Azimuthal anisotropy from OBS observations in Mahanadi offshore, India

2013 ◽  
Vol 1 (2) ◽  
pp. T187-T198 ◽  
Author(s):  
Nittala Satyavani ◽  
Mrinal K. Sen ◽  
Maheswar Ojha ◽  
Kalachand Sain
Keyword(s):  
Gas Hydrate ◽  
Seismic Anisotropy ◽  
Azimuthal Anisotropy ◽  
P Wave ◽  
Seismic Survey ◽  
Ocean Bottom Seismometer ◽  
Waveform Data ◽  
Seismic Waveform ◽  
Seismic Image ◽  
Obs Data

We have carried out an ocean bottom seismometer (OBS) survey in a grid along with multichannel seismic survey for gas hydrate exploration in the Mahanadi offshore, India. Here, we report on some interesting observations in seismic waveform data and their interpretations. These include sudden amplitude dimming in the multichannel data that is azimuth- and space-dependent and a clear manifestation of seismic anisotropy in the region. We observe significant patterns of shear wave splitting in the azimuthal gathers in the OBS data, clearly isolating the fast (S1) and slow (S2) axes of propagation in the radial azimuthal gathers. Further, amplitude nulls and amplitude maxima are observed in the transverse azimuthal gathers. These two features are diagnostic of the existence and orientation of anisotropy which is also modeled by generating full waveform synthetic seismograms. We interpret the occurrence of anisotropy to be due to the presence of fractures. The strike of this fracture set is inferred to be [Formula: see text] from the S1 and S2 orientation and variation in the P-wave amplitude with azimuth. The density of fracture network is estimated by full wave modeling of the OBS data. A good match between the synthetic and observed data is noticed for a near vertical fracture (dip angle of about 85°). The seismic image obtained from the 2D high-resolution multichannel profiles correlate well with the OBS results. Based on these analyses, we are able to delineate a fracture zone, which is linked to the near vertical faulting in the gas hydrate layers.

scholarly journals Seismic Anisotropy Within an Active Fluid Flow Structure: Scanner Pockmark, North Sea

2021 ◽  
Vol 9 ◽  
Author(s):  
G. Bayrakci ◽  
B. Callow ◽  
J. M. Bull ◽  
T. A. Minshull ◽  
G. Provenzano ◽  
...  
Keyword(s):  
Fluid Flow ◽  
North Sea ◽  
Seismic Reflection ◽  
Reference Site ◽  
Seismic Anisotropy ◽  
Stress Gradient ◽  
Azimuthal Anisotropy ◽  
Ocean Bottom ◽  
Ocean Bottom Seismometer ◽  
Seismic Reflection Data

Understanding sub-seabed fluid flow mechanisms is important for determining their significance for ocean chemistry and to define fluid pathways above sub-seafloor CO2 storage reservoirs. Many active seabed fluid flow structures are associated with seismic chimneys or pipes, but the processes linking structures at depth with the seabed are poorly understood. We use seismic anisotropy techniques applied to ocean bottom seismometer (OBS) data, together with seismic reflection profiles and core data, to determine the nature of fluid pathways in the top tens of meters of marine sediments beneath the Scanner pockmark in the North Sea. The Scanner pockmark is 22 m deep, 900 m × 450 m wide and is actively venting methane. It lies above a chimney imaged on seismic reflection data down to ∼1 km depth. We investigate azimuthal anisotropy within the Scanner pockmark and at a nearby reference site in relatively undisturbed sediments, using the PS converted (C-) waves from a GI gun source, recorded by the OBS network. Shear-wave splitting is observed on an OBS located within the pockmark, and on another OBS nearby, whereas no such splitting is observed on 23 other instruments, positioned both around the pockmark, and at an undisturbed reference site. The OBSs that show anisotropy have radial and transverse components imaging a shallow phase (55–65 ms TWT after the seabed) consistent with PS conversion at 4–5 m depth. Azimuth stacks of the transverse component show amplitude nulls at 70° and 160°N, marking the symmetry axes of anisotropy and indicating potential fracture orientations. Hydraulic connection with underlying, over pressured gas charged sediment has caused gas conduits to open, either perpendicular to the regional minimum horizontal stress at 150–160 N or aligned with a local stress gradient at 50–60 N. This study reports the first observation of very shallow anisotropy associated with active methane venting.

