Our results demonstrate that P-wave delays can reliably recover horizontal and vertical changes in the azimuth of anisotropy. However, substantial isotropic artefacts remain in the solution when only inverting for azimuthal anisotropy parameters. These isotropic artefacts are largely removed when inverting for the dip as well as the azimuth of the anisotropic symmetry axis. Due to the relative nature of P-wave delay times, these data generally fail to reconstruct anisotropic structure that is spatially uniform over large scales. To overcome this limitation, we propose a joint inversion of SKS splitting intensity with P-wave delay times. Preliminary results demonstrate that this approach improves the recovery of the magnitude and azimuth of anisotropy. We conclude that teleseismic P-wave travel-times are a useful observable for probing the 3D distribution of upper mantle anisotropy and that anisotropic inversions should be explored to better understand the nature of isotropic velocity anomalies in subduction settings.</p>

Fracture detection in a carbonate reservoir using a variety of seismic methods

Geophysics ◽  
1999 ◽  
Vol 64 (4) ◽  
pp. 1266-1276 ◽  
Author(s):  
Maria A. Pérez ◽  
Vladimir Grechka ◽  
Reinaldo J. Michelena
Keyword(s):  
Structural Changes ◽  
Horizontal Stress ◽  
Azimuthal Anisotropy ◽  
Carbonate Reservoir ◽  
P Wave ◽  
Regional Trend ◽  
Wave Data ◽  
Fracture Orientation ◽  
Geological Conditions ◽  
Maximum Horizontal Stress

We combine various methods to estimate fracture orientation in a carbonate reservoir located in southwest Venezuela. The methods we apply include the 2-D rotation analysis of 2-D P-S data along three different azimuths, amplitude‐variation‐with‐offset (AVO) of 2-D P-wave data along the same three azimuths, normal‐moveout (NMO) analysis of the same 2-D data, and both 3-D azimuthal AVO and NMO analysis of 3-D P-wave data recorded in the same field. The results of all methods are compared against measures of fracture orientation obtained from Formation microScanner logs recorded at four different locations in the field, regional and local measures of maximum horizontal stress, and the alignment of the major faults that cross the field. P-S data yield fracture orientations that follow the regional trend of the maximum horizontal stress, and are consistent with fracture orientations measured in the wells around the carbonate reservoir. Azimuthal AVO analysis yields a similar regional trend as that obtained from the P-S data, but the resolution is lower. Local variations in fracture orientation derived from 3-D AVO show good correlation with local structural changes. In contrast, due to the influence of a variety of factors, including azimuthal anisotropy and lateral heterogeneity in the overburden, azimuthal NMO analysis over the 3-D P-wave data yields different orientations compared to those obtained by other methods. It is too early to say which particular method is more appropriate and reliable for fracture characterization. The answer will depend on factors that range from local geological conditions to additional costs for acquiring new information.

scholarly journals Characterizing a decametre-scale granitic reservoir using GPR and seismic methods – A case study for preparing hydraulic stimulations

2020 ◽  
Author(s):  
Joseph Doetsch ◽  
Hannes Krietsch ◽  
Cedric Schmelzbach ◽  
Mohammadreza Jalali ◽  
Valentin Samuel Gischig ◽  
...  
Keyword(s):  
Rock Mass ◽  
Seismic Anisotropy ◽  
Shear Zones ◽  
Velocity Model ◽  
P Wave ◽  
Fracture Density ◽  
Ductile Shear Zones ◽  
Ductile Shear ◽  
Geophysical Characterization

Abstract. Ground-penetrating radar (GPR) and seismic imaging have proven to be important tools for the characterization of rock volumes. Both methods provide information about the physical rock mass properties and geology structures away from boreholes or tunnel walls. Here, we present the results from a geophysical characterization campaign that was conducted in preparation for a decametre-scale hydraulic stimulation experiment in the crystalline rock volume at the Grimsel Test Site (Central Switzerland). For this characterization experiment, we used tunnel based GPR reflection imaging as well as seismic traveltime tomography to investigate the volumes between several tunnels and boreholes. The interpretation of the GPR data with respect to geological structures is based on the unmigrated and migrated images. For the tomographic analysis of the seismic first-arrival traveltime data, we inverted for an anisotropic velocity model described by the Thomsen parameters v0, ϵ and δ to account for the rock mass foliation. Subsequently, the GPR and seismic images were interpreted in combination with the geological model of the test volume and the known in-situ stress states. We found that the ductile shear zones are clearly imaged by GPR and show an increase in seismic anisotropy due to a stronger foliation, while they are not visible in the P-wave (v0) velocity model. Regions of decreased seismic p-wave velocity, however, correlate with regions of high fracture density. For geophysical characterization of potential deep geothermal reservoirs, our results imply that wireline compatible borehole GPR should be considered for shear zone characterization, and that seismic anisotropy and velocity information are desirable to acquire in order to gain information about ductile shear zones and fracture density, respectively.

scholarly journals An analytic model for the stress-induced anisotropy of dry rocks

Geophysics ◽  
2011 ◽  
Vol 76 (3) ◽  
pp. WA125-WA133 ◽  
Author(s):  
Boris Gurevich ◽  
Marina Pervukhina ◽  
Dina Makarynska
Keyword(s):  
Seismic Anisotropy ◽  
Uniaxial Stress ◽  
Azimuthal Anisotropy ◽  
P Wave ◽  
Analytic Model ◽  
Induced Anisotropy ◽  
S Wave ◽  
Isotropic Rock ◽  
Model Predictions ◽  
Stress Induced Anisotropy

One of the main causes of azimuthal anisotropy in sedimentary rocks is anisotropy of tectonic stresses in the earth’s crust. We have developed an analytic model for seismic anisotropy caused by the application of a small anisotropic stress. We first considered an isotropic linearly elastic medium (porous or nonporous) permeated by a distribution of discontinuities with random (isotropic) orientation (such as randomly oriented compliant grain contacts or cracks). The geometry of individual discontinuities is not specified. Instead, their behavior is defined by a ratio B of the normal to tangential excess compliances. When this isotropic rock is subjected to a small compressive stress (isotropic or anisotropic), the number of cracks along a particular plane is reduced in proportion to the normal stress traction acting on that plane. This effect is modeled using the Sayers-Kachanov noninteractive approximation. The model predicts that such anisotropic crack closure yields elliptical anisotropy, regardless of the value of the compliance ratio B. It also predicts the ratio of Thomsen’s anisotropy parameters [Formula: see text] as a function of the compliance ratio B and Poisson’s ratio of the unstressed rock. A comparison of the model predictions with the results of laboratory measurements indicates a reasonable agreement for moderate magnitudes of uniaxial stress (as high as 30 MPa). These results can be used for differentiating stress-induced anisotropy from that caused by aligned fractures. Conversely, if the cause of anisotropy is known, then the anisotropy pattern allows one to estimate P-wave anisotropy from S-wave anisotropy.

scholarly journals Imaging upper mantle anisotropy with teleseismic P-wave delays: insights from tomographic reconstructions of subduction simulations

2021 ◽  
Vol 225 (3) ◽  
pp. 2097-2119
Author(s):  
Brandon P VanderBeek ◽  
Manuele Faccenda
Keyword(s):  
Subduction Zone ◽  
Upper Mantle ◽  
Large Scale ◽  
Seismic Anisotropy ◽  
Azimuthal Anisotropy ◽  
P Wave ◽  
Zone Model ◽  
Upper Mantle Anisotropy ◽  
Delay Times ◽  
Mantle Anisotropy

SUMMARY Despite the well-established anisotropic nature of Earth’s upper mantle, the influence of elastic anisotropy on teleseismic P-wave imaging remains largely ignored. Unmodelled anisotropic heterogeneity can lead to substantial isotropic velocity artefacts that may be misinterpreted as compositional heterogeneities. Recent studies have demonstrated the possibility of inverting P-wave delay times for the strength and orientation of seismic anisotropy. However, the ability of P-wave delay times to constrain complex anisotropic patterns, such as those expected in subduction settings, remains unclear as synthetic testing has been restricted to the recovery of simplified block-like structures using ideal self-consistent data (i.e. data produced using the assumptions built into the tomography algorithm). Here, we present a modified parametrization for imaging arbitrarily oriented hexagonal anisotropy and test the method by reconstructing geodynamic simulations of subduction. Our inversion approach allows for isotropic starting models and includes approximate analytic finite-frequency sensitivity kernels for the simplified anisotropic parameters. Synthetic seismic data are created by propagating teleseismic waves through an elastically anisotropic subduction zone model created via petrologic-thermomechanical modelling. Delay times across a synthetic seismic array are measured using conventional cross-correlation techniques. We find that our imaging algorithm is capable of resolving large-scale features in subduction zone anisotropic structure (e.g. toroidal flow pattern and dipping fabrics associated with the descending slab). Allowing for arbitrarily oriented anisotropy also results in a more accurate reconstruction of isotropic slab structure. In comparison, models created assuming isotropy or only azimuthal anisotropy contain significant isotropic and anisotropic imaging artefacts that may lead to spurious interpretations. We conclude that teleseismic P-wave traveltimes are a useful observable for probing the 3-D distribution of upper mantle anisotropy and that anisotropic inversions should be explored to better understand the nature of isotropic velocity anomalies particularly in subduction settings.