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>

scholarly journals OBS Data Analysis to Quantify Gas Hydrate and Free Gas in the South Shetland Margin (Antarctica)

Energies ◽  
2018 ◽  
Vol 11 (12) ◽  
pp. 3290 ◽  
Author(s):  
Sha Song ◽  
Umberta Tinivella ◽  
Michela Giustiniani ◽  
Sunny Singhroha ◽  
Stefan Bünz ◽  
...  
Keyword(s):  
Velocity Field ◽  
Wave Velocity ◽  
Gas Hydrate ◽  
P Wave ◽  
Ocean Bottom ◽  
Ocean Bottom Seismometer ◽  
S Wave ◽  
Free Gas ◽  
Hydrate Reservoir ◽  
Gas Hydrate Reservoir

The presence of a gas hydrate reservoir and free gas layer along the South Shetland margin (offshore Antarctic Peninsula) has been well documented in recent years. In order to better characterize gas hydrate reservoirs, with a particular focus on the quantification of gas hydrate and free gas and the petrophysical properties of the subsurface, we performed travel time inversion of ocean-bottom seismometer data in order to obtain detailed P- and S-wave velocity estimates of the sediments. The P-wave velocity field is determined by the inversion of P-wave refractions and reflections, while the S-wave velocity field is obtained from converted-wave reflections received on the horizontal components of ocean-bottom seismometer data. The resulting velocity fields are used to estimate gas hydrate and free gas concentrations using a modified Biot‐Geertsma‐Smit theory. The results show that hydrate concentration ranges from 10% to 15% of total volume and free gas concentration is approximately 0.3% to 0.8% of total volume. The comparison of Poisson’s ratio with previous studies in this area indicates that the gas hydrate reservoir shows no significant regional variations.

scholarly journals Imaging crustal structures through a passive seismic imaging approach in a mining area in Saxony, Germany

Solid Earth ◽  
2021 ◽  
Vol 12 (12) ◽  
pp. 2703-2715
Author(s):  
Hossein Hassani ◽  
Felix Hloušek ◽  
Stefan Buske ◽  
Olaf Wallner
Keyword(s):  
Velocity Model ◽  
Mining Area ◽  
P Wave ◽  
Seismic Survey ◽  
3D Seismic ◽  
Origin Time ◽  
Seismic Image ◽  
Passive Seismic ◽  
Microseismic Events ◽  
Image Cube

Abstract. We have used several flooding-induced microseismic events that occurred in an abandoned mining area to image geological structures close to the hypocentres in the vicinity of the mine. The events have been located using a migration-based localization approach. We used the recorded full waveforms of these localized microseismic events and have processed these passive source data as if they resulted from active sources at the known hypocentre location and origin time defined by the applied location approach. The imaging was then performed using a focusing 3D prestack depth migration approach for the secondary P-wave arrivals. The needed 3D migration velocity model was taken from a recent 3D active (controlled-source) seismic survey in that area. We observed several clear and pronounced reflectors in our obtained 3D seismic image cube, some of them related to a major fault zone in that area and some correlating well with information from the nearby mining activities. We compared our results to the 3D seismic image cube obtained directly from the 3D active seismic survey and have found new structures with our approach that were not known yet, probably because of their steep dips which the 3D active seismic survey had not illuminated. The location of the hypocentres at depth with respect to the illumination angles of those structures proved to be favourable in that case, and our 3D passive image complements the 3D active seismic image in an elegant way, thereby revealing new structures that cannot be imaged otherwise with surface seismic configurations alone.

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.

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 Imaging crustal structures through a passive seismic imaging approach in a mining area in Saxony, Germany

2021 ◽  
Author(s):  
Hossein Hassani ◽  
Felix Hloušek ◽  
Stefan Buske ◽  
Olaf Wallner
Keyword(s):  
Velocity Model ◽  
Mining Area ◽  
P Wave ◽  
Seismic Survey ◽  
3D Seismic ◽  
Origin Time ◽  
Seismic Image ◽  
Passive Seismic ◽  
Microseismic Events ◽  
Image Cube

Abstract. We have used several flooding induced microseismic events that occurred in an abandoned mining area to image geological structures close to the hypocentres in the vicinity of the mine. The events have been located using a migration-based localization approach. We used the recorded full waveforms of these localized microseismic events and have processed these passive source data as if they resulted from active sources at the known hypocentre location and origin time defined by the applied location approach. The imaging was then performed by using a focusing 3D prestack depth migration approach for the secondary P-wave arrivals. The needed 3D migration velocity model was taken from a recent 3D active (controlled-source) seismic survey in that area. We observed several clear and pronounced reflectors in our obtained 3D seismic image cube, some of them related to a major fault zone in that area and some correlating well with information from the nearby mining activities. We compared our results to the 3D seismic image cube obtained directly from the 3D active seismic survey and have found new structures with our approach that were not know yet, probably because of their steep dips which the 3D active seismic survey had not illuminated. The location of the hypocentres at depth with respect to the illumination angles of those structures proved to be favourable in that case, and our 3D passive image complements the 3D active seismic image in an elegant way thereby revealing new structures that cannot be imaged otherwise with surface seismic configurations alone.

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