scholarly journals Petrophysical constraints on the seismic properties of the Kaapvaal craton mantle root

Solid Earth ◽  
2014 ◽  
Vol 5 (1) ◽  
pp. 45-63 ◽  
Author(s):  
V. Baptiste ◽  
A. Tommasi
Keyword(s):  
Seismic Anisotropy ◽  
Azimuthal Anisotropy ◽  
Mantle Xenoliths ◽  
P Wave ◽  
Kaapvaal Craton ◽  
Compositional Heterogeneity ◽  
S Wave ◽  
Seismic Properties ◽  
Seismological Data ◽  
Cratonic Mantle

Abstract. We calculated the seismic properties of 47 mantle xenoliths from 9 kimberlitic pipes in the Kaapvaal craton based on their modal composition, the crystal-preferred orientations (CPO) of olivine, ortho- and clinopyroxene, and garnet, the Fe content of olivine, and the pressures and temperatures at which the rocks were equilibrated. These data allow constraining the variation of seismic anisotropy and velocities within the cratonic mantle. The fastest P and S2 wave propagation directions and the polarization of fast split shear waves (S1) are always subparallel to olivine [100] axes of maximum concentration, which marks the lineation (fossil flow direction). Seismic anisotropy is higher for high olivine contents and stronger CPO. Maximum P wave azimuthal anisotropy (AVp) ranges between 2.5 and 10.2% and the maximum S wave polarization anisotropy (AVs), between 2.7 and 8%. Changes in olivine CPO symmetry result in minor variations in the seismic anisotropy patterns, mainly in the apparent isotropy directions for shear wave splitting. Seismic properties averaged over 20 km-thick depth sections are, therefore, very homogeneous. Based on these data, we predict the anisotropy that would be measured by SKS, Rayleigh (SV) and Love (SH) waves for five endmember orientations of the foliation and lineation. Comparison to seismic anisotropy data from the Kaapvaal shows that the coherent fast directions, but low delay times imaged by SKS studies, and the low azimuthal anisotropy with with the horizontally polarized S waves (SH) faster than the vertically polarized S wave (SV) measured using surface waves are best explained by homogeneously dipping (45°) foliations and lineations in the cratonic mantle lithosphere. Laterally or vertically varying foliation and lineation orientations with a dominantly NW–SE trend might also explain the low measured anisotropies, but this model should also result in backazimuthal variability of the SKS splitting data, not reported in the seismological data. The strong compositional heterogeneity of the Kaapvaal peridotite xenoliths results in up to 3% variation in density and in up to 2.3% variation of Vp, Vs, and Vp / Vs ratio. Fe depletion by melt extraction increases Vp and Vs, but decreases the Vp / Vs ratio and density. Orthopyroxene enrichment due to metasomatism decreases the density and Vp, strongly reducing the Vp / Vs ratio. Garnet enrichment, which was also attributed to metasomatism, increases the density, and in a lesser extent Vp and the Vp / Vs ratio. Comparison of density and seismic velocity profiles calculated using the xenoliths' compositions and equilibration conditions to seismological data in the Kaapvaal highlights that (i) the thickness of the craton is underestimated in some seismic studies and reaches at least 180 km, (ii) the deep sheared peridotites represent very local modifications caused and oversampled by kimberlites, and (iii) seismological models probably underestimate the compositional heterogeneity in the Kaapvaal mantle root, which occurs at a scale much smaller than the one that may be sampled seismologically.

scholarly journals Petrophysical constraints on the seismic properties of the Kaapvaal craton mantle root

2013 ◽  
Vol 5 (2) ◽  
pp. 963-1005 ◽  
Author(s):  
V. Baptiste ◽  
A. Tommasi
Keyword(s):  
Seismic Anisotropy ◽  
Seismic Velocity ◽  
Body Wave ◽  
Azimuthal Anisotropy ◽  
Mantle Xenoliths ◽  
P Wave ◽  
Kaapvaal Craton ◽  
Compositional Heterogeneity ◽  
P Waves ◽  
Seismic Properties

Abstract. We calculated the seismic properties of 47 mantle xenoliths from 9 kimberlitic pipes in the Kaapvaal craton based on their modal composition, the crystal preferred orientations (CPO) of olivine, ortho- and clinopyroxene, and garnet, the Fe content of olivine, and the pressures and temperatures at which the rocks were equilibrated. These data allow constraining the variation of seismic anisotropy and velocities with depth. The fastest P wave and fast split shear wave (S1) polarization direction is always close to olivine [100] maximum. Changes in olivine CPO symmetry result in minor variations in the seismic anisotropy patterns. Seismic anisotropy is higher for high olivine contents and stronger CPO. Maximum P waves azimuthal anisotropy (AVp) ranges between 2.5 and 10.2% and S waves polarization anisotropy (AVs) between 2.7 and 8%. Seismic properties averaged in 20 km thick intervals depth are, however, very homogeneous. Based on these data, we predict the anisotropy that would be measured by SKS, Rayleigh (SV) and Love (SH) waves for 5 end-member orientations of the foliation and lineation. Comparison to seismic anisotropy data in the Kaapvaal shows that the coherent fast directions, but low delay times imaged by SKS studies and the low azimuthal anisotropy and SH faster than SV measured using surface waves may only be consistently explained by dipping foliations and lineations. The strong compositional heterogeneity of the Kaapvaal peridotite xenoliths results in up to 3% variation in density and in up to 2.3% of variation Vp, Vs and the Vp/Vs ratio. Fe depletion by melt extraction increases Vp and Vs, but decreases the Vp/Vs ratio and density. Orthopyroxene enrichment decreases the density and Vp, but increases Vs, strongly reducing the Vp/Vs ratio. Garnet enrichment increases the density, and in a lesser manner Vp and the Vp/Vs ratio, but it has little to no effect on Vs. These compositionally-induced variations are slightly higher than the velocity perturbations imaged by body-wave tomography, but cannot explain the strong velocity anomalies reported by surface wave studies. Comparison of density and seismic velocity profiles calculated using the xenoliths' compositions and equilibrium conditions to seismological data in the Kaapvaal highlights that: (i) the thickness of the craton is underestimated in some seismic studies and reaches at least 180 km, (ii) the deep sheared peridotites represent very local modifications caused and oversampled by kimberlites, and (iii) seismological models probably underestimate the compositional heterogeneity in the Kaapvaal mantle root, which occurs at a scale much smaller than the one that may be sampled seismologically.

scholarly journals Characterizing a decametre-scale granitic reservoir using ground-penetrating radar and seismic methods

Solid Earth ◽  
2020 ◽  
Vol 11 (4) ◽  
pp. 1441-1455 ◽  
Author(s):  
Joseph Doetsch ◽  
Hannes Krietsch ◽  
Cedric Schmelzbach ◽  
Mohammadreza Jalali ◽  
Valentin Gischig ◽  
...  
Keyword(s):  
Ground Penetrating Radar ◽  
Seismic Anisotropy ◽  
Shear Zones ◽  
Velocity Model ◽  
P Wave ◽  
Fracture Density ◽  
Geological Structures ◽  
Ductile Shear Zones ◽  
Ground Penetrating

Abstract. Ground-penetrating radar (GPR) and seismic imaging have proven to be important tools for the characterization of rock volumes. Both methods provide information about the physical rock mass properties and geological structures away from boreholes or tunnel walls. Here, we present the results from a geophysical characterization campaign that was conducted as part of a decametre-scale hydraulic stimulation experiment in the crystalline rock volume of the Grimsel Test Site (central Switzerland). For this characterization experiment, we used tunnel-based GPR reflection imaging as well as seismic travel-time tomography to investigate the volumes between several tunnels and boreholes. The interpretation of the GPR data with respect to geological structures is based on the unmigrated and migrated images. For the tomographic analysis of the seismic first-arrival travel-time data, we inverted for an anisotropic velocity model described by the Thomsen parameters v0, ϵ and δ to account for the rock mass foliation. Subsequently, the GPR and seismic images were interpreted in combination with the geological model of the test volume and the known in situ stress states. We found that the ductile shear zones are clearly imaged by GPR and show an increase in seismic anisotropy due to a stronger foliation, while they are not visible in the p-wave (v0) velocity model. Regions of decreased seismic p-wave velocity, however, correlate with regions of high fracture density. For geophysical characterization of potential deep geothermal reservoirs, our results imply that wireline-compatible borehole GPR should be considered for shear zone characterization, and that seismic anisotropy and velocity information are desirable to acquire in order to gain information about ductile shear zones and fracture density, respectively.

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